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:
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(10) Addressee and 10 Cds
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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 .................................................................................
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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
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15
15
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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 .....................................................................................................
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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
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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.
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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.
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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).
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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
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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.
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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.
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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
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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William S. Hart Unified High School District
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______ (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.
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PLATES