1. education
2. high school

FUGRO WEST, INC.
GEOTECHNICAL DESKTOP STUDY
NEW CASTAIC HIGH SCHOOL
ROMERO CANYON SITE, TENTATIVE TRACT 47807
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 26, 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 Romero Canyon Site,
Tentative Tract 47807, 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 Romero Canyon Site (Tentative Tract 47807), 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-1980s. The purpose of this study is to provide
a summary of geologic and geotechnical conditions that may exist at the Romero Canyon Site
based on review of existing data and on a site reconnaissance, and to provide general
preliminary geotechnical opinions relative to site development for the proposed school project.
Preliminary geotechnical opinions consist 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 44.7-acre school site pad.
Subsurface exploration was not performed for this study to verify the findings and
information presented in the 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 Union High School District
May 26, 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 Union High School District
May 26, 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
1
2
2
2
3
FINDINGS ................................................................................................................
3
2.1
2.2
2.3
2.4
Regional Geologic Conditions ..........................................................................
Local Geologic Conditions ...............................................................................
Site Conditions ................................................................................................
Earth Materials ................................................................................................
2.4.1 Alluvium (Qa) .......................................................................................
2.4.2 Colluvium (Qc) .....................................................................................
2.4.3 Landslide Deposits (Qls) ......................................................................
2.4.4 Saugus Formation (Tqs) ......................................................................
Groundwater ....................................................................................................
3
3
4
4
4
4
4
5
5
OPINIONS AND RECOMMENDATIONS .................................................................
6
2.5
3.0
3.1
3.2
3.3
General............................................................................................................
6
Preliminary Geohazard Summary ....................................................................
6
3.2.1 Landsliding ...........................................................................................
6
3.2.2 Slope Stability ......................................................................................
7
3.2.3 Fill Settlement ......................................................................................
7
3.2.4 Preliminary Recommendations if the Project Proceeds to Design at the
Romero Canyon Site ............................................................................
7
3.2.5 Seismicity.............................................................................................
8
3.2.6 Fault Rupture Potential/Ground Shaking ..............................................
8
3.2.7 Faults ...................................................................................................
8
3.2.8 Ground Shaking ...................................................................................
9
3.2.9 Preliminary 2007 CBC Design Criteria .................................................
9
3.2.10 Liquefaction .........................................................................................
10
3.2.11 Seismically Induced Settlement ...........................................................
11
3.2.12 Lateral Spreading.................................................................................
11
3.2.13 Slope Instability ....................................................................................
11
3.2.14 Hydroconsolidation ..............................................................................
12
3.2.15 Expansive Soil .....................................................................................
12
3.2.16 Flooding and Scour ..............................................................................
12
Grading Considerations ...................................................................................
12
3.3.1 Removal of Unsuitable Material ...........................................................
13
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William S. Hart Union High School District
May 26, 2010 (Project No. 3715.001)
CONTENTS-CONTINUED
Page
3.3.2 Fills ......................................................................................................
Onsite Surface Water Runoff Detention Facility ...............................................
13
13
PRELIMINARY SHALLOW FOUNDATION PARAMETERS .....................................
14
4.1
4.2
4.3
4.4
Introduction ......................................................................................................
Footing Design Criteria ....................................................................................
Modulus of Subgrade Reaction for Flat Slab-on-Ground ..................................
Sliding and Passive Resistance .......................................................................
14
14
14
15
5.0
CLOSING .................................................................................................................
15
6.0
REFERENCES .........................................................................................................
16
3.4
4.0
TABLES
1
2
Proximal Faults......................................................................................................
Summary of Preliminary 2007 CBC Seismic Design Parameters...........................
9
10
PLATES
Plate
Vicinity Map ......................................................................................................................
Preliminary Geologic Interpretation ...................................................................................
Regional Geologic Map .....................................................................................................
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William S. Hart Union High School District
May 26, 2010 (Project No. 3715.001)
1.0 INTRODUCTION
This letter report summarizes preliminary geologic and geotechnical information for the
198-acre Romero Canyon Site (Tentative Tract 47807), 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 Romero Canyon 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
RSA Associates, Inc. (RSA) performed geologic studies for development of 199.1 acres
of TT 47807 in 1989 and 1987 (RSA 1989 and 1991). Data review indicates that those studies
included advancing 18 large-diameter borings, 7 hollow-stem-auger borings, 10 test pits, and 2
trenches within the tract. According to RSA, the large-diameter borings were advanced to
evaluate hillside areas and the hollow-stem-auger borings were advanced to evaluate alluvial
areas.
Geolabs Westlake Village (GWV) performed geotechnical studies for a tank site and for
the project site (GWV 1999, 2006, and 2007). Supplemental field exploration advanced by
GWV consisted of a total of 9 hollow-stem-auger borings, 15 large-diameter bucket-auger
borings, 6 backhoe test pits, and 3 trenches.
Earth Resources, Inc. (ERI, 2009) performed a reconnaissance-level geologic study for
the Romero Canyon Site that summarized the site conditions based on previous work by RSA
and GWV and provided preliminary opinions for slope stability, seismicity, and geohazards.
1.3
PROJECT DESCRIPTION
Site development for the high school at the Romero Canyon Site is conceptual at this
time but is anticipated to consist of cut- and fill-grading of ridges and canyon area to create a
super-pad for the school site. The existing and conceptual site topography is presented on
Plate 2 - Preliminary Geologic Map. Based on our understanding of the conceptual project, site
development for the school includes the following:
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William S. Hart Union High School District
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1.4
•
An about 44.7 acre graded pad that will slope gently to the east.
•
A 2 horizontal to 1 vertical (2h:1v) cut-slope is proposed in the northwest portion of
the site that will be about 380 feet high.
•
A 100- to 170-foot-high fill slope planned along Romero Canyon Road at the eastern
property margin.
•
Fills to about 120 feet deep are planned. However, remedial grading to remove
landslide debris below planned excavation depths are likely to be extensive and
could result in fill depths of 200 feet or more to remove landslide debris.
WORK PERFORMED
The work performed for this study for the Romero Canyon 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
Romero Canyon 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.
1.4.2
Site Reconnaissance
An engineering geologist from Fugro performed a site reconnaissance of the project site
on April 12, 2010, to observe site conditions described in previous reports and to observe the
following:
•
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.
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William S. Hart Union High School District
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1.4.3
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.
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.
2.2
LOCAL GEOLOGIC CONDITIONS
Published mapping indicates that the site's hillside terrain is underlain by large landslide
complexes and non-marine bedrock of the Saugus Formation with alluvium in the canyon areas
(Plates 1 through 3). Dibblee (1993) indicates that a northwest-southeast trending anticline
(Loma Verde Anticline) axis extends through the project site from near the northwest to the
southeast corners of the property, as shown on Plate 1. GWV (2007) indicates that the site is
located on the north limb of a Loma Verde Anticline and that the regional bedding dips to the
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William S. Hart Union High School District
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east-southeast at approximately 20 degrees. GWV indicates that Saugus Formation strata also
has locally variable bedding attitudes due to channelization and cross-bedding features that
GWV did not feel were representative of the overall geologic structure.
2.3
SITE CONDITIONS
As depicted on Plate 1, the Romero Canyon site has moderately steep to steep hilly
terrain with alluvial filled valleys. Elevations range from a high of about 2,380' along the ridge at
the northwestern property limit to a low of about 1,730 feet in the southeastern portion of the site
in Romero Canyon. Total elevation change across the site is about 650 feet. Natural slopes
range from gently inclined in stream channel areas to nearly vertical in hillside areas. Previous
mapping by Barrows (1986), CDMG (2002), and GWV (2007) indicates the presence of large
landslide complexes in the hillside areas. Landslides identified by the CDMG and GWV are
highlighted in yellow on Plates 2 and 3. As described by GWV, a majority of the previously
identified landslides are on east-facing slopes.
2.4
EARTH MATERIALS
On the basis of our data review and site reconnaissance, the geologic materials exposed
at the site consist of alluvium, colluvium, landslide deposits, and bedrock of the Saugus
Formation as indicated on Plate 2.
2.4.1
Alluvium (Qa)
As depicted on Plate 2, alluvium is mapped in the Romero Canyon and tributary
drainages. GWV indicates that the alluvial sediments encountered in their explorations consist
of medium dense to dense sands, silty sands, and gravelly sands with variable gravel and
cobbles that is up to 70 feet deep. Visible pores were also reported in some samples.
2.4.2
Colluvium (Qc)
As depicted on Plate 2, colluvium is mapped primarily in draws and mantling the lower
portions of natural slopes throughout the property. GWV describes the colluvium as a mixture
of loose silty, clayey sand with variable gravel and cobbles derived from the underlying Saugus
Formation bedrock and ranges from several feet to greater than 20 feet thick in explorations.
GWV indicates that the colluvium is not suitable for support of fill or structures and will need to
be removed by site grading.
2.4.3
Landslide Deposits (Qls)
Previous studies by the CDMG (1986, 2002), RSI (1999), and GWV (2007) have
documented large-scale landsliding on the property. Those previously mapped landslides are
highlighted in yellow on Plates 2 and 3. GWV (2007) numbered the landslides (1 through 15)
and that nomenclature has been incorporated into Plate 2. Also shown on Plate 2 are potential
landslides identified by Fugro based on our aerial photographic mapping and site
reconnaissance (red areas). For this report, the landslides mapped by the previous authors and
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William S. Hart Union High School District
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Fugro have been grouped into four primary landslide complexes (Plate 3; Qls1 through Qls4) as
described in the following table.
Landslide Complex
No. & Location
Previous
Explorations
Description
Comments
Qls1/Northern portion of
site (includes GWV Qls
1,6,7,8, 9, 10 and 11)
RSI B-5, B-15;
GWV-6;
numerous other
drill holes in close
proximity.
B-5 and B-15 extended to 71
and 100 feet in Qls no base
identified. GWV-6 encountered
20 feet of Qls near top of slide.
GWV labeled entire slide mass
as geologic restricted use area.
Qls1 geometry not well defined.
Per Plate 3, landslide Qls1 within
building footprint.
GWV-7,-8,-14,18,-19, and -20
Explorations by GWV indicate
that the Qls2 complex is on the
order of 60 feet deep in the
central portion of the landslide.
Qls2 geometry generally
characterized with multiple drill
holes through the landslide mass
and into underlying bedrock. Per
Plate 3, landslide within building
footprint.
B-19,-20;GWV22, -23, -24
Explorations by RSI and GWV
indicate that Qls3 ranges from
40 to 80 feet deep.
Multiple drill holes through
landslide and into underlying
bedrock. Landslide outside of
proposed building area but could
potentially impact access road
alignment.
RSI B-1 does not indicate
landslide deposits in their drillhole log.
Fugro mapping indicates Qls4
potentially larger complex than
previously mapped. Area
underlies 130 foot high fill slope.
Additional exploration to further
evaluate subsurface conditions
and potential Qls geometry.
Qls2/Northern portion of
site; directly south of
Qls1 (includes GWV
Qls2 and 5)
Qls3/Southern portion of
site (includes GWV Qls3
and 13).
Qls4/Eastern portion of
site
2.4.4
B-1
Saugus Formation (TQs)
Bedrock of the Saugus Formation is present in the hillside areas of the property and
underlies the alluvial sediments and landslide debris at depth. GWV reported that the majority
of the bedrock materials in the eastern portion of the property consist of fine- to coarse-grained
sandstone and conglomerate that is described as friable, massive to cross-bedded, with varying
amounts of gravel, cobbles, and boulders. The western portion of the property contains a
higher percentage of claystone and clayey siltstone strata, which GWV indicated have
contributed to "major landslides."
2.5
GROUNDWATER
Groundwater was not reported in any of the explorations advanced by GSI or GWV in
previous geotechnical studies. The deepest drill hole advanced in alluvial areas is GWV's
GWV-12, which was advanced to a depth of 69 feet in the southeastern portion of the property
without encountering water. Fugro did not observe evidence of surface seeps or springs onsite
during our site reconnaissance.
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William S. Hart Union High School District
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Review of CGS (2003) indicates the depth to historically high groundwater is about 10
feet within the Romero Canyon 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.
3.0 OPINIONS AND RECOMMENDATIONS
3.1
GENERAL
Development of the Romero Canyon site will likely require significant grading and
engineering input to address several key geotechnical issues: 1) landslide removal, 2) slope
stability of the proposed 380-foot high cut-slope, and 3) settlement of up to 200-foot deep fills
beneath the building pad. Preliminary evaluations and opinions for project consideration and
recommendations for the geotechnical design-level study 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 SUMMARY
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 the preliminary geohazard evaluation presented herein.
3.2.1
Landsliding
Geomorphology observed in air photo and site reconnaissance (Plates 2 and 3)
suggests that landsliding may be more extensive on the Romero Canyon Site than previously
interpreted. Based on the data review and interpretations, the landslide complex (Qls1) in the
NW portion of site appears to contain relatively extensive deep-seated features that are
documented to be in excess of 100 feet thick and could extend to depths of greater than
150 feet. Other mapped and potential landslides (in complexes Qls-3 and -4) may be 100 feet
or more thick. Subsurface data do not exist in several of the potential landslide complexes. The
large complex in the NW portion of the site (Qls1 and Qls4) has not been adequately
characterized and the depth and extent are not known. Additional exploration will be required to
characterize and evaluate the landslide properties in sufficient detail for project design for all of
the mapped landslides depicted on Plates 2 and 3. In general, to receive site approval the
landslides will need to be removed and replaced with compacted fill to reduce the potential for
future settlement. Although not recommended, there is a potential for some landslide debris to
be left in-place if the geotechnical studies can demonstrate that the amount of future settlement
would be within project settlement requirements. Commonly, that would take dozens of
explorations, or more, at a cost of several hundred thousand dollars and those studies may not
be able to conclusively demonstrate that the settlement of the landslide mass and overlying fill
will be within project limits. In addition, leaving the landslide deposits in place will likely result in
a higher level of review by the Office of the State Architect and their geotechnical reviewer, the
California Geologic Survey, which will increase the potential for schedule impacts and delays.
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3.2.2
Slope Stability
A 380-foot-high cut-slope is planned in the NW portion of the site. Based on previous
exploration, the cut-slope could expose laterally unsupported "daylighted" geologic bedding or
create a dip-slope condition and could result in failure of the slope that could extend offsite. As
depicted on Plates 2 and 3, the proposed 380-foot-high cut-slope is in the area of landslide
complex Qls1. As described by GWV, the landsliding in the western portion of the site is likely
associated with the presence of a higher percentage claystone and clayey siltstone beds within
the Saugus Formation. A likely cause of the existing landslides is scour at the base of slopes
undercutting the slope and laterally unsupporting "daylighting" the claystone beds, which moved
downslope. A standard mitigation for laterally unsupported bedding is to re-grade the slope as a
buttressed fill to achieve acceptable factors of safety against slope instability. A buttressed fill
slope could significantly increase the amount of grading required. A typical mitigation for
laterally unsupported bedding is to construct a buttress of compacted fill that extends from the
base of the slope to near the top of slope. As a general guide, a buttress may be about one-half
as wide as the slope height, which would result in a buttress about 200-feet wide. Grading to
create a 200-foot wide buttress would extend offsite onto the adjacent property. Alternative
buttress designs may be possible, but the geologic conditions in the proposed cut-slope area
will need to be evaluated in greater detail to develop a slope configuration that meets current
grading code requirements.
3.2.3
Fill Settlement
A 100- to 170-foot-high fill slope is planned along Romero Canyon Road at the eastern
property margin (Plates 2 and 3). However, remedial grading to remove landslide debris below
the planned excavation depth could result in fill depths of 200 feet or more beneath proposed
structures. Site-specific design will need to address the estimated settlement and provide
recommendations to reduce settlement to within project requirements. Some of the mitigations
that are frequently used to help control post-construction settlement include select fill placement
in deep fills, special compaction requirements, soil additives to strengthen the soil, geogrid
reinforcement, and settlement monitoring.
3.2.4
Preliminary Recommendations if the Project Proceeds to Design at the Romero
Canyon Site
•
Extensive subsurface exploration to evaluate the presence of interpreted landslides
and to characterize landslide geometries for input into design of remedial grading for
landslide removals.
•
Subsurface exploration to characterize geometry in the vicinity of the proposed 380foot-high cut-slope along the northern portion of the site.
•
Exploration is anticipated to consist of supplemental large-diameter drill holes, test
pits, and trenches to characterize landslide distribution and geometries and
subsurface conditions in the proposed cut-slope area.
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3.2.5
•
Exploration within the large landslide complex in the NW portion of the site is
anticipated to consist of continuous coring and down-hole geophysical tools to
characterize material distribution, characteristics, and depth of slides within the
complex.
•
Subsurface exploration in alluvial areas for liquefaction and dry settlement
evaluations and for recommended removals of unsuitable materials.
•
Geotechnical design parameters and a design-level report will be required, if the
project proceeds to design.
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.
3.2.6
Fault Rupture Potential/Ground Shaking
The proposed Romero Canyon 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 within about several miles from the site. 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 1-mile south of the
project site (Plate 1) and the Santa Felicia fault is located about 1 mile north of the project site.
3.2.7
Faults
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.
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William S. Hart Union High School District
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Table 1. Proximal Faults
Approximate
Distance (mi)
Estimated
Maximum
Magnitude
(Mw)
San Gabriel
1.6
7.2
Holser
2.7
6.5
Santa Susana
4.3
6.7
Northridge (E. Oak Ridge)
6.3
7.0
San Cayetano
7.0
7.0
Oak Ridge (Onshore)
6.0
7.0
Simi-Santa Rosa
8.6
7.0
Sierra Madre (San Fernando)
11.1
6.7
Santa Ynez East
14.8
7.1
San Andreas - Whole
16.5
8.0
Verdugo
20.0
6.9
Anacapa-Dume
24.2
7.5
Sierra Madre
25.6
7.2
Fault Name
3.2.8
Ground Shaking
Review of the CGS fault database indicates that 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. A seismic hazard assessment consistent with State guidelines (CGS Note 48) should
be included as part of the project design.
3.2.9
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.
In their 2007 report, GWV assigned a site classification of Class D for the onsite earth
materials. A site class D assumes the onsite granular soils are medium dense to dense (SPT
value of 15 to 50) and the soils will not liquefy under the design earthquake. For a site Class D
to apply, potentially liquefiable alluvial deposits, colluvium, and landslide deposits will need to be
removed and replaced with compacted fill. For preliminary design purposes, the USGS
interactive webpage 'Seismic Design Values for Buildings' (USGS, 2008) was used to obtain the
seismic design criteria. Based on our data review and preliminary characterization of the
subsurface conditions and the 2007 CBC, the following values may be used for preliminary
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project planning purposes. Final design criteria will need to be based on conditions
encountered in explorations for the final design study.
Table 2. Summary of Preliminary 2007 CBC Seismic Design Parameters
2007 California Building
Code Section 1613
Seismic Parameter
Value
---
Latitude
N 34.4887°
---
Longitude
W 118.6567°
Section 1613.5.1 and Figure
1613.5(3)
Mapped Acceleration Response Parameter (Ss)
Section 1613.5.1 and Figure
1613.5(4)
Mapped Acceleration Response Parameter (S1)
Site Class B
Site Class B
1.907
0.766
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)
1.00
Section 1616.5.3 and Table
1613.5.3(2)
Site Coefficient (Fv)
1.50
Section 1613.5.3
Adjusted Acceleration Response Parameter for
Site Class D (Sms)
1.907
Section 1613.5.3
Adjusted Acceleration Response Parameter for
Site Class D (Sm1)
1.150
Section 1613.5.4
Design Spectral Response Acceleration
Parameter (SDS)
1.271
Section 1613.5.4
Design Spectral Response Acceleration
Parameter (SD1)
0.766
3.2.10 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 70 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
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ephemeral drainage course and the CGS indicates that the depth to historically high
groundwater is 10 feet. Therefore, in our opinion there is a potential for liquefaction to occur
within the alluvial sediments in the project area and to affect the 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.
3.2.11 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.
3.2.12 Lateral Spreading
The southern portion of the project site is located proximal to the Romero Canyon
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.
3.2.13 Slope Instability
As previously described in Section 3.2.1, large landslide complexes have been mapped
in the project boundaries. As depicted on Plate 3, our aerial photographic review and site
reconnaissance indicates that additional landslides are likely present on the site and will need to
be evaluated as part of the project final design. Those landslide deposits are documented to
extend to depths of over 100 feet and could extend to depths of 150 feet or more. Evaluation of
those landslides will likely require specialized exploration equipment to obtain high-quality
samples of the materials at depths of 150 to 200 feet.
The potential also exists for unfavorable geologic bedding conditions (laterally
unsupported bedding) to exist within the proposed 380-foot high east-facing cut-slope located
directly west and north of the project site. Geotechnical slope stability evaluations of the
proposed final slope conditions, geologic bedding orientations, and earth material properties will
be required as part of the design phase of the project.
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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 in the Saugus Formation for
use in slope stability evaluations for the design-level geotechnical report. Those analyses
should be consistent with CGS Special Publication (SP 117A).
In summary, we recommend that the previously mapped landslides (yellow areas on
Plate 2), potential landslides identified in the aerial photographic mapping (red areas on
Plate 2), colluvial deposits, and proposed cut areas be explored to evaluate the subsurface
geometry and material characteristics of the deposits. Based on our previous experience with
the reviewers (State of Californian Division of State Architect and California Geologic Survey),
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.
3.2.14 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.
3.2.15 Expansive Soil
Review of GWV (1999) suggests that the tested soil materials on the Romero Canyon
Site have a low shrink/swell potential. The GWV findings should be confirmed as part of the
design-level geotechnical study for the project.
3.2.16 Flooding and Scour
The project site lies adjacent to Romero Canyon. 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 (2007) reported observing pinpoint voids in alluvial and colluvial soil materials, and
oversize materials in alluvial and bedrock units. Recommendations for use of materials should
be provided in the final design report.
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 100 feet in thickness will be placed in
various areas. For those areas, special construction considerations will likely be required to
limit post-construction settlement. We recommend that extra compactive effort (minimum of
93 percent relative compaction) be utilized for fills over 40 feet deep/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 of the fill materials and alluvial
soils left-in-place (if any), 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.
3.4
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 such 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.
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Percolation areas should be set back from the base of the proposed 380-foot high cut-slope by
several hundred feet to reduce the shear strengths of buttress fills supporting the cut-slope.
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 should 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.
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
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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.
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
USDA AXJ
2K 30 to 32
1:20,000/ B&W
1959
USDA AXJ
1210 151 and 152
1:20,000/ B&W
C300
E45 -466
NP*/B&W
1989
USGS NAPP
1832 - 132 and 133
NP/Color
1969
USGS VCHC
1-34 and 35
NP/B&W
1974
USGS NAPP
6860 - 188 and 189
NP/B&W
1994
USGS GS-VDWA
1-256 and 257
NP/B&W
2002
USGS NAPP
1216 - 278 and 279
NP/Color
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 half of the Val Verde Quadrangle, Los Angeles
County, California, Landslide Hazard Identification Map #5, DMG Open-File Report 86-9.
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.
Earth Resources, Inc. (2010), Reconnaissance Geological Investigation, Proposed Romero
Canyon High School Site, portion of Tentative Tract 47807, Romero Canyon Road,
Castaic Area, County of Los Angeles, California, prepared for William S. Hart High
School District, dated January 21.
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.
Geolabs Westlake Village (GWV, 1999), Preliminary Geotechnical Assessment of Proposed
Water Tank Site, Tentative Tract No. 47807, Romero Canyon, Castaic, County of Los
Angeles, California, prepared for SIKAND, dated November 12.
_______ (2007), Grading Plan Review Report, Lots 1-31, Tract 47807-1, (Parcels 1 & 2 of PM
67132), Romero Canyon, County of Los Angeles, California, prepared for Larry
Rasmussen Trust, dated November 30.
RSA Associates (1989), Soils Engineering and Engineering geologic investigation, Proposed 79
Lot Housing Tentative Tract No. 47807, 199.1 Acres, Romero Canyon Road, Castaic
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William S. Hart Union High School District
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Area, County of Los Angeles, California, prepared for SVI Properties, Inc., dated
December 27.
______ (1991), Addendum Geotechnical Report, Proposed 77 Lot Subdivision, Tentative Tract
Nol 47807, 199.1 Acres, Romero Canyon Rd., County of Los Angeles, prepared for SVI
Properties, Inc., Dated August 2.
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PLATES