GEOTECHNICAL DESKTOP STUDY NEW CASTAIC HIGH SCHOOL HASLEY-SLOAN SITE, TENTATIVE TRACT 52475
FUGRO WEST, INC. GEOTECHNICAL DESKTOP STUDY NEW CASTAIC HIGH SCHOOL HASLEY-SLOAN SITE, TENTATIVE TRACT 52475 CASTAIC AREA, LOS ANGELES COUNTY, CALIFORNIA Prepared for: WILLIAM S. HART UNION HIGH SCHOOL DISTRICT May 2010 Fugro Job No. 3715.001 FUGRO WEST, INC. May 27, 2010 Project No. 3715.001 4820 McGrath Street, Suite 100 Ventura, California 93003-7778 Tel: (805) 650-7000 Fax: (805) 650-7010 William S. Hart Union High School District 21515 Centre Pointe Parkway Santa Clarita, California 91350 Attention: Michael Otavka Director of Facilities, New Construction Subject: Geotechnical Desktop Study, New Castaic High School Hasley-Sloan Site, Tentative Tract 52475, William S. Hart Union High School District, Santa Clarita, California Dear Mr. Otavka: Fugro is pleased to present this report summarizing preliminary geotechnical conditions for the 69.5-acre Hasley-Sloan Site (Tentative Tract 52475), being considered for William S. Hart Union High School District's (WSHUHSD) new Castaic High School. The project site is located near Castaic in an unincorporated area of Los Angeles County. The project area has been evaluated for residential development by several geologic/geotechnical consultants since the late-1970s. The purpose of this study is to provide a summary of geologic and geotechnical conditions that may exist at the Hasley-Sloan site based on review of existing data and on a site reconnaissance, and to provide general preliminary geotechnical input relative to site development for the proposed school project. Preliminary geotechnical input consists of preliminary seismic information, generalized foundation system information, and preliminary excavation and grading considerations. Site development for the high school is conceptual at this time, but is anticipated to consist of cut and fill grading of hillsides and canyon areas to create an about 48-acre school site pad. Subsurface exploration was not performed for this study to verify the findings and information presented in this data review. Subsurface exploration has been performed on the project site during previous studies conducted by others, however, additional subsurface exploration will be required if the project proceeds to design to verify the interpretations presented herein and to evaluate the site conditions in accordance with California Geological Survey Note 48 for development of the site for the high school. A member of the Fugro group of companies with offices throughout the world. William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) Thank you for the opportunity to provide geotechnical services for the WSHUHSD's new Castaic High School project. Please contact us if you have any questions regarding information presented herein. Sincerely, FUGRO WEST, INC. Lori E. Prentice, CEG Principal Engineering Geologist Reviewed by: Thomas F. Blake, G.E., C.E.G. Geotechnical Services Manager Copies Submitted: M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC (10) Addressee and 10 Cds 2 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) CONTENTS Page 1.0 2.0 INTRODUCTION ...................................................................................................... 1 1.1 1.2 1.3 1.4 Purpose ........................................................................................................... Background ..................................................................................................... Project Description........................................................................................... Work Performed .............................................................................................. 1.4.1 Data Review ........................................................................................ 1.4.2 Site Reconnaissance ........................................................................... 1.4.3 Reporting ............................................................................................. 1 1 2 2 2 3 3 FINDINGS ................................................................................................................ 4 2.1 2.2 2.3 2.4 Regional Geologic Conditions .......................................................................... Local Geologic Conditions ............................................................................... Site Conditions ................................................................................................ Earth Materials ................................................................................................ 2.4.1 Artificial Fill (af) .................................................................................... 2.4.2 Colluvium (Qc) ..................................................................................... 2.4.3 Landslide Deposits (Qls) ...................................................................... 2.4.4 Alluvium (Qa) ....................................................................................... 2.4.5 Terrace Deposits (Qt)........................................................................... 2.4.6 Saugus Formation (TQs)...................................................................... Groundwater .................................................................................................... 4 4 4 5 5 5 5 5 6 6 6 OPINIONS AND RECOMMENDATIONS ................................................................. 7 2.5 3.0 3.1 3.2 3.3 General............................................................................................................ Preliminary Geohazard Review........................................................................ 3.2.1 Seismicity............................................................................................. 3.2.2 Fault Rupture Potential/Ground Shaking .............................................. 3.2.3 Potential Surface Faulting on TT 52584 by AES .................................. 3.2.4 Ground Shaking ................................................................................... 3.2.5 Preliminary 2007 CBC Design Criteria ................................................. 3.2.6 Liquefaction ......................................................................................... 3.2.7 Seismically Induced Settlement ........................................................... 3.2.8 Lateral Spreading................................................................................. 3.2.9 Slope Instability .................................................................................... 3.2.10 Hydroconsolidation .............................................................................. 3.2.11 Expansive Soil ..................................................................................... 3.2.12 Flooding and Scour .............................................................................. Grading Considerations ................................................................................... 3.3.1 Removal of Unsuitable Material ........................................................... 3.3.2 Fills ...................................................................................................... 3.3.3 Cut Slopes ........................................................................................... 3.3.4 Cut-Fill Transition ................................................................................. M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC i 7 7 7 7 8 9 9 10 11 11 11 12 12 12 12 13 13 13 13 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) CONTENTS Page 3.4 Onsite Surface Water Runoff Detention Facility ............................................... 14 PRELIMINARY SHALLOW FOUNDATION PARAMETERS ..................................... 14 4.1 4.2 Introduction ...................................................................................................... Footing Design Criteria .................................................................................... 4.2.1 Minimum Footing Embedment ............................................................. 4.2.2 Footing Dimensions ............................................................................. 4.2.3 Allowable Bearing Pressure ................................................................. 4.2.4 Safety Factors and Transient Loads..................................................... Modulus of Subgrade Reaction for Flat Slab-on-Ground .................................. Sliding and Passive Resistance ....................................................................... 4.4.1 Sliding Resistance ............................................................................... 4.4.2 Passive Resistance .............................................................................. 4.4.3 Safety Factors ...................................................................................... 14 14 14 15 15 15 15 15 15 15 15 5.0 CLOSING ................................................................................................................. 16 6.0 REFERENCES ......................................................................................................... 17 4.0 4.3 4.4 TABLES Page 1 2 Proximal Faults...................................................................................................... Summary of Preliminary 2007 CBC Seismic Design Parameters........................... 8 10 PLATES Plate Vicinity Map ...................................................................................................................... Geotechnical Map ............................................................................................................. Regional Geologic Map ..................................................................................................... M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC ii 1 2 3 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) 1.0 INTRODUCTION This letter report summarizes preliminary geologic and geotechnical information for the 69.5-acre Hasley-Sloan Site (Tentative Tract 52475), being considered for William S. Hart Union High School District's (WSHUHSD) new Castaic High School. The project site is located near Castaic in an unincorporated area of Los Angeles County as indicated on Plate 1 - Vicinity Map. 1.1 PURPOSE The purpose of this study is to provide a summary of geologic and geotechnical conditions that may exist at the Hasley-Sloan site based on review of existing data and on a brief site reconnaissance, and to provide preliminary geotechnical information relative to site development for the proposed school project, such as potential geologic hazards, discussion of preliminary seismic design and preliminary foundation systems, and preliminary excavation and grading considerations. Because additional exploration and analyses are needed to characterize the site, the information contained herein is likely to require modification as additional site-specific data are obtained. 1.2 BACKGROUND GeoSoils, Inc. (GSI) performed geologic studies for development of TT 52475 (previously TT 34031) in 1977 and 1987 (GSI 1977 and 1987). Data review indicates that those studies included advancing 4 borings and 18 test pits within the tract. According to ERI (2009), the southern and northwest portions of the site were graded in 1986 and 1987 for development of a mobile home park, community center, and access roads. The majority of the rough grading reportedly consisted of removal and recompaction of loose alluvial soils to create 81 pads and the access roadways in the low-lying southern and northwestern portions of the site (Plate 2). Grading reportedly consisted of removal of 2 to 4 feet of alluvial soils and 4 to 6 feet of terrace deposit materials, excavation of up to 90-foot-high 2h:1v cut-slopes in selected south and west facing bedrock slopes, and maximum fill depth of about 24 feet (GSI, 1987). GSI was the geotechnical engineer of record for the mobile home project and provided a compaction report for the rough grading (1987). The proposed mobile home project was not constructed. Geolabs Westlake Village (GWV, 1999) performed a geotechnical study for development of about 47 one-acre residential lots within the low-lying and hillside areas of the project site. Supplemental field exploration advanced by GWV consisted of five hollow-stem auger borings, three 24-inch-diameter bucket-auger borings, and 18 backhoe test pits. The proposed residential project was not constructed. Earth Resources, Inc. (ERI, 2009) performed a reconnaissance-level geologic study for the Hasley-Sloan site for WSHUHSD that summarized the site conditions based on previous work by GSI and GWV and provided preliminary opinions for slope stability, seismicity, and geohazards. Allen E. Seward (AES, 2010) summarized their opinion of potential geologic/geotechnical hazards that could affect the project site based on a review of existing M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 1 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) data and on work they performed for TT 52584 approximately three-quarters of a mile to the east. 1.3 PROJECT DESCRIPTION Site development for the high school at the Hasley-Sloan site is conceptual at this time, but is anticipated to consist of cut- and fill-grading of ridges and canyon areas in the northern portion of the site and filling of the southern portion north of the Hasley Creek drainage to create a super-pad for the school site. The existing and conceptual site topography is presented on Plate 2 - Geotechnical Map. Based on our understanding of the conceptual project, site development for the school includes the following: 1.4 • An about 48-acre graded pad that will slope gently to the southwest with elevations ranging from 1344 to 1408 (Plate 2). • A 2 horizontal to 1 vertical (2h:1v) cut-slope is proposed along the northern property perimeter that will be about 55 feet high and will incorporate a 14-foot-high retaining wall. • Cut-slopes are proposed along the northern portion of the eastern property boundary that will be about 25 to 30 feet high and inclined at 2h:1v. • A 2h:1v fill slope is proposed along the southern portion of the eastern property boundary and along the southern property boundary. The fill slope will be up to about 80 feet high in the southeastern portion of the site. WORK PERFORMED The work performed for this study for the Hasley-Sloan school site consists of review of data from previous studies, site reconnaissance, preparation of this preliminary geotechnical report, and meeting attendance. Evaluations of the access road alignments that will serve the Hasley-Sloan school site and subsurface exploration, laboratory testing, and geotechnical recommendations for project design are not included in this work scope, but will be required if the project proceeds to design. The work performed for this study is for planning purposes only and is not intended for geotechnical design of the project. A site-specific geotechnical design study will be required if the project proceeds to design. 1.4.1 Data Review We reviewed readily available published geologic and geotechnical data, stereo-pair aerial photographs, and existing data provided to us by WSHUHSD and other parties for the project site. Data reviewed as part of this study are listed in the References section of this report. M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 2 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) 1.4.2 Site Reconnaissance An engineering geologist from Fugro performed a site reconnaissance of the project site on April 12, 2010, to observe onsite conditions described in previous reports and to observe the following: 1.4.3 • Surficial exposures of onsite soil and bedrock, • Evidence suggestive of onsite springs and/or seeps, • General topographic relief and terrain, • Evidence of previous onsite grading activities such as areas of cut and/or fill, • Surface water drainage courses, and • Surficial evidence suggestive of potential geologic hazards such as expansive soil, landslides, flooding, and earthquake faults. Reporting We have summarized our preliminary geologic and geotechnical discussions based on the findings from the data review and site reconnaissance, including: • Summary of past work performed at the site and findings; • Summary of the generalized soil, bedrock, and groundwater conditions; • Preliminary summary of potential geohazards (expansive soil, landslides, land subsidence [settlement], liquefaction, fault rupture, and strong ground shaking) that could affect the site based on a review of published geologic and geotechnical data; • Summary of preliminary seismic factors for use with the 2007 CBC based on the USGS website; • Presence of onsite drainage courses that may impact the site; • Preliminary foundation system types, embedment depths, and bearing capacities; • Potential for ground water seepage and dewatering; • Potential for expansive soil and potential mitigation methods; • Preliminary cut- and fill-grading considerations as well as over-excavation and recompaction discussion; • Potential underground utility issues: trenching difficulties, backfilling and dewatering; and • Recommendations for a work scope for a site-specific geotechnical design study, if the project proceeds to design. M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 3 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) 2.0 FINDINGS 2.1 REGIONAL GEOLOGIC CONDITIONS The project site is located within the eastern Ventura basin within the Transverse Ranges geologic/geomorphic province of California. The province is characterized by eastwest-trending mountain ranges composed of sedimentary and volcanic rocks ranging in age from Cretaceous to Recent. Major east-trending folds, reverse faults, and strike-slip faults reflect regional north-south compression and are characteristic of the Transverse Ranges. Several active and potentially active faults, as defined by the State of California, are proximal to the project site; however, no known active or potentially active faults recognized by the State of California or the County of Los Angeles trend toward or cross the Hasley-Sloan site. AES (1999) delineated a Building Setback in the southern portion of nearby TT 52584, located about three-quarters of a mile to the east of the Hasley-Sloan site, reportedly based on a zone of tectonic deformation that they observed during their field trenching program of that tract. AES also reported that no primary, through-going faults were identified within the zone of observed deformation and that "no direct evidence of Holocene activity was observed on the site." The approximate location of their setback is shown on Plate 3. 2.2 LOCAL GEOLOGIC CONDITIONS Published mapping indicates that the hillside terrain that comprises the northern portion of the property is underlain by non-marine bedrock of the Saugus Formation. Dibblee (1993) indicates that a synclinal axis trends southeasterly through the project site near the base of the south-facing slopes whereas CDMG (1985) depicts an east-southeasterly trending synclinal axis projecting through the low-lying area near the northwestern portion of the site as shown on Plate 3. Mapping by GWV (1999) locates the synclinal axis is as shown on Plates 3 and 4. Several restricted use areas are identified on the Los Angeles County Assessor Parcel Map for TT 52475. According to Mr. Joe Cota of ERI (2010), the areas are related to potential adverse bedding areas that may have the potential for slope instability if not mitigated. Mr. Cota also indicated that it was County of Los Angeles policy to show all areas of "geological hazards" that were not to be mitigated during project development (as was the case when the site was originally considered for development as a mobile home park) as "Restricted Use Zones" and recorded on the parcel map. The approximate locations of the restricted use zones are shown on Plate 2. 2.3 SITE CONDITIONS The project site is located northeast of the confluence of the south-draining Sloan Canyon and the easterly-draining Hasley Creek. Site access is from Sloan Canyon Road via a fair-weather crossing of Hasley Creek north of Hasley Canyon Road. Minor water flow was observed in Hasley Creek at the time of our site visit (following rainfall the previous day and the morning of our site visit). M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 4 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) The site generally consists of moderately steep hilly terrain in about the northern twothirds of the site and a previously graded low-lying area in the southern one-third of the site. Topography within the hilly terrain ranges from a high of about 1,485 feet on the ridge area at the northern property limit to about 1,360 feet near the toe of the slopes. Topography within the low-lying graded area in the southern portion of the property ranges from about 1,360 feet near the hills to about 1,315 feet along the northern bank of Hasley Creek near the southern portion of the property. Natural slopes are inclined at about 2h:1v to 2.5h:1v with locally steeper slopes. As discussed previously, portions of the site were graded over 20 years ago for a mobile home park development that was never constructed. Rough grading included construction of 2h:1v cut-slopes and fill-slopes of granular soil materials. Based on our site visit, the 2h:1v cutand fill-slopes have been eroded and gullies are common within the access roads and existing slopes. 2.4 EARTH MATERIALS On the basis of our data review and site reconnaissance, the geologic materials exposed at the site consist of artificial fill materials, colluvium, alluvium, terrace deposits, bedrock of the Saugus Formation, and potential landslide deposits as indicated on Plate 2. 2.4.1 Artificial Fill (af) The artificial fill materials are associated with previous site grading and reportedly range up to about 24 feet in thickness (GSI, 1987). GSI indicates that the fill materials were placed at relative compactions of generally 90 percent or more of maximum densities determined based on ASTM test method D1557-78. 2.4.2 Colluvium (Qc) GWV indicates that colluvial materials mantle the natural slopes and consist of about 1 to 4 feet of loose silty, clayey sand with variable gravel and cobbles derived from the underlying Saugus Formation bedrock. Based on our aerial photo review and site observations, thicker deposits of colluvial soil may exist in the hillside and valley areas indicated on Plate 2. 2.4.3 Landslide Deposits (Qls) Geomorphology suggestive of relatively minor landslide deposits was observed in aerial photographs reviewed for the project and during our site reconnaissance. Based on our preliminary interpretation of the geomorphology, it appears that the landslides likely are relatively shallow, minor features, perhaps tens of feet thick, as opposed to extensive, major deep-seated features that are in excess of a hundred or more feet thick. 2.4.4 Alluvium (Qa) Alluvial soils reportedly underlie the fill materials at depths of about 3 to 6 feet in explorations advanced by GWV in the low-lying northwestern and southern portions of the site. GWV indicates that the alluvial sediments encountered in their explorations consist of loose to M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 5 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) medium dense sands, silty sands, and gravelly sands with variable amounts of gravel and cobble materials. Minor pinpoint porosity was also reported. Granular alluvial materials are exposed within the Hasley Creek drainage channel and the near-vertical northern channel wall south of the graded area. GWV advanced three hollow-stem-auger borings in the southern portion of the site near the Hasley Creek drainage (Plate 4). Borings B-4 and B-5 reportedly met refusal on cobbles at depths of 25 and 16 feet, respectively. Boring B-3 was advanced to a depth of about 50 feet and did not penetrate the base of the alluvial sediments at that location. 2.4.5 Terrace Deposits (Qt) Review of GWV (1999) and GSI (1979) indicates that terrace deposits may underlie the artificial fill materials in the upper pad areas near the base of the south-facing bedrock slopes. GWV describes the terrace deposits as clayey, silty sand with scattered gravel and pebbles and pinpoint porosity. 2.4.6 Saugus Formation (TQs) Bedrock of the Saugus Formation is present in the hillside areas in the northern portion of the property and underlies the alluvial sediments at depth in the low-lying areas of the property. Based on data review, the majority of the bedrock materials consist of fine- to coarsegrained sandstone that is described as friable, massive to cross-bedded, with varying amounts of gravel and cobbles. The sandstone is reportedly interbedded with lesser amounts of silty sandstone, sandy siltstone, and siltstone. GWV also reports encountering thin silt lenses and clay beds within the Saugus Formation bedrock. 2.5 GROUNDWATER Groundwater was not reported in any of the explorations advanced by GSI or GWV in previous site investigations. Based on our data review, the deepest boring advanced on the property is GWV's boring B-3, which was advanced to a depth of 50 feet northeast of the Hasley Creek fair-weather crossing (Plate 4). We did not observe evidence of surface seeps or springs onsite during our site reconnaissance. Review of CGS (2003) indicates the depth to historically high groundwater is about 10 feet within the Hasley Creek drainage in the site vicinity. The County of Los Angeles (1990) indicates the depth to ground water is in excess of 30 feet in the project area. Surface water was observed flowing in Hasley Creek at the time of our field reconnaissance following a period of rainfall. Hasley Creek is apparently an ephemeral stream and we understand that the fair-weather crossing can become impassible due to water flow as a result of heavy rainfall. M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 6 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) 3.0 OPINIONS AND RECOMMENDATIONS 3.1 GENERAL Preliminary evaluations and opinions for project consideration and recommendations for the scope of future geotechnical design-level studies are provided below. Additional sitespecific subsurface exploration, laboratory testing, and geotechnical evaluations will be required, if the school project proceeds to design at this site. 3.2 PRELIMINARY GEOHAZARD REVIEW Potential geohazards that may exist within the project area are summarized based on our data review and on our field reconnaissance of the project site. Subsurface exploration was not performed for this study, but should be performed as part of the design phase of work. 3.2.1 Seismicity The project site is located within a seismically active area and the potential exists for strong ground motion to affect the school project during the design lifetime. In general, the primary effects will be those phenomena associated with shaking and/or ground acceleration. Those effects are typically mitigated through appropriate design and construction procedures. The design-level geotechnical report should provide site-specific geotechnical seismic design criteria in accordance with CGS Note 48 and 2007 CBC. 3.2.2 Fault Rupture Potential/Ground Shaking The proposed Hasley-Sloan school site is not located within an Alquist-Priolo fault rupture hazard zone and no known active or potentially active faults cross the project site. However, the project is proximal to a number of faults that are considered active or potentially active by the California Geologic Survey (CGS) including the Holser and San Gabriel faults, which are located about 1 mile and 2.5 miles from the site, respectively. Faults within the CGS database considered as active or potentially active within about a 25-mile radius of the project site and their estimated maximum earthquake magnitudes are listed in Table 1. In addition to the CGS faults listed in Table 1, the County of Los Angeles (1990) considers the Santa Felicia fault and two unnamed fault strands proximal to the project site as being potentially active. The unnamed fault strands are located about 0.8-mile northwest of the project site and the Santa Felicia fault is located about 2.7 miles north of the project site. M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 7 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) Table 1. Proximal Faults Approximate Distance (mi) Estimated Maximum Magnitude (Mw) Holser 1.0 6.5 San Gabriel 2.5 7.2 Santa Susana 2.7 6.7 Northridge (E. Oak Ridge) 4.8 7.0 Oak Ridge (Onshore) 6.0 7.0 San Cayetano 6.5 7.0 Simi-Santa Rosa 7.1 7.0 Sierra Madre (San Fernando) 10.2 6.7 Santa Ynez East 15.7 7.1 San Andreas - Whole 18.0 8.0 Verdugo 18.8 6.9 Anacapa-Dume 22.6 7.5 Sierra Madre 24.4 7.2 Fault Name 3.2.3 Potential Surface Faulting on TT 52584 by AES AES (1999) delineated a Building Setback in the southern portion of TT 52584, located about three-quarters of a mile to the east of the Hasley-Sloan site, reportedly based on a zone of tectonic deformation of Saugus bedrock materials that they observed during their field trenching program on that site. However, AES reported that no primary, through-going faults were identified within the zone of observed deformation and that "no direct evidence of Holocene activity was observed on the site" (Holocene activity is considered active faulting by the State of California). AES reports that the setback was based on the observed deformation and the association with folds oriented with regional structure and considers the deformation to be fault-related. Based on our review of the AES trench logs, the observed deformation generally consists of shearing within mudstone and claystone units, and fracturing within granular sandy bedrock materials. The degree of observed shearing and fracturing appears to be concentrated near synclinal and anticlinal axes and seems to decrease with distance from the axes. It seems likely to us that the shearing observed along the finer-grained mudstone and siltstone units and fracturing within the granular units is probably related to flexural-slip movement in the fold-hinge areas as the bedrock materials were deformed by regional compression rather than being of primary seismogenic origin. That opinion is suggested by the apparent decrease in occurrences of shearing and fracturing with distance from the fold axes and the apparently discontinuous nature of the observed shears. Therefore, we do not interpret the shearing and fracturing to be related to primary faulting as interpreted by AES. Further, AES reports that there was no direct M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 8 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) evidence of Holocene fault activity and that the AES-interpreted faults on TT 52584 were not observed to be through-going, so they may not even project to the proposed school site. The Hasley-Sloan site is located about three-quarters of a mile west of TT 52584 explored by AES. Review of published data is not suggestive of faulting on or trending toward the site and we did not observe lineaments on or trending toward the Hasley-Sloan site in our aerial photographic review. Therefore, at this time, it appears that the potential for primary seismogenic fault rupture to affect the project site is likely to be low. However, the potential for faulting at the site will need to be evaluated further, if the project proceeds to design. 3.2.4 Ground Shaking As summarized in Table 1, there are numerous active or potentially active faults within a 25-mile radius of the site that have a potential to generate strong ground motion. Probabilistic seismic hazard studies by the CGS (CGS website) indicate that the project area could experience horizontal ground accelerations (10 percent probability of exceedance in 50 years) of about 0.69g. 3.2.5 Preliminary 2007 CBC Design Criteria The proposed school project should be designed to resist the forces generated by earthquake shaking in accordance with local design practice. The seismic design procedures outlined in Section 1613A of the California Building Code (CBC) are designed to meet the intent and requirements of ASCE 7. It appears that much of the Hasley-Sloan site is situated on soils that are considered to meet the criteria for Site Class D. The USGS interactive webpage 'Seismic Design Values for Buildings' (USGS, 2008) was used to obtain seismic design coefficients for Site Class D materials. Based on our data review and preliminary characterization of the subsurface conditions and the 2007 CBC, the following values may be used for preliminary project planning purposes. M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 9 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) Table 2. Summary of Preliminary 2007 CBC Seismic Design Parameters 2007 California Building Code Section 1613 Seismic Parameter Value --- Latitude N 34.4632° --- Longitude W 118.6522° Section 1613.5.1 and Figure 1613.5(3) Mapped Acceleration Response Parameter (Ss) Site Class B 2.263 Section 1613.5.1 and Figure 1613.5(4) Mapped Acceleration Response Parameter (S1) Site Class B 0.686 Section 1613.5.2 and Table 1613.5.2 Soil Profile Type SD Section 1613.5.3 and Table 1613.5.3(1) Site Coefficient (Fa) 1.0 Section 1616.5.3 and Table 1613.5.3(2) Site Coefficient (Fv) 1.5 Section 1613.5.3 Adjusted Acceleration Response Parameter for Site Class D (Sms) 2.263 Section 1613.5.3 Adjusted Acceleration Response Parameter for Site Class D (Sm1) 1.029 Section 1613.5.4 Design Spectral Response Acceleration Parameter (SDS) 1.508 Section 1613.5.4 Design Spectral Response Acceleration Parameter (SD1) 0.686 3.2.6 Liquefaction Liquefaction is generally described as the sudden loss of soil strength because of a rapid increase in soil pore water pressure due to cyclic loading during a seismic event. In order for liquefaction to occur, three general geotechnical characteristics should be present: 1) groundwater must be present within the potentially liquefiable zone; 2) the potentially liquefiable soils are typically granular and the grain size distribution commonly falls within a relatively specific range; and 3) the potentially liquefiable soil must be of low to moderate relative density. If those criteria are met and strong ground motion occurs, then those soils may liquefy, depending upon the intensity and cyclic nature of the strong ground motion. Liquefaction that produces surface effects generally occurs in the upper 40 to 50 feet of the soil column, although the phenomenon can occur deeper than 100 feet. Groundwater was not encountered within the alluvial sediments within 50 feet of the ground surface by GWV based on the data review performed for this study. However, the southern portion of the site is underlain by alluvial sediments that are located within an ephemeral drainage course and the CGS indicates that the depth to historically high groundwater has been as shallow as about 10 feet deep. Therefore, there is a potential for liquefaction to occur within the alluvial sediments in the project area and for them to affect the M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 10 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) project elements, if not mitigated during design and construction. Liquefaction is not likely to occur in areas underlain by Saugus Formation bedrock. The design-level geotechnical report should provide analyses of liquefaction potential in areas underlain by alluvial sediments and an assessment of total and differential settlement that could occur as a result of a liquefaction event. Mitigations should be provided as appropriate based on the findings. 3.2.7 Seismically Induced Settlement Seismically induced settlement or consolidation due to ground shaking can occur in soils that are loose, soft, or that are moderately dense but weakly cemented. Based on our review of the existing data, the potential exists for the alluvial sediments and terrace deposits to be susceptible to seismically induced settlement. Typically, bedrock materials are not likely to be susceptible to seismically induced settlement. The design-level geotechnical report should provide analyses of seismic settlement of unsaturated soil materials and planned artificial fill, and an assessment of total and differential settlement that may be expected at the site. 3.2.8 Lateral Spreading The southern portion of the project site is located proximal to the Hasley Creek drainage and the potential exists for earthquake-induced lateral spreading to affect the project site. The geotechnical design report should provide an analysis of the potential for lateral spreading of the proposed site and provide potential mitigations, if necessary. 3.2.9 Slope Instability Landsliding can be common in the Saugus Formation and landslides have been mapped in the project vicinity. We observed geomorphology suggestive of colluvium and minor landslide deposits in our aerial photographic review and site reconnaissance near the locations indicated on Plate 2. Based on our preliminary geomorphic interpretation of the potential subsurface conditions and landslide distributions, it appears that the onsite landslides are likely to be relatively shallow features, as opposed to extensive deep-seated features. Also, based on review of the conceptual grading plan (CCL, 2010), it appears that the majority of potential landslide and colluvial deposits are likely to be removed during the proposed grading. The potential also exists for unfavorable geologic bedding conditions to exist within the natural hillsides or to be exposed as part of the proposed grading operations. Review of the conceptual grading plan indicates that the majority of the potentially unfavorable bedding conditions (not including the south-facing slopes at the northern portion of the property) that would be temporarily exposed during grading operations would be buttressed as part of the canyon fill process. Geotechnical evaluations of the temporary and final slope conditions and geologic bedding orientations will be required as part of the design phase of the project. M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 11 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) We recommend that the potential landslide and colluvial deposits indicated on Plate 2 be explored to verify their presence and to characterize the geometries of the deposits. We recommend that all landslide deposits, colluvial deposits, and unsuitable soil material be removed to competent bedrock during project development to reduce the potential for settlement, hydroconsolidation, and/or slope instability to affect the project. Weak clay layers are described on boring logs by GWV (1999). It appears to us that the existing data contain insufficient strength testing to evaluate the residual strength for alongbedding conditions for use in slope stability evaluations. Additional residual strength tests and Atterberg limits tests should be performed on clayey units within the Saugus Formation for use in slope stability evaluations for the design-level geotechnical report. 3.2.10 Hydroconsolidation Based on review of GWV (1999), the potential exists for hydroconsolidation to occur within the fill and within underlying soil materials above the groundwater level. The design-level geotechnical report should evaluate the hydroconsolidation potential as a result of future increases in soil moisture content or an increase in the groundwater level over time that could occur in the future. 3.2.11 Expansive Soil Review of GWV (1999) suggests that the tested soil materials on the Hasley-Sloan site have a low shrink/swell potential. The GWV findings should be confirmed as part of the designlevel geotechnical study for the project. 3.2.12 Flooding and Scour The project site lies adjacent to the confluence of the Sloan Canyon and Hasley Creek drainages. Currently, the low-lying portions of the site are located within areas that we have been told were identified by the County of Los Angeles (1990) and the Federal Emergency Management Agency (FEMA, 2008) as a 100-year flood zones. The potential for flooding and scour should be evaluated by the appropriate design professional, if the project proceeds to design. 3.3 GRADING CONSIDERATIONS Based on review of existing data, the majority of the onsite soils appear to be suitable for use as artificial fill material. It may be possible to perform selective screening to segregate finer and coarser grained materials for various project uses. Slopes should be constructed and benched in accordance with County requirements and water should not be allowed to flow over slopes in an uncontrolled manner. M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 12 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) 3.3.1 Removal of Unsuitable Material GWV (1999) reports observing pinpoint voids in terrace deposit and alluvial soil materials encountered by their explorations. Further, the artificial fill materials onsite were placed over 20 years ago without further project construction. Also, extensive erosion and gullying has occurred in areas. Therefore, the existing fill materials and unsuitable terrace deposits and alluvial sediments should be removed to competent subgrade before constructing the proposed school site. 3.3.2 Fills Fill is typically placed in uniform lifts, not exceeding 8 inches in loose state, and compacted to a minimum of 90 percent relative compaction per ASTM D1557. Review of the conceptual grading plan indicates that fill in excess of 40 feet in thickness will be placed in various areas. We recommend that extra compactive effort (minimum of 93 percent relative compaction) be utilized for fills over 40 feet depth/thick to help reduce the collapse-settlement potential of the deep fill. Additionally, the lower portions of the fills should be constructed using granular materials to help limit the amount of settlement of the fill. Clayey material should be mixed with granular soils and placed on the outer portions of fill slopes to help reduce the potential for erosion and provide for plant growth. The design-level geotechnical report should provide an analysis of the settlement potential assessing the elastic compression and consolidation settlement of the Saugus Formation, left-in-place alluvial soils, and the compacted fill as appropriate. The report should also provide recommendations for a settlement monitoring program for the project following construction of the fills prior to construction of facilities on-site. 3.3.3 Cut Slopes Based on our review of GWV (1999), the potential exists for laterally unsupported bedding and weak clay materials to be exposed in the 2h:1v south-facing cut-slopes proposed in the northern portion of the site as well as in other areas of the site. The design-level geotechnical report should evaluate the proposed temporary and final slope configurations relative to the proposed grading, geologic bedding orientations, and soil strength parameters. Additional field exploration, laboratory testing, and stability analyses likely will be required as part of the design level effort. 3.3.4 Cut-Fill Transition Where site grading will create cut-fill transitions, the cut portion should be overexcavated and backfilled with compacted fill to minimize the potential for differential settlement resulting from dissimilar foundation materials to affect proposed facilities. The lateral and vertical extent of the overexcavation will depend on the geometry of the transition and the consolidation properties of the earth materials. M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 13 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) 3.4 ONSITE SURFACE WATER RUNOFF DETENTION FACILITY We understand that an onsite surface water runoff detention facility is included in the project design as required by the County. Due to limited space available to provide a separate detention basin, Lund Engineering has indicated that the detention basin area likely will be located within the athletic fields and may be designed to temporarily impound water for percolation into the subsurface. We anticipate that fill materials derived from onsite soil materials will be primarily silty to clayey sand placed at minimums of 90 to 93 percent relative compaction (per ASTM D1557) depending on the depth of the fill materials. On that basis, as a preliminary estimate, we suspect that percolation rates may be about 1 inch per hour or less within compacted fill materials. The geotechnical design-level report should provide evaluations of the percolation potential of fill materials likely generated from onsite soil materials based on additional field percolation testing, laboratory testing, review of published data, and experience with similar projects. From a geotechnical perspective, water percolation into fills and on or near slope areas is not recommended due to the potential for hydroconsolidation and slope instability to occur. As preliminary input to siting the detention basin facility, we recommend that the containment facility be located no closer than 3h:1v projection from the toe of adjacent fill slopes. Detention facilities should also be setback from the base of cut-slope buttress fills by a 2h:1v projection from the toe of the buttress fill. Based on information presented in GWV (1999) for the HasleySloan site, we estimate the setback from the cut-slope would be approximately 50 to 100 feet. The geotechnical design-level report should evaluate the percolation of the contained storm water runoff into the subsurface and provide geotechnical mitigations to limit the adverse effects of percolation of the water. 4.0 PRELIMINARY SHALLOW FOUNDATION PARAMETERS 4.1 INTRODUCTION Typically, the structural loads for the proposed facilities can be supported on continuous (wall) or isolated (spread) footings assuming that the new facilities will be placed on a properly engineered compacted fill pad. Locally accepted, good quality construction techniques should be utilized. The parameters suggested below should be verified by further analyses, if the project proceeds to design. 4.2 FOOTING DESIGN CRITERIA 4.2.1 Minimum Footing Embedment Continuous wall and isolated column footings be founded in and on compacted fill. For at-grade (or near at-grade) foundations, the minimum embedment depth (relative to the adjacent finished final grade or slab elevation, whichever is lower) is typically about 24 inches. M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 14 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) 4.2.2 Footing Dimensions Minimum footing widths of 18 and 24 inches are commonly used for wall and column footings, respectively. The footing thickness should be determined by the structural engineer, but commonly is not less than 12 inches thick. 4.2.3 Allowable Bearing Pressure Assuming footing elements are embedded to at least the minimum recommended depths noted above, wall and column footing elements typically are proportioned for dead load plus probable maximum live loads using a maximum net (pressure in excess of existing overburden pressures) allowable bearing pressure of 1,500 psf. 4.2.4 Safety Factors and Transient Loads The recommended value for allowable bearing pressure provides a factor of safety against shear failure in excess of 2.5. A one-third increase in the allowable bearing pressure may be used for transient loads such as seismic or wind forces. 4.3 MODULUS OF SUBGRADE REACTION FOR FLAT SLAB-ON-GROUND Flat on-grade floor slabs are commonly designed using a flat slab on an elastic halfspace analogy. A modulus of subgrade reaction of 200 pounds per cubic inch may be used for preliminary planning purposes. The modulus value is for a 1-foot-square plate and must be corrected for mat size and shape, assuming a cohesionless subgrade. 4.4 SLIDING AND PASSIVE RESISTANCE 4.4.1 Sliding Resistance Ultimate sliding resistance generated through a soil/concrete interface can be estimated by multiplying the total dead weight structural loads by a coefficient of 0.3. 4.4.2 Passive Resistance Passive resistance can be developed from lateral bearing of footings bearing against compacted backfill or undisturbed native materials. Passive resistance can be estimated using an equivalent fluid density of 300 pcf. 4.4.3 Safety Factors Sliding resistance and passive pressure are commonly used together without reduction when used with the safety factors recommended below. Minimum factors of safety of 1.5 and 2.0 are suggested for foundation overturning and sliding, respectively, where sliding resistance and passive pressure are used together. The safety factor for sliding can be reduced to 1.5 if passive pressure is neglected. For seismic conditions, the factors of safety for overturning and sliding may be reduced to 1.1. M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 15 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) 5.0 CLOSING This report has been prepared for the exclusive use of WSHUHSD for planning purposes only. The information presented herein should not be used for design and construction of the project. We note that subsurface exploration was not performed as part of this preliminary site characterization and geohazards study. Subsurface exploration, laboratory testing, and geotechnical analyses should be performed as the project proceeds to design. Although information contained in this report may be of some use for other purposes, it may not contain sufficient information for other parties or uses. If any changes are made to the project as described in this report, the conclusions and recommendations in this report shall not be considered valid unless the changes are reviewed and the conclusions and recommendations of this report are modified or validated in writing by Fugro. In performing our professional services, in our opinion, we have used generally accepted geologic and geotechnical engineering principles and have applied that degree of care and skill ordinarily exercised, under similar circumstances, by reputable geotechnical engineers currently practicing in this or similar localities. No other warranty, express or implied, is made as to the professional advice included in this report. M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 16 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) 6.0 REFERENCES Aerial Photographs Reviewed Year Flight Frames Scale/Color 1952 AXJ 2K - 29 to 32 1:20,000/B&W 1959 AXJ 12W-153; 19W 181 to 183 1:20,000/B&W C300 E47-48 NP* / B&W 1969 USGS GS-VCHC 1-19 & 20 NP / color 1989 USGS NAPP 1832 - 132 & 133 NP / B&W 1994 USGS NAPP 6860 - 188 & 189 NP / B&W 2002 USGS NAPP 1216 - 278 & 279 NP / color NP = not provided on photograph Allen E. Seward (2010), Geologic/Geotechnical Report, Preliminary Site Assessment with Respect to Potential Geologic/Geotechnical Hazards, Tentative Tract 52475, Hasley Canyon and Sloan Canyon, Castaic, County of Los Angeles, California, prepared for Larry Rasmussen, dated February 23. ______ (1999), Geologic and Geotechnical Report, Vesting Tentative Tract 52584, Castaic, California, Prepared for Hasley Canyon Land Company, LLC, dated January 15. California Division of Mines and Geology (2002), Seismic Hazard Evaluation for the Val Verde 7.5-Minute Quadrangle, Los Angeles and Ventura Counties, California, Seismic Hazard Zone Report 073. ______ (1986), Landslide Hazards in the East ½ of the Val Verde Quadrangle, Los Angeles County, California, Landslide Hazard Identification Map #5, DMG Open-File Report 86-9. Cota, Joseph (2010), email communication dated April 6. Earth Resources, Inc. (2009), Reconnaissance Geological Investigation, Proposed HasleySloan High School Site, 69.5± Acre Parcel, Tentative Tract 52475, APN 3247-043-011, Hasley Canyon Road & Sloan Canyon Road, Castaic Area, County of Los Angeles, California, dated December 17. Federal Emergency Management Agency (FEMA, 2008), National Flood Insurance Rate Map Number 06037C0800F available on website http://www.msc.fema.gov, effective date September 26. GeoSoils, Inc. (GSI, 1979), Geologic and Soil Engineering Review of 1"=40' Scale Grading Plan, Tract 34031, Hasley Canyon County of Los Angeles, prepared for Trega, Incorporated, dated April 3. M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 17 William S. Hart Unified High School District May 27, 2010 (Project No. 3715.001) ______ (1987), Rough Grading Correction Punchlist, Lots 1-81, Tract 34031, Hasley Canyon, California, dated January 12. ______ (1989), Update Geologic Report, Tract 47797 (Formerly Tract 34031) Hasley Canyon, Los Angeles County, California, dated August 24. Geolabs Westlake Village (GWV, 1999), Geotechnical Investigation of Tentative Tract No. 52475, Hsley Canyon and Sloan Canyon, Castaic, County of Los Angeles, California, dated November 30. County of Los Angeles (1990), Technical Appendix to the Safety Element of the Los Angeles County General Plan, Hazard Reduction in Los Angeles County, December. M:\WP\2010\3715.001\HASLEY\HSRPT5-27-10.DOC 18 PLATES
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