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HomeMy WebLinkAboutTB Draft SEIR Geotechnical Appendix 01-1996G ° ~ ~ ~MP~L~~~ti~'Js~~ t ! ~~MOG~C~~VJ~~Q~ ~~i~aC~~ ~C~~~~~ GOO ~~ ~l~ PG°3~ ° ~o C~0 t~G°3~° e ~ -~ ~? ~PC~~~~~C~ ~ e ~ W,~]D D~~1~~0~~~~(~U` aC~aG°~~~MC~~~ C~'~~~~~~~~~ GE4TECHNICAL APPENDIX SCH Number: 95092027 Prepared for the CITY OF SOUTH SAN FRANCISCO by WAGSTAFF AND ASSOCIATES Urban and Environmental Planners in association with Harlan Tait Associates, Engineering Geologists January 1996 WPS11S4810SE1RlCGV-GE0.548 REPORT "AREA D" SLOPE STABILITY ANALYSIS AND REMEDIATION PHASE I, TERRABAY PROJECT. SOUTH SAN FRANCISCO, CALIFORNIA JULY 1995 PREPARED FOR: SunChase GA Calif I, Inc. 6001 North 24th Street, Suite A Phoenix, Arizona 85016 1866-002 i- ~ - ~ ~ - - ~" /_,~ T ...` Geo/Resource Consultants, Inc. GEOLOG ISTS~'ENG1 N EERS/ENVIRO NMENTAL SCI ENTISTS 505 BEACH STREET, SAN FRANCISCO. CALIFORNIA 941 t 3 REPORT AREA "D" SLOPE STABILITY A1~IALYSIS AND REMEDIATION PHASE I, TERRABAY PROJECT CITY OF SOUTH SAN FRANCISCO, CALIFORNIA PREPARED FOR: t r-, SUN CHASE GA CALIF I, INC. 6001 NORTH 24TH STREET, SUITE A PHOENIX, ARIZONA 85016 PREPARED BY: r~ u ,t :J :~ U GEO/RESOURCE CONSULTANTS, IlYC. 505 BEACH STREET SAN FRANCISCO, CALIFORNIA 94133 JULY 1995 GRC PROJECT NO.1866-002 ,r GeoJResource Consultants, Inc. ~ ~i Geo/Resource Consultants, Inc. GEOLOGISTS /ENGINEERS ! ENVIRONIAENTAL SCtENTt5T5 1 7uly 26, 1995 1866-002 SunChase GA Calif I, Inc. 6001 N. 24th Street, Suite A ~` Phoenix, Arizona 85016 5oa e..rr San Frandxo. Gilonra 94133 (t15)77'S~1T7 fAX (It5) 77rr23S.9 Re9bnat o~ ~«,. GNOrr.a -,~.. w.9.., RE: AREA "D" SLOPE STABILITY ANALYSIS AND REMEDIATION `^ PHASE I, TERRABAY PROJECT ' SOUTH SAN FRANCISCO, CALIFORNIA Ladies and Gentlemen: u !_. Transmitted herein aze the results of our slope stability analysis of the referenced site. During the course of our study, we have conferred with you and representatives of the City of South San Francisco. We have evaluated several remedial alternatives for Area D. The new remediation plan is based in part on new test borings and updated readings from the existing piezometers, inclinometers and hydraugers. The preferred remedial alternative consists of removing the upper 10 to 20 feet of Area D, constructing a keyway at the base of the slope, and providing subdrainage improvements. With this alternative, slopes of Area D meet or exceed generally accepted stability criteria. The repair will provide protection for the proposed homes, storm drains and debris basin, and adjoining HCP lands. We refer you to the contents of our report for details. It has been our pleasure to work with you on this important project. If you have any questions concerning our findings, please call. r ~ Sincerely, ~ GEO/RESOURCE CONSULTANTS, INC. -, ., ~---~- _ Alan D. Tryhorn, Senior Vice President `J Glenn Romig 1 , Geotechnical Engineer l.1 ,v ADT,'ES~ /GR.csc cc: City of South San Francisco (3 copies) c~:-ar~.t3 ~~ ~ /U Eric S. Ng, P.E., G~ Principal Engineer TABLE OF CONTENTS .1 PACE - 1.0 INTRODUCTION 1 1.1 BACKGROUND 1 1.2 PURPOSE OF THIS INWESTIGATION 1 1.3 SCOPE OF SERVICES 2 ;. 2.0 FIELD INVESTIGATION 4 ~ 2.1 FIELD EXPLORATION 4 2.2 EXPLORATORY BORINGS 4 2.3 MONITORING OF PIEZOMETERS, INCLINOMETERS " - AND HYDRAUGERS 5 3.0 GEOLOGIC CONDITIONS ~ 3.1 SURFACE CONDITIONS 7 3.2 SUBSURFACE CONDITIONS 7 -- 4.0 FINDINGS AND CONCLUSIONS 10 ~ 4.1 EXPLORATORY BORINGS 11 4.2 PIEZOMETER AND INCLINOMETER RESULTS 11 F' 4.3 EXTENT AND DEPTH OF LANDSLIDE I2 ~.. 5.0 RECOMIvIENDATIONS 14 5.1 PREVIOUS WORK 14 5.2 STABILITY EVALUATION 15 5.3 RECObEvvIEENDED ALTERNATIVE 18 5.4 SITE MONITORING 19 ~ ~ 6.0 LIMITATIONS 21 L ~ ,v Geo/Resousce Consultants, inc. FIGURES _ FIGURE 1 SITE VICIIVITY MAP ' FIGURE 2 AND FIGURE 3 TEST BORING LOGS FIGURE 4 UNIFIED SOIL CLASSIFICATION SYSTEM FIGURE 5 HYDRAUGER LOCATIONS FIGURE 6 GEOLOGIC CROSS SECTION A-A' FIGURE 7 ~--~ GEOLOGIC CROSS SECTION B-B' ~ FIGURE 8 RAINFALL AND GROUNDWATER LEVELS PLATE 1 GEOTECFiNICAL MAP (IN POCKED TABLES TABLE 1 WATER LEVEL READINGS 1 TABLE 2 HYDRAUGER READINGS C1 TABLE 3 SUMMARY OF STABILITY RESULTS APPENDICES ~ APPENDIX A INCLINOMETER DATA U APPENDIX B SLOPE STABILITY RESULTS U ~ ,~ Geo/Resource Consultants. Inc. 1.0 INTRODUCTION This report summarizes the results of our supplementary geologic and slope stability evaluation - for an area previously known as "Landslide D" in the Terrabay project, South San Francisco, California. This area is located towards the north side of the Phase 1 development, as shown in Figure 1. k~ ~ 1.1 BACKCTROtJND Terrabay is a residential development that will consist of two phases of single-family houses and townhouse units. Site grading for the Phase 1 azea was completed in 1989. Area "D" was (~ recognized as an upper area of relatively shallow, active landslides; however, the overall slide _, r mass was believed to be inactive. An initial remedial measure for Area D was developed by 1 building atie-back wall to overcome translational slide movement and localized slumping. • .. However, during the construction of the tie-back wall, lower sheared and weaker clay surfaces were discovered. The presence of the lower shear planes below the wall presented a bigger and • ~ deeper slide potential in the area and made the constructed wall a less effective remedial measure, than originally planned. -. ~ In 1992, Leighton and Associates, Inc. (L.&A) together with the Geotechnical Engineer for the (~ project, PSC Associates (PSC), provided supplementary investigation of Landslide D. The U evaluation in their report focused on the potential for movement of this slide, its effect on the development, and possible remedial measures. Several alternative measures were prepared and they will be discussed later in this report 1.2 pi TitPOSE OF THIS INVESTIGATION The purpose of this study was to re-evaluate proposed remedial measures for Area D in view of L safety and constructability, particularly taking into account the significant rainfall that occurred in the winter of 1994/1995. The study was to maintain the safety standards that were established U ~ ~`1~ ,,r Geo/Resource Consultants, Inc. _~ ~~ ~. ;. July 26, 1995 1866-002 Page 2 by the City and are recognized as typical in the industry. The study was initiated by SunChase pursuant to its obligations as developer. Our conclusions are based on reviewing previous data accumulated by others, as well as our own field investigations which supplemented those data with two new test borings and recent readings from the on-site piezometers, hydraugers and inclinometers. 1.3 ~('OPE OF SERVICES The scope of our services, as stated in our proposals to ..you dated October 10, 1994, and modified in subsequent meetings, consisted of the following: (" A. Review of reports by PSC and L&A; B. Discussion and meeting with City Engineers and Consultants regarding the history and concerns of the site development; 1. C. Field reconnaissance and advancing two test borings to supplement and update previous ~ data within the toe of the landslide area; L [ j D. Collecting readings between November 1994, and July 1995, on the piezometers, hydraugers and inclinometers that were previously installed on the site; C E. Evaluation of the remedial alternatives as proposed by the previous consultants and _ recommendation of alternatives that may be most appropriate for the current development; L (' F. Summary our findings in this report. ~ ~~~ ,r Geo/Resource Consultants, Inc. ~: r 0 c c u v July 26, 1995 1866-002 Page 3 During the course of our investigation, we have discussed this project with Mr. Gary Parikh of Parikh Associates who had substantial participation of the investigation in the Terrabay development. We also consulted with Mr. John Gibbs, City Project Manager, and with Dr. Eric McHuron, City consultant, who both had performed substantial review on previous project work for the City. L GD99:1E66-R y Geo/Resource Consultants, Inc. July 26, 1995 1866-002 Page 4 2.0 FIELD INVESTIGATION ,--. 2.1 FIELD EXPL.OR_ATION ' During the period from October 1994 to February 1995, GRC geologists and engineers f• periodically visited the Terrabay site to observe and map geologic conditions. Our reconnaissance focused on surficial features such as scarps, ground cracks, hummocky ground bulges, seepage, and rocky outcrops in the vicinity of Area D. We used the previous mapping ~t contained in the L&A report of January 28, 1992, as a base, and modified those maps as appropriate. Features were mapped at a scale of 1 inch equals 40 feet, and were taped or paced from topographic and cultural features shown on the map. ` Additional groundwater levels and flow data were collected during spring and summer, 1995, by ;`- our geologists and Dr. McHuron. ~s 2.2 F7~LORATORY BORINGS t! ~ Two borings (BH-1 and BH-2) were drilled at the toe area of the mapped landslide as shown on Geotechnical Map, Plate 1. 'The boring locations were selected to supplement existing subsurface data and to help in defining the lower extent and geometry of the slide mass. ~~ C Frequent rainstorms delayed the exploratory drilling until mid-December. on December 20, { ~ 1994, attempts were made to access lots 104, 105, 106, and 107 with afour-wheel drive truck t ~ mounted auger, but were unsuccessful. On December 29 and 30, 1994, we drilled two borings, using a rotary wash, track-mounted drill rig. Boring BH-1 was drilled on lot 105 to a depth of `- 64.2 feet, and BH-2 was drilled on lot 106, to a depth of 49.5 feet. ;~ L: V c~:tas~A ~ „~ Geo/Resource ConsuttaMs, Mc. ~~ July 26,1995 1866-002 d Page 5 Both borings were continuously logged by our engineering geologist, and were sampled with drive samplers and, in selected intervals, continuously with a 36-inch-long Pitcher Barrel sampler. Samples were extruded in the field, examined, and placed in core boxes for transport to our laboratory. Additionally, Dr. McHuron examined the core samples in the GRC laboratory. ~ The logs of these borings are presented as Figures 2 and 3. The soils are described in accordance with the Unified Soil Classification System, as summarized in Figure 4. 2.3 MONITOIZTNG OF PIEZOMETERS. INCLINOMETERS AND HYDRAUGERS Previous geotechnical investigations by PSC and L&A resulted in the establishment of a number ,, of instrumentation stations across Area D. These stations are in the form of groundwater level observation wells (piezometers) and slope movement detection casings (inclinometers). Near- horizontal drains (hydraugers) were installed at various locations as an aid in lowering groundwater levels, and they also serve as groundwater monitoring points_ The locations of all piezometers and inclinometers are shown on Plate 1. Locations of the hydraugers are shown on Figure 5. i We collected groundwater level readings from 17 piezometers over an eight-month period (from November 1994 to July 1995), using a Solinst electric sounder. The result of these readings, t presented as depth below the ground surface, are shown in Table 1. ~ Measurements from seven inclinometers were made on two separate oc~sions, December 1 and ~-, 15, 1994 by Parikh Consultants. These readings were performed by the same individuals using similar techniques and equipment as the previous measurements dating back to 1989 and 1991. This was done to provide comparable data sets, and to avoid wide variations possibly induced by `~ methods, equipment type and different operators. Two readings were performed to provide j ' confidence in the interpretation of inclinometer results. Readings were made on two axes within L each inclinometer casing. One axis was along the longitudinal direction of the slide and the other 1~ `~ GD99:1 E66R (131 8 .~ GeoJResource Consultants, Inc. 1 ~ July 26,1995 1866-002 Page 6 along the transverse direction. A graphical display of previous and current measurements is presented along with a reference historical reading in Appendix A. _ The flow rate from 16 hydraugers was measured, from a total of 22 located in the upper slide, ~ lower slide, and building pad levels. The gravity flow from these hydraugers was measured using a stop watch and graduated glass beaker, over a short duration. To be consistent with previous reports, the flow rates were calculated on the basis of number of gallons per day. Thus, the calculated flow rates presented in Table 2 should be considered approximate. +r. l ~, ~~ G ~~ ~. 8 ~~~ ,~ Geo/Resource Consultants, Inc. ~~ July 26, 1995 1866-002 -~ Page 7 3.0 GEOLOGIC CONDI'T'IONS -~ 3.1 Si 1RFACE CONDITIONS r; ~.r Area D occupies a portion of the Terrabay Phase I development that includes moderate to steep, _ southeast-facing natural slopes, steep, south-facing slopes, and level building pads. The slopes drain to a series of lined ditches and a natural drainage which empties into a concrete debris basin. Most of the ground surface is covered with grasses, and some shrubs. Clusters of water- loving plants mark a number of locations, particulazly at seepages, drainage courses, and - hydrauger dischazge points. r ' ~ The most prominent geomorphic feature of the area is the subdued, hummocky topography of shallow and deep-seated landslide deposits. The slide mass is flanked on both sides by graded bedrock slopes. Grading has exposed Franciscan Complex sandstone and melange (mainly sheazed shale), as well as alluvium and slope debris. 3,2 Si TRST TRFACE CONDITIONS + Descriptions of subsurface geologic units are based largely on the summary given in the I.&A j report, 1992, and modified by our site observations. Area D is characterized by landslide terrane underlain by a variable thickness of Quaternary-age ~-. unconsolidated surfiicial materials, which are in turn underlain by older bedrock of the Franciscan Complex. The Franciscan Complex consists of either predominantly (graywacke) sandstone or predominantly melange (a sheared, deeply weathered mixture of mostly shale, siltstone, and ~' chert). In the study area, these two bedrock units are typically separated by the inactive Hillside (' fault. U ` } ~ ~'~~ ,r Geo/Resource Consultants, Inc. July 26, 1995 1866-002 ~~ Page 8 The unconsolidated deposits mantling Franciscan bedrock are geologically young deposits derived from older geologic materials on or neaz the site. Topsoil, fill, colluvium, alluvium, slope debris, debris flow deposits, sand lenses, and landslide deposits are all surficial materials which cover the bedrock. The areal distribution of the unconsolidated units is shown on the r~, ,y Geotechnical Map (Plate 1). Deposits of artificial fill (Map symbol A.f) were mapped neaz the toe of Area D. These fill materials were placed during grading for the sheaz key of a buttress fill at the toe of Area D, upslope of the pads of Lots 105 and 106. Fill was derived from on-site soils and placed under - the observation and testing of PSC. Most of the upper 40 feet of materials within Area D are composed of debris which has been transported downslope primarily as slopewash or colluvium and debris flows, and possibly, '-', wind-blown sands. These materials are composed of mixtures of gravel, sand, silt, and clay. ~ ~ U r debris flow de sits bol aze redominantl li ht brown sand cla with PPe Po (MaP ~ Qd) P Y g Y Y gravel. The upper debris flow deposits are generally moist and stiff. ~.; ~ The upper debris flow deposits include discontinuous stringers of light brown silty fine sand. The sand deposits are thickest neaz the upper reaches of the landslide. These sand stringers may j be portions of, or derived from, the Colma sand. . ~~~ In the central portion of the slope, the upper debris flow deposits are underlain by a generally ~ continuous layer of silty clay ranging in thickness from approximately 2 to 15 feet. The clay is ~ characteristically light brown with olive to light gray clay stringers with minor amounts of gravel. Discrete sheaz surfaces were observed within this clayey unit. Additionally, the clay L' layer was mapped in a portion of the key bottom. This sheared clay typically marks the basal G rupture surface of the landslide. In the toe azeas, the clay layer (as well as the upper debris flow Q ~ ~~~ ,r Geo/Resource Consultants, Inc. -' July 26, 1995 1866-002 Page 9 deposits) interfingers with the undifferentiated alluvium/debris flow deposits. This clay layer, as reported in the I.&A study, does not exist in the lateral mazgins of the landslide deposits. The lower debris flow and colluvial deposits .(Map symbol Qd2) which underlie the above n 1,~ described clay layer are predominantly reddish-brown clayey gravel with minor amounts of sand _ and boulders. These materials are interpreted as debris flow deposits which accumulated in the '.,; deeper portions of the ancient valley. The unit includes material previously called the valley infill deposits. These deposits do not appeaz to be involved in the potential landslide mass. Undifferentiated alluvium and debris flow deposits (Map symbol Qal and Qd), located in the tce ~ ~ area of the slope, is a mixture of stiff gravelly clays to dense clayey sands. These materials are interpreted as alternating, overlapping sequences of debris flow deposits from the surrounding ' hills and water-carried alluvium de sited in the stream channel. Po Underlying the surficial materials are bedrock units of Jurassic to Cretaceous-age Franciscan Complex, which consists in this area of predominantly sandstone (graywacke) or predominantly l_: melange. Melange is typically a weak, highly fractured to sheared mixture of rock types, mainly ,~ shale and siltstone. The sandstone graywacke is generally fractured and moderately weathered and varies from hard and strong to weak. l~ P, ~i u u ~ ~:~~ ~ ,r Geo/Resource Consultants, tnc. July 26, 1995 1866-002 ~~ Page 10 4.0 FINDINGS AND CONCLUSIONS Based on the additional test borings and the latest readings from the piezometers, hydraugers and • inclinometers, we summarize. our conclusions below. These conclusions form the basis of our recommendations in the subsequent section. o The extent and the depth of the slide has been further refined. The two borings provided an indication that the slide zone is less extensive and shallower towazds the toe area than . estimated in the LBtA (1992) report. ~~ o No significant slope movement was detected by the inclinometers between the previous - reading (10/23/91) and the recent readings (12/1/94 and 12/15/94). This provides some confidence that Area D has remained stable within this period at its current configuration `' following the site grading. ,. '~ o Piezometer readings during a portion of the very wet '94= 95 winter season indicate higher water levels than indicated in previous reports. Shallow piezometers typically have responded directly to infiltration of rainfall, rising dramatically after prolonged rain, and falling during periods when rainfall ceased. This is especially evident in the July 14, +' 1995, readings where most of the shallow piezometers have fallen to previous lower Y { levels. Deeper piezometers show very slow changes relative to rainfall, as would be ! expected We expect that some portion of the slope area, such as the upper area behind ~ the existing retaining wall, may be saturated and more prone to localized slope 1 movement. ~ o Water flow from hydraugers was similaz to shallow piezometers in that the flow rates 1 varied with rainfall patterns. Hydraugers appear to be effective in dewatering portions of the slide mass, and thus confirm assumptions in earlier reports. { GD99:IE66-R 11 ~ ,v Geo/Resource Consultants, Inc. `~ ~ July 26, 1995 _ 1866-002 Page 11 :~ o The existing retaining wall, while deemed to be not effective against deep slope movement, probably provides some resistance against localized near-surface movement. The wall probably increases the general stability of the area towazds the twQ ends of the retaining wall. In addition, it provides a catchment fence to prevent occasional loose -; boulders from reaching lower levels. . 4.1 FXPT.(~RATORY BORINGS ! - The two exploratory borings, BH-1 and BH-2, were located to provide information on the extent ~~ and depth of slide debris in the toe azea. BH-1 provided positive information on a shallower ~--~ slide depth than previously shown, while BH-2 confirmed the general type of soil and rock i ~ materials. r i ~ As illustrated in Figure 2, Log of Boring BH-1, we found a thin, gray silty clay layer at a depth ' of 16.8 feet. This clay, which resembled clay described from the central portion of the slide, was i in abrupt contact with black-stained sandy and gravelly clay. The dip of the contact was about 10 degrees from horizontal. Although the core sample was not oriented, the slide plane geometry as determined from previous work suggests that the slide plane does not continue to plunge down at that location, but rises back toward the ground surface. 4.2 "~~^~~TER ND INCLINOMETER RESULTS r y Based on our review of the piezometric (water level) data we collected during this investigation and on previous data, we believe that at least two distinct perched groundwater zones exist within ~ the slopes of Area D. An upper, and apparently discontinuous, zone of groundwater in the (; colluvium and debris flow deposits above the basal rupture surface, is generally quite responsive ~ to large amounts of rainfall. The response of selected piezometric levels to the '94-'95 winter 1 ~ GD~'~~ ~ ,r Geo/Resource Consultants, fnc. July 26, 199s 1866-002 ., Page 12 rains is displayed in Table 1 and in Figure 8. After nearly twenty consecutive days of rain, some of the upper zone water levels had risen to an average of 5 to 20 feet below the ground surface. In a three-day period between two readings (February 2 and s, 199s), no rain was recorded on site, and most of the piezometers recorded a slight drop in water levels. This pattern is consistent r; ~,~ with previous observations and may reflect more permeable sandy soils close to the surface. A _ _ lower zone of groundwater is at or near the bedrock surface and is separated from the upper zone, at least in part, by the clay unit at the slide plane. During the high precipitation period, water levels in the lower zone did not appear to rise dramatically. In contrast to the upper piezometers, s two piezometers screened below the slide plane level (LSDP- 6B and 7B) continued to show a - slight rise even after rainfall ceased. This is typical of deeper water bearing units. ~~ • The results from inclinometers are consistent with previous readings, typically slowing movement in the upper several feet, probably attributable to surficial soil creep or irregularities '~ in the inclinometer casing. The magnitude of deflections observed in the graphical plots, which i. are presented in Appendix A, generally fall within the measurement tolerance of the instruments, and in our opinion, can be considered "noise". Based on inclinometer results, we believe there ~. has been no movement of the inactive Area D landslide since the previous instrumentation period • in 1991. ~•, j 4.3 FA'T' AND DEPTH OF I,ANDSL.IDF. 1 Area D is occupied by a lazge, inactive landslide feature that has developed through translational r, movement of older debris flow deposits and colluvium. Shallow debris flows and rotational ti landslides of more recent age have occurred within the lazger mass and at its margins. According to the extensive supplemental geotechnical investigation performed by L,&A (1992), the depth of i~ the large inactive landslide is defined by a basal rupture surface approximately 4s to s0 feet deep f ~ in the central portion of the slide. The rupture surface is at the base of a 2 to 15-foot-thick clay L ~S ` ] ~ ~`~~ ,r GedResource Consultants, Mc. r~ July 26, 1995 1866-002 ,r Page 13 layer. The rupture surface is not well defined at the lateral mazgins of the slide, or at the tce area., r i where the slide mass was removed through ancient alluvial processes. r The lateral limits of the large slide are indicated on the Geotechnical Map, Plate 1, and are - ~ modified from the L&A report in the tce area. R/e interpret the thin (0.2 feet thick) layer of clay in BH-1 to represent a basal rupture surface. This relationship is illustrated on Figure 6, Cross- Section A-A, and Figure 7, Cross-Sectioa B-B'. Based on additional readings and test results, the depth of this feature suggests a shallower slide plane, and more limited toe extent than estimated in the I,&A report. r- t. r ;_ ' ~ GD99:1b66•R ~ ,v Geo/Resource Consultants, tnc. .,,, ~~~ July 26, 1995 1866-002 Page 14 5.0 gECOM1I~NDATIONS 5.1 PRRVIOt1S WORK ~~ In the previous report by L&A, several remedial measures were examined. The options presented included construction of a large toe buttress, placement of toe fill, removal of upper slide materials, and a laterally loaded drilled pier system. All these options were designed to increase the factor of safety against potential slope movement. The static factor of safety used for design was 1.5, which was consistent with the design basis for other slopes in the Terrabay -- project ~' '' Alternative 1 (^ The proposed remedial measure in the LBtA report, referred to in this report as Alternative 1, 'c consisted of construction of two large sheaz keys, one above and one below the existing tieback retaining wall, together with some removal of overburden above the wall, and subsurface 1 drainage improvements. I` We have evaluated this proposed remedial measure from both technical and construction feasibility aspects. From a technical standpoint, the proposed repair scheme would minimize future slope instability, Provided that the shear keys are extended below the failure planes, the L i keys are constructed with adequate subsurface drainage, and that adequate compactive effort is -. used in fill placement. However, from a construction standpoint, we have several concerns regazding the proposed system. First of all, the shear keys involve extensive earthwork which requires deep excavation on the ' ~ order of 50 to 60 feet in some areas. Temporary excavation requires open cut slopes on the order of 1:1 to 1.5:1 (Horizontal to Vertical). Such a deep cut (50 to b0 feet) in a marginally stable U L' ~ ~t~ ,~ Geo/Resource Consultants, inc. July 26, 1995 1866-002 Pagc t S landslide slope generates a concern about the overall hillside stability during construction. In the past, we understand that a similaz slope problem was developed during the repair of "Landslide R" (immediately southwest of "Area D") and major failures occurred on the slide plane. ,f ; Second, the location and geometry of the failure plane within the slide zone are based on data points from various borings. The actual slide plane configuration within the slope can only be verified during excavation. It is possible that deeper excavation would be required in some areas. If this were to occur, a steeper cut would be necessary to achieve the sheaz key construction, adding to the difficulty of constructing the shear key. 1~ Third, extensive cuts in the slide toe area would require either partial or total removal of the ' ~ existing debris basin. Such a change would also require a modified design of the debris catchment system. r ~. 5.2 STA>~IL.ITY EVALUATION ~ -. We evaluated several remedial alternatives during the course of our work. In order to assess the feasibility of each alternative, we performed stability analyses of several of them. The computer . program "TSLOPE", furnished and applied by Dr. Robert Pyke, was used for the stability j calculations. A summary of the results is presented in Table 3, and brief description of 1, alternatives 2, 3 and 4 is described below. Copies of the computer printouts from the 1 ~ recommended alternative (Alternative 4) are presented in Appendix B. 4 ' The soil strength parameters used in our stability analysis are identical to those used by L&A in their January 28, 1992 report. We also assumed simiiaz groundwater conditions. -~ l' ~ c~:~a~-a „v Geo/Resource Consultants, Mc. r+~ ' July 26, 1995 _ 1866-002 Page 16 We understand from the City of South Saa Francisco that a factor of safety of 1.5 is required for ' static stability for landslide evaluation. In our opinion, a factor of safety of 1.1 is generally used in practice for seismic stability analysis. r• . ~ The alternatives evaluated in addition to the I,&A repair scheme, developing a setback from the existing unrepaired slope (Alternative 2), included construction of a soil buttress at the toe of i : slope (Alternative 3), and a combination of significant removal of slide mass and construction of a shear key at the tce of slope (Alternative 4). -~ ~ternative 2 Our stability analysis indicates that the existing slope does not meet the required factor of safety of 1.5. A setback could be established to protect the development below the slopes of Area D by elimination of 7 to 8 lots within a reasonable setback zone. This would protect residents; however, it would not remove the need for future maintenance due to a potential landslide, or ~ mitigate the impact on underground infrastructure. Alternative 3 j Constructing an earth buttress at the base of Area D would improve the static and seismic factors r, of safety to approximately 1.3 and 1.0, respectively. The static factor of safety for this ~ -~ alternative is in the range often accepted for a development such as this, in our opinion, with no - structures actually being built on it A small amount of movement of the slope during seismic -' ground shaking would be possible with this alternative. However, the movements would likely be small. This alternative would also result in eliminating 7 to 8 lots, and would require relaxing V the requirement for a static factor of safety to under 1 S. it ~~ ~ ,r Geo/Resource Consultants, Inc. July 26;1995 1866-002 Page 17 Alternative 4 r( Removing a thickness of approximately 10 to 20 feet of the slide mass and constructing a shear key at the bottom of the slide would meet the static and seismic factors of safety above those ~-i commonly required for this type of development. This alternative would entail cutting to a relatively uniform slope, beginning below the upper concrete drainage ditch, and cutting only !~ within the Area D limit. Twenty feet of soil would be removed at the lower portion of the slope, decreasing generally to 10 feet in the upper portion, with less at the very top. Where competent bedrock is encountered within the cut, such as within the west edge of the slope, the depth of the - - cut can be reduced. )n addition, a sheaz key similaz to that proposed in the L&A repair scheme ~ -' (Alternative 1) would be constructed at the base of the slope. However, this shear key would require an unsupported cutslope about 20 feet lower in height because of the proposed removal of - 20 feet of the slide mass, significantly reducing the potential for safety hazards to develop during ' construction. This proposed remedial scheme is presented on Figure 6, Geologic Cross Section A-A'. The static and seismic factors of safety for this alternative aze approximately 1.6 and 1.2, respectively. We also briefly evaluated the sensitivity of the stability analysis to variations in groundwater conditions. When raising the assumed groundwater level for this alternative by 7 ~~ feet, to the final graded ground elevation, the static and seismic factors of safety were reduced to 1.5 and 1.1, respectively. In our opinion, a groundwater level this high would be highly unusual. However, the analysis shows that even with such high groundwater, the stability remains within (~ the range required for the development. The details of the stability analysis results aze presented l! in Appendix B of this report. i• ~ - ' In our opinion, the fourth alternative is the most desirable alternative, from a safety and constructability viewpoint. In addition, this alternative is the only one that actually reduces the ~ potential landslide driving forces. The computed factor of safety of this alternative is equivalent ~'1 to the L&A Plan, which has previously been accepted by the City. u ~ ~~~ ,r Geo/Resource Consultants, Inc. July 26, 1995 18b6-002 Page 18 5.3 RFrO1ViMENDED AT.TEIZNATNE The repair should be designed and constructed in accordance with the following criteria. ~;~ 1. The area and depth of the slide mass removal and shear key aze shown on the Geotechnical Map, Plate 1., and the cross section, Figure 6. The final repair plan should be developed by a ` ' Civil Engineer, working closely with our office; more detailed earthwork criteria will be provided by our office. The surface drainage features in the area that will be removed, should be replenished. In addition, to the extent possible, the final ground surface should be sloped in an easterly direction to divert debris/boulders to the swale and debris basin at the ~ eastern edge of the slope, away from the developed lots. Alternatively, a debris catchment fence could be constructed at the base of repair. r' 2. The keyway should extend to Elevation 270 feet, as shown on Figure 6. We anticipate that at this depth the key will be below the slide planes. Exploration pits should be dug in advance ` of the keyway excavation to confirm that this depth is adequate. 3. A subdrain should be placed in the base of the keyway, and extend up to Elevation 310 feet. The subdrain should consist of a 12-inch perforated pipe, perforations placed down, t~ embedded in a 3 foot width of Caltrans specification Class 2 permeable material. (~ Alternatively a composite geofabric/crushed rock scheme can be used. The 3 foot width of t• j Class 2 rock should be extended up as fill is placed in the keyway. A solid pipe should be ~, used to direct drained water to a suitable discharge point. 4. On-site soil may be used to backfill the keyway. The fill should be compacted neaz the ~ optimum water moisture and at a relative density of at least 92 percent, as determined by ASTM Test D-1557, latest edition. L lJ ~ ~ie~ ,,r Geo/Resource Consultants, tac. July 26, 1995 1866-002 Page 19 5. The keyway will extend partially under lots 105 and 106. This will result in up to 30 feet of fill underlying portions of these lots, and cut surfaces beneath other portions. Because of the ~ :~ variability in soil conditions across these lots, we recommend that the Geotechnical Engineer of record for the lots prepare specific recommendations for them. The eazlier ,~ ~ recommendations will likely not be adequate. In addition, to reduce the potential for ~-- settlement of the keyway fill, structural fill placed on tbe lots should be compacted at a ' relative compaction of 95 percent relative compaction (ASTM 155. 6. Drawings should be developed, showing the existing piezometers, slope indicator casings, hydraugers, subsurface drains and other underground pipes within the repair area. Those ~~l features which are desired to be kept in operation should be identified in the contract documents. A contract mechanism to protect these items during construction should be developed as appropriate. 7. The topsoil from the graded azea should be stripped, to the extent possible, and stockpiled. ~ This topsoil can then be spread over the final graded surface, to promote growth of vegetation on the repair surface. C 8. We should be retained to work with the Civil Engineer and your other consultants in ~- ~~ developing plans for the repair, to review the final plans and specifications developed, and to (~ observe and test the earthwork portion of the repair during construction. Li (~ 5.4 SITE MO>`TITOIZiNG U We strongly recommend that periodic monitoring be performed at Area D currently and u throughout the construction period. This includes further readings of hydraugers, inclinometers and piezometers. We also recommend that new instrumentation be installed upon the completion of the repair so that future monitoring of the slope is feasible. We would work closely with your 3J GD99:1 E66-R ~ ,r Geo/Resource Consultants, inc. f i ~. r ~i I J 1`~ J L July 26, 1995 1866-002 Page 20 Civil Engineer regarding the locations of the new monitoring device and the frequency of future measurement. GD99:tE66-R ;r GeoJResource Consultants, Inc. t July 26, 1995 1866-002 Page 21 6.0 LIMITATIONS This report is based upon the services we provided in conducting the study for the specific purposes of geological evaluation, as described herein. The scope of services associated with this report was developed specifically for the client in light of their risk management preferences. This study was performed with the skill and care ordinarily exercised by members of the `; profession practicing under similaz conditions at the same time, and in the same or similaz locality. No other warranty, either expressed or implied, is made or intended. This report is subject to certain limitations that may or may not be noted in the report itself. In addition, recognize that the passage of time affects the information provided in the report Our opinions relating to site conditions aze based upon information that existed at the time our conclusions were formulated. As we are sure you can appreciate, site conditions can change rapidly, such as seasonally, or in some cases, overnight. Please understand that the services provided for in this project were limited to the specific requirements of the client; the limited scope of service allows our firm to form no more than an opinion of the actual conditions at the site. This letter confers upon no third party the right to rely upon the information contained in the report. No other pazty is entitled to rely on the report unless our express written consent is first obtained. Please contact us if you have any questions r~ or concerns regazding the information contained in the report or these limitations. U 1.: L ~ ~'-a~-a ,~ Geo/Resource Consultants, Ina FIGURES L' r ~1 ~~ u l L t { ~ ,~ Geo/Resource Consultants, Inc. r i l . f" u ,o r G u r r s~ r- ~~ i. t~ r 1 lJ F, U r u i L L~ a r ' ~ N ~ C d r ~ N ~ r.. r r ~, ~ ~ ~ y 0 d ~ ~ v _G d d ^= N m U ~ ~ y D Vf 0-Tl 5- 10' LOG OF BORIN6 BH-1 EQtripment Solid Flight Auger/Rotary Yash Elevation 297±ft. Date 12/29/94 SANDY CLAY (CL) brown; soft, wet; rock fragments @ 2' augered to 10' depth SILTY SAND (SM) red brown; medium stilt; wet; some scattered gravel; rocky @ 5' CLAYEY GRAVEL (GC) red brown; low plastic fines; medium stiff; gravel is fine to 2", angular switched to rotary dulling @ 10' hard dulling @ 13' color change to gray @ 14' SB~H PB PB PB PB PB PB PB PB PB PB 66 15~ PB-1 PB-2 20 PB-3 PB-4 Pe-s 25 PB-6 30 PB-7 PB-6 35 PB-9 PB-10 l PB 1 I I I 140 ~~ GeotResource Consultants, lnc. ~~~ Geologists 1 Engineers - Enviroronentsl Scientists Job No. 1866-002 Appr:~ZDate ~ set casing to 12' GRAVELLY CLAY (CL-GC) mottled gray and yellow brown; very stiff--hard; moist; gravel is fine, some coarse sand; mostly sandstone and volcanic rock poss~le slide plane @ 16.8' angular to sub rounded rock obstruction @ 17' .15' gray clay seam -stiff, wet, w/sharp lower contact; dips about 10 degrees gravel @ 21 ' -moderately hard; very strong gravel Ca 23.5' CLAYEY SANDY GRAVEL (GC) light olive gray, mottled w/orange brown dulled out to 26 2' hard dulling @ 27.5'-30' cuttings are hard silty sand, gray @ 31.0' chance to red-brown to 32.5', then mottled w /gray and minor y allow washed away 32.5 + to 33.4' CLAYEY SAND (SC) yellow brown mottled w/olive gray and orange brown; very fine sand, some medium; some weathered sandstone gravel LOG OF BORING BH-1 TERRABAY AREA D FIGURE 2a i ~ r~ /'~ ~ w -~ N .. ~ N ~~ ~~ ~~ ~ 0 .,r ~'' v d , 0 V D qr Vf m NZ E p N P8 PB-11 40~ SAND AND GR AYQ r= ,~ s (, r i i c r U 45 50 PB PB LOG OF BORIN6 BH-1 Equipment SohdFlight Auger/Rotary wash Elevation 297 ± ~- Date 12!29/94 increase in gravel hard dulling (~ 45'-47.8' cuttings are sandstone olive black to olive brown; hard; very strong some zones of hard dnlting ~ 50'-54.5' 5 5 easy dulling C~ 54.5' cuttings are green gray and red-brown, clayey hard dulling (d 57'-575' ~ 59' -clay, yellow gray; plastic 6O ~s ssss s FRANCISCAN MEt.ANGE ssss s s s sandstone and stltstone s s s olive brown w/clay seams PB-12 ssss ss s light olive gray PB-13 ssss s 65 Boring terminated ~ 642' 70 75 l 1 I { I I 8o..1L_ GeolResource Consu{tams, Inc. ~~ Geologists f Engineers f Environmental Scientists Job No 1866-002 pppr:~D Date ~~~- LOG OF BORING BH-1 FIGURE TERRABAY 2 ~J AREA D i =~ i~' t ~. I~ J .. ~ LOG OF BORIN6 BH-2 6 ~ ~ Equipment Rotary Mash n ~ 297±ft. 12/30/94 Elevation Date 0 GRAVELLY SANDY CLAY (CL) red-brown; stiff; moist to wet; bw plasticity; grades to brown, minor red-brown, mottled rocky dolling Q 4.5' 5 mostly sandstone, gavel is olive black and red-brown; subangular to subround; deeply to moderately weathered coarse black sand; triable, probable Mn02 10 --{ ~ rocky dolling Q 10.5' 1 S rocky dolling Q 15.7' hard drilling Q 17'-18' fractured ehert Q 18.4' SANDY CLAY (CL) w/some gravel orange brown mottled w/light gay; stiff; moist 20 to wet; light gray cby, less sand, more plasti~n near-veritcal contact from 18.9'-26.5' 25 30 hard dotting Q 26.5' -sandstone boulder, olive gray; very strong; moderately hard hard dolling Q 32'-34 ± ,then easier dolling to 36' cuttings from 34'-36' in gray clay 35 refusal fi rock (boulder) @ 36' w/Pitcher sorrel sampler CLAYEY SAND (SC) AND GRAVEL (GC) olive gay w /yellow brown; mottled; dense 40 _~L GeolResource Consultants, Inc. ~~ Geoloflists f Enflineers ! Environanentd Scientists Job No. 1866-002 pppr:~~Date 7 !s' K a ~' ~r v N N ~ N ~ a o a `~ V1 m E U D lA Z S8cH 78 SS<H 77 PB PB-1 PB PB-2 PB PB-3 pB PB-4 PB PB-5 PB LOG OF BORING BH-2 TERRABAY AREA D FIGURE 3a r ,.i r L1 -. ~? u ~ ~' ,• N .. L06 OF BORIN6 BH-2 a N ~ ~ ~ ~ S ` ~ ~ .s EQuipment Rotary tifash m E c°v o ... v, ~ o ~ Elevation 297 *_ tt. Date 12/30/94 Pe PB-6 40 hard dulling Q 41 ' •asy dulling Q 42'-43' :. hard dulling Q 44' 45 Bulk 5 0 FRANCISCAN MELANGE dark gray shale Boring terminated Q 49.5' 55 60 65 70 75 t I I I 1 I 8O_.!j_ GeolResource Consultants, Inc. ~~~ Geologists 1 Engineers ! Envirotunentd Scientists Job No. ~ 866-002 Appr:~Date ~ ~ LOG OF BORING BH-2 TERRABAY AREA D FIGURE 3b fi t ~i •ti• I r i 1 ~: L -- u L j L! ~T~JIFIED SOIL CLASSIFICATION SYSTEM MAJOR DIVISIONS SOIL DESCRIPT.I.ON clean gravels GW t~~ Yell Graded Gravels, Gravel -Sand Mixtures GRAVELS with little or ` ~ ~,, „ over half of nv fines GP Poorly Graded Gravek, Gravel -Sand Mixtures ~ J coarse traction g o ~ o larger than gravels with G(~ Silty Gravek, Poorly Graded Gravel -Sand - S~1t Mixty-es c No. 4 sieve over 1296 ~ z ~ fres ~ Clayey Gravels, Poorly Graded GraVl1-Sand-Clay MvcturK Q L ~ ~ clean sands SW r=' Yen Graded Sands Gravelly Sands ' $ SANDS with Mtle or o ~ ~ over halt of f ti nO fines SP Poorly Graded Sands, Gravelly Sands coarse rac on fmr than sands Mrith SM Silty Sands, Poorly Graded Sand - Silt Mixtures ° No. 4 sieve over 1296 fxks SC Clayey Sands, Poorly Graded Sang - Ciay Mixtures } SILTS AND CLAYS ML Silts, Very Fine Sands, Silty or Clayey Fine Sands v ~ °o C N liquid limit less than 50 . CE Low Plasticity Clays, Sandy or Slty Clays N c A OL Low Plasticity Organic Silts and Clays W t 2 ~ ~ SI LTS AND CLAYS MH Micaceous ~ Diatomaceous Silts, Volcanic Ash, Elastic S, z N 4 c liquid limit greater than 50 CH ~ ,~y ~l,y ~,y Plastic' s -Fat s ., n OH .. . • • • ... High Plasticity OrgSnie Sits and Clays ° HIGHLY ORGANIC SOILS Pt Peat an0 Other Fibrous Organio Soils KEY TO SAMPLES Modified California Indicates depth of sampling with no recovery Hydraulically pushed Shelby Tube Indicates depth of Standard Penetration Test and 2" sample KEY TO TEST DATA Shear Strength, psf Confining Pressure or Normal Load, psf TxW 750 (2600) Unconsolidated Undrained Triaxial TxCU 540 (2600) Consolidated Undrained Triaxial TxCD 800 (2600) Consolidated Drained Triaxial DS 500 (2000) Direct Shear ~u ~2 400 Shear Strength obtained from Unconfsxd Compression Test P 1 =Plasticity Index C =Consolidation Test ~~~~ Geo/Resource Consultants, Inc ~~~E SOL CLASSIFICATION CHART .~, K~~ 4 .a, sn~,Er. a,w rn~nc+aoo, G~.~rcnrr~aa,o~ AND Job No. ~~°02 Appr.~ Date ~ KEY TO TEST DATA wT~..w,~.. rronw ~ - f l . L L t U ~, 1 \ 1 ~ _ ~ ~~~~ ~~ . / ~ ~ ~ ' ~~ ;' ~ I ti . , -~~ ~ \ ~; / ` ;: moo. ~ ~ ~ ~ ~ ) ci ~ ~ _=- ~~ ~, . ti ' ,, --~ ' '' ' /~ •h~ • ~~ , 1 /: ;~ , ~ti ~! ~ r F •, c ~` ~. f ~ 2 - ~' .. T- ~ %' ~ ~ ~~ ~ ~r ~ ~ i j ~ / • 2yT.0 . O ~~ N I 27L0 •~ I s . 4 A ,. ~~ ' ~~~ Geol Resource Consultants, Inc. oEa~oa~s / e+ar~ ~ e+ra sc~e+nsrs OD6 oEACH STiBcT. tAN FAAF1Ct9C0, CALF~OfOAA Mib Job No. X866-002 ApIX. ~T Date ~ 9~ Rcu~ l HYDRAUGER LOCATION MAP AREA D - TERRABAY PROJECT - SOUTH SAN FRANCISCO, CALIFORNIA ~ - ' '-"" "' 'T' ELEVATION IN FEET ' '-"" "' 'T' ELEVATION IN FEET ~---L_._L.__l I I I I I ~ i ~ ~ ~ i i i ~8 i ~~ i ~ / r // / ~ ~ // ~g / ~r ,/ ~ ~ i /~ ~ ~ o / / .i• 4 ~ 3 / /:.. ~ • ~ a~ ~ / ::~~ ~o / ~: ~ :r ~ / :. /g /i •::~ / N /v ~ ' " / :/ mm~ / / ::. o l ~ Il• ;48 ~ / .~ :. :: ~ ~ o // is : ' • a m~ ~/ N io• / ~ ~ m / ~ a / ~ / ~ ~ / to / ': i // / / m~mm / O ~ / O~n~•yi / C/ ~>~~ ~/ ~ // S ~/ ~`~ i / ~/ ~ / / / / ~ l l I V • / ~ - / ~ i N / ~m • / mm ~ / ::" ~ • .:" ~ n / „~ J ~ ~' , ~~ I ~ J ~~ . ~~m e I {{Q~} I `S I 'D ~ T q ~ g ~ ~' ~~ ~' ~~m ~ / • '• ~ ~ o 1, ~ mw 1 S o I c ~ ~ ~ ~ ^v m n~ ~ ~ ~ / O ~ ~ ~ y • , < C ~ I • ~ ~~ - ~~ ~~o~ ~~ ~~~ g R / ~~ m ~~~°z o~s'> a ~ r V 1 ~Q=o ~ s ~ ~ s ~ ~ ~ ~s ~ ~ ~ ' ~ ~ ~0-~ ; Z ELEVATION IN FEET ~ m G~ T_ C 37 T s ~ 1 :i J ..~ _1 C' f? J F~ W \ ~ z '~ Z } Z ~-. • p U~ ~ J -~ ~ n . ~ i o /O l~ J I .. / ' I ~ 1 ~ N O ' O '~ l E ~ ~ ~ ' m ,e • . .- ,, ~ :>: ~, o . < ~~ 1 ~ . ' Z Z` N W ~ ~ ~ I ~` Q Q as ~ ~ I f ao , - • ,~~~`~ f f N ~ / ~` O o ~..- r N WW ~ J ~ 1 Q Q I as 1 J ~ / m ~ ~ ~ ~ ~ ~7 N N N m s s d O ~p O O dD ~ C ~ O .m.. .~ Q ,i =o` a c • mg ~ ~~ o o ~ z W 'r tl) Q T ~- oZ[ a U O c m O J C m ¢ U F~~ `: W Q U G ~w~ ~ ¢°a ° ~ ~ V Q F ~ Z CV ~ ~~a cZ ~F~ ~W< c~° v~~ m~~ iw~ Z a~~ W ~~qq Ct~= 08~ C~3o~ ~ ~. 0 •- J W Q t~1 a R ~ o 1 o R 3 S $ ~Wo u. p ~ ~ o ~ 3a N ~ , Q } 0 ~ ? Q7 ~ ~ 'l m ~ ~ Q FLI. Z N ~ -+ aQ Q W~ O R t J J ¢ ~ ~ LL ~ Z ~ Z o o ~ G~ ~ N Za ~ ~ ~ U ~ N J ~ Z ~ 3 ~ U ~ ~ O p V R~ ~ '~~ ~ O~~ MM W ~ O li ~ a ~S~ ~ n ~a~ , J t J 1 N _ 1 ., ~ I ~ ~ ~ ~ ~ i 0 1 r I I II t 1 ~ - ~ ~ 1 1 1 1 I 111 1 ~ ~ 1 ' J W ~ 11 1 I I ii 1 I ~ ~ ~ 'g ~ ~ I II 1 ~_ 11 1 a ~ I I I II I g ~ 1 I I II I ~~ o II 1 I I II 1 -~ ~ ~ tmva do saw ~ M or~noa~ nno~a w.a3a TABLES r r -. t_. r [~ ~ ,r Geo/Resource Consultants, Inc. i -~ i-( ~{ r f? lJ u 0 ~aa a J W ~ ~ FaA W F ~" ~~ N N O O N N ~ ~O .--~ N N M et M .r O~ ~ 00 ~O ~. ~,. tV r-. (V rr vy N M M ~ N vi 00 ,C oC r. N h ~ h ~C OC rr h ,..~ N h ~ tr'f ~ C~ e+1 ~. ~ ~~~. t" fs r. N ~ ~p N f~ M M N ~ ~ OC ~p [~ .-~ f`~ y~ O~ M ~ h er ~C ~' ,..~ M h O ~ e+7 ~ ~ ' : ~ L~ ~ M ~ 111 N V1 M O~ N ~/'1 ~ O Oh Q~ ~ Q h V1 h N OC ~ ~p N ~ ~ h M ~ ~ GG' ~p M .-+ .--r 00 C~ r. ~ ~ .r .--i 00 O {~' h h n h C 00 .~ O~ et C ~C e+'f OC try N N et vi N t~ .--~ v1 N v1 v ~ s W ,may ~7 k + ~. •• rZ M vii M N O N c+1 dam' O ~ O N O I~ 'ct ~ N h ~ ~C .~ [~ ~y [~ ~O N ly .mot ~s. . „' t, ~O ~O N C ~': h N N ~ 'met N t~ ~ V~1 ~ ~ ~ N Ga M ~ N N ~O V~ }' rr 00 ~! ~< v 'f N N ~ h Q N ~ ~~'! h ran N ~ h N ~ ~ V~'f l~ I~ t~ N N ~ O L~ N 00 ~ ~' O. M M [~ ~ M ~ h 1I1 O , e'n M [~ l~ l~ O~ et V) 00 O~ ~ O '~t ~O ~p ~ ~-i N b ~ ~ ~ M et 00 h N ~ ~ ~ ~ O y~ h .-r .-. ^ Q' .. b [ ~ N M ~ h ~ ~O ~ ~ 06 0~0 oV,O ~i ~ '.~ ~ V a a a a a p„ d. a a a a a ~'' c. ~ 00 c .~ ~" ~ a« °4 TABLE 2 HYDRAUGER READINGS AREA D - TERRABAY r r ~~ V [: u ~~ i i 1 W Measured Flan Rats _, Hydrauger Number {estimated ia;$all°zts k~'~~~..... ... <''~'>`">>`..:..... 1,(20(9Sr~ . . :.: S ~ Ki $f?l~S , ?151 .:.~..~ ....:. .......... v .....ii -..- lv:: Upper Slide # 11 Steady flow, leaking SS # 18 ~ 1040 # 19 180 # 20 335 # 21 Steady thin flow 30 # 22 125 175 160 Lower Slide # 5 Moist No drip # 6 Moist No drip # 7 Dry Dry # 8 Steady trickle 50 50 # 9 20 40 30 # 10 30 50 SO # 25 Drip 25 No flow # 26 90 175 150 # 27 90 125 8S # 28 135 230 190 # 29 Flow 145 140 # 30 Drip Slow drip Slow drip # 31 l?ulsing drip Slow drip No drip Pad Level # 15 20 # A (lot 109) 25 30 # B (lot 109) Dry Notes: Recent slope work @ 14/15 has covered hydraugers; #14 was not located. Calculated flows were rounded to the nearest Sgpd. ~ cns~:ts~rz .r Geo/Resource Consultants, Inc. TABLE 3 SifMMARY OF STABILITY RESULTS AREA D - TERRABAY -. .~ lJ r^ [ul 1~ t 1 , iJ Remedial Sfitic Factror- Seismic Factor Scope a€, ' Potential for:. Constructability Alternative of Safety of Safety Constnxxion ProblEms 1) Recommended 1.6 Not presented Extensive -Rebuild High Leighton and Associates debris basin, deepening Plan keyway at base of slope, and providing second key at midslope area 2) Setback from 1.1 to 12 *w/o - No appr~eciabte Low Existing Slope groundwater construction. 3) Construct Tce 1.3 to 1.4 0.99 to 1.03 Approx. 30,000 cubic Low Buttress yards of material required 4) Preferred Alternative 1.6 1.2 Approx. 45,000 cubic Moderate Remove 10 to 20 feet of yards removed from Slide Mass and slope, approx. 20,000 Construct Shear Key** cubic yards reworked for key at base of slope, install drain in base of keyway, replace surface drains, repair hydroaugers. ' All cases, other than this one, included a groundwater level similar to that assumed by L&A. * * See attached stability section showing location of keyway and depth of soil to be removed y 1 ~ ,r Geo/Resource Consultants, Inc. APPENDIX A i~~ Sterling Pacific Job No. 95125.10 July 24, 1995 Page 3 General Site GeoloQy General site geology consisted of various types of surficial soils (slopewash, slope debris, topsoil, colluvium, etc.) overlying formational materials consisting of melange, sandstone, shale and metamorphic and igneous rocks in a variety of weathering conditions. The presence of several landslides, in-filled buried valleys, and geologic contacts both depositional and fault related in nature have added to the complexity of the geology and grading conditions. The geologic conditions encountered are considered to be substantially as anticipated in the "Geotechnical Engineering Investigation Report for the proposed Terrabay Village and Terrabay Park, Neighborhoods A and B" . The areas of significant difference were in landslide "C" where a buried valley was encountered, and in area of the cut slope along Pazkridge/Skypazk (Goatfarm Cut Slope). These areas aze discussed in~ further detail in the following text and in the referenced reports. Slope Stability During grading cut slopes were observed by representatives of both PSC and RFA. Based on our slope stability analysis and field observation, it is our opinion that the cut and fill slopes aze grossly stable against deep seated failures. Natural slopes prone to surficial instability have had debris basins, debris walls or debris fences installed or grading performed to reduce future instability related problems. Surficial slope stability of properly vegetated and irrigated graded slopes should be adequate. Slopes which are not planted such as slopes for split level lots are more susceptible to erosion. Maintenance of drainage devices on slopes, vegetation and proper watering techniques are imperative to future slope performance. Landslides/Buried Valleys Three areas with landslides were addressed during the grading of the Village. Landslide "A" was located on the west side of the Village at the end of Baycrest Way. Remediation in this azea consisted of the excavation of a deep keyway and construction of a high density buttress fill with a minimum relative compaction of 95 percent. As designed, a drain system was installed in the 11L Sterling Pacific Job No. 95125.10 July 24, 1995 Page 4 keyway and within the backcut for the buttress fill. This system of drains discharges into the adjacent controlled drainage systems. Landslide "B" is located to the north of Highcrest Lane. To aid in the stabilization of the soils associated with this area, a tie back retaining wall designated Landslide "B" Retaining Wall was constructed. Memos issued on October 21, 1989 by Jacobs Associates and on October 25, 1989 by PSC Associates recommended a reduction in pier depth to a minimum embedment of 8 feet into "hard" bedrock and 18 feet below the landslide plane. These recommendations were approved by Roger Foott Associates in a memo dated October 27, 1989. The area below the wall received additional fill soils to attain the proposed finished grades and to aid in stabilizing the slide. The area above the wall was graded to help seal the surface against infiltration of surficial water. Landslide "C" involved extensive grading to remove and replace the majority of the slide materials. A deep keyway and a detailed drainage system were included in the construction as part of the buttress design. A separate memorandum presenting the construction details and the stability calculations was prepared separately entitled "Landslide "C" Repair Program date October 11, 1989" . In addition, a separate memo from Roger Foott Associates was issued which accepted the repair work for Landslide "C" . A soil nailing wall was installed in the area of Lots 162 through 168. This wall was adesign- built-system by Schnabel Construction. The wall was constructed to provide sufficient space for the proposed buildings and to maintain the street width. (A difference between field survey and the proposed grading resulted in loss of some horizontal distance). The wall consisted of drilled shafts with 10 foot long bars grouted into place. The bars were tied into a shotcreted wall face providing support for the vertical cut. Drainage behind the wall face was provided by vertical strip drains which were connected to a horizontal collector drain at the base of the wall. This system is discharged to a controlled drainage device. Maximum wall height is on the order of 8 feet with approximately 7 feet or less exposed. Retaining wall "E" was constructed along the eastern end of the site along the north side of Hillside Boulevard from approximately Stations 139+40 to 149+59. The wall was extended from the original plans to adjust for some topographic and survey differences which were recognized during construction. The wall varies in height to a maximum of approximately 4'9". The wall is topped by a V-ditch which is discharged through vertical drains into controlled drainage devices. I ~~ Sterling Pacific Job No. 95125.10 July 24, 1995 Page 5 Rock Fill/Boulder Fill A deep canyon located at the west end of the Village was designated for a rock fill area due to its depth relative to the proposed finished grade of the fill. The original plan was a controlled boulder fill program using oversize rocks (3 foot minus). Grading began with the removal of loose surficial soils, vegetations and other deleterious materials. A canyon subdrain was installed and a toe key drain was installed. As fill was placed in this area, loose material and vegetation were removed from the sites of the canyon and benches were cut. Prior to placing any fill the bottom of the canyon was covered by a layer of filter fabric. When the fill operation was started, it was observed that the available oversize rocks were more weathered than anticipated and were breaking down during compaction resulting in a significant quantity of fines. Therefore, a change in the design and construction was implemented subsequent to a program of Field Plate Bearing Tests. The Plate Bearing Test program is discussed in a subsequent section of this report. The change included removal of the original fill followed by placing a lift of oversize rock mixed with fill from the 'PG & E cut'(18 inch minus material). Abundant water was added to aid in filling the voids and in breaking down the oversize rock. Placement, blending and compaction was performed by heavy equipment such as Caterpillar D-8 bulldozers, 825 compactors and loaded 631 and 637 scrapers. It was then graded to a crown, directing flow into perimeter drains and was then covered by a blanket drain which consisted of a 6-inch layer of gravel covered by a layer of filter fabric. These blanket drains were placed at regular vertical increments and locations as the fill progressed. Various minimum compaction standards were utilized for different zones of fill depending on overburden. Fills of more than 40 feet in thickness were compacted to at least 98 percent relative compaction, fills of 20 to 40 feet were compacted to at least 95 percent relative compaction and fills of 20 feet or less were compacted to at least 90 percent relative compaction. Plate Load Bearing Tests As discussed above, a plate load bearing testing program was performed on the rock fill to aid in determining the effectiveness of the compaction procedures and required densities. The testing procedure included the preparation of a test fill at a variety of moisture contents and relative compactions. The test were performed on a test pad approximately 60 feet in length, I ~~ Sterling Pacific Job No. 95125.10 July 24, 1995 Page 6 25 feet in width and 4 feet in thickness. The plate load tests were conducted using a series of plates to distribute the load to an 18-inch diameter bearing plate. The reaction load was provided by a Caterpillar D-8 bulldozer. The jacking system consisted of a 25-ton ram with a dial gauge read out system. Prior to starting the testing procedure the gauge and ram were calibrated by Consolidated Engineering Laboratories. Deflection measurements were provided by a dial gauge and a 10-foot long reference beam. Tests were performed on materials placed at three different conditions of: 98 percent relative compaction at 7 percent moisture content; 94 percent relative compaction at 9.4 percent moisture content; and 98 percent relative compaction at 13 percent moisture content (saturated for over 3 days). Simulated overburden pressures of 9,000 pounds were applied during each test. Settlements were observed to be permanent with virtually no rebound observed on relaxing of loads. The data is summarized below: PERCENT MOISTURE PERCENT RELATIVE SETZ'L.EMENT AT COMPACTION 9,000 psf 7% 9% 13% 98 % 0.06 INCH 94 % 0.14 INCH 98% 0.18 INCH These settlements aze well within the tolerable limits for typical wood-framed construction. The plate load testing program was approved by RFA. Debris Basin Debris basin No. 1 was constructed in substantial conformance with the plans and specifications in the northwest corner of Terrabay Village. The debris basin was designed and constructed to aid in controlling runoff and potential debris from the upslope drainage. Additional information utilized for the design of the debris basins was presented in the report entitled "Debris Flow, Potential Debris Flow Areas, Debris Flow Paths, Potential Debris Paths and Estimated Volume of Debris Materials in Storm Drainage Basins", File No. A82103-04-I prepazed by PSC Associates and dated February 15, 1983. i~~ Sterling Pacific Job No. 95125.10 July 24, 1995 Page 7 Current Site Conditions PSC geotechnical engineers visited [he subject site on December 22 and 29, 1994 and July 24, 1995, to perform a reconnaissance of current conditions. The following is a summary of our observations: 1. Minor erosional gullies have formed in several areas, particularly on cut slopes and along some of the temporary drain pipes installed for winterization. Ponded water was observed on some building pads. 2. Severe erosion has occurred on Lot 70, one of the proposed model home locations. A 1-to 2-foot wide gully, about 5 to 6 feet deep has developed in the slope between the upper and lower pads. 3. A small landslide has developed on the cut slope along the north side of Highcrest Lane, just west of the intersection with Northcrest Drive. The slide does not appear to be impacting any proposed building lots or pavements. The nearest lots to this slide are about 50 feet away to the east and across the street to the south. 4. A small slump on the slope to the west of the Debris Basin No. 1 access road was noticed. This slump has been depositing soil into the concrete ditch, which had recently been cleaned out. 5. At the east end of Highcrest Lane, the street pavement has settled around a manhole at the south curb above Lots 163 and 164 and the soil nailing wall which supports this end of the street. Some cracking of the curb and sidewalk was also noticed. The pattern of the cracking and settlement seems to indicate settlement of the manhole backfill upon saturation from infiltrated rainwater. No evidence of movement or cracking of the adjacent soil nailing wall was observed. Conclusions and Recommendations 1. Based on our observations and test results during the mass grading work, it is our professional opinion that the mass grading work was performed in substantial conformance with the geotechnical recommendations presented in our reports. Necessary ~ 1~ Sterling Pacific ]ob No. 95125.10 July 24, 1995 Page 8 modifications which were made during site grading were approved by the City and their consultant prior to performing these modifications. 2. Foundations should be constructed in accordance with the conclusions and recommendations presented in the report entitled "Foundation Recommendations Terrabay Village" reported by PSC Associates, Inc., dated February 13, 1991. Our recent review of site conditions indicate that these recommendations are still valid for the proposed development as currently planned and designed. 3. The small landslides, the distressed pavement on Highcrest Lane, and large gullies noted in this report, plus any more that may develop prior to development should be repaired in accordance with the grading recommendations presented in our previous reports. 4. Even though most of the loose rocks above the cut slope have been removed, yearly reconnaissance of large rocks above these slopes should be performed. Any unstable rocks should be removed or stabilized. 5. A program of annual monitoring and maintenance of slopes, subdrains, debris basins, and erosion control measures should be implemented until development is completed. 6. Some of the building pads are covered with grass and shrubs. Near surface soils have undergone several cycles of wetting and drying since the rough grading. Final site grading consisting of clearing of vegetation, scarification and recompaction of the upper several inches of the pad soils will be required prior to the start of foundation construction. 7. All final site grading and foundation construction must be observed and tested by a representative of PSC. The conclusions and recommendations contained in this report are contingent on this provision. 8. Any changes to the final grading or foundation plans should be reviewed by our office. 9. Good surface drainage is imperative to the future performance of the site. Positive measures should be taken to properly finish grade the building pads after the structures and other improvements are in place to reduce the potential for differential soil movement, erosion and subsurface seepage. Drainage water from the lot and adjacent I ~~ Sterling Pacific Job No. 95125.10 July 24, 1995 Page 9 properties should be directed off the lot and to the street away from the foundations and the top of the slopes. Experience has shown that even with these provisions, a shallow groundwater or subsurface water condition can develop in areas where no such water condition existed prior to site development. Proposed subdrainage systems around the structures should help alleviate such conditions. 10. It is recommended that homeowners be provided with a copy of the attached Appendix A "Suggested Guidelines for Maintenance of Hillside Homesites for Slope Stability and Erosion" or a similar document. This helps the individual homeowner to understand the importance of hillside maintenance. ' Limitations Our professional opinions and recommendations contained herein were made in accordance with generally accepted geotechnical engineering principles and practices and are based on our previous work for the project and a site reconnaissance and the assumption that the soil conditions do not deviate from the observed conditions. All work done is in accordance with generally accepted geotechnical engineering principles and practices. No warranty, expressed or implied, of merchantability or fitness, is made or intended in connection with our work by the furnishing of oral or written reports or findings. The recommendations and conclusions contained herein shall be considered valid only if PSC Associates, Inc. is retained to review any changes to the plans and to monitor and test all geotechnical related construction, including final site grading, repairs of existing erosion and landslides and foundation construction. If these services are performed by others, the conclusions and recommendations contained herein will be considered null and void and invalid. 'This report has been prepared for the proposed Terrabay Village to assist in the current evaluation of the property and to assist the architect and engineer in the design of this project. Fn the event any changes in the design or location of facilities are planned, or if any variations or undesirable conditions are encountered during construction, our conclusions and recommendations shall not be considered valid unless the changes or variations are reviewed and our recommendations modified or approved by us in writing. i~~ Sterling Pacific Job No. 95125.10 July 24, 1995 Page 10 This report is issued with the understanding that it is the owner's responsibility to ensure that the information and recommendations contained herein are called to the attention of the designer for the project, and that the necessary steps are taken to see that the recommendations are carried out in the field. Should ownership of this property change hands, the new owner should be informed of the existence of this report. The findings in this report are valid as of the present date. However, changes in the conditions of the property can occur with the passage of time, whether they result from legislation or from the broadening of knowledge. Accordingly, the findings in this report might be invalidated wholly or partially, by changes outside of our control. Therefore, this report is subject to review by the controlling governmental agencies and is valid for a period of one year. Respectfully submitted, PSC ASSOCIATES, INC. Daniel . O'Conn 11, P.E., G.E. Principal Engineer DPO:mc Enclosures: References Appendix A c:\wps l~rpt<om\95 vs IO.n4 -~~ LIST OF REFERENCES 1. "Additional Details for use in the Final Site Grading Plans for Neighborhoods A and B, Terrabay Development, South San Francisco, California', Job No. 83103.10, prepared by PSC Associates, Inc., dated September 27, 1984. 2. "Additional Slope Stability Analyses, Terrabay Development, South San Francisco, California, A Development by W.W. Dean & Associates for Resources Engineering and Management, Grading Design for Terrabay Village and Terrabay Park (Neighborhoods A and B)", Job No. A83103-01, prepared by PSC Associates, Inc., dated March 20, 1984. 3. "Final Report -Slope Monitoring Services at the Tie-back Soldier Beam Retaining Walls for Landslides 'B' and 'D", Job No. 83103.31, prepared by PSC Associates, Inc., dated November 20, 1987. 4. "Geotechnical Engineering Investigation Report, Grading Design for the Proposed Terrabay Village and Terrabay Park, Neighborhoods A and B, Terrabay Development, South San Francisco, California", Job No. A83103-01, prepared by PSC Associates, Inc., dated November 15, 1983. 5. "Geotechncial Engineering Investigation, Proposed Tieback Retaining Walls at Landslides "B" and "D", Terrabay Development, South San Francisco, California", Job No. 83103.31, prepared by PSC Associates, Inc., dated July 31, 1985. 6. "Geotechnical Engineering Investigation, Recommendations for Tieback Retaining Walls at Landslides "B" & "D", Terrabay Development, South San Francisco, California, Addendum -1", Job No. 83103.31 prepared by PSC Associates, Inc., dated May 15, 1989. 7. "Geotechnical Engineering Investigation Recommendation for Tieback Retaining Walls at Landslide "B" and "D"", Job No. 83103.31, prepared by PSC Associates, Inc., dated May 15, 1989. 8. Letter of "Clarification for Seismic Design Considerations Tie-back Walls at Landslides "B" and "D"", Job No. 83103.31, prepared by PSC Associates, Inc., dated June 26, 1989. i~~ LIST OF REFERENCES (Cont.) 9. Memorandum Regarding "Landslide "C" Repair Program", prepared by PSC Associates, Inc., dated October 11, 1989. 10. "Debris Flow, Potential Debris Flow Paths, Potential Debris Flow Paths and Estimated Volume of Debris Materials in Storm Drainage Basins", Job No. A82103-04-I, prepared by PSC Associates, Inc., dated February 15, 1983. 11. "Foundation Investigation for Proposed Terrabay Village, South San Francisco, California", Job No. 89102.11, prepared by PSC Associates, Inc., dated February 13, 1991. 12. "Geotechnical Review, Terrabay Village, Terrabay Park, Recreation Center, Terrabay Development, South San Francisco, California", Job No. 94127.10, prepared by PSC Associates, Inc., dated December 30, 1994. 13. "Slope Monitoring Services at the Tie-back Soldier Beam Retaining Walls for Landslides 'B' and 'D'", Job No. 83103.31, prepared by PSC Associates, Inc., dated February 23, 1987. 14. "Supplemental Geotechnical Evaluation of Completed "Goat Farm" Cut Slopes, Terrabay Development -Phase I, South San Francisco, California", Job No. 89140.20, prepared by PSC Associates, Inc., dated November 29, 1991. I ~4 APPENDIX A INCLINOMETER DATA tl ~ ,,v Geo/Resource Consultants, inc. ~~ ~~ r t t. f i d ~ ~ m ~ Z N V1 as "~ Ca.) ~ =' ~~ W N N F- '' W p ai! ~~~ 0 ~ Z N J O U Z o m tl~ N Z ~- ~ ~ ~ Qua V ~ ~ r ~ ~ N Q W r' N H ~ oil 0 ~ Z N_ ...I O ~..~ ~ Z O H S i ~. ~ ~ = s ~ ±. p y o s e 0 .. i d 0 e L A e o e g i Q t 4 _a 4 • ~ ~ ~ ~ e °w w °w w $ ~ ~ S ~ w O ~• I r- Q ~ r ~ 00 N_ ? ~ ~! ~~ ~~~ Qr..` V N O ~ ~ N r r Q CW ~ r G ~ O N Z_ c J '- U o Z Q ~ !"' ~ 1 {n m r ~ N Z ~ ,~ (nmd Q ~ ~ U~~ N Q ~ r'N r W o~C ~ ~ ~ o~ N Z 0 J v o z g i ~ ° 4 ~ ~ • 0 e a 4 s o a . 8 z ~ 4 ~ n ~~y S a e a a ` e ~ ~ ~ ~ w ~i w n ~ _ ~ H d W O • s a v i r-- a N .~ ~ o~ m ~ . N Z ~ t0 °~ Q~O~f U~'~ ~o W ~ aid ~ ~ as ~ co Z N J ~ U p Z t- N a r- c~ m a C7 N Z 07! Q ~ ~ .- r. U~'~ WoN ~O~ ~~~ O e Z N J ~ U Z ~ a 4 ~ s e e a a ~ ~ ~ :~ .. ": _ z d 0 a a <• s ~ e _ 8 ~ '~ S v q a v 1~ T • s _ a ~ ~ ~ s e s s J r• i r-~ ~} O~ ~ ~ ~ Z N d! No ac~et ~~ U"'~ o .- ~~o W O N r W O °~ ~ '~ ~ ~ _Z N J ~ Z ~ Qf d (/~ w ~ N Z~y" W yo Q ~ !~ ~ ~ Vow ~ r. O W O N a- W O ~ ~~~ ~ ~ Z N J ~ U Z ~ a a ~ g ° 4 ~ ~ ~ e a e ~~= e a e ,. .. ,. 0 i `~ v w a e 3 .... .... .... .... .... .... .... .... C Q .... .... .... ..... .... .... .. e a _ _ ................... ....:....~..,.......:........,........,........,........,...... a x 0 1 J • -, ~~ ~; J ~~ M ~ r ~ ~ ~ (' N Z ~ Q ~ d r 07 ~~ U ~o ~ O N W ~ r- ~~~_ O ~ Z N J ~ U p Z ~ M a ~"' OY r (' N Z ~ y~~ ~, a~~ U~•- ~o ~ O N W r- W ~ °~ ~ d o~ O N Z ~ o U p Z -- S a 4 S v "a e s W 0 • a 4 e e o i 4 ~ ~< 0 e } e - ~ s ^~ .. .. s i t 1, t. . f~' f it U r ~ N ~ ~ ~ C7 N Z V) c~ ~ V N ~~ OC o ~ W ~ ~ F" O ~ ~ LL as O N o U p Z t- N ~ C7 N r V1 as ~ Q N ~ ~ O O W ~ ~O~ ~~~ O N o U p Z t- a e ~ z ~ o Q P a ~ ± ~ ~ v e o ~~ e ~ o e a ~ s s d 0 e = 4 a a a ; e 0 S s e } e o ~ ,. o o s v s 0 l L F N ~ N ~ Z ~~ ~ ~ d . ~ t0 V N ~ ~ O N W ~ .- E"' p ai! ~~~ ~ ,. Z J O U p Z f- N ~ N ~ N () N Z ~ ~ e~ t,/~ o~ ~ Q~c~ V N ~ ~ O j W ~ .- F"O~! W~^ ~~~ ~ ~ J o U p Z f- a i a ~ z • s ~ r • e t 0 v e e- s a x s• s .. e d v ...._...._ .............. a 'a e __ - ~ ~ ° 4 ~ •. _ ~ o _ e _ } t e • ~ n w °w w = = i ~ i ~ n w ~ i ~ i g Y W~ G u APPENDIX B SLOPE STABILITY RESULTS ALTERNATIVE 4 Slope stability analysis was based on SPENCER's method using Computer Code TLLOPE developed by TAGA Engineering Software Services in Lafayette, California. The cases evaluated aze listed below. • - (1) Static Condition -low water table, 7 feet below existing ground. (2) Seismic Condition -low water table, 7 feet below existing ground. (;) Static Condition -high water table, at finished ground surface. (4) Seismic Condition -keyway backslopes high water table, at finished ground surface. 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O C a m ~ N ~lmoom~om ~~ omma r/ rl e'1 i~ OHNNN Om~O10b O O eel eel ed N rl ~-1 rl e'1 ~+ .INeI•a M a M m ~q M N a N as ~~ocnrovrr+~oovF.~+ la WWAAA00.4.-I.AN W lIn ~O FWO.-INn1vW m ~o~o~o~v~oc-rFe~rv O pW O W AW AO l~1 rl R1 QIA .~ •AC~IC~.i~00UfAC+1PN v OIA ADO Nt~Nmlr1W m 'O~i NNNNNNNNNNe~~I 0 0 0 0 0 0 0 0 0 0 0 0 Waooooo0000000 '~ m m HoaDaDaDaoaDaDmmaD d~ouinrin~nm~tia~n~fai - • y N .i ~-1 .-1 .~ ri .~1 .-1 ri .i .~ rl - i -i . p~ O eel 1!~ PI v ~O CO v N l~ r1 O Ga~i ! b v PI NI W f nl O A O N~fCDrIVLL'lrlmiAO {~ gmAA~O ; h~NPI ~oFnNAev.im~neno NONiffmOlrl~OArlvhO ~~OAW[~F~OIAVVRINN vPr11AAR1l~rl1AA1~F N N R1 R1 of V v h W W tp tp ~ d d o ,~ N e.~ sin ~o e~ m A o .i ~~ ~~ m ~, N 1p rl O O as 00 0 v v v 'f ~ ~ ~N Q ~' a ~ a~ m ~a u ADDENDUM "AREA D" SLOPE STABILITY ANALYSIS AND REMEDIATION PHASE I, TERRABAY PROJECT SOUTH SAN FRANCISCO, CALIFORNIA SEPTEMBER 1995 1866-002 PREPARED FOR: SunChase GA Calif I, Inc. 6001 North 24th Street, Suite A Phoenix, Arizona 85016 i ~' r- ~ ,~ ~ ~ s .~- i .~ Geo/Resource Consultants, Inc. GEOLOGISTS/ENGINEERS~ENV)RONMENTALSCIENTISTS 505 BEACH STREET, SAN FRANCISCO, CALIFORNIA 94113 Geo/Resource Consultants, Inc. GEOLOGISTS ENGINEERS 'ENVIRONMENTAL SCIENTISTS Corpcra:e lieadgw~rs sos Beach Street San Francesco. California 94133 (415) 775-3177 FAX (415) 7752359 °eg~onal OH:CeS An20na Cahfoma Hawau Yryr+a J r, _~ r '_~ u September 5, 1995 1866-002 SunChase GA Calif I, Inc. 6001 North 24th Street, Suite A Phoenix, Arizona 85016 Attention: Mr. Larry Harris RE: SUPPLEMENTARY REMEDIAL MEASURES AREA "D" SLOPE STABILITY PHASE 1 TERRABAY PROJECT SOUTH SAN FRANCISCO, CALIFORNIA Ladies and Gentlemen: This letter report presents additional remedial measures to enhance the stability of Area "D" in the Terrabay project referenced above. In our report dated July 26, 1995, Geo/Resource Consultants, Inc. (GRC) evaluated five alternatives for remediation including the alternative proposed by Leighton and Associates (L&A). Out of these five alternatives, we believe Alternative 4, which consists of removing a thickness of 10 to 20 feet of slide material across the entire slide and constructing a shear key at the bottom of the slope, is the most desirable alternative. This alternative actually removes some of the driving forces from the potential slide and improves the constructability of the required sheaz keys.. The computed factor safety of this alternative is equivalent to the previous L&A plan, which has been accepted by the City. Upon further review by various parties, including L&A, while Alternative 4 satisfies the static factor of safety requirement of 1.5 for considering potential slide movement below the bottom sheaz key, it did not meet the requirement in considering shallower slides above the bottom sheaz key. To achieve the same level of factor of safety for all modes of potential failure, we propose to lower the groundwater level within the existing inactive slide deposits to below the slide plane, approximately 20 feet below the finished slope grade. To accomplish this goal, GRC recommends two additional drainage keyways be established within the existing inactive slide deposits. These two drainage keyways will be located across the entire width of Area D and extend to below the slide plane, at the approximate locations shown on the profile attached as Figure 1. The depth of the drainage keyway excavations is anticipated to be about 20 feet, with temporary construction slopes to be determined during grading. Drainage will be provided by placing a minimum 6-inch-diameter perforated pipe covered by Caltrans Class II permeable drain rock, or Z GD100:t866-L1 September 5, 1995 1866-002 Page 2 ,. alternatively, wrapped by crushed rock and filter fabric, and sloped to drain to the Swale immediately north of Area "D". The keyways will be constructed of compacted fill, placed at 90% relative compaction in accordance with our report. The locations of these two cross slide drainage keyways were selected to effectively intercept subsurface water infiltration in the remaining potential slide mass. The placement of these two keyways also enhances overall stability by replacing the slide plane and slide mass with engineered fill. Based on the above drainage measures added to the Alternative 4, the safety factors were computed to be 1.8 for static condition and 1.3 for seismic condition for potential slides above the bottom shear key. The details of the analytical data are shown in the appendix of this letter report. In our opinion, placement of these keyway drains will eliminate the need for additional hydraugers; existing hydraugers, where salvaged during construction, may be rehabilitated and used. Selected piezometers and inclinometers should be preserved for long term monitoring of Area "D". We trust that we have provided the information you need at this time. Very truly yours, GEO/RESOURCE CONSULTANTS, INC. Jam'" Glenn A. Romig, P.E., G.E. Geotechnical Engineer ~~ ~ ~ /v -~,- Eric S. Ng, P.E., .E. Principal Engineer Alan D. Tryho .G. Senior Vice President ADT/ESN/GAR:csc Attachment: Figure 1 Appendix .1 '~ c~nioo:ts66-tt ,,~ Geo/Resource Consultants, Inc. ,~ a i r t: r ~_ r -~ U v ygrMlpN1 i i i I 4 i i i! i! i i i i ~~ I y a.'~ i ~~ s i ,,.; ~T ~ ~ ~', i ys a i ~~~ ~ ~ I ' ~ :: ,', .„~ •• _~ / : _: ~3, ~F ~ .:~:i ; f~ ~€ ; ,.. ~ ~ ,- q . ~: ~ / / ~ ~ . fi/ ~ a / ~ 7 // 1 / /~ a. H i i Y i i t i I i i t t 3 8 s j i' ~~ I 1 i _, APPENDIX SLOPE STABII.ITY RESULTS ALTERNATIVE 4 Slope stability analysis was based on SPENCER's method using Computer Code TLLOPE developed by TAGA Engineering Softwaze Services in Lafayette, California. The cases evaluated aze listed below. 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O ~ I O O~ , m ~ ~ y. i F- W N N W UW3 U N ~. ~ lL ~ rC~ • ~ 2 H or ~j~ w Fn ~ ~=- v=i w ~'" ~ Z r ~ f~~ ~~ 0 o d ~' ~ ~~` Z +'~ :~ J ~ ' i ~`~ l~ ,~ ~~ O O ~' Oeb- N 00'OOb- -. y Y ~ S r `7 `y .Y ) ~ w O ~ { ~ ib ~ ~ ~ {1{ ~ 1 ~ LL. vt o -r, E N ~ 1 ~ J ~~ N C W o ~ n 0 C O ..~ m OO"oa~- w ObZ- TOTAL P.F79 GRADING REPORT SHOWING SUMMARY OF TESTING AND OBSERVATION SERVICES DURING THE MASS GRADING OF 'I'ERRABAY VII,LAGE AT TERRABAY DEVELOPMENT, SOUTH SAN FRANCLSCO, CA FOR Sterling Pacif c Management Services, Inc. 6001 N. 24th Street, Suite A Phoenix, AZ 85016 n1~ ASSOCIATES INC Geotechnfca/ & Env/ronmental Consultants Construction Materla/s Test/ng ServJces CORPORATE HEADQiJARTE1tS 1185 Terra Bella, P.O. Box 699 Mountain View, CA 94042-0699 Ph: (41~ 969-1144 • Fax: (41~ 969-5523 Job No. 95125.10 July 24, 1995 GRADING REPORT SHOWING SLTMIVIARY OF TESTING AND OBSERVATION SERVICES DURING THE MASS GRADING OF TERRABAY VII.,LAGE AT TERRABAY DEVELOPMENT SOUTH SAN FRANCISCO, CALIFORNIA FOR STERLING PACIFIC MANAGEMENT SERVICES, INC. Introduction This report presents a summary of the observations and testing services provided by PSC Associates, Inc. (PSC) during the mass grading operations for the proposed Terrabay Village at the Terrabay Development in the City of South San Francisco. Our services were primarily provided between June, 1989, and December, 1990. We also performed recent observations of site conditions in December, 1994, and July, 1995, as descn~bed in this report. The grading contractor for the mass grading phase of the project was Piombo Construction Company. Project plans were prepared by C-REM Engineers (now known as KLH Engineers, Inc.) and are entitled "Stage I Grading, Rough Grading Plans for Terrabay Development", dated April 17, 1989. Grading was performed in substantial conformance with the project plans and Stage I Grading Specifications, also by C-REM. Any modifications were approved by the City during the construction. A representative of Roger Foott Associates (RFA), the City's geotechnical consultant, was present on a full time basis during the mass grading. RFA also reviewed and approved the various geotechnical engineering designs and construction methods. RFA was recently acquired by GEI Consultants. Project Description Terrabay Village (Village) is one of the two residential units completed during the Phase I grading of the Terrabay Development. The Village is located at the west end of the Terrabay site. It is bounded by Hillside Boulevard Extension on the south, the South San Francisco Drive Entrance on the east, open space to the north and the Hillside School to the west. Grading for the Village entailed the construction of 169 building pads and several interior streets. Elevations on the site range from a low of approximately 190 feet (NiSL) near the southeast corner of the site to a high of approximately 525 feet (MSL) located near the north central portion of the site. Prior to grading, the natural topography of the site sloped towards the south, with two main drainage courses located on the east and west ends. Fill slopes constructed during the grading of the Village were at inclinations of 2:1 (horizontal to vertical), with maximum vertical distance of about 160 feet between the toe and top of fill slopes, and maximum thickness of about 60 I ~~ Sterling Pacific Job No. 95125.10 July 24, 1995 Page 2 feet. Cut slopes constructed had maximum heights on the order of 80 and 40 feet with inclinations of 2:1 and 1 3/4:1 (horizontal to vertical), respectively. Additional site grading partially filled the major eastern drainage course and a desilting basin was installed at the upper grading extent in the western drainage course. Geotechnical conditions within the Village were presented prior to grading in the "Geotechnical Engineering Investigation Report, Grading Design for the Proposed Terrabay Village and Terrabay Park, Neighborhoods A and B, Terrabay Development, South San Francisco, California, Job No. A83103-01", prepared by PSC Associates (Reference 1). Actual conditions were reviewed during the grading and supplemental geotechnical investigations and analyses were performed as the conditions encountered warranted. Several areas with special geotechnical or design conditions were encountered during grading. These areas included landslide stabilization or removal, rock cuts, retaining walls, high density fills, toe keys and subsurface drains. In general, the extent of these areas was indicated in the previously referenced report or aze discussed in this report. These areas are shown on the Grading Operation Guidelines Index Map, which is part of the C-REM rough grading plans. Grading Summary Grading generally began with the removal of brush and vegetation from the areas to be graded. Loose surficial soils were removed to firm natural ground in areas to receive fill. Prior to placing fill, the exposed ground surface was scarified, moisture conditioned and compacted. Fill soils derived from onsite cutting operations were then placed and compacted in layers until design elevations were obtained. During the grading operations, compaction procedures were observed and in-place density tests (ASTM D 2922-81) were performed to evaluate the relative compaction of the fill. Field observations and the results of the in-place density tests indicated that the fill was generally compacted to design specifications of at least 90 percent or 95 percent relative compaction depending on the specific requirements. Laboratory tests were performed on samples of the materials used for fill to evaluate moisture-density relationships, optimum moisture content and maximum dry density (ASTM D-1557-78). Results of all field and laboratory tests aze tabulated. in our project files and are available for review. I ~~ GRADING REPORT SHOWING SUMMARY OF TESTING AND OBSERVATION SERVICES DURING THE MASS GRADING OF 'I'ERRABAY PARS AT TERRABAY DEVELOPMENT, SOUTH SAN FRANCISCO, CA FOR Sterling Pacific Management Services, Inc. 6001 N. 24th Street, Suite A Phoenix, AZ 85016 n1~ Geotechn/ca/ & Envlionmenta/ Consultants Construction Mate~la/s Testing Services CORPORATE HEADQUARTERS 1185 Terra Bella, P.O. Box 699 Mountain View, CA 94042-0699 Ph: (415) 969-1144 • Fax: (415) 969-5523 Job No. 95125.10 July 27, 1995 GRADING REPORT SHOWING SitMMARY OF TESTING AND OBSERVATION SERVICES DURING THE MASS GRADING OF 'I'ERRABAY PARK AT TERRABAY DEVELOPMENT SOUTH SAN FRANCISCO, CALIFORNIA FOR STERLING PACIFIC MANAGEMENT SERVICES, INC. Introduction This report presents a summary of the observations and testing services provided by PSC Associates, Inc. (PSC) during the mass grading operations for the proposed Terrabay Park at the Terrabay Development in the City of South San Francisco. Our services were primarily provided between June, 1989, and December, 1990. We also performed recent observations of site conditions in December, 1994, and May, 1995, as described in this report. The grading contractor for the mass grading phase of the project was Piombo Construction Company. Project plans were prepared by C-REM Engineers (now known as KLH Engineers, Inc.) and are entitled Stage I Grading, Rough Grading Plans for Terrabay Development", dated April 17, 1989. Grading was performed in substantial conformance with the project plans and Stage I Grading Specifications, also by C-REM. Any modifications were approved by the City during the construction. A representative of Roger Foott Associates (RFA), the City's geotechnical consultant, was present on a full time basis during the mass grading. RFA also reviewed and approved the various geotechnical engineering designs and construction methods. RFA was recently acquired by GEI Consultants. Terrabay Park encompasses an area which includes Landslide "D", a major landslide that has not yet been repaired. A supplemental investigation report by Leighton and Associates dated January 28, 1992, presented a scheme to repair the slide. Another firm, GeoResource Consultants, performed a recent investigation of the slide. These reports should be consulted for conclusions and recommendations regarding the stability of the slide area and development of the adjacent lots. Proiect Descriution Terrabay Park is one of the two residential units that was mass graded as part of the Phase I grading. Grading was performed to provide building pads for 125 single-family residential units and the interior streets. Terrabay Park is located in the northwestern portion of the site. It is bounded on the north by open space, on the south by the alignment of South San Francisco -~~ Sterling Pacific Job No. 95125.10 July 27, 1995 Page 2 Drive, on the west by Terrabay Village and on the east by the alignment of Gree~azk Terrace and undeveloped land. General site topography slopes towards the south. Elevations within the site range from a high of approximately 500 feet (MSL) near the east side of Terrabay Park to a low of approximately 210 feet (MSL) located on the south side of the site adjacent to South San Francisco Drive. Prior to grading, the site was dissected by three main drainage courses which all trended southeast. Grading resulted in the construction of fill slopes with maximum vertical distance of approximately 210 feet or less at inclinations of 2.0 to 1.0 (horizontal to vertical) and cut slopes of 60 feet or less in height with inclinations of 1.5 to 1.0 (horizontal to vertical) or less. The thickness of fill was generally less than 30 feet. Geotechnical conditions within the Park were presented prior to grading in the "Geotechnical Engineering Investigation Report, Grading Design for the Proposed Terrabay Village and Terrabay Park, Neighborhoods A and B, Terrabay Development, South San Francisco, California, Job No. A83103-01", prepared by PSC Associates. Actual conditions were reviewed during the grading and supplemental geotechnical investigations and analyses were performed as the conditions encountered warranted. Several areas with special geotechnical or design conditions were encountered during grading. These azeas included landslide stabilization or removal, rock cuts, retaining walls, high density fills, toe keys and subsurface drains. In general, the extent of these areas was indicated in the previously referenced report or are discussed in this report. These areas are shown on the Grading Operation Guidelines Index Map, which is part of the C-REM rough grading plans. Grading Summary Grading generally began with the removal of brush and vegetation from the areas to be graded. Loose surficial soils were removed to firm natural ground in areas to receive fill. Prior to placing fill, the exposed ground surface was scarified, moisture conditioned and compacted. Fill soils derived from onsite cutting operations were then placed and compacted in layers until design elevations were obtained. During the grading operations, compaction procedures were observed and in-place density tests (ASTM D 2922-81) were performed to evaluate the relative compaction of the fill. Field observations and the results of the in-place density tests indicated that the fill was generally compacted to design specifications of at least 90 percent or 95 percent relative compaction i~~ Sterling Pacific Job No. 95125.10 July 27, 1995 Page 3 depending on the specific requirements. Laboratory tests were performed on samples of the materials used for fill to evaluate moisture-density relationships, optimum moisture content and maximum dry density (ASTM D-1557-78). Results of all field and laboratory tests are tabulated in our project files and are available for review. General Site Geology General site geology consisted of various types of su~cial soils (slopewash, slope debris, topsoil, colluvium, etc.) overlying formational materials consisting of melange, sandstone, shale and metamorphic and igneous rocks in a variety of weathering conditions. The presence of several landslides, in-filled buried valleys, and geologic contacts both depositional and fault related in nature have added to the complexity of the geology and grading conditions. The geologic conditions encountered are considered to be substantially as anticipated in the "Geotechnical Engineering Investigation Report for the proposed Terrabay Village and Terrabay Park, Neighborhoods A and B" . The areas of significant difference were in landslide "D" and ^R" where buried valleys were encountered. These areas are discussed in further detail in the following text and in the referenced reports. The presence of a shear zone was determined during investigation of the site in the vicinity of Parkridge Circle and Sky Park Circle. This shear zone is believed to be associated with the Hillside Fault and may represent the northern most splay of the fault zone. However, the named trace of the Hillside Fault was previously mapped by M.G. Bonilla (1971, USGS MF-311) 500 to 900 feet to the south of this shear zone. No evidence to suggest that this shear zone should be classified as an active fault was observed during the grading of the site. However, this site (as well as any other site in this area of California) should be considered to be subject to strong ground motion as a result of future seismic events originating on any of the nearby major faults. im act of this shear zone on the project is its potential to impede subsurface water The primary p flow. This has been recognized and whenever possible appropriate drainage measures have been implemented during the grading operation. No other faults or indications of faults were observed during grading. I ~~ Sterling Pacific Job No. 95125.10 July 27, 1995 Page 4 Sloe Stability During grading cut slopes were observed by representatives of both PSC and RFA. Based on our slope stability analysis and field observation, it is our opinion that the cut and fill slopes other than those in the vicinity of Landslide "D" and the adjacent lots, are grossly stable against deep seated failures. Natural slopes prone to surficial instability have had debris basins, debris walls or debris fences installed or grading performed to reduce future instability related problems. Surficial slope stability of properly vegetated and irrigated graded slopes should be adequate. Slopes which are not planted such as slopes for split level lots are more susceptible to erosion. Maintenance of drainage devices on slopes, vegetation and proper watering techniques are imperative to future slope performance. Site Monitoring Slopes which exhibited seasonal seepage were perforated with horizontal drains (hydraugers). Slope inclinometers were placed in several of the major cut slopes, in Landslide "R" and Landslide "D" to provide monitoring of slope stability. Piezometers were also located in Landslide "D" to monitor water levels. Monitoring of Landslide "D" has been performed by Leighton Associates and GeoResource Consultants as part of their supplemental investigations. Landslides/Buried Valleys Two areas of landslides were addressed during the grading of the Terrabay Park. Landslide "D" is located on the north side of the site near the western terminus of Parkridge Circle. A tieback retaining wall was proposed to support the upper 6-8 feet of surficial movements and to help contain potential debris flows. A detailed description of the design parameters, analysis and construction of the wall are presented in the report entitled "Geotechnical Engineering Investigation Recommendations for Tieback Retaining Walls at Landslide "B" and "D", Terrabay Development" prepared by PSC Associates and dated May 15, 1989. This report was superseded by supplemental reports by others, as described below. i ~~ Sterling Pacific Job No. 95125.10 July 27, 1995 Page 5 A supplemental geotechnical investigation report of the landslide by Leighton & Associates dated January 28, 1992, presented a scheme to repair the slide. Another geotechnical consulting firm, GeoResource Consultants, recently performed a new evaluation of the landslide. PSC has not been involved in any recent evaluation of the stability of Landslide "D" , and therefore this report does not include any conclusions or recommendations regarding the potential impact of Landslide D on the development. These supplemental investigations should be followed for further development of this portion of the project. The area designated as Landslide "R" was removed during grading down to the underlying contact with the weathered rock melange. A keyway and buttress fill were then constructed tq the proposed design grades. Construction included the installation of a drain at the heel of the keyway and the placement of a network of horizontal drains at approximately 10 foot vertical increments. Additional blanket drains were placed in areas where seepage was present. The drains were connected to the controlled drainage system. The analyses and construction in the Landslide "R" area is covered in the report entitled "Geotechnical Report on Grading Work to Repair "Buried Valley"/Landslide "R", Terrabay Development, Phase 1" prepared by PSC Associates, dated December 10, 1990. Retaining Walls Due to field survey differences, the proposed wall at the rear of the Lots 281 and 282 R-as eliminated. Instead, a retaining wall above lots 281 and 282 was constructed. The wall consists of 21 inch diameter drilled piers with steel H-beams in concrete. The maximum depth of the piers is approximately 16 feet below finished grade and the height of the wall is 7.5 feet or less. A layer of geotextile fabric was placed against the back of the wall and Class II permeable drain rock was placed to within approx•T~hee allfacing consisted of concrete lagging placed within constructed at the top of the wall the flanges of the H-beams. Canvon Underdrains A series of subsurface canyon drains were installed in the three main drainages which traversed the site with a southerly trend prior to grading. The canyons were cleaned out to firm native material and a fabric wrapped gravel drain was installed. The drains consist of approximately i ~~ Sterling Pacific Job No. 95125.10 July 27, 1995 Page 6 9 cubic feet of 1-1/2 inch minus gravel per lineal foot with Mirafi 140 or an approved equivalent. Additional subsurface drains in the Park included two curtain drains which extend from the termini of Pazkridge Circle and Skypazk Circle to the intersection with Pazkgrove Drive and from the back of lot 228 to the western terminus of Baypazk Circle. A subdrain was extended from Landslide "R" near the northeast comer of lot 309 to the previously mentioned curtain drain neaz northwest comer of lot 235. Debris Basins Three debris basins were constructed in Terrabay Park. All three are located on the north side of the site at the mouth of the main drainage courses. Debris basin "2A" is located on the east side of retaining wall at landslide "D" and to the east of the terminus of Pazkridge Circle. Debris~basin "2C" is located to the north of Parkridge Circle across from lots 311 and 312. The third debris basin is designated "3A" and is located north of Skypazk Circle across from lot 323. The debris basins were designed and constructed to aid in controlling runoff and potential debris from the upslope drainage. Additional information utilized for the design of the debris basins was presented in the report entitled "Debris Flow, Potential Debris Flow Areas, Debris Flow Paths, Potential Debris Paths and Estimated Volume of Debris Materials in Storm Drainage Basins", File No. A82103-04-I prepared by PSC Associates and dated February 15, 1983. Present Site Conditions PSC engineers visited the subject site on May 25, 1995 to perform a reconnaissance of the present conditions. On the same day, a total of 14 test pits were excavated on the building pads, including the 4 model lots, and at some of the distressed areas, using a rubber tired backhce. On the pads, the pits were only excavated to a depth of one foot or less to observe the depth of vegetation and to allow testing of pad compaction at that depth to compare with the surface readings. Deeper excavations were made on the slopes below Lot 203 and between Lots 294 and 317 to explore conditions of the shallow landslides. I ~4 Sterling Pacific Job No. 95125.10 July 27, 1995 Page 7 The following is a summary of our observation from the May 25, 1995 reconnaissance along with remedial recommendations: 1. Most of the building pads are covered with native grass and occasional weeds or small brush. Our excavations indicate the native grass does not have extensive roots and most of the organic material can be lost from the pads during fine grading or removed from the pads by scraping the surface with a blade from a bulldozer or grader, and should not require stripping to remove the roots. 2. Our field density tests show the relative compaction of the near surface soils in the area tested less than the specified 90 percent. This is likely due to the effects of weather changes since the rough grading. In most areas tested the moisture content was well above optimum due to the heavy rains from the past few months. These tests indicate the pad surfaces will need to be scarified and recompacted prior to construction. Some drying and conditioning of the soils may be required after scarification, prior to recompaction. 3. Significant erosion was observed on a few building pads and cut slopes above the development. Substantial gullies have developed on the cut slopes on the south side of Green Park Terrace, the east side of the Water Tank Access Road, and west of the end of Parkgrove Drive. Large gullies, several feet in width and depth, have developed along the temporary drain pipes placed for winterization at the southwest corner of Lot 264, and on the slope leading between Lots 239 and 242 down to Lots 231 and 232. Each of these areas were observed last December, with some increase in severity noted during the present reconnaissance. Also, a new area of severe erosion was noted on the slope between the upper and lower pad of Lot 267. Erosion of the cut slope above Lot 269 was also observed. The erosion on the pad areas should be repaired by overexcavating all loose material and replacement to original grades as a compacted, engineered fill. After the gullies are filled back in, the surface drainage above should be redirected to avoid a reoccurrence of the erosion. Erosion on the cut slopes seems to be occurring primarily in the sandstone portion of the bedrock despite the use of jute mesh as a preventative measure. The erosion seems to be caused by the runoff of rainfall directly on the slopes, and not from the slopes above. i~~ Sterling Pacific Job No. 95125.10 July 27, 1995 Page 8 Also, no vegetation has been established on these slopes. Therefore, a better surface treatment is needed to reduce future erosion in these areas. The slopes should first be trimmed below the depth of the gullies, rather than trying to fill in the gullies. Then a surface treatment either with erosion control blankets or sprayed-on stabilization mixtures could be used. Selection of the most appropriate method should be based on aesthetic considerations and with consultation of a local erosion control expert. 4. The erosion has resulted w fiches of silt was not d especially o Lots 2692 671 266, Accumulation of up to a fe 238, 237, 232, 231, 305, and 302. This silt should be removed from the pads pnor to construction. 5. Small shallow landslides have occs Franc sco DDrive was byes rv~edllast December,sl~ below Lot 203 adjacent to South test pit showed this slide toad area above The slide s about 30 t Osfeet wide,dand concentrated runoff on the p extends about 10 to 15 feet above the toe of the slope. A small landslide was detected in our latest reconnaissance on the slope between Lots 294 and 317. Our test pit sho es bedrock t i b s sab lut 30 to 35a feet wideeeand extends weathered and sheared melang from the toe to the top of the slope. These two slides can be repaired by removal and replacement as engineered fill with subdrain systems installed, following the recommendations in our report for grading design dated November 15, 1983. 6. The shallow landslide previously noted on the cut slope above Parkridge Circle just west. of Debris Basin 2C has become larger, and a second slide ~ ~~ 8 ed,~ lide is~a west. Anew landslide has occurred on the cut slope abo shallow failure at the contact between sandstone and shale bedrock, and is about 10 to 15 wide, extending 30 to 40 feet upslope above the bench at midslope. 1 ~~ Sterling Pacific Job No. 95125.10 July 27, 1995 Page 9 The slide areas in the cut slopes are not presently impacting the lots. The debris basin area fence has been damaged by one Tide areaslcan be repaired by removing the slide unpact the road in this area. These s debris and replacement with oversize rocks and boulders. Conclusions and Recommendations 1. Based on our observations and test results during the mass grading work, it is our professional opinion that the mass grading work, with the exception of Landslide "D° was performed in substantial conformance with the geotechnical recommendations presented in our reports. Necessary modifications which were made during site grading were approved by the City and their consultant prior to performing these modifications. The recent reports by Leighton & Associates and GeoReources should be consulted for the current status and proposed repair scheme for Landslide "D" and the adjacent lots. 2. Foundations should be constructed in accordance with the conclusions and recommendations presented in the report entitled "Foundation Recommendations Terrabay Park" reported by PSC Associates, Inc., dated May 15, 1990. Our recent review of site conditions indicate that these recommendations are still valid for the proposed development as currently planned and designed. 3. The small landslides and large gullies noted in this report, plus any more that may develop prior to development should be repaired in accordance with the grading recommendations presented herein and in our previous reports. 4. Even though most of the loose rocks~ab a slo es should be performed. mAny unstable reconnaissance of large rocks abo Pe rocks should be removed or stabilized. 5, A program of annual monitoring and maintenance of slopes, ~evb elo mentbiss ompleted erosion control measures should be implemented until p Subdrains should be checked for proper functioning and repaired or cleaned out if necessary. i~~ Sterling Pacific Job No. 95125.10 July 27, 1995 Page 10 6. Some of the building pads are covered with grass and shrubs. Near surface soils have undergone several cycles of wetting and drying since the rough grading. Final site grading consisting of clearing of vegetation, scarification and recompaction of the upper several inches of the pad soils will be required prior to the start of foundation construction. 7. All final site grading and foundation construction must be observed and tested by a representative of PSC. The conclusions and recommendations contained in this report are contingent on this provision. 8. Any changes to the final grading or foundation plans should be reviewed by our office. 9. Good surface drainage is imperative to the future performance of the site. Positive measures should be taken to properly finish grade the building pads after the structures and other improvements are in place to reduce the potential for differential soil movement, erosion and subsurface seepage. Drainage water from the lot and adjacent properties should be directed off the lot and to the street away from the foundations and the top of the slopes. Experience has shown that even with these provisions, a shallow groundwater or subsurface water condition can develop in areas where no such water condition existed prior to site development. Proposed subdrainage systems around the structures should help alleviate such conditions. 10. It is recommended that homeowners be provided with a copy of the attached Appendix A "Suggested Guidelines for Maintenance of Hillside Homesites for Slope Stability and Erosion" or a similar document. This helps the individual homeowner to understand the importance of hillside maintenance. Limitations Our professional opinions and recommendations contained herein were made in accordance with generally accepted geotechnical engineering principles and practices and are based on our previous work for the project and a site reconnaissance and the assumption that the soil conditions do not deviate from the observed conditions. All work done is in accordance with generally accepted geotechnical engineering principles and practices. No warranty, expressed i~~ Sterling Pacific Job No. 95125.10 July 27, 1995 Page 11 or implied, of merchantability or fitness, is lmdadme s r intended in connection with our work by the furnishing of oral or written reports or f g The recommendations and conclusions contained herein shall be considered valid only if PSC Associates, Inc. is retained to review any changes to the plans and to monitor and test all geotechnical related construction, including final site grading, repairs of existing erosion and landslides (other than Landslide D) and foundation construction. If these services are performed by others, the conclusions and recommendations contained herein will be considered null and void and invalid. This report does not include an evaluation of the stability of Landslide "D" and the adjacent lots, or a method of repairing this slide. Our previous reports for Landslide "D" have been superseded by the supplemental investigation reports by Leighton & Associates and GeoRource Consultants. These supplemental reports should be consulted for development of this part of the site. PSC hereby disclaims liability or responsibility for Landslide "D" and the adjacent lots. This report has been prepared for the proposed Terrabay Park to assist in the current evaluation of the property and to assist the architect and engineer in the design of this project. In the event any changes in the design or location of facilities are planned, or if any variations or undesirable conditions are encountered during co a ortioariations areureviewed and o~urerecommendations be considered valid unless the Chang modified or approved by us in writing. This report is issued with the understanding that it is the owner's responsibility to ensure that the information and recommendations co stems are rtaken to see thaththe recomrnendarions arer for the project, and that the necessary p carried out in the field. Should ownership of this property change hands, the new owner should be informed of the existence of this report. The findings in this report are valid as of the present date. However, changes in the conditions of the property can occur with the passage of time, whether they result from legislation or from the broadening of knowledge. Accordingly, the findings in this report might be invalidated i ~~ Sterling Pacific Job No. 95125.10 July 27, 1995 Page 12 wholly or partially, by changes outside of our control. Therefore, this report is subject to review by the controlling governmental agencies and is valid for a period of one year. Respectfully submitted, PSC ASSOCIATES C. aniel P. O'Connell, P.E., G.E. Principal Engineer DPO:mc Enclosures: References Appendix A c:1wp51\rp«om\9512510.727 ~FESSIp~,~,. r.,~ ~~ ~ 11L LIST OF REFERENCES 1. "Additional Details for use in the Final Site Grading Plans for Neighborhoods A and B, California", Job No. 83103.10, prepared Terrabay Development, South San Francisco, by PSC Associates, Inc., dated September 27, 1984. 2. "Additional Slope Stability Analyses, Terrabay Development, South San Francisco, California, A Development by W.W • Dean & Associates for Resources Engineering and 1~~lanagement, Grading Design for Terrabay Village and Terrabay Park (Neighborhoods A and B)", Job No. A83103-01, prepared by PSC Associates, Inc., dated March 20, 1984. 3. "Final Report -Slope Monitoring Services at the Tie-back Soldier Beam Retaining Walls for Landslides 'B' and 'D", Job No. 83103.31, prepared by PSC Associates, Inc., dated November 20, 1987. 4. "Geotechnical Engineering Investigation Report, Grading Design for the Proposed Terrabay Village and Terrabay Park, Neighborhoods A and B Ted b baPy DeAssoca tes, South San Francisco, California", Job No. A83103-01, p p y Inc., dated November 15, 1983. 5. "Geotechnical Engineering Investigation, Proposed Tieback Retaining Walls at Landslides "B" and "D" , Terrabay Developmtest~ Inc u dated July 311S 1985 . ~ifornia" , Job No. 83103.31, prepared by PSC Associa , Recommendations for Tieback Retaining Walls (. "Geotechnical Engineering Investigation, California, at Landslides "B" & oD r83103~31ayr pared by PSCSAssocS tes~inc lsdated May 15, Addendum -1", Job N P 1989. 7. "Geotechnical Engineering Investigation Recommendation for Tieback Retaining Walls at Landslide "B" and "D"", Job No. 83103.31, prepared by PSC Associates, Inc., dated May 15, 1989. g. Letter of "Clarification for Seismic Design Considerations Tie-back Walls at Landslides "B" and "D"", Job No. 83103.31, prepared by PSC Associates, Inc., dated June 26, 1989. i ~~ LIST OF REFERENCES (Cont.) 9. "Debris Flow, Potential Debris Flow Paths, Potential Debris Flow Paths and Esrtimat~ Volume of Debris Materials in Storm Drainage Basins", Job No. A82103-04-I, p p by PSC Associates, Inc., dated February 15, 1983. 10. "Geotechnical Review, Terrabay Village, Terrabay Park, Recreation Center, Terrabay Development, South San Francisco, California", Job No. 94127.10, prepared by PSC Associates, Inc., dated December 30, 1994. 11. "Slope Monitoring Services at the Tie-back Soldier Beam Retaining Walls for Landslides 'B' and 'D'", Job No. 83103.31, prepazed by PSC Associates, Inc., dated February 23, 1987. 12. "Foundation Investigation for Proposed Terrabay Park, South San Francisco, California"•, Job No. 89102.11, prepazed by PSC Associates, Inc., dated May 15, 1990. -~~ APPENDIX A 1 ~~ SiJG~'I~D GUIDE r~ N~I'EI~PiCE OF HIIZ.SIDE HC~E'SI~'E5 FQR SLOPE STABILI'T'Y AND ~I~d StJOGFS'1'ID ~R N~I~NCE OF HIZZSII~ F~CMESl'I'~ FL7R SI1~PE STABII~I'Y AND EEtO6ION During the wet weather season, hca~~eown~-rs such as Y~lf. livirq in houses placed on fill (man-placed earth) or in the vicinity of excavated (cut) of ~ concerned abort the vcmdition of their building site. fill slopes, minimize the likelihood In general, modern design and connstzvvction practices codes of the local of serious larxlslidirg (slope failure) • ~ grades filled land, jurisdictions (cities and axinties~~ru~onlare among the most stringent in excavation, terracing arxi slope have been corLStrvc'ted In addition, most hillside levels the cotmtty. rofessional standards. Zherefore, the concern of the according to critical p sl drainage provisions hameawner should be directed toward maw ~s- and facilities so that they will perfozm as designed. the following general' bons and simple prat-autions are p~~ to help you Properly Main Your hillside hcenesite. Please refer to the attac3ied diagram for an i.l.lustration of terms - The general public often regards the natural terrain as stable at work firma." This is, of course, an erroneous Nature is always altering the landscape. Hills and mcxuztains ~ the valleys and lo~wlad (erosion, landsliding, creeping soil et cetera) ~ ~~ leveling the collect these products. thus the natural process terrain. Periodically (war millions of years) major land movements rebuild mountains and hills arxi these processes begin over again. In scm~e areas these proses-ses are very slow and in others they occur at a relatively rapid rate. 'Ihe development of hillsides for residential ~ sit~.erand to min~mize the possible, to enhance the natural stability probability of instability resulting from the gr~~J necessary to provide et cetera. Zhis has been dyne by the developer hcanesites, streets, yards. ~r~g ;nvestigations. and designers on the basis of geologic ~ a d drainage p~~i~ and Hover, in order to be successful. the hce~eowner. facilities must be maintained by Y'~~ As a he~neowner you are aecustc~-ed to maintainincl Yom' h~% that is, y+ou expect to paint Your hhese periodically, clean out clogged plumbing, raga-ir roofs, et cetera. MaintP~ance of a hillside hamesite must be considered on an even more serious basis because neglect can result in serious corLSeq~iences. In most cases, lot arxi site maintenance can be provided along with normal care of the grcnmds anr3 larr3scaping • Any e°sts of maintenance are far cheaper to you than repair aftP-r neglect. nl~ DON' 1. DON'T alter lot grading without ~e~~ ~1Oe. The ~~ ~°~' on yowr lot were designed to carry away water rtu~off to a place where it can be safely distributed. 2 . DON'T block or alter ditches whidz have been graded arcxu~d your Yvotise or the lot pad. Ztlese shallow ditches have been put there for the p~u.~pose of quickly rear~ving water towatrl the driveway, street or others positive outlet. 3. DON'T block or alter ditr3les or drains. If several banes rely on the same facilities, it is a good idea to cdieck with your r~ghbors- ~''~~' bacJ~d up on their property may everYtually reach you. Water bacJ~,ed up in surface drains will overflow arxi infiltrate slopes which leads to instability. Maintain the grcxmi surface upslope of lined ditches to ensure that surface water is collecfied in the ditch arr3 is riot pPnn;teed to collect behind or flora wader the lining. (See detail sketch on the attached diagram). 4. DON'T permit water to collect or porxi anywhere on Your' lot. Such water' will either seep into the ground causing tuiwanted saturation, or will overflow onto slopes and begin erosion. once erosion is started, it is difficult to control arxi severe damage may result rather quickly. 5. DON'T direct water over slopes even where this may seem a good way to prevent porxiing. This ten7s to cause erosion and slope instability. Dry wells are sometimes used to get rid of excess water when alter means of disposing of water are not readily available. I~owever, such facilities should be planned arx3 located by a qualified engineer. 6. DON'T let water pond against fourx~ations, r+etainirx3 walls and basement walls. 'Ilzese walls are built to withstand the orriinazy moisture in the grcxuxi and, where necessary - are a~anied by su~idra.; ns to carry of f excess subsurface water. However, excel-s surface water aaLSt be directed away from these structures roof drains, gutters or down spouts to pY; ~;,'~ 7. DON'T connect for that purpose. subsurface drains which may not have been designed Ir~tead, either collect the water in lined ditdzes or w;~erforated pipes and conduct it to a storm drain, paved road or suitable area of natural graurl. mere such d~annel flow is direc.'ted onto natural grcxax3 it must be converted to sheet flow Mess a suitable natural dzannel exists. g, DON'T discharge surface water into septic tanks or leaching fields. Not only are septic tanks constrvcl~ for a different purpose, but they wi11 tend, because of their '~~, ~ acuminate additional water from the gtYxmd during a heavy rain. Overloading them artificially during the rainy season is bad f'ran a slope stability starx~oint arri is doubly dangerous since their overflow can pose a serious health hazard. We generally reco®errl that the use of septic tanks be disoorrtirrued as soon as sewers are grade available. 9. DON'T place loose soil or debris over the sides of slopes. Ir.~ose soil soaps ~ ~,,~~ more readily than ca~acteci f i11. It is not ~ac'~d ~ the same strength as the slope itself and will terra to slide when laden with water arr3 may even affect the soil beneath it. Zhe sliding may clog terrace drains below or may cause additional damage in w~ea]cenis~g the slope. If you live below a slope, be sure that loose fill is not chn~ed above your property. 10. DON'T over-irrigate slopes or leave a hose or sprinkler mm~ing unattended on or near a slope. G cover and other vegetation will r,eguire moisture during the hot sumoer months, but during the wet season irrigation can cause grcxind cover to pull loose, which not only destroys the cover, but also starts serious erosion. 11. DON'T try to conpact earth in trenches by flooding with water. Not only is flooding the lease efficient way of compactirxl fine grained soil, but this could saturate and redtii~ the bearing capacity of• sLtQporting soils. 12 . DON'T change surface grade behind retaini tx~ walls or against bu.ildincJ walls because this would incrp-are the lateral loading on the walls, which could result in damage to such walls. In conclusion, your neighUor's slope, above or below your Property, is as important to you as the slope that is within your property lines. For this reason, it is desirable to develop a ~exative attitude ~~~~ hillside maintenance and we re~eryd developing a "good neighbor" policy. 5hculd conditions develop off your property which are urx3esirable frcaa indications given above, n~ action should be taken by you to ensure that prcanpt remedial measures be taken. n1~ 'Ihe following several pages P guidelines for the general mainter~x~e of hillside residential property. ~e frequerxY of attention to st~h measures depPSr]s upon both the corxliticazs at each site arxi th,e nature of the iaq~ravements on the property Recc~er~dations aarztained in the Soil Report apply to future site improvements, but we advise that you include ~ltation of a qualified professional in the planning, design and ccnstructicaz of any improvements. Such improvements including patios, swiaming pools, decks, etc. as well as ~; t direr structures. Plans ar~d details of the i~rovements to your property sha~ld be kept so that the maintenance procedures on the following Pages may be readily followed. A continuing Program of maintenance is necessary for assurance of minim~l_ difficulties. However, forces of nature acting in the future, mi afurictioning of improvements and othex asPec-'ts may cause results whidi ~~d ~ ;n~peo•~,ed by qualified professionals. Attentica'i to this ~ Y~ responsibility in order that any necessary corrective or preventative measures may be applied as soon as possible and to the extent required. Zize qualities of hillside living are superior, but in order to ~- ~~II each owner must accept the fact that the dynamic natural forces which formed the hills will contisrue to alter them. Zhe developer, his design consultants, arxi the constructors are powerless to c3~ange this. Only by your pnx3ent future actions and diligent inspec-;tion arxi maintenance can problems be kept to a minurn~. n1~ Most luuoi ~~le lot problems are associated with water. Ur~trollea water from a broken Pipe, septic tank or during wet weather causes most damage. Wet weather is the principal time of slope problems in California because the rainfall is quite variable anti may be or prolatxied. 'Ilzerefore, drainage and erosion control are iaportant asPec-'ts of h~~esite stability and the prwisions built into the developed lot must not be altered without oa~et.ent Professional advice. a of the provisions must be carried cut to a-ssui"e their v~tirn~d oPemtion. Zherefore, we Offer the following list of "Dogs" and "Don'ts" as a guide to you: DO 1. (heck roof drains, gaffe-rs arxi dam spouts to be sure they are clear. Deperraing on your location, if you cio not have roof gutters and down spouts, you may wish to install them because roofs and their wide space can shed tr~endaus quantities of water. Without gutters or other adequate drainage provisions, water falling frcffi the eaves collects against the fau~dation arxi basemerYt walls which is Lu~desirable. 2. Clear drainage ditches and d~erk the3n frequently during the re~Y' season. AsJc your neighbors to dp likesrise. 3, ~~k interceptor (brew) ditches at the tap of slopes to be sure that they are clear and that water will Holt Overflow the slope, causing erosion. 4. Be sure that all drain outlets arxi weep-holes are open arxi clear of debris, vegetation and other material which could block the3n in a storm. If blockage is evident, have it cleared. 5. Check for loose fill above and below Your Property ~ Y~ live on a slope or terrace. 6. Limit watering during the rainy season when little irrigation is required. Crver-saturation of the grauxi can cause major subsurface damage. 7. Watch for water bac3aip inside the house at sump drains and toilets, since this irxiicates drain or sewer bloc~mge. 8. Watch for wet spots oa7 Your property- ~~ may be natural seeps or an indication of a broken water or sewer line. In either case, obtain ccs~~petent advice regarding the proble3n and its correction. 9 . E~tercise ord; ~w precautica~. Your' house and building site were to meet ~ which should protect against most natural Provided you do your part in maintaining th~a. n~4 TERMS USED ON HILLSIDE HOMESITES ~ 13 4 8 4 11 Q n ~ ~ ~ r,r- 3 _~~ ~__~ ~..J f__~~ 9 ~ r -~ .~ ~~~ . r ~.-i -~ s TYPICAL SLOPE SECTION HOT TO SCALE NOTA710NS: O NATURAL GROUND SLOPE O ORIGINAL GROUND SURFACE O FILL SLOPE O CUT SLOPE O FIIL COYPACTED TO ENGINEERING SPEC IfICAT10NS AND BENCHED INTO FIRY GROUND. ® ROOF GUTTER O DOWNSPOUT CO`INECTEO TO AN UNPERFORATED PIPE OR LINED DITCH WATER COLLECTION STSTEY. US DRAINAGE SCALE OR DITCH O9 SUBORAIN (PERfORATED PIPE AND/OR PE RYE ABLE NATERIAt). OO SUBDRAIN DISCHARGE (UNPERFORATED PIPE). 11 DRAINAGE TERRACE AND DITCH (SEE DETAIL) O2 BR011 DITCH 13 LINED DRAINAGE DITCH (SEE DETAIL) 14 RETAINING 1fAll 15 VEEP-HOLES THROUGH RETAINING 1I ALL 16 BERY TO DIRECT EATER OFF SLOPE it OR T2 ~~v 'ter 6p~ pi ---- ~ ./ •. •. . ~ POOR DRAINAGE If GROUND IS ll[E 13 OOTTFD IINE i DETAIL NOT TO SCALE ',`