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31 (261) Susana Barrios From:Rick Moyer <dr.rickmoyer@gmail.com> Sent:Monday, To:Public Comment; Nicholas J. Taylor; Kristin Pelletier Subject:\[EXTERNAL\] Local landslide information Attachments:E and S 1996 Report Volume 1.pdf; Santiago_Landslide 2005 Study GRABEN.pdf Some people who received this message don't often get email from Learn why this is important Warning: This email originated from outside the City of Anaheim. Do not click links or open attachments unless you recognize the sender and are expecting the message. Dear Mayor Aitken and members of the City Council: According to the Deer Canyon Geology and Soils Report, "There have been historic landslides within eastern Anaheim. In 1993, the Santiago Landslide occurred following a major El Nino rain event a bluff slid and prompted the evacuation of dozens of families. This event destroyed over 30 homes and impacted over 200 other structures in the vicinity". Unfortunately this is a gross misrepresentation of the Santiago Landslide which actually began in 1992. It is incomprehensible that such a misrepresentation would occur in this day and age when there is so much information available about the Santiago Landslide. This misrepresentation makes the entire Soils report suspect. For example, in 2023 the City of Anaheim paid for an updated geotechnical report of the Santiago landslide by Tetra-Tech. According to their engineers the landslide almost reactivated a couple of years ago. They estimate that if our dewatering system shuts down (the current assessment sundowns and we will run out of money in 2025), it will take about 18 months before we have another landslide. Below is a link to their report. This report is also available on a website dedicated to the landslide that the City maintains. Search Anaheim.net/SGHAD. The City of Anaheim is very involved with our landslide issue and Mr. Emami personally attends most of the board meetings. SGHAD Geotechnical Evaluation and Groundwater Impact Web 7-18-23.pdf The main SGHAD website is www.santiagoghad.org. It has a lot of information, a good GIS map and is up to date. There is a link to a video created by one of the homeowners a short time following the landslide. In 1996 Eberhart and Stone published a 7 volume report about the Santiago landslide. I've attached Volume 1 which provides great detail about the landslide and local geology, the other volumes contain maps and technical information and are available along with many other documents on the SGHAD website. An interesting study about the landslide was published in 2005 for a professional journal, noting the presence of a graben. The authors reviewed old aerial photos of the area (pre-development) and noted the presence of the ancient landslide that was involved with the Santiago landslide (bottom right of page 12). It might be worth investigating whether or not the geotechnical consultants for the Deer Canyon project reviewed aerial photography as part of their assessment. Had this been done by the pre- 1 development consultants in our area further up the hill, the City would have either not allowed any development or mandated certain mitigation efforts to stabilize the earth prior to development. ENGEO is the SGHAD manager, the President is Uri Eliahu (ueliahu@engeo.com). Best to all, Rick. -- Dr. Rick Moyer 2 The Santiago Landslide and Associated Ridge-top Graben (Sackungen): Implications for Paleoseismic Landslide Studies PHILIP L. JOHNSON WILLIAM R. COTTON Cotton, Shires & Associates, 330 Village Lane, Los Gatos, CA 95030 Key Terms:Ridge-top Graben, Sackungen, Landslide, Paleoseismology, Rainfall ABSTRACT Some recent paleoseismic studies have focused on datingridge-topgrabendepositstoevaluatethetiming of paleoseismic events. By contrast, our study of the Santiago landslide demonstrates that ridge-top gra- bens also can be associated with aseismic, deep-seated landsliding. The Santiago landslide in Anaheim Hills, California, failed during the winter of 1992–1993 in response to elevated groundwater conditions asso- ciated with intense rainfall. The head of the active landslide included a zone of extensional deformation along the bounding ridgeline. Interpretation of histor- ical, aerial photographs indicates that the active landslide is a re-activated, ancient, deep-seated, trans- lational landslide and an associated ridge-top graben. Large-diameter borings within the ridge-top graben encountered thick colluvium, steeply dipping colluvi- um-ﬁlled fractures, and shears with normal offsets. In contrast to the rupture surface within the central part of the landslide, the basal rupture surfaces in the graben area had signiﬁcantly less gouge. We interpret this contrast in gouge development as an indication that the ridge-top graben developed later than the original landslide by upslope progression of the deformation. Our limit-equilibrium, slope-stability analyses indicate that either high groundwater or seismicgroundmotioncouldhavepreviouslyactivated the ancient landslide and ridge-top graben. Because colluvial deposits preserved within the ridge-top graben and produced by these two different types of triggeringeventscouldbemisinterpretedasrepresent- ing the late Quaternary paleoseismic record, these features are not useful for paleoseismic studies unless aseismic activation can be clearly precluded. INTRODUCTION The Santiago landslide, located in the Anaheim Hills area of the northern Santa Ana Mountains, California (Figure 1), is an active landslide that produced extensional deformation within the adjacent part of the upslope bounding ridgeline. Initial movement of the landslide causedminorcracksinroadsurfacesduring1992(Barrows et al., 1993). This was followed in January 1993 by major episodes of landslide movement following intense rainfall in December 1992 and January 1993 (Slosson and Larson, 1995). Initial investigations (McLarty and Lancaster, 1999a) concluded that elevated groundwater conditions triggered landslide movement and that the maximum displacement was approximately 1 ft (0.3 m). A zone of extensionalground cracks wasmappedalongthe ridgeline at the head of the landslide in January 1993; these ground- crackdatawereincorporatedintoourengineeringgeologic map (Figure 2). Movementof the landslide andopening of the associated ridge-top graben occurred during a seismi- cally quiescent period when groundwater levels were ele- vated (Barrows et al., 1993). Thus, re-activation was related to groundwater conditions associated with intense rainfall rather than strong seismic shaking. There are several different interpretations for the origin of ridge-top grabens (sackungen). Some have hypothe- sized that sackungen develop by slow, gravitational defor- mation of ridgelines (Tabor, 1971; Varnes et al., 1989; Bovis and Evans, 1995; McCalpin and Irvine, 1995; and Thompson, 1997). Others have interpreted sackungen de- velopment as a response to loss of buttressing and to stress relief associated with late Pleistocene deglaciation (Bovis, 1982; Agliardi et al., 2001; Kellogg, 2001a; and Smith, 2001). A third hypothesis is that these grabens open in response to strong seismic shaking and ridge-top shatter (Beck, 1968; Clague, 1979; Wallace, 1984; Morton and Sadler, 1989; and Kellogg, 2001b). One argument for the seismic origin of sackungen is the abundance of these features in some regions with high rates of seismic activity (Radbruch-Hall, 1978; Hart, 2001). Several studies in the SantaCruzMountainsofnorthernCaliforniafollowingthe 1989 Loma Prieta earthquake (Ponti and Wells, 1991; NolanandWeber,1998)reportedapparent,activeopening of sackungen in response to strong seismic shaking. McCalpin (1999) trenched across a ridge-top graben in central Nevada that was active during two historical Environmental & Engineering Geoscience, Vol. XI, No. 1, February 2005, pp. 5–15 5 earthquakes and found evidence of four prehistoric, graben-openingeventsthattheauthorinterpretedaspaleo- seismic in origin. McCalpin and Hart (2001) interpreted sackungendepositsintheSanGabrielMountainsofsouth- ern California as paleoseismic in origin and compared graben-opening events with paleoseismic events recorded at nearby fault trench sites. Jibson(1996)suggestedthatsackungenmightbeuseful as paleoseismic sites only if landsliding that resulted from highgroundwatercanbeanalyticallyprecluded.However, few studies have analytically demonstrated a seismic origin for these features. If ridge-top grabens in seismi- callyactiveregionsdevelopandactivatesolelyinresponse to strong seismic ground-shaking, the in-ﬁll deposits would indeed provide a paleoseismic record. However, if these features also activate by aseismic landsliding resulting from elevated groundwater, then their use in paleoseismic studies would be severely limited. GEOLOGIC SETTING The geology of the Anaheim Hills area is characterized by a northward-dipping section of sandstone and siltstone oftheMiocene-agePuenteFormation(Schoellhameretal., 1981). Our ﬁeld mapping of the Santiago landslide and adjacent parts of the Anaheim Hills indicates that bedding dips range from 78 to 258 to the north, and strikes range from northeast to northwest (Figure 2). The Santiago landslideapparently failedalongasurface alignedroughly parallel or sub-parallel to bedding within the Puente Formation. The Santiago landslide occurred within the Soquel Member and uppermost part of the La Vida Member of the Puente Formation. The sandstone of the Soquel Member consists of multiple, ﬁning-upward sequences of very coarse- to medium-grained sandstone that is poorly cemented and weak; the sandstone is locally interbedded with siltstone. The siltstone of the underlying La Vida Member is interbedded with very thin beds of ﬁne- grained, ripple-laminated sandstone. These rocks were deposited in a submarine fan environment within the rapidly subsiding Los Angeles basin during Miocene time (Critelli et al., 1995; Bjorklund et al., 2002). Beginning in Pliocene time, compressional uplift of the Santa Ana Mountains (Gath and Grant, 2003) produced tilting of the Tertiary sedimentary section in the Anaheim Hills area. Recent mapping of a series of ﬂuvial terraces directlywestoftheAnaheimHillsshowedthatupliftofthe Figure 1. The Santiago landslide is located in the northern Santa Ana Mountains of southern California. 1 ft ¼0.3048 m. Figure 2. Geologic map of the Santiago landslide and surrounding region. Base map shows the topography during 1993 with elevations in feet above mean sea level. 1 ft ¼0.3048 m. ! Johnson and Cotton Environmental & Engineering Geoscience, Vol. XI, No. 1, February 2005, pp. 5–156 The Santiago Landslide: Implications for Paleoseismic Landslide Studies Environmental & Engineering Geoscience, Vol. XI, No. 1, February 2005, pp. 5–15 7 Santa Ana Mountains has continued into Quaternary time (Gath and Grant, 2002). It is hypothesized that blind thrust faults are responsible for this active uplift, and that these faults might produce large-magnitude earthquakes. Strong seismic shaking in the Anaheim Hills area results primarily from earthquakes on nearby strike-slip and thrust faults. Major earthquakes on the Elsinore and Whittier faults are capable of producing peak ground accelerations in the range of 0.39 to 0.46 g in the Anaheim Hills (Boore et al., 1997). The Elsinore fault has a recurrence interval for large-magnitude (ground-ruptur- ing)earthquakesofapproximately200years(Treimanand Lundberg, 2003). The Whittier fault has a recurrence interval of 760 (þ640,274) years (Working Group on California Earthquake Probabilities, 1995). Earthquakes on nearby blind thrust faults, such as the Puente Hills blind thrust system (Shaw et al., 2002), can produce peak ground accelerations in the range of 0.2 to 0.3 g in the Anaheim Hills. Dolan et al. (2003) identiﬁed at least four large-magnitude(Mw 7.2to7.5),Holoceneearthquakeson the Puente Hills system. Because strong seismic ground shaking likely affected the Anaheim Hills area repeatedly during late Quaternary time, it could have contributed to landsliding and opening of the ridge-top graben. Mass grading of the Anaheim Hills area during the 1970s ﬁlled drainage valleys and excavated spur ridges to developlevelbuildingpadsandroadsforresidentialdevel- opment.ThetoeoftheSantiagolandslidelieswithinapart ofthedevelopmentwherethetopographywashighlymod- iﬁed by grading. Although less-extensive grading was completed along the northeast-trending ridgeline at the head of the landslide, the subtlest geomorphic features were obliterated or substantially altered. GEOMORPHIC EVALUATION We evaluated the pre-development geomorphology by interpreting stereo pairs of historical aerial photographs. Drainage development and incision followed the uplift of the Anaheim Hills area, and several large, deep landslides failed into the incised valleys. These landslides are depicted on our photogeologic map (Figure 3). Over time, erosion and drainage incision modiﬁed the morphology of these deep landslides. The upper part of one of these Quaternary landslides (landslide A in Figure 3) coincides with the lower part of the modern Santiago landslide. Drainage incision has dissected the body of the ancient landslide and partially obscured the morphology. However, the dissected head scarp of the ancient landslide is still clearly evident. Directly southeast of the head scarp of landslide A, a well-developed graben with a prominent, northwest- facing scarp and more-subdued, southeast-facing scarp crosses the ridge obliquely. Between these scarps is an elongate depression that forms the axis of the graben and has a dark appearance on the aerial photographs. We interpret the dark tones within the graben as evidence of lush vegetation, perhaps grasses, that ﬂourished in the thick colluvium that ﬁlled the depression. This contrasts with the sparser vegetation on the surrounding parts of the ridge where the soil is thin. The head of the Santiago landslide correlates closely with the ridge-top graben that is visible in the historical aerial photographs. Based on the map relationships with landslide A and the ridge-top graben, we hypothesize that the graben developed as a result of the upslope progres- sion of landsliding during late Quaternary time. SUBSURFACE INVESTIGATION OF THE SANTIAGO LANDSLIDE We conducted a subsurface investigation consisting of downhole logging of 18 large-diameter bucket auger borings drilled within the head, toe, and body of the Santiago landslide and within adjacent areas off the land- slide. The boring locations are shown in Figure 2. The borings drilled within the body and toe of the landslide encountered a basal rupture surface with a well-developed gouge bounded by highly polished, striated surfaces (Figure 4). The thickness of the basal rupture gouge in these borings ranges from 0.1 to 3 ft (0.03 to 0.9 m). By contrast, the borings within the ridge-top graben encountered a basal rupture surface with less gouge development; the thickness of the observed clay gouge ranges from 0.1 to 1 in. (0.25 to 2.5 cm). In the ridge-top graben area, numerous open fractures, colluvium-ﬁlled fractures, and steeply dipping shears with normal offsets were encountered above the basal rupture surface. The log of boring LD-3 (Figure 5) provides a good example of the geology exposed within borings in the ridge-top graben area. In the upper 3 ft (0.9 m), the boring encountered artiﬁcial ﬁll placed during mass grading. Belowtheﬁllisan11ft(3.4m)thickdepositofcolluvium. Below the colluvium, the sandstone has abundant open fractures and colluvium-ﬁlled fractures that widen up- ward;oneofthesefractureshasadistinctnormaloffset.At a depth of 35 ft (10.7 m), the polished and striated basal rupture surface of the landslide has a relatively thin, 0.1 to 1.0 in. (0.25 to 2.5 cm) thick, clay gouge. In boring LD-2, we encountered thick colluvium and colluvium-ﬁlled fractures up to 1.5 ft (0.5 m) wide that extend to a depth of approximately 24 ft (7.3 m). These fractures strike roughly parallel to the ridge-top graben. The fractures in the ridge-top graben area apparently ﬁlled with colluvium after earlier graben- opening events. RAINFALL AND GROUNDWATER The Santiago landslide and associated ridge-top graben failed during a period of intense rainfall during December Johnson and Cotton Environmental & Engineering Geoscience, Vol. XI, No. 1, February 2005, pp. 5–158 1992 and January 1993. Fifteen inches (38 cm) of rain, 102% of the 14.7-in. (37.3-cm) average annual rainfall for Orange County, fell during those two months (Figure 6). Rainfall during the previous year was also above average andundoubtedlycontributedtoelevatedgroundwatercon- ditions. Figure 6 shows the yearly rainfall record for two nearby rainfall stations. Rainfall during the winter of 1992–1993 was exceptionally high when compared with the rainfall record from preceding years. Piezometer data from Eberhart and Stone (1996) indicate that groundwater levels were elevated within the landslide mass and surrounding area at the time of failure (Figure 7). Subsequent installation of dewatering wells and horizontal drains has lowered groundwater Figure 3. A photogeologic map of ancient landslides in the vicinity of the Santiago landslide. Topographic base shows conditions before mass grading. The elevations are in feet above mean sea level. 1 ft ¼0.3048 m. The Santiago Landslide: Implications for Paleoseismic Landslide Studies Environmental & Engineering Geoscience, Vol. XI, No. 1, February 2005, pp. 5–15 9 levels and substantially improved the stability of the landslide mass (McLarty and Lancaster, 1999b). The Santiago Landslide has not moved since completion of the dewatering system. Little historical data are available regarding the groundwater conditions in the Anaheim Hills area before development. Early geotechnical investigations did not include installation of piezometers to evaluate the ground- water levels. Exploratory borings drilled before develop- mentwerefew,widelyspaced,andgenerallyshallow;most did not encounter groundwater. However, earlier inves- tigationsoftheSantaAnaMountainsshowedspringsinthe area of landslide A (Schoellhamer et al., 1954). Permeable strata that are exposed in the vicinity of the Santiago landslide can be traced up-dip through the subsurface to a south-facing, anti-dip slope where recharge occurs. If landslides and associated ridge-top graben in the Anaheim Hills area are currently being activated during wet winters (such as 1992–1993), then it is highly likely thattheywouldhavebeenactivatedduringtheevenwetter periods of late Quaternary time. Several paleoclimatic studies conducted in southern California have found evi- dence of wet periods during late Pleistocene and Holocene time. Templeton (1964, described in Stout, 1977) evalu- ated the latest Pleistocene rainfall history of southern Cal- ifornia using dendrochronologic analysis of cypress samplesrecoveredfromtheLa Breatar pitsandconcluded that average annual precipitation during a late Pleistocene wet period (14.90 ka to 14.89 ka) ranged from two to ﬁve times the current average annual rainfall for Los Angeles. Quade et al. (2003) studied wetland deposits in southern Nevada and found evidence for three late Pleistocene- to-Holocenewetperiods,datedat ,26.3to16.4ka,14.5to 12.3 ka, and 11.6 to 9.5 ka, when groundwater recharge and discharge from desert springs was high. Miller et al. (2001) studied fan development of debris ﬂow at Silurian Lake in the Mojave desert and found evidence of a wet period between 6.5 and 6.3 ka. Owen et al. (2003) dated latero-frontalmorainesintheSanBernardinoMountainsof southern California and found four glacial advances dated at 20 to 18 ka, 16 to 15 ka, 13 to 12 ka, and 9 to 5 ka; the authors further concluded that these glacial advances occurred during periods of increased winter precipitation and decreased summer temperatures. Clearly, the late Quaternary climate included wet periods when ground- water recharge rates were relatively high, increasing the probabilityofaseismicactivationofdeep-seatedlandslides and ridge-top grabens. LIMIT-EQUILIBRIUM ANALYSES Although the late Quaternary paleohydrologic and paleoseismic conditions that resulted in the upslope progression and graben development are not known directly from this study, we evaluated the contribution of both strong seismic shaking and elevated groundwater by performing limit-equilibrium slope-stability analysis on cross section A-A9. Speciﬁcally, we used the pre-grading proﬁle and modeled landslide A and the ridge-top graben as a single block with a tension crack at the upslope end. These analyses were performed using three different groundwater levels (Figure 8). The highest groundwater Figure 4. Photograph looking upward at the basal rupture surface of the Santiago landslide in boring LD-15 located within the central portion of the landslide. Note the polished, striated, upper-bounding surface that overlies a thick, cohesive gouge. Johnson and Cotton Environmental & Engineering Geoscience, Vol. XI, No. 1, February 2005, pp. 5–1510 Figure 5. Log of boring LD-3 within the ridge-top graben. Note the thick accumulation of colluvium and steeply dipping, colluvium-ﬁlled fractures. 1 ft ¼0.3048 m. The Santiago Landslide: Implications for Paleoseismic Landslide Studies Environmental & Engineering Geoscience, Vol. XI, No. 1, February 2005, pp. 5–15 11 level corresponds to the level at the time of failure during January 1993. The lowest groundwater level approx- imates conditions during a dry period, when groundwater levels were below the rupture surface of the landslide. The third groundwater level is a hypothetical intermedi- ate piezometric surface used to complete the analysis. To evaluate displacement resulting from seismic ground motion, we used a computer program by Jibson and Jibson (2002) that incorporates the methods of both Newmark (1965) and Bray and Rathje (1998). The seismic-displacement input-parameters are provided on Table 1, and the results of our analysis are shown on Table 2. DISCUSSION Our subsurface observations support the hypothesis that a well-developed, ridge-top graben is present at the head of the Santiago landslide. The presence of a thick accumulation of colluvium is not easily explained in a ridge-top setting without graben development. Open and colluvium-ﬁlled vertical fractures, as well as shears having normal displacements, also indicate a history of extensional deformation of the ridgeline. A representative cross section (A-A9, Figure 7) shows our interpretation of the subsurface relationships between landslide A, the ridge-top graben, and the Santiago landslide. The development of a thick, basal rupture gouge in the body and toe of landslide A implies that either this ancient landslide has experienced considerably more displacement than the ridge-top graben or that the basal rupture surface followed a weak bed that thinned toward the ridge-top. The development of a thick, basal rupture gouge would require repeated displacements that would total more than the approx- imately 1-ft (0.3-m) maximum displacement that was recorded during the 1992–1993 event. The geomor- phology of the ancient landslide that is visible in aerial photographs before development also implies that landslide A has experienced considerable, cumulative displacement. Thus, we propose a model in which landslide A originally failed without involving the ridgeline and development of the graben followed as a result of upslope progression of the landsliding. This progression likely resulted from development of a steep head scarp and loss of lateral support along the ridgeline. Our limit-equilibrium analysis conﬁrms that at the highest groundwater level, the landslide and graben acti- vated, as they did during January 1993; the calculated seismic displacements at this groundwater level are large (3.2 m to 8.1 m). At the intermediate and low ground- Figure 6. Rainfall records for stations located within 5 miles (8 km) of the Santiago landslide. 1 in.¼25.4 mm. Johnson and Cotton Environmental & Engineering Geoscience, Vol. XI, No. 1, February 2005, pp. 5–1512 water levels, our analysis shows that the landslide and graben remain static unless triggered by seismic ground motion. Therefore, our analysis shows that past activation of landslide A and the associated ridge-top graben likely occurred in response to either high groundwater or strong seismic ground motion. CONCLUSIONS AND IMPLICATIONS FOR PALEOSEISMIC STUDIES The Santiago landslide and the associated ridge-top graben provide an example of the re-activation of a ridge- top graben by aseismic landsliding related to elevated groundwater conditions. Based on our limit-equilibrium analyses, both elevated groundwater and strong seismic shaking have likely triggered previous movement epi- sodes of the landslide and the ridge-top graben. Colluvial wedges preserved within the ridge-top graben and produced by these very different triggering events would be indistinguishable and could be misinterpreted as representing the late Quaternary paleoseismic record. Therefore, we conclude that ridge-top graben deposits should be used to date paleoseismic events only if the potential for activation by aseismic landsliding associated Figure 7. Cross section A-A9 illustrating the subsurface relationships between the Santiago landslide and landslide A. Also note the high groundwater levels during January 1993 when the Santiago landslide was active. 1 ft ¼0.3048 m. Figure8.Generalized cross sectionA-A9 usedfor limit-equilibrium analyses. Thetopographicproﬁledepicts conditionsbeforemassgrading.Thestatic factors of safety (FS) for the three groundwater conditions are: 1.0 for high (H), 1.15 for intermediate (I), and 1.3 for low (L) groundwater conditions. 1ft¼0.3048 m. The Santiago Landslide: Implications for Paleoseismic Landslide Studies Environmental & Engineering Geoscience, Vol. XI, No. 1, February 2005, pp. 5–15 13 with intense rainfall and high-groundwater conditions can be clearly precluded. ACKNOWLEDGMENTS William F. Cole, Christopher J. Sexton, Randall W. Jibson, and an anonymous reviewer provided comments that improved this manuscript. John Coyle, Dale Marcum, and John Wallace assisted greatly with data collection. Tim Sneddon assisted with limit-equilibrium analyses. Julia Lopez and Noli Farwell drafted the ﬁgures. REFERENCES AGLIARDI, F.; CROSTA, G.;AND ZANCHI, A., 2001, Structural constraints on deep-seated slope deformation kinematics:Engineering Geology, Vol. 59, No. 1–2, pp. 83–102. BARROWS, A. G.; TAN, S. S.;AND IRVINE, P. J., 1993, Damaging landslides related to the intense rainstorms of January–February 1993, southern California:California Geology, Vol. 46, No. 5, pp. 123–131. BECK, A. 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M., 1999a, Evaluation and mitigation of the Santiago Landslide, Anaheim Hills, Anaheim, California, in Association of Engineering Geologists, 42nd Annual Meeting Program with Abstracts, p. 78. MCLARTY,M.AND LANCASTER, J. M., 1999b, Groundwater control of stability, Anaheim Hills, Anaheim, California, in Association of Table 1.Seismic ground motion parameters. Fault Distance to Fault (km) Mw PGA* (g) Elsinore 6.5 6.7 0.46 Whittier 6.6 6.8 0.39 *Boore et al. (1997). Table 2.Seismic-displacement calculation results. Groundwater Level Ky Estimated Displacement High 0.001 126 to 310 in. (3.2 to 7.9 m) 1 319 in. (8.1 m) 2 Intermediate 0.03 70 to 172 in. (1.8 to 4.4 m) 1 34 in. (0.9 m) 2 Low 0.055 43 to 138 in. (1.1 to 3.5 m) 1 19 in. (0.5 m) 2 1Estimated displacements by method of Bray and Rathje (1998). 2Estimated displacements by method of Newark (1965). Johnson and Cotton Environmental & Engineering Geoscience, Vol. XI, No. 1, February 2005, pp. 5–1514 Engineering Geologists, 42nd Annual Meeting Program with Abstracts, pp. 78. MILLER, D. M.; YOUNT, J. C.;AND MAHAN, S. A., 2001, Mid-Holocene debris ﬂow and lake stand events at Silurian Lake, Mojave Desert, California:Geological Society America, Abstracts with Programs, Vol. 33, No. 3, pp. A–70. MORTON,D.M.AND SADLER, P. M., 1989, The failings of the Pelona Schist: Landslides and sackungen in the Lone Pine Canyon and Wrightwood areas of the San Gabriel Mountains of southern California. In Morton, D. M. and Sadler, P. M. (Editors), Landslides in a Semi-arid Environment with Emphasis on the Inland Valleys of Southern California: Inland Geological Society, Riverside, CA, Publication 2, pp. 301–322. NEWMARK, N. M., 1965, Effects of earthquakes on dams and embankments:Geotechnique, Vol. 15, No. 2, pp. 139–160. NOLAN,J.M.AND WEBER, G. E., 1998, Evaluation of coseismic ground cracking accompanying the earthquake: trenching studies and case histories. In Keefer, D. K. (Editor),The Loma Prieta, California, Earthquake of October 17, 1989–Landslides: U.S. Geological Survey Professional Paper 1551-C, pp. 145–163. OWEN, L. A.; FINKEL, R. C.; MINNICH, R. A.;AND PEREZ, A. E., 2003, Extreme southwestern margin of late Quaternary glaciation in North America: Timing and controls:Geology, Vol. 31, No. 8, pp. 729–732. PONTI,D.J.AND WELLS, R. E., 1991, Off-fault ground ruptures in the Santa Cruz Mountains, California: Ridge-top spreading versus tectonic extension during the 1989 Loma Prieta earthquake: Bulletin Seismological Society America, Vol. 81, No. 5, pp. 1480–1510. QUADE, J.; FORESTER, R. M.;AND WHELAN, J.F., 2003, Late Quaternary paleohydrologic and paleotemperature change in southern Nevada. In Enzel, Y.; Wells, S.G.; and Lancaster, N. (Editors), Paleoenvironments and Paleohydrology of the Mohave and Southern Great Basin Deserts: Geological Society of America, Boulder, CO, Special Publication 368, pp. 165–188. RADBRUCH-HALL, D. H., 1978, Gravitational creep of rock masses on slopes. In Voight, B. (Editor),Rockslides and Avalanches 1, Natural Phenomena: Elsevier, Amsterdam, The Netherlands, pp. 607–657. SCHOELLHAMER, J. E.; YERKES, R. F.; KINNEY, D. M.;AND VEDDER,J. G., 1954,Geologic Map of the Northern Santa Ana Mountains, Orange and Riverside Counties: U.S. Geological Survey Pro- fessional Oil and Gas Investigations Map OM-154. SCHOELLHAMER, J. E.; VEDDER, J. G.; YERKES, R. F.;AND KINNEY,D. M., 1981,Geology of the Northern Santa Ana Mountains: U.S. Geological Survey Professional Paper 420D, 107 p. SHAW, J. H.; PLESCH, A.; DOLAN, J. F.; PRATT, T. L.;AND FIORE, P., 2002, Puente Hills blind thrust system, Los Angeles, California: Bulletin Seismological Society America, Vol. 92, No. 8, pp. 2946–2960. SLOSSON,J.E.AND LARSON, R. A., 1995, Slope failures in southern Cali- fornia: rainfall threshold, prediction, and human causes:Environ- mental Engineering Geoscience, Vol. 1, No. 4, pp. 393–401. SMITH, L. N., 2001, Columbia Mountain landslide: Late glacial emplacement and indications of future failure, northwestern Montana, USA:Geomorphology, Vol. 41, pp. 309–322. STOUT, M. L., 1977, Radiocarbon dating of landslides in southern California:California Geology, Vol. 30, No. 5, pp. 99–105. TABOR, R. W., 1971, Origin of ridge-top depressions by large-scale creep in Olympic Mountains, Washington:Geological Society America Bulletin,Vol. Vol. 82,, pp. 1811–1822. THOMPSON, S. C., 1997, Probable gravitational (nontectonic) origin for two conspicuous ridge-top scarps in southern Coast Mountains, British Columbia:EOS, Transactions, American Geophysical Union,Vol. 78, No. 17, suppl., pp. S316–S317. TREIMAN,J.AND LUNDBERG, M. (compilers), 2003, Elsinore fault zone, Glen Ivy section,in Quaternary Fault and Fold Database of the United States: U.S. Geological Survey Open-File Report 03-417. VARNES, D. J.; RADBRUCH-HALL, D. H.;AND SAVAGE, W. Z., 1989, Topographic and Structural Conditions in Areas of Gravitational SpreadingofRidgesintheWesternUnitedStates: U.S. Geological Survey Professional Paper 1496, 28 p. WALLACE, R. E., 1984,Fault Scarps Formed During the Earthquakes of October 2, 1915, in Pleasant Valley, Nevada, and Some Tectonic Implications: U.S. Geological Survey Professional Paper 1274-A, 33 p. WORKING GROUP ON CALIFORNIA EARTHQUAKE PROBABILITIES, 1995, Seismic hazards in southern California-probable earthquakes, 1994 to 2024:Bulletin Seismological Society America, Vol. 85, No. 2, pp. 379–439. The Santiago Landslide: Implications for Paleoseismic Landslide Studies Environmental & Engineering Geoscience, Vol. XI, No. 1, February 2005, pp. 5–15 15 July 5, 2023 Project No. TET 22–236E Geotechnical Engineering ● Engineering Geology GEOTECHNICAL EVALUATION AND GROUNDWATER IMPACT ASSESSMENT SANTIAGO LANDSLIDE ANAHEIM HILLS Anaheim, California Prepared for: City of Anaheim, Department of Public Works 200 S. Anaheim Boulevard, Suite 276 Anaheim, California 92805 Prepared by: Tetra Tech 21700 Copley Drive, Suite #200 Diamond Bar, California 91765 21700 Copley Drive, Suite #200 * Diamond Bar, CA 91765 * Tel: 909-860-7777 Project No. TET 22-236E July 5, 2023 Mr. Rudy Emami Director of Public Works City of Anaheim, Department of Public Works 200 S. Anaheim Boulevard, Suite 276 Anaheim, California 92805 Subject: GEOTECHNICAL EVALUATION AND GROUNDWATER IMPACT ASSESSMENT SANTIAGO LANDSLIDE Anaheim Hills City of Anaheim, California Dear Mr. Emami: Tetra Tech is pleased to submit this report that presents the results of our geotechnical evaluation and groundwater impact assessment for the Santiago Landslide of Anaheim Hills, in the City of Anaheim, California. This study relied primarily on readily available past geotechnical and groundwater monitoring data by others as well as new data from existing groundwater monitoring wells and inclinometers installed during 1992/1993 Santiago Landslide emergency response efforts. From our review, geologic, engineering and hydrogeologic analyses were performed to evaluate the effects of groundwater on the stability of the landslide within the Santiago Geologic Hazard Abatement District (SGHAD). Results of analyses were then used to assess the impacts of the recent 2022/2023 rainfall season, as well as the potential for renewed landslide movement if the current dewatering operations are terminated. This report includes a brief summary of the history of the events that took place during the 1992/1993 Santiago Landslide emergency response efforts, the establishment of the SGHAD, and implementation of the Plan of Control adopted by the SGHAD. This report also includes our monitoring efforts, engineering, geologic and hydrogeologic analyses, and includes a summary of our findings, conclusions, and recommendations. The accompanying plates and appendices include compilations of collected data, geologic and hydrogeologic maps and exhibits, and graphical output of our analyses. We appreciate the opportunity to provide our professional services on this project. If you have any questions regarding this report or if we can be of further service, please do not hesitate to contact the undersigned. Respectfully submitted, Tetra Tech BAS, Inc. Patrick M. Keefe, CEG Supervising Engineering Geologist Peter Skopek, PhD, GE Principal Distribution: Addressee (pdf by email REmami@anaheim.net) Mr. Carlos Castellanos (pdf by email CCastellanos@anaheim.net) Filename: 2023-07-05 SGHAD Geotechnical Evaluation and Groundwater Impact RPT .docx City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 i TABLE OF CONTENTS Page 1. INTRODUCTION AND PURPOSE ................................................................................... 1 2. THE SANTIAGO LANDSLIDE ......................................................................................... 2 3. GEOLOGIC HAZARD ABATEMENT DISTRICT AND PLAN OF COTNROL ............ 4 3.1. HISTORY OF SGHAD GROUNDWATER EXTRACTION ...................................... 6 3.1.1. VERTICAL DEWATERING WELLS .............................................................................. 6 3.1.2. HORIZONTAL DRAINS .............................................................................................. 7 3.2. HISTORY OF SGHAD GROUNDWATER MONITORING ...................................... 8 3.3. HISTORY OF SGHAD INCLINOMETER MONITORING ..................................... 10 4. GEOLOGIC AND HYDROGEOLOGIC SETTING ........................................................ 12 4.1. LOCAL GEOLOGY ................................................................................................... 12 4.2. GROUNDWATER ..................................................................................................... 12 4.3. WATERSHED BOUNDARY .................................................................................... 13 4.4. GROUNDWATER RECHARGE............................................................................... 15 4.4.1. RAINFALL .............................................................................................................. 15 4.4.2. IRRIGATION ............................................................................................................ 16 4.4.3. EVAPOTRANSPIRATION .......................................................................................... 16 4.4.4. INFILTRATION AND HYDRAULIC CONDUCTIVITY ................................................... 17 4.5. GROUNDWATER DISCHARGE ............................................................................. 17 5. CURRENT MONITORING BY TETRA TECH .............................................................. 19 5.1. OBSERVATION WELLS AND STANDPIPE PIEZOMETERS .............................. 19 5.2. INCLINOMETERS .................................................................................................... 19 6. HYDROGEOLOGIC AND ENGINEERING ANALYSES ............................................. 20 6.1. GROUNDWATER FLOW AND GROUNDWATER MONITORING ..................... 20 6.2. GROUNDWATER LEVELS ..................................................................................... 20 6.3. SLOPE STABILITY................................................................................................... 21 6.4. GROUNDWATER LEVEL EFFECT ON STABILITY ............................................ 22 6.5. POTENTIAL EFFECT OF DEWATERING SHUT-OFF .......................................... 24 7. CONCLUSIONS AND RECOMMENDATIONS ............................................................ 26 7.1. SGHAD FUNDING AND PROJECTED REVENUE RESOURCES ........................ 26 7.2. IMMINENT THREAT OF LANDSLIDE REACTIVATION.................................... 26 7.3. SENSITIVITY OF GROUNDWATER CONTRIBUTIONS ..................................... 26 7.4. WATER CONSUMPTION ANALYSIS BY ANAHEIM PUBLIC UTILITIES ....... 26 7.5. OUTDOOR WATER RESTRICTIONS ..................................................................... 27 8. LIMITATIONS .................................................................................................................. 29 9. SELECTED REFERENCES ............................................................................................. 30 City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 ii FIGURES, PLATES & APPENDICES FIGURES Figure 1 – Approximate Boundaries of Santiago Landslide Figure 2 – SGHAD Boundaries Figure 3 – Generalized Aquifer Conditions Figure 4 – SGHAD Watershed Contribution Limits Figure 5 – Annual Rainfall Records Over Past 30 years for Anaheim Hills PLATES Plate 1 – SGHAD Facilities Location Plan Plate 2 – Geologic Map Plate 3 – Geologic Cross-Sections 1 – 1’ through 4 – 4’ Plate 4 – Groundwater Profiles Through the Landslide Mass APPENDICES Appendix A – Cumulative and Incremental Inclinometer Surveys Plots for SI-4 Appendix B – Groundwater Elevation Plots with Monthly Precipitation Appendix C – Graphic Relationships of Various Groundwater Surfaces (Plates C-1 through C-4) Appendix D – Slope Stability Analyses Appendix E – Water Consumption Analysis for the Santiago Geological Hazard Abatement District (Anaheim Public Utilities Department, dated June 29, 2023) City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 1 1. INTRODUCTION AND PURPOSE Due to the heavy rainfall that occurred during January through March of 2023 (17.5 inches), the City of Anaheim contracted with Tetra Tech to assess geotechnical conditions within the Santiago Geologic Hazard Abatement District (SGHAD). Our services included a detailed assessment of readily available past geotechnical and groundwater monitoring data by others and collection of new groundwater and inclinometer data from the existing monitoring devices. With this data, geologic, engineering, and hydrogeologic analyses were performed to evaluate the impacts of the recent 2022/2023 rainfall as well as the potential for renewed landslide movement if the current dewatering operations are terminated. This report includes a brief summary of the history of events that took place during the 1992/1993 Santiago Landslide emergency response efforts, the establishment of the SGHAD, and implementation of the Plan of Control (POC) adopted by the SGHAD. This report also summarizes our monitoring efforts, engineering, geologic and hydrogeologic analyses, and includes a summary of our findings, conclusions and recommendations. The attached plates and appendices include compilations of collected data, geologic and hydrogeologic maps and exhibits, and graphical output of our analyses. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 2 2. THE SANTIAGO LANDSLIDE The Santiago Landslide is a dormant landslide that encompasses roughly 25 acres in the Anaheim Hills area of Anaheim, California. The Santiago Landslide is one of the largest landslides to affect urban public property within Orange County. The landslide mass encroaches within 60 residential properties and public and private streets. The limits of the landslide were defined at the ground surface (now obscured) as a discontinuous zone of extensional ground cracks along the head scarp of an upslope-bounding ridgeline and local compressional ground deformation (bulges) along the poorly developed landslide toe. The basal slip surface of the landslide has been interpreted to reach maximum depths on the order of 200 feet (Eberhart & Stone, Inc, 1996). Initial movement of the landslide was recognized as relatively small cracks in road surfaces during the summer months of 1992. Movement of the landslide mass accelerated in January 1993 following intense rainfall in December 1992 and January 1993. This episode of ground movement resulted in development of more open ground cracks, damage to multiple underground utility lines, damage to swimming pools and private residences, and deformation and development of ground cracks within several public and private streets (E&S, 1996). By mid-February 1993, extensive dewatering efforts coordinated by the City’s geotechnical consultant, Eberhart & Stone, Inc., (E&S) successfully arrested further landslide movement. The approximate limits of the Santiago Landslide are shown in Figure 1. Figure 1. Approximate Boundaries of Santiago Landslide City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 3 The primary factors that have been attributed to the 1992/1993 landslide mobilization include: • North-facing hillside terrain in conjunction with unfavorably north-dipping geologic structure and south-dipping ancient faults. • Pre-existing planes of weakness within the underlying bedrock. • Elevated groundwater. In essence, the landslide was triggered by buoyancy forces due to a rise in groundwater that reduced the shear strength of the already relatively weak bedrock that resulted in the landslide movement in a downslope direction along the unfavorably dipping bedrock bedding. Due the lateral extent and depth of the landslide, combined with its location within a moderately to heavily developed urban environment, conventional stabilization repair methods such as earth buttressing, engineered tiebacks, or other permanent mechanical ground stabilization methods were deemed impracticable and even unfeasible. Therefore, continuous groundwater dewatering to reduce the pore pressures and associated monitoring programs were implemented to manage the groundwater conditions and stabilize the landslide. Whereas this method of mitigation is common and effective, it did not stabilize the landslide with a sufficient margin of safety that would forgo requirements for permanent maintenance and monitoring. This approach was deemed acceptable because an important benefit of this stabilization method is that it can always be scaled up to increase the amount of dewatering in case the monitoring indicates unfavorable increase in groundwater levels. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 4 3. GEOLOGIC HAZARD ABATEMENT DISTRICT AND PLAN OF COTNROL The Anaheim City Council formed the SGHAD on March 16, 1999 under the authority of the California Public Resources Code, Division 17, Section 26500 et seq. with the approval of City of Anaheim Resolution 99R-50. Anaheim and the SGHAD ultimately entered in an agreement on June 10, 1999 that required the City of Anaheim to: a) transfer and assign all rights and interests that it held in specified Dewatering Facilities, b) provide the SGHAD access to such Dewatering Facilities, and c) transfer $3.5 million to the SGHAD for the purpose of construction, acquisition, operation, maintenance, and repair of Dewatering Facilities and for the purpose of monitoring, abating and/or stabilizing the Santiago Landslide. The SGHAD’s obligation under the Agreement was to assume sole and total responsibility for all ownership, control, operational, maintenance, and repair responsibilities relating to the Dewatering Facilities. Specifically, the agreement requires the SGHAD to operate, maintain, and repair all or part of the Dewatering Facilities, as well as any additional new or replacement facilities that may be constructed or installed in a manner within its discretion which will control groundwater levels to prevent reactivation and/or to abate movement of the Santiago Landslide (Benink, 2021). As part of the agreement, a “Plan of Control” (POC) was established to allow the SGHAD to permanently monitor and maintain the Santiago Landslide. The primary objective of the POC is to achieve groundwater elevations within the SGHAD no higher than those recorded on October 5, 1994 each and every year at the onset (October 15) of the annual rainy season. Annual reporting requirements of the POC include documenting the inspection and monitoring results for the preceding rain year by November 1 of each year and addressing both the mechanical state of the dewatering system and evaluate preparedness for the up-coming rainfall year, particularly groundwater elevations compared to the October 5, 1994 maximums (POC, 1999). The POC includes provisions for retention of a “Primary Geologic/Geotechnical Consultant”, a “Pump/Well Contractor” and a “Review Geologic/ Geotechnical Consultant” to provide required technical expertise and recommendations to the SGHAD Board. 1) The role of the Primary Geologic/Geotechnical Consultant (currently ENGEO) is to:  conduct groundwater monitoring in monitoring wells and piezometers,  perform inclinometer surveys once per year,  compile pump discharge volumes and report and analyze findings. 2) The role of the Pump/Well Contractor (currently Charles King Company) is to:  service pumps,  monitor performance and,  report to the Primary Consultant. 3) The role of the Review Geologic/Geotechnical Consultant (currently ENGEO) is to assist the SGHAD Board in reviewing reports and activities of the Primary Consultant and Pump/Well Contractor. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 5 According to the POC, “It has been determined from hydrogeologic analyses that the rising groundwater in the Santiago landslide area was primarily due to local recharge of applied landscape irrigation. Therefore, properties contributing to the destabilizing elevated groundwater include those that are contiguous with and tributary to properties directly underlain or damaged by Santiago Landslide movements of 1992-93. These “tributary” properties influenced original destabilization of the Santiago Landslide by groundwater recharge and their on-going recharge water must be withdrawn by the GHAD dewatering system. Therefore, these “tributary” properties should be included in the GHAD, along with the properties within the surface distress boundary.” Properties included within the SGHAD encompass an area of approximately 152 acres that includes 305 residential properties with public and private streets as shown in Figure 2. Figure 2. SGHAD Boundaries City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 6 3.1. HISTORY OF SGHAD GROUNDWATER EXTRACTION 3.1.1. Vertical Dewatering Wells Based on our review of available data, a total of 39 vertical dewatering wells (DW) were installed during the emergency response efforts of 1993. Dewatering well DW-38 was drilled by others for the purpose of attaining Conditions of Approval for the Point Premier residential development that was constructed at the western terminus of Avenida de Santiago, well after the 1993 emergency operations. Dewatering well DW-40 was reportedly never drilled (Amec 2018). Metered discharge volumes and the pumping water level are recorded at each of the dewatering wells on an approximately monthly basis by the Charles King Company (CKC). CKC’s logs also include notes regarding any required equipment replacements or other maintenance that is performed. Table 1 presents and inventory of each dewatering well with notes as reported by ENGEO (2023). Table 1 Inventory of Dewatering Wells Well No. Drilled Depth (ft) Notes DW-1 55 No records. Not included in the active dewatering system, per the 1999 POC DW-2 50 Currently operational DW-3 52 DW-4 50 DW-5 50 DW-6 49 DW-6 was abandoned and replaced by DW-41 DW-7 50.5 Currently operational DW-8 50 DW-9 49 DW-10 132 DW-10 was abandoned and replaced by DW-39 DW-11 50 No records. Not included in the active dewatering system, per the 1999 POC DW-12 unknown Currently operational DW-13 40 No records. Not included in the active dewatering system, per the 1999 POC DW-14 90 Currently operational DW-15 90 DW-16 75 DW-17 89 DW-18 57.5 DW-19 52 DW-20 90 DW-21 50 No records. Not included in the active dewatering system, per the 1999 POC DW-22 89 Currently operational DW-23 unknown Non-operational. Requires maintenance due to blockage in discharge pipes DW-24 unknown No records. Not included in the active dewatering system, per the 1999 POC DW-25 unknown Non-operational. Requires maintenance due to blockage in discharge pipes DW-26 94.5 Currently operational DW-27 100 No records. Not included in the active dewatering system, per the 1999 POC DW-28 300 Currently operational DW-29 200 DW-30 300 City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 7 DW-31 200 Currently operational DW-32 300 DW-33 200 DW-34 200 DW-35 200 DW-36 330 DW-37 290 DW-38 unknown Drilled by others, no pump installed. Not included in the active dewatering system DW-39 360 Currently operational DW-40 N/A Not drilled DW-41 360 Currently operational As reported by ENGEO, the approximate annual pump rates for all operating dewatering wells over the last 7-year period are presented in Table 2 below: Table 2 Dewatering System Pump Rates Calendar Year Approximate Annual Dewatering System Yield (gallons/yr) 2022 14,928,500 2021 12,979,700 2020 13,578,000 2019 13,615,200 2018 13,305,200 2017 12,734,800 2016 11,764,500 3.1.2. Horizontal Drains A total of 87 horizontal drains were constructed in association with the Santiago Landslide emergency response efforts of 1993. The horizontal drains consist of small diameter boreholes (typically 2 to 3 inches) that were drilled into the face of a slope at a slight upward angle to allow gravity flow and discharge of groundwater. Slotted PVC casing/pipe was then inserted into the hole to collect any groundwater that enters the hole and to transmit the collected water out to the slope face. The outer 5 to 10 feet of the casing was typically solid to allow a seal to be placed around the pipe at the slope face to force the groundwater flow into the pipe and discharge to a collection gallery at the slope face (AMEC, 2017). At the time of the initial emergency response efforts, essentially all of the horizontal drains were reportedly flowing after they were installed. About a month after the January 20, 1993 landslide movement, the estimated volume of discharge from the horizontal dewatering system far exceeded City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 8 the pumping discharge from the vertical dewatering wells for several weeks and peaked at an average of 170,000 gallons per day in late February – early March 1993 (Wood, 2018). Over time, the outflow from the horizontal drains diminished steadily to the point where most drains no longer continued to flow except during periods of high rainfall. By October of 2005, flow was observed and measured in 9 of the drain outlets, and then diminished to flow from only 5 of the 37 horizontal drains in June 2006 (Wood, 2018). Based on ENGEO’s 2022 Annual Monitoring report, only 3 of the 87 horizontal drains were flowing at the time of their last site visit. The combined approximate outflow total from the 3 flowing drains was measured to be about 1,490 gallons per day in May of 2022. The approximate location of all the horizontal drains installed within the SGHAD are shown on the SGHAD Facilities Location Plan – Plate 1. 3.2. HISTORY OF SGHAD GROUNDWATER MONITORING A total of 37 observation wells and 5 nested standpipe piezometers were installed to form the monitoring system. The observation wells (P-wells) typically consist of small diameter (2 to 4-inches) slotted PVC pipe that was inserted into a drilled borehole. Observation wells P-1 through P-28 are reportedly composed of slotted PVC pipe with an end cap installed to the full depth of each borehole (E&S, 1996). The annular space around the PVC pipe was backfilled with No.4 sand to allow free flow of groundwater into the pipe. Upon completion of each well, the casing was capped with a flush-mount well box embedded in concrete to provide a nominal seal at the top of the well. Observation wells P-22 B, P-22C, P-25B, P-25C, P-26B, P-26C, PA, PT-A, and PT-B were apparently drilled by others after the emergence response efforts. No details regarding slotted pipe and annular space backfill for these wells were available for review during this study. The standpipe piezometers (PZ-wells) are comprised of “nested” piezometers intended to provide information on possible differences in water pressure that may be present at various depths in the strata that comprise the underlying bedrock. The nested piezometers have slotted casing installed only in a specific interval within a borehole with bentonitic seals above and below to isolate that specific interval. Each of the PZ-1 through PZ-5 installations include two standpipes and the letter designations represent slotted monitoring intervals that are separated by bentonite seals. The approximate location of each observation well and standpipe piezometer is indicated on the attached SGHAD Facilities Location Plan – Plate 1. Details of each monitoring well are presented in Table 3. Initially, annual monitoring reports were prepared by AMEC Earth & Environmental (AMEC) and their predecessor companies, followed more recently by ENGEO. The last formal annual monitoring report prepared by ENGEO was dated October 31, 2022 and summarized recorded groundwater depths acquired on December 21, 2021. More current groundwater depths were collected by ENGEO on January 31, 2023 but were not included in a formal monitoring report. Raw groundwater data from the January 2023 recording date was presented to Tetra Tech in an Excel spreadsheet for use in this report. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 9 Table 3 Inventory of Groundwater Observation Wells & Standpipe Piezometers Well No. Drilled Depth (ft) Well Notes P-1 100 Screened total depth per E&S P-2 100 P-3 100 P-4 100 P-5 60 P-6 60 P-7 80 P-8 70 P-9 70 P-10 70 P-11 100 P-12 101 P-13 100 P-14 50 P-15 100 P-16 50 P-17 60 P-18 50 P-19 100 P-20 104 P-21 101 P-22 100 P-22B Unknown Unknown installation details P-22C Unknown P-23 200 Screened total depth by E&S P-24 190 P-25 200 P-25B Unknown Unknown installation details P-25C Unknown P-26 200 Screened total depth by E&S P-26B Unknown Unknown installation details P-26C Unknown P-27 260 Screened total depth by E&S P-28 200 PZ-1C 200 Screened interval: 138’-143’ by E&S PZ-1D 200 Screened interval: 183’-188’ by E&S PZ-2A 195 Screened interval: 86.5’-91.5’ by E&S PZ-2C 195 Screened interval: 182’-197’ by E&S PZ-3B 236 Screened interval: 126’-131’ by E&S PZ-3C 236 Screened interval: 201’-206’ by E&S PZ-4B 204 Screened interval: 108’-113’ by E&S PZ-4C 204 Screened interval: 161’-166’ by E&S PZ-5B 214 Screened interval: 95’-100’ by E&S PZ-5C 214 Screened interval: 193’-198’ by E&S PA Unknown Unknown installation details PT-A Unknown PT-B Unknown City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 10 3.3. HISTORY OF SGHAD INCLINOMETER MONITORING A total of 10 inclinometers (SI’s) that varied in depth from 180 feet to 260 feet were installed by E&S during the Santiago Landslide emergency response efforts of 1993. The approximate location of each inclinometer is indicated on the attached SGHAD Facilities Location Plan – Plate 1. The inclinometers typically consist of thick-walled PVC casing that is inserted and grouted in place in a drilled borehole. The inside of the casing includes longitudinal slots in the 4 quadrants of the casing. An inclinometer probe with guide wheels that fit in the slots is lowered into the casing and precise measurements of the pipe inclination are measured at 2-foot intervals along the depth of the casing. A baseline set of measurements is compared to subsequent sets of measurements, and any displacement of the casing can be interpreted from the change in casing inclination. Inclinometers can typically detect displacements as small as about 0.01 inches. From February of 1993 through March of 2000 the inclinometers were monitored by E&S. The results of these monitoring efforts indicated that very small offsets had occurred in inclinometers SI-4, SI-5 and SI- 9. No discernable offsets were reported within the remaining inclinometers. After March of 2000 E&S purchased a new inclinometer probe, field data collector, and computer software package for data evaluation. As reported by Wood (name successor of AMEC), their review of the E&S annual reports from 2000 through 2003 indicated the following: • Only 9 of the inclinometer casings were monitored as the damage that occurred to the SI-7 during its installation apparently made monitoring unreliable with the possibility that the probe might become lodged in the casing. • Data from inclinometers SI-4, SI-5 and SI-9 were evaluated to determine if any significant additional movements had occurred. The data from the remaining casings would be evaluated only if significant movements were recognized in SI-4, SI-5 and SI-9. • Additional small incremental movements that decreased in magnitude through time were reported in inclinometers SI-5 and SI-9 through 2002. • By the end of the 2002-2003 monitoring year the magnitude of ground movements observed in SI-4, SI-5 and SI-9 appear to have subsided to levels below the ability of the of the instrumentation to reliably detect movement. The existing inclinometers, with the exception of SI-7, were re-surveyed by Wood in July of 2004 to establish a new baseline for comparison with future surveys. Wood then performed annual monitoring of the each of the 9 inclinometers between 2004 and July 2008. Comparison of the 2005, 2006, 2007 and 2008 surveys with the 2004 baseline data indicated that no significant landslide movement had occurred over that time period. Following the 2008 measurements, Wood recommended that annual inclinometer monitoring be discontinued until a substantial rise in the groundwater levels was observed. In 2012, Wood recommended that annual inclinometer measurements be performed at SI-4, which was considered to be the most sensitive installation with regard to detecting possible future landslide movement. The final set of measurements by Wood was collected on December 18, City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 11 2017 and review of the processed results showed no evidence considered indicative of landslide movement. Cumulative and incremental plots of Wood’s inclinometer surveys for SI-4 are included in Appendix A. In 2019, ENGEO replaced Wood as SGHAD Primary Geologic/Geotechnical consultant. In March 2019, ENGEO established new baseline readings for SI-2, SI-3, SI-4, SI-6, SI-8, SI-9, and SI-10 and followed by a second monitoring event in November and December 2019. Inclinometers SI-1 and SI-7 were reported damaged and no longer capable of monitoring. Based on the March 2019 survey (final inclinometer survey by ENGEO), no significant displacement indicative of soil movement was identified by ENGEO. No inclinometer plots were presented by ENGEO. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 12 4. GEOLOGIC AND HYDROGEOLOGIC SETTING 4.1. LOCAL GEOLOGY The Santiago Landslide is located in the Anaheim Hills area of the northern Santa Ana Mountains. The northern Santa Ana Mountains are a dominate feature in the northern Peninsular Ranges of coastal southern California and form the southeast margin of the Los Angeles basin. The mountains expose the most complete section of the late Mesozoic and Cenozoic sequence that underlies the north part of the Peninsular Ranges physiographic province (Schoellhamer et al., 1981). The Santiago Landslide, as mapped by various consultants, is largely underlain by northward- dipping sequences of sandstone and siltstone assigned to the Soquel Member and the underlying La Vida Member of the Upper Miocene-age Puente Formation. These rocks were deposited in a marine environment within the rapidly subsiding Los Angeles basin during Miocene time. Other bedrock units beneath the landslide area include basaltic volcanic rock assigned to the middle Miocene El Modeno Volcanics and sandstone and conglomerates assigned to the middle Miocene Topanga Formation. Surficial units that generally mantle the bedrock include topsoil/colluvium, alluvium and artificial fills associated with various residential developments in the area. The Santiago Landslide occurred within the Soquel Member and uppermost part of the La Vida Member of the Puente Formation. The Soquel Member consists primarily of sequences sandstone that are poorly cemented and weak and is locally interbedded with siltstone. The La Vida Member consists of siltstone that is interbedded with very thin beds of fine-grained sandstone. The landslide failure has been interpreted to have failed along weak sandstone and siltstone bedding planes in the bedrock units. Several bedrock faults are also believed to have formed the boundaries of the landslide, as well as may significantly control vertical and horizontal migration of groundwater beneath the slide mass. The limits of the Santiago Landslide and the distribution of the major geologic units and faults in the area, based on mapping by E&S (1996), are shown the attached Geologic Map – Plate 2. Geologic Cross-Sections 1 -1’ though 4 – 4’, as presented within the E&S 1998 report, are presented on the attached Geologic Cross-Sections – Plate 3. 4.2. GROUNDWATER The groundwater regime beneath the SGHAD is complex and occurs as both unconfined and confined aquifer conditions that are locally interconnected by fractures. The concept of confined and unconfined aquifer conditions is shown on Figure 3. The groundwater conditions beneath the SGHAD are largely controlled by bedrock stratigraphy, geologic structure (i.e., bedding and faulting), hydrogeologic characteristics of the geologic unit, and sources of water. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 13 Figure 3. Generalized Aquifer Conditions (From: National Groundwater Association, 2007) Evidence of high groundwater and running springs at the ground surface has been well documented within the Anaheim Hills areas by various sources long before significant urbanization of the area in the late 1960’s and early 1970’s (E&S, 1996). As urbanization of Anaheim Hills progressed to its current state, the surface water infiltration increased dramatically in response to increased landscape irrigation. The resulting surface water contributions combined with the complex geologic environment created the framework for historic high groundwater levels beneath the SGHAD prior to installation of the current dewatering system. 4.3. WATERSHED BOUNDARY The watershed contribution limits are defined for this study as the land area that channels rainfall and irrigation water from higher to lower elevations that eventually flows inside the SGHAD limits. The watershed limits shown in Figure 4 are formed by high points such as prominent hills or ridges. The SGHAD watershed area shown in Figure 4 is roughly 211.4 acres. This area is approximately 60 acres larger than the SGHAD. Of the watershed total area, approximately 85.1 acres is open space natural slopes, approximately 57 acres are irrigated slopes and yards, and approximately 69.2 acres are impervious improvements that include residential structures, streets, and sidewalks that typically control water runoff to storm drains. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 14 Figure 4. SGHAD Watershed Contribution Limits City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment June 23, 2023 15 4.4. GROUNDWATER RECHARGE Infiltration of surface water and subsurface inflow are the primary sources of recharge to the groundwater system within the SGHAD limits. Subsurface groundwater flow beneath much of the watershed boundary defined by this study and entirely beneath the SGHAD is directed to the north and northeast (E&S, 1986). The volume of groundwater recharge is difficult to quantify short of completing a very complex and rigorous water budget. However, based on our review of roughly 30 years of monitoring data collected during the emergency response operations and subsequent SGHAD reporting, we concur with various other consultant’s that groundwater recharge is markedly influenced by seasonal rainfall and year-long irrigation. 4.4.1. Rainfall Annual rainfall records over the past 30 years for the Anaheim Hills area are presented in Figure 5. From the data presented, the annual totals have averaged about 14.5 inches. However, episodic weather systems have produced annual rainfall variations ranging from as low as 2.8 inches to as much as 32.8 inches. Though annual precipitation totals are unpredictable, historical rainfall records can provide a reasonable median and extreme rainfall total that can be considered when evaluating recharge potential from rainfall. It is noted that within Southern California approximately 92% of seasonal rainfall events occur between the months of November and April. Note: Rainfall totals shown for 2022-2023 do not extend beyond May 2023. Sources: AMEC 2017 Annual Report Table V "Summary of Estimated Annual Rainfall Totals 1895 to 2017" County of Orange Water Data Portal (hydstraocpublicworks.com/web.htm) Anaheim, CA Precipitation/Rainfall Data (rainharvestcalculator.com/Rainfall/CA/Anaheim/92806) Figure 5. Annual rainfall records over the past 30 years for Anaheim Hills 0 5 10 15 20 25 30 35 Rainfall (Inches)July through June Yearly Rainfall -1992 to 2023 City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 16 4.4.2. Irrigation Groundwater contributions due to irrigation are affected by individual homeowners or tenants, board members comprising a homeowner’s associations (HOA’s), and departments at the municipal level. For example, the extent or type of landscaping for individual properties, community common space, or public right-of-ways often varies from extremes that can be described as “sparse desert landscape” to “tropical oasis”. Depending on the landscape practices made by individuals, the amount of water, type of irrigation equipment, method of irrigation, duration of irrigation, and when not to irrigate varies dramatically. Other factors, such as location and size of property can also influence decisions regarding landscaping and irrigation practices. If we assume the typical homeowner irrigates their lawn and garden for 15 to 20 minutes 3 times per week, approximately 1 inch of water will have been applied to the lawn and garden space in 1 week, or 4 inches of water over the month. Assuming a modest decrease in irrigation during the rainy season, the annual irrigation by a typical homeowner would be roughly 46 inches each year. According to the 1996 E&S report, residential irrigation for moderate to densely landscaped properties can be equated to roughly 50 inches to 140 inches of rainfall each year which is 3.5 to 10 times the average annual rainfall. When comparing recharge potential from rainfall and irrigation, an overriding factor to consider is that irrigation is typically applied at lower rates to limit runoff and enhance infiltration, particularly on sloping terrain. Precipitation, on the other hand, is often characterized by periods of high intensity rainfall that often results in more runoff than recharge. To further exemplify the impacts of irrigation to groundwater recharge, E&S (1996) report discusses January of 1995 when groundwater levels are reported to had risen in response to heavy rains during the 1994/1995 rainy season. However, the groundwater levels continued to rise through the 1995 dry summer months which could be explained only by the effects of irrigation. 4.4.3. Evapotranspiration A natural process that can offset the impacts of surface water recharge, no matter what the source, is referred to as evapotranspiration. In simple terms, evapotranspiration is the combined process of water surface evaporation, soil moisture evaporation, and plant transpiration. Depending on several factors that include, but are not limited to, climate, soil type, and amount of vegetation to name a few, the rate of evapotranspiration in the Orange County region can vary from roughly 2 to as much as 6.5 inches/month, or up to 46 inches per year (California Water Resources, 2012). Ideally, to optimize water management and limit ground saturation and groundwater recharge due to landscape irrigation, the rate of irrigation should be roughly equivalent to the rate of evapotranspiration. However, in many Southern California neighborhoods, the rate of landscape irrigation often considerably outweighs the rate of natural evapotranspiration. For example, the Santiago Landslide initially mobilized in the summer of 1992 following several drought years that were recorded prior to landslide mobilization (E&S, 1996). Though landslide movement was clearly intensified by the 1992/1993 winter rainy season, landscape irrigation is believed to have been the primary contributor to the rise in groundwater that triggered the 1992-1993 Santiago City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 17 Landslide event. As summarized earlier in Section 3 of this report “…the rising groundwater in the Santiago landslide area was primarily due to local recharge of applied landscape irrigation (POC, 1999)”. 4.4.4. Infiltration and Hydraulic Conductivity Water applied to the pervious ground surface through rainfall and irrigation enters the subsurface through the process of infiltration. Surface water infiltration is controlled by the rate and duration of water application, physical properties of the earth material, slope gradient, vegetation, and surface roughness. Generally, if the supply of water is greater than the soil’s ability to infiltrate, excess water will either accumulate on the ground surface or become runoff. On the other hand, if water is applied at lower rates, runoff is considerably reduced, and infiltration is significantly enhanced. For saturated flow, i.e., flow that occurs when all pore spaces of the earth materials are completely filled with water, hydraulic conductivity can be readily quantified through laboratory and/or in-situ field testing. For example, the compacted soil that comprises fill materials beneath the SGHAD typically possess hydraulic conductivity in the range of 10-6 cm/sec which can be correlated to an infiltration rate of approximately 12 inches/year. Similarly, the hydraulic conductivity of the bedrock materials beneath the SGHAD have been reported to possess values ranging from 10-5 to 10-6 cm/sec and thus can produce recharge rates on the order of 12 to 120 inches/year. To put this information into perspective and to assess the potential impacts of irrigation and rainfall on groundwater recharge potential, we can assume a hydraulic conductivity of 60 inches/year, with 14 inches/year due to rainfall and 46 inches/year due to irrigation. Based on a Google Earth assessment, the irrigated lawns and decorative landscape areas within SGHAD range from roughly 2,000 sqft to 9,000 sqft, thus, the average recharge potential for each irrigated residential property (avg. 5,000 sqft) can be roughly estimated at 515 gallons per day. At 305 residences within the SGHAD, the total recharge potential is roughly 157,000 gallons per day or 38 million gallons annually, assuming only 8 months of irrigation during the year. If we account for an annual evapotranspiration rate of 46 inches/year, the recharge potential could be adjusted to about 9 million gallons annually. Certainly, if we consider higher recharge rates, the HOA slopes within the SGHAD, and other residential properties not included within the SGHAD but within the watershed contribution area shown on Figure 4, the potential recharge volume increases substantially. Conversely, other factors such as decreasing permeability through the unsaturated vadose zone could limit recharge potential. While the values presented above are based on rough estimates and simplified assumptions, they are clearly reflective of the large recharge potential due to irrigation and rainfall. 4.5. GROUNDWATER DISCHARGE Groundwater discharge can be primarily attributed to evapotranspiration, seepage from natural springs, subsurface outflow, and mechanical dewatering methods. Based on the history of groundwater extraction summarized in Section 3 of this report, discharge from dewatering wells appears to dominate the discharge regime within the boundaries of the SGHAD and was determined to be responsible for ceasing ground movement during the 1993 landslide event. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 18 Groundwater discharge rates from dewatering operations within the limits of the SGHAD have been credited with discharging upwards of 11 to nearly 15 million gallons of groundwater each year over the last decade. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 19 5. CURRENT MONITORING BY TETRA TECH Due to the amount of rainfall that occurred during January through March of 2023 (cumulative total of 17.5 inches), the City of Anaheim contracted with Tetra Tech to provide geotechnical consulting services to assess groundwater conditions and monitor inclinometers within the SGHAD. 5.1. OBSERVATION WELLS AND STANDPIPE PIEZOMETERS Tetra Tech initiated monthly groundwater monitoring beginning in April 2023. Groundwater level measurements for each of the monitoring wells (P and PZ) were recorded and compared to previous groundwater measurements recorded from 2009 through January 2023 by AMEC and ENGEO. Groundwater elevation plots (hydrographs), including monthly precipitation totals, were prepared for each monitoring well and are herein included in Appendix B. Groundwater elevations between January 31, 2023 (date of last measurement by ENGEO) and April 4, 2023 (date of initial measurement by Tetra Tech) correspond to the rainfall events that produced 17.5 inches of rainfall. A review of the groundwater data during this period shows increased groundwater elevations by up to 13.8 feet in 27 of the monitoring wells. Continued monitoring of the horizontal drains was not included as part of this study due to low output yields reported by ENGEO (2022). 5.2. INCLINOMETERS Eight of the original 10 inclinometers (excluding to SI-1 and SI-7) were surveyed by Tetra Tech in April 2023 to establish new baselines for comparison with future surveys. Establishment of new baseline surveys were necessary since digital files provided by ENGEO for earlier 2019 surveys could not be readily merged with our inclinometer readings. At the time of this report, no subsequent inclinometer readings have been collected, therefore, no conclusions regarding any landslide movements can yet be developed. A supplemental report will be prepared at a later date once additional data has been collected. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 20 6. HYDROGEOLOGIC AND ENGINEERING ANALYSES 6.1. GROUNDWATER FLOW AND GROUNDWATER MONITORING North-facing descending topography with underlying bedrock units that dip to the north dominates the Anaheim Hills area as indicated on the Geologic Map – Plate 1. Several prominent north to northeast trending faults are also present and/or inferred beneath the study area. Under saturated flow conditions, topography and geologic structure (bedding and faulting) can be dominant factors that impact groundwater flow. For example, some of the high angle ancient faults identified by E&S were interpreted as local groundwater barriers as well as possible conduits to transmit vertical groundwater flow. In addition, portions of the higher elevations along the southerly ridgetop are underlain by a plunging syncline structure or folded bedrock units that form a funnel-shaped structural depression that directs subsurface flow toward the landslide. Groundwater models were developed for this study utilizing the April 2023 monitoring well data summarized within Section 5.1 of this report. By accurately plotting locations and elevations of the groundwater surfaces measured from each monitoring well, contour lines representing surfaces of equal elevation were created to depict the groundwater flow direction and gradient beneath the SGHAD. The elevations utilized to create the groundwater models do not differentiate between unconfined or confined aquifers, although fracture porosity is believed to provide connectivity between aquifers. By overlaying the April 2023 groundwater surface on the January 1993, February 1993, October 1994 and August 1995 groundwater contour models prepared by E&S (1996), the current groundwater conditions can be readily assessed to determine where current elevation fall beneath or exceed the elevations of critical threshold levels identified within the E&S (1996) report and/or established by the POC, and further discussed in Section 6.2 below. Graphic presentations showing these relationships are presented in Appendix C. Cross-sectional profiles showing the various groundwater levels through the body of the landslide mass are presented on Plate 4. 6.2. GROUNDWATER LEVELS Analyses were performed as a part of this study to evaluate the effect of various groundwater levels on the stability of the landslide. The landslide cross-sections, configuration, stratigraphy, groundwater levels and material properties were taken directly from the seminal E&S (1996) report that was the basis for the design and implementation of the current monitoring and dewatering program. The report established several key groundwater conditions that need to be considered in evaluation of the performance of the dewatering system and its effects on the landslide stability, i.e., for assessment of the potential for landslide movement re-initiation. Specifically, the following reference groundwater levels were established in the E&S (1996) report: A. Groundwater at Landslide, i.e., groundwater at the time of landslide in January 1993 before any groundwater control measures were implemented and landslide was still moving is associated with a Factor of Safety of less than 1. This groundwater level is unacceptable as it signifies a failure, ground movements, and ongoing damage. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 21 B. Groundwater at Movement Threshold, i.e., groundwater level in February 1993 when the landslide movement halted. The associated Factor of Safety is expected to be slightly above 1. This groundwater level is also unacceptable as it signifies an incipient failure. C. Target Groundwater Level, i.e., groundwater level achieved in October 1994 when the dewatering system was fully established and functional. This groundwater level is intended to be maintained as it is deemed to provide an acceptable margin against re-initiation of landslide movements. The slope stability Factors of Safety associated with this groundwater level are greater than 1 and this groundwater level and the associated Factors of Safety are referred to as “desirable”. D. Limit Groundwater Level, i.e., groundwater level achieved in August 1995 when the dewatering system was operating with some routine inefficiencies. This groundwater level is considered the highest acceptable groundwater before the dewatering system needs to be augmented to prevent re-initiation of landslide movements. The slope stability Factors of Safety associated with this groundwater level are greater than 1 and this groundwater level and the associated Factors of Safety are referred to as “acceptable”. 6.3. SLOPE STABILITY The 1996 E&S analyses for the aforementioned groundwater levels were herein reproduced and matched utilizing Slope/W slope stability software to form a basis for subsequent evaluation and comparison of the actually observed, future, and any other investigated groundwater levels. The summary of the slope stability analyses is provided in Table 4 below and the printout of the analyses are included in Appendix D. Table 4 Summary of Slope Stability Analyses Groundwater: Landslide (January 1993) Movement Threshold (February 1993) Target “desirable” (October 1994) Limit “acceptable” (August 1995) Current (April 2023) Cross-Section (slip surface) Appendix page 1-1’ Appendix D-1a~1e 0.98 (0.96) 1.02 (1.01) 1.12 (1.09) 1.10 (1.08) 1.09 2-2’(upper) Appendix D-2a~2e 1.02 (1.04) 1.09 (1.09) 1.22 (1.23) 1.16 (1.17) 1.20 3-3’(upper) Appendix D-3a~3e 0.91 (0.99) 1.01 (1.09) 1.19 (1.28) 1.09 (1.19) 1.08 4-4’(upper) Appendix D-4a~4e 0.98 (1.10) 1.09 (1.21) 1.25 (1.41) 1.20 (1.40) 1.14 Notes: Factors of Safety values in parenthesis are the values reported in the E&S (1996) report. Factors of Safety in red indicate values below “desirable” Factor of Safety. Factors of Safety in bold red indicate values below “acceptable” Factor of Safety. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 22 The key observations based on these analyses are provided below. A. The analyses presented in the E&S (1996) report were satisfactorily reproduced and matched. The numerical difference between the values presented in the E&S (1996) report and the herein reproduced values are attributed to the use of different slope stability assessment method as well as to the small differences in the geometry configuration due to digitization of the geologic cross-sections from the E&S report. B. The margin of safety expressed as the increment of the Factor of Safety over 1 for the Target and Limit Groundwater levels is 0.12 to 0.25 and 0.09 to 0.20, respectively. Typically, a margin of 0.5 or at least 0.3 is required for a slope to be considered acceptable or marginally acceptable, respectively. This observation highlights that both the “desirable” and “acceptable” Factors of Safety are lower than what would be conventionally considered acceptable. C. All Factors of Safety for the current groundwater condition (April 2023) are below the “desirable” Factor of Safety values and all of the Factors of Safety, except for Cross-Section 2-2’, are even below their respective “acceptable” Factor of Safety values. This analysis is concerning as it indicates that the landslide in its current state is less stable than what was established as minimally acceptable by the E&S (1996) report. 6.4. GROUNDWATER LEVEL EFFECT ON STABILITY The groundwater levels within the landslide area are monitored by vertical observation wells at 37 locations. At 5 locations the groundwater is monitored by nested piezometers at 2 levels for a total of 42 measurement points. It is notable that during the groundwater monitoring conducted by Tetra Tech personnel for this study, the groundwater was above the Target Level in at least 20 locations and above the Limit Level in at least 10 locations. The groundwater elevations measured during this study are at/near its highest level in at least 13 locations. This is a concerning observation as higher groundwater signifies a lower stability. It is, however, recognized that the individual groundwater measurement points do not sufficiently describe the measure of stability. This is why we have developed a groundwater parameter to be correlated with the slope stability, i.e., a Factor of Safety. This groundwater parameter characterizes the overall quantity of the water affecting the slope stability and is determined separately for each cross-section as the saturated area above the slip surface. This Groundwater Index was calculated for all above-mentioned reference groundwater levels and calibrated against additional groundwater levels in the range between the Limit Groundwater Level and the Target Groundwater Level. This relationship of the Groundwater Index and the respective Factors of Safety for each cross-section is presented in Chart 1 below. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 23 Chart 1 shows the current Factor of Safety for each cross-section as the large solid dot and the Factor of Safety at the Limit Groundwater Level, i.e., “acceptable” Factor of Safety, as the large open dot. The chart also allows a visual assessment of the margin of safety relative to the initiation of the landslide that is signified for each cross-section by a colored dash. In short: • the large open dot signifies acceptable condition; if the large solid dot is below the open dot, the Factor of Safety is unacceptable; • the distance between the large solid dot and the dash signifies the margin of safety before the landslide could re-initiate. The chart can also be used to estimate the Factor of Safety for any investigated groundwater level, e.g., immediately after completion of the groundwater survey. The key observation from this chart is that the current margin of safety relative to the expected initiation of the landslide is less than 0.1 and, therefore; it does not take much increased groundwater to approach a condition when re-initiation of the landslide is possible. Chart 1. Factor of Safety as a function of GW Index. The large solid dot shows the current (April 2023) stability; the open dots show the stability at Limit Groundwater Level. The color dashes show where landslide movements are expected to be initiated. Cross-Section 4-4' y = -7E-06x + 1.2434 Cross-Section 2-2' y = -9E-06x + 1.2285 Cross-Section 3-3' y = -8E-06x + 1.1914 Cross-Section 1-1' y = -2E-05x + 1.1223 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 0 10,000 20,000 30,000 40,000 50,000 Factor of SafetyGW Index City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 24 6.5. POTENTIAL EFFECT OF DEWATERING SHUT-OFF Currently, groundwater levels are managed through the existing dewatering system. It is of particular interest to estimate how long it would take for the groundwater to rise sufficiently to re-initiate the landslide movements if the existing dewatering system was shut off. Whereas performance of a rigorous analysis would be extremely difficult and likely unreliable and subjective because of the uncertainty and variability of virtually all input parameters, a simple empirical assessment was instead performed as described below. The rationale of this assessment is as follows: 1. It took about 20 months during 1993 and 1994 to lower the groundwater from the level at Landslide to the Target Level. 2. The dewatering pumping system is reported (E&S, 1996) to had removed about 34.5 million gallons (MG) of groundwater during this period. 3. Based on the geometry of the slip surface and the respective reference groundwater surfaces, we can easily calculate the volume of the saturated landslide masses under each of the reference groundwater surfaces (Column 1 in Table 5 below). 4. The volume of water available for release, i.e., movable water, from a soil mass can be calculated by multiplying the soil mass volume by the effective porosity. Knowing that the removal of 34.5 MG of groundwater by dewatering (Column 3 in Table 5) resulted in lowering of the groundwater from the level at Landslide to the Target Level we also know that the associated dewatered landslide mass volume is about 1.7 Mcy, i.e., 3 – 1.3 = 1.7 Mcy (Column 1 in Table 5) and the effective porosity can be back-calculated. This effective porosity is back-calculated as 0.1, which matches well with the values typical for fine- grained materials. 5. Knowing that the current groundwater levels are similar to the Limit Groundwater Level we can calculate that the water volume needed to achieve the Groundwater Level at Movement Threshold is about 50 – 34 = 16 MG (Column 4 in Table 5). 6. Noting that the dewatering pumping system is typically removing about 1 MG of groundwater per month, if the pumping is shut off, it will take about 16 months to reach the February 1993 Groundwater Level at Movement Threshold, when the landslide movement was halted and the landslide movement is therefore likely to re-initiate. By similar calculation it would then take 26 months to reach the groundwater level at the time of the landslide (Column 5 in Table 5). City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 25 Table 5 Landslide Mass and Groundwater Quantities Column #: 1 2 3 4 5 Groundwater Level Volume of Landslide Mass below specified groundwater level Volume of Movable Water effective porosity neff = 0.1 Volumes of water to achieve specified scenarios: Mcy MG At landslide (1/20/1993) 3.0 60 60 – 26 = 34 MG of groundwater removed during initial dewatering to Target GW level 60 – 34 = 26 MG of infiltration water to reach the groundwater level At Landslide At movement threshold (2/15/1993) 2.5 50 50 – 34 = 16 MG of infiltration water to reach the groundwater level At Movement Threshold Target (10/5/1994) 1.3 26 Limit (8/9/1995) 1.5 34 Notes: Mcy = 1,000,000 cy MG = 1,000,000 gallons City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 26 7. CONCLUSIONS AND RECOMMENDATIONS 7.1. SGHAD FUNDING AND PROJECTED REVENUE RESOURCES In 1999, the initial SGHAD capital was approximately $3,500,000. Recent financial projections have estimated that the SGHAD funds will be exhausted by the end of the 2023 calendar year if no additional sources of revenue are secured. Tetra Tech understands that at least 2 attempts were recently made to approve a Board Resolution to establish an annual assessment to provide continued funding for maintaining and monitoring of the dewatering system, both of which were voted down by the SGHAD residents. 7.2. IMMINENT THREAT OF LANDSLIDE REACTIVATION In the event that the SGHAD defaults on its obligation to maintain and effectively monitor the dewatering system, the Santiago Landslide and possible surrounding properties will be in imminent threat of landslide reactivation due to the resulting groundwater rise if no other stabilizing measures are implemented. 7.3. SENSITIVITY OF GROUNDWATER CONTRIBUTIONS From this study, we can conclude that recharge and discharge of groundwater within the limits of the SGHAD are paramount factors that are key to the stability of the Santiago Landslide. For more than 30 years, dewatering and monitoring efforts have proven to effectively manage the groundwater regime during periods of intense rainfall and near constant irrigation and thus have successfully mitigated re-activation of the Santiago Landslide. This study has shown just how sensitive the Santiago Landslide is to groundwater fluctuations. Under the current groundwater environment, the landslide is marginally stable. When groundwater levels rise, the Factor of Safety against landslide re-activation decreases. Tetra Tech’s estimates presented in Section 6.5, provide rough timeline for groundwater levels to rise to past critical threshold limits and ultimately to levels that will induce re-activation of the landslide if the dewatering system is shut off. Rainfall and irrigation water have been identified as the primary contributors to surface water infiltration within and around the SGHAD. Since options to mitigate infiltration due to rainfall are limited, emphasis needs to be placed on irrigation controls in the event dewatering operations are terminated until such time that a permanent solution can be implemented. 7.4. WATER CONSUMPTION ANALYSIS BY ANAHEIM PUBLIC UTILITIES Concurrent with this study, the Anaheim Public Utilities (APU) conducted an analysis of water usage in the SGHAD. The analysis utilized data from 305 residential water meters within the SGHAD, and 10 residential water meters from a residential development (Point Premier) located outside the limits of the SGHAD but within the elevated limits of the watershed contribution boundaries shown on Exhibit 4. The purpose of this study was to determine a residential water consumption baseline that allows for sufficient household water consumption with a portion of the City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 27 allocation intended for modest outdoor irrigation uses. If implemented within the study area, the residential water consumption baseline would provide the framework to significantly reduce outdoor water use and hence, reduce groundwater recharge potential within the SGHAD. A copy of APU’s water consumption analysis for the SGHAD is included within Appendix E. The following presents a brief summary of their analysis. Data presented by APU is measured in hundreds of cubic feet (HCF), where 1 HCF is equivalent to approximately 748 gallons of water. Water meter data for the study area was collected in January 2023 to establish the average total household water consumption (indoor and outdoor) during prolonged periods of precipitation and cooler weather when many residents were likely not be using outdoor irrigation. Based on a review of this data, the average household water consumption (indoor and outdoor) was approximately 11 HCF/meter/month. Water meter data was also collected for the very dry January 2022 (7.84 annual rainfall total in 2022). Based on this review, an average household water consumption of 17 HCF/meter/month was calculated for the month of January 2022. Over the entire 2022 calendar year, the average water consumption was estimated to be 24 HCF/meter/month, with a summer peak in July 2022 at 29 HCF/meter/month. Based on the presented analysis, APU recommended a residential water consumption of 15 HCF/meter/month which includes a modest water allocation for outdoor uses such as hand watering of trees and drip irrigation. Since this monthly recommended water consumption value is almost double the minimum health and safety needs for residents as established by the California Department of Water Resources in their December 2021 State Water Project Allocation Notice for California Water Contractors, the finding of the APU analysis was considered reasonable. From this study, APU concluded that if a monthly water usage target of 15 HCF/month/meter is established and enforced for the SGHAD property homeowners, a monthly water savings of up to 2 million gallons or 24 million gallons annually could be achieved. Assuming that these savings are reflective of mostly irrigation recharge reduction and comparing these values to the approximate annual yield from the existing dewatering system presented in Figure 5, it appears that the reduced water consumption program could reduce groundwater recharge potential, and thus reduce the demand for dewatering. 7.5. OUTDOOR WATER RESTRICTIONS If it is determined that no other means are available to effectively dewater the SGHAD, as a minimum, water restrictions to outdoor landscaping should be considered by the City of Anaheim. While water restrictions are in no way intended to be an equivalent replacement to the current dewatering system, future outdoor water restriction can provide extended time for alternative planning prior to potential landslide re-activation. Obviously, the amount of time that could be extended through water restrictions is dependent on the extent of successful implementation of water restrictions. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 28 As a minimum, water restrictions should be implemented to restrict groundwater within the SGHAD by a minimum of 1.5 times the annual dewatering discharge rates averaged over the last 7 years, or roughly 20 million gallons/year. This is intended to account for the highly complex variables that can affect the recharge over such a large and geologically complex area. Furthermore, outdoor irrigation should be completely suspended during rainfall events and should not resume until such time that ground has sufficiently dried. Specific criteria for suspension and continuation of outdoor watering during the rain months, along with tips for reducing outdoor irrigation for landscaping can be found on the California Department of Water Resources website at https://saveourwater.com/News- and- Events/Latest-News/Rainfall-is-Here. The extent of the irrigation restrictions should include all properties within the watershed contribution limits identified through this study and shown on Figure 4, which encompasses the SGHAD and also includes approximately 60 additional acres of surrounding open space and developed properties. Regardless of the implementation of the irrigation restrictions and if any reduction in the dewatering system is reduced or stopped, the groundwater monitoring within the SGHAD should be continued and provided for geotechnical review on at least a monthly basis. Such groundwater data could then be compared to the Groundwater Indices developed in Chart 1 in Section 6.5 of this report, to assess the stability of the Santiago Landslide. The findings of the independent APU study summarized herein provides a reasonable approach for implementation of an outdoor water reduction program. If the outdoor water reduction program is implemented and enforced to meet the anticipated outdoor water reduction goals, the time frame for landslide re- activation can be prolonged to allow for implementation of alternative planning and further mitigation measures. However, due to the inherent complexities to accurately estimate how long it would take for the groundwater to rise sufficiently to re-activate the landslide, water restriction options cannot be considered a solution for landslide mitigation and must be accompanied by a comprehensive groundwater monitoring program until a permanent solution is developed. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 29 8. LIMITATIONS The conclusions, recommendations and opinions expressed in this report are based on Tetra Tech’s review of background documents and on information obtained from this study. The findings of this report may, therefore, need to be re-evaluate and revised as more data become available. This document is intended to be used only in its entirety. No portion of the document, by itself, is designed to completely represent any aspect of the project described herein. Tetra Tech should be contacted if the reader requires additional information or has questions regarding the content, interpretations presented, or completeness of this document. Reliance by others on the data presented herein or for purposes other than those stated in the text is authorized only if so permitted in writing by Tetra Tech. It should be understood that such an authorization may incur additional expenses and charges. Tetra Tech has endeavored to perform its evaluation using the degree of care and skill ordinarily exercised under similar circumstances by reputable geotechnical professionals with experience in this area in similar soil conditions. No other warranty, either expressed or implied, is made as to the conclusions and recommendations contained in this report. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 30 9. SELECTED REFERENCES AMEC Foster Wheeler, 2017, Annual Monitoring/Dewatering Summary, November 1, 2016 Through October 31, 2017, Santiago Landslide Area, Anaheim Hills, Anaheim, California, Project No. IR13164620, December 18, 2017. Benink & Slavens LLP, 2021, Demand for Arbitration, May 7, 202. Bjorklund, T.; Burke, K.; Zhou, H. W.; and Yeats, R. S., 2002, Miocene rifting in the Los Angeles basin: Evidence from the Puente Hills half-graben, volcanic rocks, and P-wave tomography: Geology, Vol. 30, No. 5, pp. 451–454. California Department of Water Resources, 2023, Save our Water, https://saveourwater@water.ca.gov. California Department of Water Resources, 2012, Reference Evapotranspiration Zones, California Irrigation Management Information System, dated January 2012, https://cimis.water.ca.gov Cretelli, S.; Rumelhart, P. E.; And Ingersoll, R. V., 1995, Petrofacies and provenance of the Puente Formation (middle to upper Miocene), Los Angeles Basin, southern California: Implications for rapid uplift and accumulation rates: Journal Sedimentary Research, Vol. A65, No. 4, pp. 656–667. Eberhart and Stone, Inc., 1996, Santiago Landslide, Anaheim Hills, Anaheim, California: unpublished consultant report, City of Anaheim, Office of City Attorney, W.O. 165140.69, Volume I through VIIb, June 28, 1996, 69 p. ENGEO, 2019a, 2019 1st Quarter Monitoring and Documentation, Santiago Geologic Hazard Abatement District (GHAD), Anaheim Hills, Anaheim, California; Project Number 14174.000.000; March 28, 2019. ENGEO, 2019b, 2019 2nd Quarter Monitoring and Documentation, Santiago Geologic Hazard Abatement District (GHAD) Anaheim Hills, Anaheim, California; Project Number 14174.000.000; July 3, 2019. ENGEO, 2019c, Limited Reconnaissance and Document Review, Slope Above Smokewood Circle, Anaheim, California; Project Number 14174.000.000; June 6, 2019. ENGEO, 2020a, Annual Monitoring Report – February 2019 through January 2020, Santiago Geologic Abatement District (GHAD), Anaheim, California; Project Number 14174.002.019; April 21, 2020 (Revised June 5, 2020). ENGEO, 2020b, 2020 Monitoring Event and Documentation No. 1, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.019; April 21, 2020. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 31 ENGEO, 2020c, 2020 Monitoring Event and Documentation 2nd Quarter, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.019; July 27, 2020. ENGEO, 2020d, Horizontal Drains Monitoring and Documentation, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.019; July 27, 2020. ENGEO, 2020e, 2020 Annual Monitoring Report, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.020. November 6, 2020. ENGEO, 2020f, 1ˢᵗ Semi-Annual Vertical Well Observation Report Fiscal Year 2020/21 Monitoring Event and Documentation, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.020; October 27, 2020. ENGEO, 2020g, October 2020 – Monthly Comparison of Groundwater Discharge Volumes, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.020; October 27, 2020. ENGEO, 2021a, November 2020 – Monthly Comparison of Groundwater Discharge Volumes, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.020; January 14, 2021. ENGEO, 2021b, December 2020 – Monthly Comparison of Groundwater Discharge Volumes, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.020; January 14, 2021. ENGEO, 2021c, January 2021 – Monthly Comparison of Groundwater Discharge Volumes and Post –Rain Event Horizontal Drains, Vertical Piezometers, and Monitoring Well Observation Report, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.020; April 14, 2021. ENGEO, 2021d, February 2021 – Monthly Comparison of Groundwater Discharge Volumes, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.020; April 14, 2021. ENGEO, 2021e, March 2021 – Monthly Comparison of Groundwater Discharge Volumes, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.020; April 14, 2021. ENGEO, 2021f, April 2021 – Monthly Comparison of Groundwater Discharge Volumes, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.020; May 28, 2021. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 32 ENGEO, 2021g, May 2021 – Monthly Comparison of Groundwater Discharge Volumes, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.020; June 1, 2021. ENGEO, 2021h, June 2021 – Monthly Comparison of Groundwater Discharge Volumes, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.020; July 6, 2021. ENGEO, 2021i, July 2021 – Monthly Comparison of Groundwater Discharge Volumes, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.021; September 3, 2021. ENGEO, 2021j, August 2021 – Monthly Comparison of Groundwater Discharge Volumes, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.021; September 15, 2021. ENGEO, 2021k, September 2021 – Monthly Comparison of Groundwater Discharge Volumes, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number 14174.002.021; September 28, 2021. ENGEO, 2021l, October 2021 – Monthly Comparison of Groundwater Discharge Volumes, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California; Project Number14174.002.021; October 27, 2021. ENGEO, 2021m, Annual Monitoring Report, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.021. October 29, 2021. ENGEO, 2021n, Pre-Rain Season Piezometers and Monitoring Wells Observations Report, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.021. November 18, 2021. ENGEO, 2021o, Monthly Comparison of Groundwater Discharge Volumes – November 2021, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.021. December 17, 2021. ENGEO, 2022a, Post-Rain Event Piezometers and Monitoring Wells Observations Report, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.021. February 9, 2022. ENGEO, 2022b, Monthly Comparison of Groundwater Discharge Volumes – December 2021, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.021. February 10, 2022. ENGEO, 2022c, Monthly Comparison of Groundwater Discharge Volumes – February 2022, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.021. March 8, 2022. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 33 ENGEO, 2022d, Monthly Comparison of Groundwater Discharge Volumes – March 2022, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.021. April 18, 2022. ENGEO, 2022e, Monthly Comparison of Groundwater Discharge Volumes – April 2022, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.021. April 28, 2022. ENGEO, 2022f, May 2022 Monthly Comparison of Groundwater Discharge Volumes and Horizontal Drain Observation Report, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.021. June 21, 2022. ENGEO, 2022g, Monthly Comparison of Groundwater Discharge Volumes – June 2022, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.021. July 8, 2022. ENGEO, 2022h, Monthly Comparison of Groundwater Discharge Volumes – July 2022, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.022. September 1, 2022. ENGEO, 2022i, Monthly Comparison of Groundwater Discharge Volumes – August 2022, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.022. September 8, 2021. ENGEO, 2022j, Monthly Comparison of Groundwater Discharge Volumes – September 2022, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.022. October 25, 2022. ENGEO, 2022k, Monthly Comparison of Groundwater Discharge Volumes – October 2022, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.022. October 25, 2022. ENGEO, 2022l, Annual Monitoring Report, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.022. October 27, 2022. ENGEO, 2023a, Monthly Comparison of Groundwater Discharge Volumes and Post-Rain Events Vertical Piezometers and Monitoring Well Observation Report – January 2023, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.022. February 13, 2023. ENGEO, 2023b, Monthly Comparison of Groundwater Discharge Volumes – February 2023, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.022. March 15, 2023. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 34 ENGEO, 2023c, Monthly Comparison of Groundwater Discharge Volumes – May 2023, Santiago Geologic Hazard Abatement District (GHAD), Anaheim, California. Project Number 14174.002.022. May 24, 2023. Gath, E. M. and Grant, L. B., 2002, Is the Elsinore Fault responsible for the uplift of the Santa Ana Mountains, Orange County, California: Geological Society America, Abstracts with Programs, Vol. 34, No. 5, pp. A–87. Hevesi, J.A., and Johnson, T.D., 2016, Estimating spatially and temporally varying recharge and runoff from precipitation and urban irrigation in the Los Angeles Basin, California: U.S. Geological Survey Scientific Investigations Report 2016–5068, 192 p., http://dx.doi.org/10.3133/sir20165068. Johnson, Philip, L., and Cotton, Willian, R., 2005, The Santiago Landslide and Associated Ridge- top Graben (Sackungen): Implications for Paleoseismic Landslide Studies, Environmental & Engineering Geosciences, Vol. XI, No. 1, February 2005, pp. 5 – 15. McLarty, Mark, W., and Clark, Donald, W., 1998, Geomorphology & Geology Defining the Santiago Landslide 1992 – 1993, dated October 6, 1998. McLarty, Mark, W., and Clark, Donald, W., 1999, Plan of Control Prepared for Proposed Santiago Geologic Hazard Abatement District, Anaheim Hills, Anaheim, California, dated February 22, 1999. Morton, P. K., and Miller, R.V., 1973, Geologic Map of Orange County, California, California Department of Conservation, Division of Mines and Geology, CDMG Preliminary Report 15. Schoellhamer, J. E.; Yerkes, R. F.; Kinney, D. M.; And Vedder, J.G., 1954, Geologic Map of the Northern Santa Ana Mountains, Orange and Riverside Counties: U.S. Geological Survey Professional Oil and Gas Investigations Map OM-154. Schoellhamer, J. E.; Vedder, J. G.; Yerkes, R. F.; And Kinney, D.M., 1981, Geology of the Northern Santa Ana Mountains: U.S. Geological Survey Professional Paper 420D, 107 p. Tan, Siang S., 1992, Landslide Hazards in the North Half of the Black Star Canyon Quadrangle, Orange and Riverside Counties, California, California Department of Conservation, Division of Mines and Geology, CDMG Open-File Report 90-10. Wood, 2018, Annual Monitoring/Dewatering Summary, November 1, 2017 Through October 31, 2018, Santiago Landslide Area, Anaheim Hills, Anaheim, California, Project No. IR13164620, December 24, 2018. City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 35 PLATES 21700 Copley Drive, Suite 200, Diamond Bar, CA 91765PROJECT NO.:DATE:DRAWN BY:TETRA TECHTEL 909.860.777 www.tetratech.comGRAPHIC SCALE:1''=TET 22-236E JUNE 2023DLPLATE 1SANTIAGO LANDSLIDESGHAD FACILITIES LOCATION PLAN500100 200100'NOTE: Base map produced by AMEC, Dec 2012 20 22 Tps Tps Tps Tps f 18 27 15 10 15 20 24 12 8 40 18 38 10 22 18 33 11 33 19 Tps Tps Qls Tps 69 14 30 30 10 53 86 54 54 50 25 Tt?19 11 22 24 25 22 23 6 27 25 5 10 9 16 6 10 16 18 20 28 14 Temb Tps Qal Qls Tt Tplv Tps af 16 16 19 12 10 33 19 27 Tt Tt Tt Tt 35 25 14 18 18 41 14 10 32 17 23 19 19 20 26 Qal Tplvss 40 Tplvss af Tps af af Tps af af af af af af 18 af 22 16 17 17 69 80-85 Tplvslt Tplvslt Tplvslt Tplvslt Tplvslt Tplvslt Tplvslt Tplvslt Tplvslt Tplvslt 30af af af af af Tplvslt Tplvslt Tplvslt ??????? ? ? ????????Tplvss Tplvss Tplvss Tplvss Tplvss Tplvss ???? ?????Tt Tt TtTt Tt Tplvss Tplvslt ? Tplvslt ?? Tplvslt af af af af af af af af af af af af af af af Tps Tps Tps af af Tplvss af Tplvslt Tplvslt Tps 27 ? 20 40 ?Tplvslt Tt afaf Temb Temb Temb ?11 af Tps 27??af Tplvslt Tps 47 25 65 42 32 20 70 30-40 54-74 40 60 29 @934.7' @933.5' @933.5' @931.2' @931.1' @929.5' @928.5' @928' @926.9' @925.5' @925.5' @921.5' TD = 25' (916.5' elev) 26-35 @932' @928.5' 23 @925.3' 44 @925' 19 @922.0' 25 @920.5' 48 TD = 25' (869' elev) 80 5 5 5 5 36 54 33 20 20 17 20 @938' @935' @932' @929.6' @927' @920.5' @911.8' @910' @907.5' @906.5' @905.5' TD = 50' (892' elev) 5 @917.5'23 @903.2' @904.7' 25 @900' Tplvslt 20 18 24 21 @922' @908' TD = 75' (869' elev) 2 2' 3 Tplvslt 1 1' 4' 4 3'???? ? Qal RESISTANT SANDSTONE BED Temb ?TplvssGEORGETOWNT-1 T-2 14 T-3 45 42 @1038' @1033.2' @1031.7' @1029.2' @1027' @1023.4' @1023' @1021.3' @997' @992.8' @991' @988' @986.8' @984.5' @980' @980' @976' 34 45 25 32 45 45 60 66 45 68 30 28 27 27 25 TD = 82' (972' elev) 35 @1009' 22 @997' 27 @991' 21 @984' 47 @993' TD = 100' (918' elev) B-1 B-5 B-6 18 B-4 B-3 B-2 SI-7 SI-2 T-4 SI-6 SI-5 T-5 SI-9 SI-10 SI-1 A.D.S FAULTA.D.S FAULTB-7 (Incipient Toe)RIMWOODFAULTZONEaf SI-3 SI-4 SI-8 DW-11 DW-18 DW-8 DW-15 DW-9 DW-14 DW-7 DW-20 DW-5 DW-29 DW-6 DW-4 DW-17 DW-3DW-10 DW-1 DW-13 DW-22 DW-35 DW-19 DW-2 DW-27 DW-34 DW-33 DW-16 DW-12 DW-31 DW-21 DW-30 DW-32 DW-28 DW-26 P-10 P-18 P-16 P-28 P-20 P-15 P-6 P-5 P-1 P-21 P-19 P-26 P-25 P-27 P-17 P-9 PZ-5 P-13 P-11 P-23 P-14 P-8 P-12 PZ-2 P-3 P-7 P-22 P-24 PZ-3 P-4 PZ-1 PZ-4 P-2 DW-38 DW-36 DW-39 DW-41 B-8 DW-37 DEVELOPMENT OF TOE UNCERTAIN RELATIVELY MINOR DISPLACEMENT OBSERVED Pressure RidgeLandslide ToePressureRidgeLandslide HeadscarpTension CracksTension CracksHeadscarpHeadscarpM.W.D SANTIAGO LATERAL TRENCH TUNNEL DEVELOPMENT OF LATERAL MARGIN UNCERTAIN NO GROUND DISPLACEMENT OBSERVED DEVELOPMENT OF LATERAL MARGIN UNCERTAIN NO GROUND DISPLACEMENT OBSERVED FAULTP-26BP-26C P-22BP-22C P-25B P-25C PT-APT-B P-A 21700 Copley Drive, Suite 200, Diamond Bar, CA 91765 PROJECT NO.:DATE:DRAWN BY: TETRA TECH TEL 909.860.777 www.tetratech.com GRAPHIC SCALE:1''= TET 22-236E JUNE 2023 DL PLATE 2 SANTIAGO LANDSLIDE GEOLOGIC MAP500100 200 100' LEGEND (LOCATIONS APPROXIMATE) P-28 DW-42 T-4 Tps af 24 60 B-1 SI-5 PZ-5 Tplv Temb Qls Qal Tt Artificial Fill Alluvium Landslide Debris Puente Formation - Soquel Member Puente Formation - La Vida Member (slt - siltstone, ss - sandstone) El Modeno Volcanics - Basalt Topanga Formation Observation Well Piezometer/Core Boring Dewatering Well Exploratory Boring Utility Excavation Trench Inclinometer Bedding Attitude (dashed where buried or encountered within the M.W.D.) Fault or Shear Attitude ? Geologic Contact (queried where uncertain) ? Fault (soild where well located, dashed where approx. located, querried where uncertain, dotted where concealed by fill) 65 Anticline (Solid where well located, dashed where approx. located, queried where continuation or existence uncertain, arrow indicates direction of plunge) ? ?Syncline (Solid where well located, dashed where approx. located, queried where continuation or existence uncertain, arrow indicates direction of plunge) Santiago Landslide (E&S) (Boundry as defined by mapped ground deformations, landslide features labeled where defined) 4'4 Geologic Cross-Section NOTE: Geologic information modified from Eberhart & Stone, 1996 4 4' 900 800 700 1000 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400P-11 (proj. 48')P-2 (proj. 63')P-19 (proj. 89')P-10 (proj. 90')P-6 (proj. 110')P-4P-23 (proj. 23')P-21 (proj. 95')P-15P-6 (proj. 66')P-1 (proj. 20')900 800 700 1000 1100 1000 110033' 940 840 740 1000 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 940 840 740 1040 1140 1040 1140 1 1' 960 860 760 1000 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 960 860 760 1060 1160 1060 1160 2 2' 940 840 740 1000 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 940 840 740 1040 1140 1040 1140 P-5 (proj. 215')P-1 (proj. 170')P-20 (proj. 20')Vassar Cir. Serrano Ave.Georgetown Cir.Ave. De Santiago Serrano Ave. Serrano Ave.Leafwood Dr. Rimwood Dr. Ave De Santiago Ave De Santiago Georgetown Cir. Ave De Santiago Ave De Santiago TET 22-236E DL PLATE 3 SANTIAGO LANDSLIDE GEOLOGIC CROSS SECTIONS 1-1' THROUGH 4-4'500 100 200 100' JUNE 2023 21700 Copley Drive, Suite 200, Diamond Bar, CA 91765 PROJECT NO.:DATE:DRAWN BY: TETRA TECH TEL 909.860.777 www.tetratech.com GRAPHIC SCALE:1''=NOTE: Cross-Sections modified from Eberhart & Stone, 1996 4 4' 900 800 700 1000 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400P-11 (proj. 48')P-2 (proj. 63')P-19 (proj. 89')P-10 (proj. 90')P-6 (proj. 110')P-4P-23 (proj. 23')P-21 (proj. 95')P-15P-6 (proj. 66')P-1 (proj. 20')900 800 700 1000 1100 1000 110033' 940 840 740 1000 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 940 840 740 1040 1140 1040 1140 1 1' 960 860 760 1000 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 960 860 760 1060 1160 1060 1160 2 2' 940 840 740 1000 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 940 840 740 1040 1140 1040 1140 P-5 (proj. 215')P-1 (proj. 170')P-20 (proj. 20')Vassar Cir. Serrano Ave.Georgetown Cir. Serrano Ave. Serrano Ave.Leafwood Dr. Rimwood Dr. Ave De Santiago Ave De Santiago Georgetown Cir. Ave De Santiago Ave De Santiago Ave De Santiago TET 22-236E DL Plate 4 SANTIAGO LANDSLIDE GROUNDWATER PROFILES THROUGH LANDSLIDE MASS 500 100 200 100' JUNE 2023 21700 Copley Drive, Suite 200, Diamond Bar, CA 91765 PROJECT NO.:DATE:DRAWN BY: TETRA TECH TEL 909.860.777 www.tetratech.com GRAPHIC SCALE:1''= Landslide failure surface Current groundwater - April 2023 Groundwater at landslide failure - January 1993 Target groundwater - October 1994 Limit groundwater - August 1995 LEGEND City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 36 APPENDIX A Cumulative and Incremental Plots of Inclinometer Surveys for SI-4 (Wood, 2018) City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 37 City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 38 City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 39 Appendix B Groundwater Elevation Plots (Hydrographs) with Monthly Precipitation 0123456789109409509609709809901,0001,0101,0201,0301,040Rainfall (in)Elevation (ft)P-1Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-1 012345678910800810820830840850860870880890900Rainfall (in)Elevation (ft)P-2Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-2 012345678910800810820830840850860870880890900Rainfall (in)Elevation (ft)P-3Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-3 012345678910800810820830840850860870880890900Rainfall (in)Elevation (ft)P-4Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-4 012345678910830840850860870880890900910920930Rainfall (in)Elevation (ft)P-5Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-5 012345678910800810820830840850860870880890900Rainfall (in)Elevation (ft)P-6Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-6 012345678910810820830840850860870880890900910Rainfall (in)Elevation (ft)P-7Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-7 012345678910790800810820830840850860870880890Rainfall (in)Elevation (ft)P-8Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-8 012345678910780790800810820830840850860870880Rainfall (in)Elevation (ft)P-9Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-9 012345678910790800810820830840850860870880890Rainfall (in)Elevation (ft)P-10Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-10 012345678910780790800810820830840850860870880Rainfall (in)Elevation (ft)P-11Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-11 012345678910770780790800810820830840850860870Rainfall (in)Elevation (ft)P-12Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-12 012345678910780790800810820830840850860870880Rainfall (in)Elevation (ft)P-13Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-13 012345678910790800810820830840850860870880890Rainfall (in)Elevation (ft)P-14Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-14 012345678910800810820830840850860870880890900Rainfall (in)Elevation (ft)P-15Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-15 012345678910740750760770780790800810820830840Rainfall (in)Elevation (ft)P-16Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-16 012345678910800810820830840850860870880890900Rainfall (in)Elevation (ft)P-17Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-17 012345678910700710720730740750760770780790800Rainfall (in)Elevation (ft)P-18Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-18 012345678910900910920930940950960970980Rainfall (in)Elevation (ft)P-19Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-19 0123456789109409509609709809901,0001,0101,0201,0301,040Rainfall (in)Elevation (ft)P-20Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-20 0123456789109209309409509609709809901,0001,0101,020Rainfall (in)Elevation (ft)P-21Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-21 0123456789109009109209309409509609709809901,000Rainfall (in)Elevation (ft)P-22Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-22 012345678910800810820830840850860870880890900Rainfall (in)Elevation (ft)P-22BRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-22B 012345678910880890900910920930940950960970980Rainfall (in)Elevation (ft)P-22CRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-22C 012345678910850860870880890900910920930940950Rainfall (in)Elevation (ft)P-23Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-23 012345678910850860870880890900910920930940950Rainfall (in)Elevation (ft)P-24Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-24 0123456789109209309409509609709809901,0001,0101,020Rainfall (in)Elevation (ft)P-25Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-25 012345678910830840850860870880890900910920930940950Rainfall (in)Elevation (ft)P-25BRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-25B 0123456789109109209309409509609709809901,0001,0101,020Rainfall (in)Elevation (ft)P-25CRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-25C 0123456789109109209309409509609709809901,0001,010Rainfall (in)Elevation (ft)P-26Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-26 012345678910830840850860870880890900910920930940950Rainfall (in)Elevation (ft)P-26BRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-26B 0123456789109009109209309409509609709809901,000Rainfall (in)Elevation (ft)P-26CRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-26C 012345678910850860870880890900910920930940950Rainfall (in)Elevation (ft)P-27Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-27 012345678910800810820830840850860870Rainfall (in)Elevation (ft)P-28Rainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-28 0123456789109809901,0001,0101,0201,0301,0401,0501,0601,0701,080Rainfall (in)Elevation (ft)P-ARainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWP-A 0123456789109809901,0001,0101,0201,0301,0401,0501,0601,0701,080Rainfall (in)Elevation (ft)PT-ARainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWPT-A 0123456789109009109209309409509609709809901,000Rainfall (in)Elevation (ft)PT-BRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWPT-B 012345678910760770780790800810820830840850860Rainfall (in)Elevation (ft)PZ-1CRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWPZ-1C 012345678910720730740750760770780790800810820Rainfall (in)Elevation (ft)PZ-1DRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWPZ-1D 012345678910810820830840850860870880890900910Rainfall (in)Elevation (ft)PZ-2ARainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWPZ-2A 012345678910710720730740750760770780790800810820830840850Rainfall (in)Elevation (ft)PZ-2CRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWPZ-2C 012345678910840850860870880890900910920930940Rainfall (in)Elevation (ft)PZ-3BRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWPZ-3B 012345678910770780790800810820830840850860870880890Rainfall (in)Elevation (ft)PZ-3CRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWPZ-3C 012345678910850860870880890900910920930940950Rainfall (in)Elevation (ft)PZ-4BRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWPZ-4B 012345678910800810820830840850860870880890900Rainfall (in)Elevation (ft)PZ-4CRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWPZ-4C 012345678910750760770780790800810820830840850Rainfall (in)Elevation (ft)PZ-5BRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWPZ-5B 012345678910660670680690700710720730740750760770780790800810820830840850Rainfall (in)Elevation (ft)PZ-5BRainfall DataStandpipe BottomGW @LandslideTarget GWLimiting GWPZ-5C City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 40 Appendix C Graphic Relationships of Various Groundwater Surfaces (Plates C-1 through C-4) © 2023 Microsoft Corporation © 2023 Maxar ©CNES (2023) Distribution Airbus DS © 2023 Microsoft Corporation © 2023 Maxar ©CNES (2023) Distribution Airbus DS 800800 850900950810820830840860870 880890910920930940960970 950920930940960970 980850 900820830840860870880890910 920920 2 2' 3 1 1' 4' 4 3'990 1000 970 980 990 1010 21700 Copley Drive, Suite 200, Diamond Bar, CA 91765 PROJECT NO.:DATE:DRAWN BY: TETRA TECH TEL 909.860.777 www.tetratech.com SANTIAGO LANDSLIDE N:\02 - PROJECTS\2022\TET 22-236E (4552-0236) Santiago Landslide GHAD - Anaheim Hills\06 Drafting\GW - Slip Surface Contours\Groundwater Contours.dwg 6/13/2023 3:52 PMLEGEND CURRENT GROUNDWATER SURFACE CONTOUR CURRENT GW BELOW GW AT LANDSLIDE JANUARY 20, 1993 GROUNDWATER SURFACE CONTOUR 0 GRAPHIC SCALE:1''= 50 100' 100 200FEBRUARY 2023 GROUNDWATER ELEVATIONS AND GROUNDWATER ELEVATIONS AT LANDSLIDE (1/20/1993) APPROXIMATE LIMITS OF LANDSLIDE, MODIFIED BY TETRA TECH RELATIONSHIP OF CURRENT GW AND GW AT LANDSLIDE TET 22-236E CLM PLATE C1JUNE 2023 © 2023 Microsoft Corporation © 2023 Maxar ©CNES (2023) Distribution Airbus DS © 2023 Microsoft Corporation © 2023 Maxar ©CNES (2023) Distribution Airbus DS 800850900950780790810820830840860 870 880 890910920930940960950920930940960970 980850 900820830840860870880890910 920920 2 2' 3 1 1' 4' 4 3'990 1000 960 970 980 990 21700 Copley Drive, Suite 200, Diamond Bar, CA 91765 PROJECT NO.:DATE:DRAWN BY: TETRA TECH TEL 909.860.777 www.tetratech.com SANTIAGO LANDSLIDE N:\02 - PROJECTS\2022\TET 22-236E (4552-0236) Santiago Landslide GHAD - Anaheim Hills\06 Drafting\GW - Slip Surface Contours\Groundwater Contours.dwg 6/13/2023 3:50 PMLEGEND CURRENT GROUNDWATER SURFACE CONTOUR FEBRUARY 15, 1993 GROUNDWATER SURFACE CONTOUR 0 GRAPHIC SCALE:1''= 50 100' 100 200 FEBRUARY 2023 GROUNDWATER ELEVATIONS AND GROUNDWATER ELEVATIONS AT MOVEMENT THRESHOLD (2/15/1993) APPROXIMATE LIMITS OF LANDSLIDE, MODIFIED BY TETRA TECH RELATIONSHIP OF CURRENT GW AND GW AT MOVEMENT THRESHOLD TET 22-236E CLM PLATE C2JUNE 2023 CURRENT GW BELOW GW AT MOVEMENT THRESHOLD CURRENT GW 0' TO 15' ABOVE GW AT MOVEMENT THRESHOLD GW AT MOVEMENT THRESHOLD NOT ESTABLISHED © 2023 Microsoft Corporation © 2023 Maxar ©CNES (2023) Distribution Airbus DS © 2023 Microsoft Corporation © 2023 Maxar ©CNES (2023) Distribution Airbus DS 850 810 820 830 840 860 8708708809 0 0 950 88089091 0 92 0 93 0 940 960 970950920930940960970 980850 900820830840860870880890910 920920 2 2' 3 1 1' 4' 4 3'990 LEGEND CURRENT GROUNDWATER SURFACE CONTOUR CURRENT GW BELOW TARGET GW OCTOBER 5, 1994 GROUNDWATER SURFACE CONTOUR CURRENT GW 0' TO 20' ABOVE TARGET GW 40' TO 60' ABOVE TARGET GW 60' TO 80' ABOVE TARGET GW 80' TO 100' ABOVE TARGET GW 100' TO 120' ABOVE TARGET GW 0 GRAPHIC SCALE:1''= 50 100' 100 200 20' TO 40' ABOVE TARGET GW TARGET GROUNDWATER NOT ESTABLISHED FEBRUARY 2023 GROUNDWATER ELEVATIONS AND TARGET GROUNDWATER ELEVATION (10/5/1994) APPROXIMATE LIMITS OF LANDSLIDE, MODIFIED BY TETRA TECH RELATIONSHIP OF CURRENT GW AND TARGET GW TET 22-236E CLM PLATE C3JUNE 2023 21700 Copley Drive, Suite 200, Diamond Bar, CA 91765 PROJECT NO.:DATE:DRAWN BY: TETRA TECH TEL 909.860.777 www.tetratech.com SANTIAGO LANDSLIDE N:\02 - PROJECTS\2022\TET 22-236E (4552-0236) Santiago Landslide GHAD - Anaheim Hills\06 Drafting\GW - Slip Surface Contours\Groundwater Contours.dwg 6/13/2023 3:49 PM © 2023 Microsoft Corporation © 2023 Maxar ©CNES (2023) Distribution Airbus DS © 2023 Microsoft Corporation © 2023 Maxar ©CNES (2023) Distribution Airbus DS 850 90 0 950 810 820830 840 860 870 88 0 8 9 0 910 920 930 940 960 97 0950920930940960970 980850 900820830840860870880890910 920920 2 2' 3 1 1' 4' 4 3'990 21700 Copley Drive, Suite 200, Diamond Bar, CA 91765 PROJECT NO.:DATE:DRAWN BY: TETRA TECH TEL 909.860.777 www.tetratech.com SANTIAGO LANDSLIDE N:\02 - PROJECTS\2022\TET 22-236E (4552-0236) Santiago Landslide GHAD - Anaheim Hills\06 Drafting\GW - Slip Surface Contours\Groundwater Contours.dwg 6/13/2023 3:49 PMLEGEND CURRENT GROUNDWATER SURFACE CONTOUR CURRENT GW BELOW LIMIT GW AUGUST 9, 1995 GROUNDWATER SURFACE CONTOUR CURRENT GW 0' TO 20' ABOVE LIMIT GW 40' TO 60' ABOVE LIMIT GW 60' TO 80' ABOVE LIMIT GW 80' TO 100' ABOVE LIMIT GW 100' TO 120' ABOVE LIMIT GW 0 GRAPHIC SCALE:1''= 50 100' 100 200 20' TO 40' ABOVE LIMIT GW LIMIT GROUNDWATER NOT ESTABLISHED FEBRUARY 2023 GROUNDWATER ELEVATIONS AND LIMIT GROUNDWATER ELEVATIONS (8/9/1995) APPROXIMATE LIMITS OF LANDSLIDE, MODIFIED BY TETRA TECH RELATIONSHIP OF CURRENT GW AND LIMIT GW TET 22-236E CLM PLATE C4JUNE 2023 City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 41 Appendix D Slope Stability 0.98Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7608008408809209601,0001,0401,0801,1201,1601,200Elevation (ft)7608008408809209601,0001,0401,0801,1201,1601,200Description: Static Stability at Time of Failure, Jan 1993Analysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 1-1'Color NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Failure Surface127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33Rimwood FaultFault ZoneAve. De SantiagoRimwood Dr.D-1a 1.02Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7608008408809209601,0001,0401,0801,1201,1601,200Elevation (ft)7608008408809209601,0001,0401,0801,1201,1601,200Description: Static Stability Post Failure, Feb 1993Analysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 1-1'Color NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Failure Surface127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33Rimwood FaultFault ZoneAve. De SantiagoRimwood Dr.D-1b 1.10Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7608008408809209601,0001,0401,0801,1201,1601,200Elevation (ft)7608008408809209601,0001,0401,0801,1201,1601,200Description: Static Stability GW Limit Level, 1995Analysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 1-1'Color NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Failure Surface127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33Rimwood FaultFault ZoneAve. De SantiagoRimwood Dr.D-1c 1.12Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7608008408809209601,0001,0401,0801,1201,1601,200Elevation (ft)7608008408809209601,0001,0401,0801,1201,1601,200Description: Static Stability Target GW, 1994Analysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 1-1'Color NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Failure Surface127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33Rimwood FaultFault ZoneAve. De SantiagoRimwood Dr.D-1d 1.09Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7608008408809209601,0001,0401,0801,1201,1601,200Elevation (ft)7608008408809209601,0001,0401,0801,1201,1601,200Description: Static Stability Current GWAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 1-1'Color NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Failure Surface127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33Rimwood FaultFault ZoneAve. De SantiagoRimwood Dr.Ave. De SantiagoRimwood Dr.D-1e 1.02Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7407808208609009409801,0201,0601,100Elevation (ft)7407808208609009409801,0201,0601,100Description: Static Stability at Time of Failure, Jan 1993, Upper FailureAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 2-2'Rimwood FaultFault ZoneFault ZoneSerrano Ave.Leafwood Dr.Ave. De SantiagoTiebackColor NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Af - Artificial Fill120 150 20Failure Surface127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33D-2a 1.09Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7407808208609009409801,0201,0601,100Elevation (ft)7407808208609009409801,0201,0601,100Description: Static Stability Post Failure, Feb 1993, Upper FailureAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 2-2'Rimwood FaultFault ZoneFault ZoneSerrano Ave.Leafwood Dr.Ave. De SantiagoTiebackColor NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Af - Artificial Fill120 150 20Failure Surface127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33D-2b 1.16Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7407808208609009409801,0201,0601,100Elevation (ft)7407808208609009409801,0201,0601,100Description: Static Stability GW Limit Level, 1995, Upper FailureAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 2-2'Rimwood FaultFault ZoneFault ZoneSerrano Ave.Leafwood Dr.Ave. De SantiagoTiebackColor NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Af - Artificial Fill120 150 20Failure Surface127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33D-2c 1.22Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7407808208609009409801,0201,0601,100Elevation (ft)7407808208609009409801,0201,0601,100Description: Static Stability Target GW, 1994, Upper FailureAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 2-2'Rimwood FaultFault ZoneFault ZoneSerrano Ave.Leafwood Dr.Ave. De SantiagoTiebackColor NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Af - Artificial Fill120 150 20Failure Surface127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33D-2d 1.20Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7407808208609009409801,0201,0601,100Elevation (ft)7407808208609009409801,0201,0601,100Description: Static Stability Current GW, Upper Failure SurfaceAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 2-2'Rimwood FaultFault ZoneFault ZoneSerrano Ave.Leafwood Dr.Ave. De SantiagoTiebackColor NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Af - Artificial Fill120 150 20Failure Surface127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33D-2e 0.91Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7407808208609009409801,0201,0601,100Elevation (ft)7407808208609009409801,0201,0601,100Description: Static Stability with GW at Failure, Jan 1993, Upper FailureAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 3-3'Fault ZoneFault ZoneRimwood FaultSerrano Ave.Georgetown Cir.Ave. De Santiago Ave. De SantiagoColor NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Af - Artificial Fill120 150 20Failure Surface 127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33D-3a 1.01Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7407808208609009409801,0201,0601,100Elevation (ft)7407808208609009409801,0201,0601,100Description: Static Stability with GW Post Failure, Feb 1993, Upper FailureAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 3-3'Fault ZoneFault ZoneRimwood FaultSerrano Ave.Georgetown Cir.Ave. De Santiago Ave. De SantiagoColor NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Af - Artificial Fill120 150 20Failure Surface 127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33D-3b 1.09Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7407808208609009409801,0201,0601,100Elevation (ft)7407808208609009409801,0201,0601,100Description: Static Stability GW Limit Level, 1995, Upper FailureAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 3-3'Fault ZoneFault ZoneRimwood FaultSerrano Ave.Georgetown Cir.Ave. De Santiago Ave. De SantiagoColor NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Af - Artificial Fill120 150 20Failure Surface 127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33D-3c 1.19Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7407808208609009409801,0201,0601,100Elevation (ft)7407808208609009409801,0201,0601,100Description: Static Stability Target GW, 1994, Upper FailureAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 3-3'Fault ZoneFault ZoneRimwood FaultSerrano Ave.Georgetown Cir.Ave. De Santiago Ave. De SantiagoColor NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Af - Artificial Fill120 150 20Failure Surface 127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33D-3d 1.08Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7407808208609009409801,0201,0601,100Elevation (ft)7407808208609009409801,0201,0601,100Description: Static Stability Current GW, Upper Failure SurfaceAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 3-3'Fault ZoneFault ZoneRimwood FaultSerrano Ave.Georgetown Cir.Ave. De Santiago Ave. De SantiagoColor NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Af - Artificial Fill120 150 20Failure Surface 127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33D-3e 0.98Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7007407808208609009409801,0201,060Elevation (ft)7007407808208609009409801,0201,060Description: Static Stability with GW at Failure, Jan 1993, Upper FailureAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 4-4'A.D.S. Fault ZoneFault ZoneRimwood FaultSerrano Ave.Vassar Cir.Georgetown Cir.Ave. De SantiagoColor NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Af - Artificial Fill120 150 20Failure Surface127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33D-4a 1.09Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7007407808208609009409801,0201,060Elevation (ft)7007407808208609009409801,0201,060Description: Static Stability with GW post Failure, Feb 1993, Upper FailureAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 4-4'A.D.S. Fault ZoneFault ZoneRimwood FaultSerrano Ave.Vassar Cir.Georgetown Cir.Ave. De SantiagoColor NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Af - Artificial Fill120 150 20Failure Surface127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33D-4b 1.20Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7007407808208609009409801,0201,060Elevation (ft)7007407808208609009409801,0201,060Description: Static Stability with GW Limit Level, 1995, Upper FailureAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 4-4'A.D.S. Fault ZoneFault ZoneRimwood FaultSerrano Ave.Vassar Cir.Georgetown Cir.Ave. De SantiagoColor NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Af - Artificial Fill120 150 20Failure Surface127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33D-4c 1.25Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7007407808208609009409801,0201,060Elevation (ft)7007407808208609009409801,0201,060Description: Static Stability Target GW, 1994, Upper FailureAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 4-4'A.D.S. Fault ZoneFault ZoneRimwood FaultSerrano Ave.Vassar Cir.Georgetown Cir.Ave. De SantiagoColor NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Af - Artificial Fill120 150 20Failure Surface127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33D-4d 1.14Distance (ft)0100200300400500600700800900 1,000 1,100 1,200 1,300 1,400Elevation (ft)7007407808208609009409801,0201,060Elevation (ft)7007407808208609009409801,0201,060Description: Static Stability Current GW, Upper Failure SurfaceAnalysis Type: Morgenstern-PriceOptimize Critical Slip Surface Location: YesDate: 06/02/2023Anaheim HillsSantiago LandslideSection 4-4'A.D.S. Fault ZoneFault ZoneRimwood FaultSerrano Ave.Vassar Cir.Georgetown Cir.Ave. De SantiagoColor NameUnit Weight (pcf)Effective Cohesion (psf)Effective Friction Angle (°)Af - Artificial Fill120 150 20Failure Surface127 100 11Tplv - La Vida Member, Puente Formation124 100 15Tps - Soquel Member, Puente Formation127 100 33D-4e City of Anaheim Project No. TET 22-236E Geotechnical Evaluation & Groundwater Impact Assessment July 5, 2023 42 APPENDIX E Water Consumption Analysis for the Santiago Geological Hazard Abatement District (Anaheim Public Utilities Department, dated June 19, 2023)