HomeMy WebLinkAbout4.F._Geology-SoilsIV. ENVIRONMENTAL IMPACT ANALYSIS
F. GEOLOGY/SOILS
INTRODUCTION
This section of the Draft EIR describes geology and soils in the project area. Where appropriate, this
section provides project level analysis for the Phase 1 Precise Plan, and program level analysis for the
remainder of development proposed by the Gateway Business Park Master Plan. A regulatory framework
is also provided in this section describing applicable agencies and regulations related to the geology and
soils.
Preparation of this section used data from various sources. These sources include (1) the City of South
San Francisco General Plan, the East of 101 Area Plan, and the Gateway Business Park Master Plan, (2)
Geotechnical reports for adjacent properties at 180 and 200 Oyster Point Blvd, completed by Treadwell
& Rollo; (3) Review of USGS Open File Reports (OFR) of the area, including a map of the bedrock
geology (USGS OFR 98-354, 1998), Quaternary Geologic Map, including liquefaction susceptibility
(USGS OFR 97-715, 1997), and Landslide Map (USGS OFR 97-745 C); (4) Review of Official
California Geologic Survey (CGS) (formerly the California Division of Mines and Geology (CDMG))
Maps, including the South San Francisco Alquist-Priolo (A-P) Earthquake Fault Zone Map (1982), and
Fault Activity Map of California (1994); (5) Review of government websites, including the Association
of Bay Area Government’s (ABAG) website (www.abag.gov) for a summary of hazards ranging from
liquefaction to seismic landsliding; (6) Review of Project Description; (7) Review of the California
Geological Survey’s Probabilistic Seismic Hazard Assessment; (8) Review of the 2007 Working Group
on California Earthquake Probabilities Uniform California Earthquake Rupture Forecast, Version 2,
USGS Open-File Report 2007-1437; (9) Review of the East of 101 Area Plan of the City of South San
Francisco, as well as all other applicable ordinances and regulations; (10) Site Reconnaissance by our
Senior Engineering Geologist and Staff Geologist; and (11) Review of the geotechnical report prepared
for the site by Treadwell and Rollo.
No comment letters related to geology and soils were received in response to the June 16, 2008 Notice of
Preparation (NOP) or the October 22, 2008 Revised NOP circulated for the project. The NOP and
comment letters are included in Appendix A of this Draft EIR.
ENVIRONMENTAL SETTING
Regional Seismicity
The site lies in the tectonically active Coast Ranges Geomorphic Province of Northern California, on the
east side of the San Francisco Peninsula. Development of the northwest trending ridges and valleys in the
vicinity, including the Santa Cruz Mountains and San Francisco Bay, are controlled by active tectonism
along the boundary between the North American and Pacific Tectonic Plates, the San Andreas Fault
System. Area faults have predominantly right-lateral strike-slip (horizontal) movement, with lesser dip-
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City of South San Francisco October 2009
slip (vertical) components of displacement. Horizontal and vertical movement is distributed on the
various fault strands within a fault zone. Throughout geologic time the fault strands experiencing active
deformation change in response to regional shifts in stress and strain from plate motions. Within 15 miles
of the project site there are three major active faults that display large right-lateral strike-slip offsets, the
San Andreas Fault, the San Gregorio Fault, and the Hayward Fault.
The nearest known active fault is the San Andreas Fault, located approximately 3.5 miles (6 km)
southwest of the site. Other nearby active faults include the Hayward Fault located 15 miles (24 km)
northeast, the San Gregorio Fault (a.k.a. the Seal Cove Fault) located approximately 11 miles to the
southwest, the Calaveras Fault located approximately 26 miles northeast and the Concord/Green Valley
Fault located approximately 30 miles northeast.
Other faults are nearer than the San Andreas but are not considered active since they show no evidence of
Holocene rupture or movement during the past 11,000 years. These include the Serra and City College
Faults mapped approximately 3.1 miles southwest and 2.8 miles north of the site, respectively. The
nearest mapped fault of any type is the Hillside Fault, the trace of which passes through the southern
portion of the project site. However, this fault is buried beneath Holocene age hillslope deposits and
Pleistocene age alluvium that have not been offset by fault movements, therefore is not subject to
1
development restrictions under the Alquist-Priolo Earthquake Fault Zoning Act.
Seismicity of the project region has resulted in several major earthquakes during the historic period,
including the 1868 Hayward Earthquake, the 1906 San Francisco Earthquake, and most recently, the 1989
Loma Prieta Earthquake. According to ABAG, violent ground shaking, Modified Mercalli Intensity
(MMI) IX is possible in response to a large earthquake along the nearby San Andreas Fault. A major
2
rupture of the Hayward Fault is expected to produce strong ground shaking, MMI VII.
Regional Geology
The site is located near the western margin of the San Francisco Bay, a submerged valley in the Central
Coast Ranges of California. This area is characterized by northwest trending mountain ranges and valleys
oriented sub-parallel to faults of the San Andreas Fault System. In the San Francisco Bay Area, Tertiary
strata commonly rest in angular unconformity on rocks of the Franciscan complex, which is composed of
weakly to strongly metamorphosed greywacke (sandstone), argillite, limestone, basalt, serpentinite, and
chert. The rocks of the Franciscan complex are ancient Jurassic oceanic crust and deep marine (pelagic)
deposits added onto the edge of the North American Continent and metamorphosed as a result of tectonic
processes. Late Jurassic to Late Cretaceous sedimentary deposits overlay these deposits. Deposits of these
rocks may be found outcropping along San Bruno Mountain in the project vicinity. Little metamorphosed,
high-pressure, low-temperature metamorphic minerals are common in the Franciscan complex, but there
are also high grade metamorphic blocks in sheared but relatively un-metamorphosed argillite matrix
which reflect the complicated history of the Franciscan.
1
Geologic and geotechnical Investigation Gateway and Oyster Point Boulevard, South San Francisco, CA
Treadwell and Rollo, September, 2008.
2
Association of Bay Area Governments (ABAG) Shaking Intensity Map for Future Earthquake Scenarios
Gateway Business Park Master Plan IV.F. Geology/Soils
Draft Environmental Impact Report Page IV.F-2
City of South San Francisco October 2009
These rocks have been offset by movement along the San Andreas Fault System, which traverses the
Santa Cruz Mountains prior to heading offshore in Southern Daly City. Several northwest trending and
structurally controlled valleys dissect the San Francisco Peninsula. Colma Creek defines one such valley,
with the project site near its opening to the bay. During the Quaternary Period of rising and falling sea
level in response to patterns of global glaciation these valleys were repeatedly incised and then backfilled
with sediment to form the suite of alluvial deposits that can be found today, including the Pleistocene
Colma Formation. Along the bay margin, deposits of Holocene “Bay Mud” deposited during the past
11,000 years, during which time the Bay has filled with seawater, can be found, as well as marsh deposits,
and other fine grained sediment accumulated by currents along the shore.
Site Geology and Soils
According to a recent map of bedrock geology, Cretaceous aged sheared rocks of the Franciscan
Formation underlie the southern portion of the site while Quaternary aged slope debris and ravine fill
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underlie most of the northern portion of the site. The Pleistocene aged Colma Formation underlies the
very northernmost portion of the site. The sheared rock of the Franciscan Complex consists of small to
large fragments of hard rock in a matrix of tectonically sheared shale and siltstone. The matrix is
generally coherent and firm, but in places may be soft, especially where deeply weathered. The
Quaternary slope debris and ravine fill consists mainly of silty to sandy clay and locally sandy silt to
clayey sand or gravel. The Colma Formation is described as friable, well sorted, fine to medium grained
sand containing a few beds of sandy silt, clay, and gravel throughout most of the area, but also as sandy
clay and silty sand in the project vicinity. Project site and vicinity geology and soils are shown in Figure
IV.F-1.
A geotechnical investigation was performed for the site by Treadwell and Rollo, including a subsurface
investigation consisting of twelve boreholes drilled through existing pavement sections to depths of 18.5
to 60 feet below existing ground surface (BGS). Under the pavement sections the site was covered by
sandy clay and clayey sand fill materials to depths of 2 to 13 feet. Native material under the fill consisted
of dense to very dense clayey sand, sand with clay and highly weathered shale. The clayey sand and sand
with clay were encountered to the maximum depth explored, except for three boreholes which terminated
in weathered shale and mudstone.
Landsliding and Slope Stability
Slope steepness is generally the dominant factor governing slope stability, depending upon soil and
bedrock conditions. Steep slopes greater than 50 percent are especially prone to landslides in areas of
weak soil and/or bedrock. There is a cut slope along the southeastern border of the project site, up to a
former railroad grade. The cut has a sloe of approximately 1.5 : 1 (horizontal : vertical) and is cut into
Franciscan mélange. The southern extent of this cut is approximately 10 feet high, while the northern
3
Bonilla, M. G. Preliminary Geologic Map of the San Francisco South 7.5’ Quadrangle and part of the Hunter’s
Point 7.5’ Quadrangle San Francisco Bay Area, California. Plot derived from USGS Open-File Report 98-354.
1998
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City of South San Francisco October 2009
extent is approximately 20 feet high, indicating the gentle slope upward to the south. There are also
retaining walls along the northeast portions of the site holding up excavations related to new construction
in the adjacent parcel. Natural slope stability is not expected to be an issue; however, attention must be
paid to stability of cut slopes.
Expansive Soils
Soil expansion is a phenomenon in which clay and silt soils expand in volume as a result of an increase in
moisture content, and shrink in volume upon drying. Changes in soil volume as a result of moisture
fluctuations, including seasonal fluctuations, can cause damage to concrete slabs, foundations and
pavements. Expansive soils are generally identified by use of two types of soil tests. Expansion index
tests determine the potential for expansion of soils. Soils with expansion indices greater than 20 have a
potential for damaging site improvements. Atterberg limits testing, including liquid limit and plastic limit
testing, is another type of physical properties test used to determine the plasticity index and the potential
for soil expansion. Soils with plasticity indices of 12 and above are considered to be expansive. Atterberg
limits tests were performed on two samples from 11.5 feet and 18.5 feet BGS as part of the geotechnical
investigation performed by Treadwell and Rollo. These tests indicated plasticity indices of 6 and 18,
respectively. This indicated that sandy clay soils at depth are slightly to moderately expansive. However,
these materials are sufficiently deep as to preclude their having an effect on proposed building
foundations and Treadwell & Rollo did not identify expansive soils as a hazard at the project site.
Primary Seismic Hazards – Surface Fault Rupture
A number of active and potentially active faults are present in the region. According to criteria of the
State of California Geological Survey, active faults have experienced surface rupture within the last
11,000 years (Holocene Period). The Alquist-Priolo Earthquake Fault Zoning Act of 1972 initiated a
program of mapping active and potentially active faults (faults with displacement within Quaternary time
– the last 1.6 million years). According to the program, active faults must be zoned and development
projects within the Earthquake Fault Zones investigated to establish the location and age of any faulting
across the development site. Active and potentially active faults along the San Francisco Peninsula have
undergone extensive investigation in the past. ABAG has summarized results from many of these studies
to quantify the potential impact to certain areas, while the California Geological Survey has established
Earthquake Fault Zone (EFZ) boundaries. According to these maps, the proposed development is not
located within an EFZ.
The nearest EFZ is for the San Andreas Fault, located slightly more than 3 miles southwest of the site.
The Hillside Fault is mapped as passing through the southern portion of the site (See Figure IV.F-1).
However, this fault has not shown evidence of rupture in at least the last 2 million years, and is not
considered at risk of surface rupture, therefore the risk of ground rupture within the project boundaries is
considered very low.
Gateway Business Park Master Plan IV.F. Geology/Soils
Draft Environmental Impact Report Page IV.F-4
Legend
0
2000
1000
Feet
Source: Bonilla, 1998, Geologic Map of the South San Francisco 7.5’ Quadrangle and Part of the Hunters Point 7.5’ Quadrangle, US Geological.
Figure IV.F-1
Site Geology
City of South San Francisco October 2009
Figure IV.F-1: Project Site Geology 11x17 (back)
Gateway Business Park Master Plan IV.F. Geology/Soils
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City of South San Francisco October 2009
Secondary Seismic Hazards
Ground Shaking
The San Francisco Bay Area is a seismically active region. The project site and region will likely be
subjected to strong to violent seismically induced ground shaking within the design life of the
development.
According to a recent study completed by the Working Group on California Earthquake Probabilities
(WGCEP), which assesses the probability of earthquakes in the San Francisco Bay Area, there is a 63
percent probability that an earthquake of Richter Magnitude 6.7 or greater will strike between 2007 and
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2037.
The intensity of ground shaking will vary with the distance and magnitude of the earthquake causing the
ground shaking. The maximum intensity ground shaking expected to occur at the site would be a
Modified Mercalli Intensity level of IX (violent) in response to an earthquake of equivalent magnitude to
the 1906 earthquake (7.9) on the San Andreas Fault. An earthquake of magnitude 6.8 on the Hayward
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fault would be expected to produce strong ground shaking equivalent to Modified Mercalli Intensity VII.
Peak ground accelerations for the site with a 10 percent probability of being exceeded in a 50-year period
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are approximately 59 percent of the acceleration due to gravity (g). Actual ground motions resulting
from ground acceleration may be amplified or dampened depending on the underlying geologic materials.
Deep soft soils tend to amplify waves whereas shallow soils overlying hard bedrock tends to dampen
shaking intensity. With relatively dense soils at the project site, no amplification of seismic waves is
anticipated.
Seismically Induced Liquefaction
Liquefaction is the temporary transformation of saturated, cohesionless soil into a viscous liquid as a
result of ground shaking. Liquefaction potential was examined for the site in geotechnical reports for
adjacent properties. According to those reports, the risk of liquefaction was determined to be very low.
7
According to ABAG, soils at the site have a very low susceptibility to liquefaction. Liquefaction
potential was examined for the site in the geotechnical investigation by Treadwell and Rollo for the
subject property. According to this analysis, site soils are sufficiently dense and contain sufficient clay
content such that the risk of liquefaction was determined to be very low.
4
The Uniform California Earthquake Rupture Forecast, Version 2, 2007 Working Group on California
Earthquake Probabilities, 2008. obtained from http://pubs.usgs.gov/of/2007/1437/
5
Association of Bay Area Governments, www.abag.ca.gov , 2005.
6
California Geologic Survey, http://www.consrv.ca.gov/CGS/rghm/pshamap/pshamain.html .
7
ABAG website, www.abag.ca.gov
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City of South San Francisco October 2009
Seismically Induced Densification
Dynamic densification or ground subsidence can occur when dry cohesionless soils collapse as a result of
seismic shaking. This may be particularly true of unconsolidated sandy fill, or ground overlying hollow
areas due to caves, mines, or areas with excessive groundwater removal. Since these conditions do not
occur at the site, dynamic densification is not considered a hazard.
Seismically Induced Lurch Cracking
Lurching is the sudden swaying, rolling, or spreading of the ground during a strong earthquake. Lurch
cracking is the development of fissures or cracks on slopes overlain by weak soils. Sandy soils at the site
are sufficiently dense, and lurch cracking is not considered a hazard.
Seismically Induced Landslides
Seismically induced slope failure is another secondary seismic hazard. During earthquake induced ground
shaking, unstable slopes can fail, causing landslides and debris flows. The project site has a natural slope
downward toward the south. However, the project site has been leveled for existing development, and
along the southeast border of the project site there is an approximately 1.5:1 (horizontal:vertical) cut slope
ranging from approximately 10 feet high at the southwestern end to approximately 25 feet high at the
northeastern end. This steep slope could be susceptible to seismically induced landslides.
Regulatory Setting
Federal
National Pollutant Discharge Elimination System (NPDES) Phase I (General Construction Activity
Stormwater Permit)
A Stormwater Pollution Prevention Plan (SWPPP) prepared in compliance with an NPDES Permit
describes the project site, erosion and sediment controls, runoff water quality monitoring, means of waste
disposal, implementation of approved local plans, control of post-construction sediment and erosion
control measures and maintenance responsibilities, and non stormwater management controls.
Dischargers are required to inspect construction sites before and after storms to identify stormwater
discharge from construction activity, and to identify and implement controls where necessary.
State
California Building Code
Title 24 of the California Code of Regulations, also known as the California Building Standards Code,
sets minimum requirements for building design and construction. The 2007 version of the California
Building Standards Code are effective as of January 1, 2008. The California Building Standards Code is a
compilation of three types of building standards from three different origins:
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Building standards that have been adopted by state agencies without change from building
standards contained in national model codes;
Building standards that have been adopted and adapted from the national model code
standards to meet California conditions; and
Building standards, authorized by the California legislature, that constitute extensive
additions not covered by the model codes that have been adopted to address particular
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California concerns.
In the context of earthquake hazards, the California Building Standards Code’s design standards have a
primary objective of assuring public safety and a secondary goal of minimizing property damage and
maintaining function during and following seismic events. The 2007 version of the California Building
Standards Code differs significantly from the previous versions of the code. The 2007 code assigns a
seismic design category (SDC) to each structure. The SDC is assigned as a means of capturing both the
seismic hazard, in terms of mapped acceleration parameters (spectral values), site class (defining the soil
profile), and the occupancy category (based on its importance or hazardous material contents). The SDC
affects design and detailing requirements as well as the structural system that may be used and its height.
The previous versions of the code captured these requirements simply based on the location’s seismic
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zone and proximity to active faults.
Alquist-Priolo Earthquake Fault Zoning Act
The California Legislature passed the Alquist-Priolo Earthquake Fault Zoning Act in 1972 to mitigate the
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hazard of surface faulting to structures for human occupancy. The Act’s main purpose is to prevent the
construction of buildings used for human occupancy on the surface trace of active faults. The Act
addresses only the hazard of surface fault rupture and is not directed toward other earthquake hazards.
Local agencies must regulate most development in fault zones established by the State Geologist. Before
a project can be permitted in a designated Alquist-Priolo Earthquake Fault Zone, the city or county with
jurisdiction must require a geologic investigation to demonstrate that proposed buildings would not be
constructed across active or potentially active faults.
California Seismic Hazards Mapping Act
The California Seismic Hazards Mapping Act of 1990 (California Public Resources Code Sections 2690-
2699.6) addresses seismic hazards other than surface rupture, such as liquefaction and seismically
induced landslides. The Seismic Hazards Mapping Act specifies that the lead agency for a project may
withhold development permits until geologic or soils investigations are conducted for specific sites and
8
California Building Standards Commission website at http://www.bsc.ca.gov/title_24/default.htm
9
Bonneville, David New Building Code Provisions and Their Implications for Design and Construction in
California (abstract), 2007, obtained from
http://www.consrv.ca.gov/cgs/smip/docs/seminar/SMIP07/Pages/Paper12_Bonneville.aspx
10
California Division of Mines and Geology, 1997 revision, Fault-Rupture Hazard Zones in California, DMG
Special Publication 42.
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mitigation measures are incorporated into plans to reduce hazards associated with seismicity and unstable
soils. The Seismic Hazards Mapping Act became effective in 1991 to identify and map seismic hazard
zones for the purpose of assisting cities and counties in preparing the safety elements of their general
plans and to encourage land use management policies and regulations that reduce seismic hazards. The
intent of this Act is to protect the public from the effects of strong ground shaking, liquefaction,
landslides, ground failure, or other hazards caused by earthquakes. In addition, the California Geological
Survey’s Special Publication 117, Guidelines for Evaluating and Mitigating Seismic Hazards in
California, provides guidance for the evaluation and mitigation of earthquake-related hazards for projects
in designated zones of required investigations.
Local
City of South San Francisco Municipal Code
The City of South San Francisco Municipal Code Title 15 includes information on the Construction
Codes and Amendments adopted by the City of South San Francisco. This includes the California
Building Code, among other codes used in construction in the City of South San Francisco. The
California Building Code Vol. 1 and 2, 2007 Edition, including the California Building Standards, 2007
Edition, published by the International Conference of Building Officials, and as modified by the
amendments, additions and deletions set forth in Title 15 was adopted by reference as the building code of
the City of South San Francisco on January 1, 2008.
City of South San Francisco Hazard Mitigation Plan
The City of South San Francisco has adopted the Association of Bay Area Governments Local Hazard
Mitigation Plan as the Hazard Mitigation Plan (HMP) for the City by resolution 65-2006, on August 16,
2006. The HMP has been designed to identify the areas where people or structures may have higher
vulnerability to earthquakes, flood, wildland fires, and other natural hazards. The Plan identifies policies
and actions that may be implemented by the City to reduce the potential for loss of life and property
damage in these areas based on an analysis of the frequency of earthquakes, floods, wildland fires and
landslides in terms of frequency, intensity, location, history, and damage effects. The Plan serves as a
guide for decision-makers as they commit resources to reduce the effects of natural hazards.
City of South San Francisco General Plan Update
The General Plan Update Health and Safety Element includes a section on Geological and Seismic
Hazards. This section identifies geotechnical and geologic impacts to the general City of South San
Francisco area. The most recent General Plan update was completed in October 1999.
The 1999 South San Francisco General Plan Health and Safety Element contains policies designed to
minimize the risks associated with development in areas of seismic hazards. As such, the South San
Francisco General Plan, Health and Safety Element, has set forth specific guidelines with respect to site
treatment and building design and the unique geological hazards of the area. The South San Francisco
General Plan, Health and Safety Element, policies are as follows:
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Policy 8.1-I-1: Do not permit special occupancy buildings, such as hospitals, schools and other
structures that are important to protecting health and safety in the community, in
areas identified in Figure 8-2.
Policy 8.1-I-2: Steep hillside areas in excess of 30 percent grade should be retained in their
natural state. Development of hillside sites should follow existing contours to the
greatest extent possible. Grading should be kept to a minimum.
Implementing Policies 8.1-I-1 and 8.1-I-2 refer to Figure 8-2 (General Plan Policies for Seismically
Sensitive Lands), of the South San Francisco General Plan, Health and Safety Element.
East of US 101 Area East of 101 Area Plan
In 1994 the City of South San Francisco developed the East of 101 Plan with the overall goal of
recognizing the unique character of the East of 101 Area and to guide and regulate development in a
manner which protects and enhances the area’s physical, economic and natural resources, while also
encouraging appropriate development in the area. As such, the East of 101 Plan Chapter 10, Geotechnical
Safety Element, has set forth specific guidelines with respect to site treatment and building design and the
unique geological hazards of the area. The East of 101 Geotechnical Safety Element policies are as
follows:
GEO-1 The City shall assess the need for geotechnical investigations on a project-by
project basis on sites in areas of fill shown of Figure 17 (of the East of 101 Area
Plan), and shall require such investigations where needed.
GEO-2 Where fill remains under a proposed structure, project developers shall design
and construct appropriate foundations.
GEO-3 Given the extensive use of the area for industrial and waste disposal purposes,
investigation both by drilling and by examination of historic aerial photographs
shall be conducted by project developers to determine if landfills exist under the
project site prior to construction.
GEO-4 Project developers shall design developments on landfills and dump sites to deal
safely with gas produced by the decomposition of the buried garbage. Inorganic
soil capping over landfills shall be thick enough that excavation for repair of
existing utilities or installation of additional utilities does not penetrate to buried
garbage.
GEO-5 If hazardous fill, such as garbage organics, is encountered it shall be
appropriately disposed by a project developer during construction. This material
shall not be used for either structural fill or grading fill. However, other uses may
be possible, such as landscaping around vegetation if the fill has a high organic
content. If no acceptable use is found on-site, the hazardous fill should be
properly disposed off-site.
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GEO-6 Where a landfill or dump occurs under a proposed structure, project developers
shall design and construct appropriate foundations.
GEO-7 New slopes greater then 5 feet in height, either cut in native soils or rock, or
created by placing fill material, shall be designed by a geotechnical engineer and
should have an appropriate factor of safety under seismic loading. If additional
load is to be placed at the top of the slope, or if extending a level area at the toe
of the slope requires removal of part of the slope, the proposed configuration
shall be checked for an adequate factor of safety by a geotechnical engineer.
GEO-8 The surface of fill slopes shall be compacted during construction to reduce the
likelihood of surficial sloughing. The surface of cut or fill slopes shall also be
protected from erosion due to precipitation or runoff by introducing a vegetative
cover on the slope or by other means. Runoff from paved or other parts of the
slope shall be directed away from the slope.
GEO-9 Steep hillside areas in excess of 30 percent grade shall be retained in their natural
state. Development of hillside sites should follow existing contours to the
greatest extent possible and grading should be kept to a minimum.
GEO-10 In fill areas mapped on Figure 17 (of the East of 101 Area Plan), a geotechnical
investigation to determine the true nature of the subsurface materials and the
possible effects of liquefaction shall be conducted by the project developer
before development.
GEO-11 Development shall be required to mitigate the risk associated with liquefaction.
GEO-12 Structural design of buildings and infrastructure shall be conducted according to
the Uniform Building Code and appropriate local codes of practice, which
specify procedures and details to reduce the effects of ground shaking on
structures.
GEO-13 Development within the preliminary boundary of the Coyote Point hazard area,
as depicted on Figure 15 (of the East of 101 Area Plan), shall be reviewed by a
geotechnical engineer. Fault trenching may be required on individual
development sites where feasible and determined necessary by the engineer. No
structure for human occupancy shall occur within 50 feet of identified active
faults, unless a geotechnical investigation and report determine that no active
branches of that fault underlie the surface.
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City of South San Francisco October 2009
ENVIRONMENTAL IMPACTS
Methodology
As described in the section introduction, sources were consulted to document and analyze the local
geology and geologic risks on the project site included City of South San Francisco plans and policies, the
Gateway Business Park Master Plan, geotechnical reports for the adjacent properties, USGS Maps, review
of government websites, including the Association of Bay Area Government’s (ABAG) website
(www.abag.gov), and a site reconnaissance.
According to CEQA guidelines, exposure of people or structures to major geological hazards is a
significant adverse impact. The basic criterion applied to the analysis of impacts is whether construction
of the project will create, or be founded, on unstable geologic conditions that would last beyond the short-
term construction period. The analysis of geological hazards is primarily based on the degree to which the
site geology could produce hazards to people, structures, and the environment from earthquakes, fault
rupture, landslides, soil creep, expansion and settlement or other geologic events.
Thresholds of Significance
The following thresholds of significance are based on Appendix G of the 2006 CEQA Guidelines. For
purposes of this Draft EIR, implementation of the proposed project could result in potentially significant
impacts from geology and soils if the proposed project would result in any of the following:
Expose people or structures to potential substantial adverse effects, including the risk of loss,
injury or death involving:
Rupture of a known earthquake fault, as delineated on the most recent Alquist-Priolo
Earthquake Faulting Zoning Map issued by the State Geologist for the area or based on
other substantial evidence of a known fault.
Strong seismic ground shaking.
Seismic-related ground failure, including liquefaction and landslides.
Result in substantial soil erosion or the loss of topsoil.
Be located on a geologic unit or soil that is unstable, or that would become unstable as a result of
the project, and potentially result in on-or off-site landslide, lateral spreading, subsidence,
liquefaction or collapse.
Be located on expansive soil, as defined in Table 18-1-B of the Uniform Building Code (1994),
creating substantial risks to life or property.
Have soils incapable of adequately supporting the use of septic tanks or alternate waste water
disposal systems where sewers are not available for the disposal of waste water.
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Project Impacts
Impact IV.F-1: The proposed project would not expose people or structures to potential substantial
adverse effects, including the risk of loss, injury or death involving rupture of a known earthquake
fault, as delineated on the most recent Alquist-Priolo Earthquake Faulting Zoning Map issued by the
State Geologist for the area or based on other substantial evidence of a known fault.
According to the latest available fault maps, the site is not contained within an Alquist-Priolo Earthquake
Fault Zone boundary. Published geologic maps show the Hillside Fault as crossing the project site,
however this fault shows no evidence of activity for at least the past 2 million years. Therefore rupture of
no impact
a known earthquake fault has on the project and no mitigation measures are required.
Impact IV.F-2: The proposed project would expose people or structures to potential substantial adverse
effects, including the risk of loss, injury or death involving exposure to strong seismic ground shaking.
The proposed project is located in the seismically active San Francisco Bay Area and there is a high
probability that the proposed development would be subjected to strong to violent ground shaking from
an earthquake during its design life. Strong seismic ground shaking is considered a potentially significant
impact. The project applicant shall require that construction of buildings on the project site adhere to the
requirements of building code provisions and current foundation-engineering principles designed to
minimize earthquake-induced impacts to safety and the structural integrity of buildings. Implementation
of these requirements as described in Mitigation Measures IV.F-2.1 through 2.3 would ensure proper
less than significant
foundation and structural design, thereby decreasing this impact to a level of.
Mitigation Measure IV.F-2.1 California Building Code Requirements
The project applicant shall ensure that the project development during all phases of the Precise and
Master Plan meets requirements of the California Building Code Vol. 1 and 2, 2007 Edition, including the
California Building Standards, 2007 Edition, published by the International Conference of Building
Officials, and as modified by the amendments, additions and deletions as adopted by the City of South
San Francisco, California to reduce impacts from strong seismic ground shaking. As new development
occurs over the project site from the Precise Plan and subsequent phases of the Master Plan, this
development would meet the current requirements existing at each phase of the project. Incorporation of
seismic construction standards would reduce the potential for catastrophic effects of ground shaking, such
as complete structural failure, but will not completely eliminate the hazard of seismically induced ground
shaking.
Mitigation Measure IV.F-2.2 Foundation Engineering and Construction
The project applicant shall ensure that proper foundation engineering and construction shall be performed
during all phases of the Precise and Master Plan in accordance with the recommendations of a Registered
Geotechnical Engineer or Civil Engineer experienced in geotechnical design and a Registered Structural
Engineer or Civil Engineer experienced in structural design to reduce impacts from strong seismic ground
shaking. As new development is proposed over the project site from the Precise Plan and subsequent
phases of the Master Plan, each development would require geotechnical evaluation and the preparation
Gateway Business Park Master Plan IV.F. Geology/Soils
Draft Environmental Impact Report Page IV.F-14
City of South San Francisco October 2009
of specific recommendations for each phase of the project based on the site specific location and proposed
building design.
The structural engineering design shall incorporate seismic parameters as outlined in the 2007 California
Building Code. The project Geotechnical Investigation shall establish the seismic design parameters, as
determined by the geotechnical engineer in accordance with requirements of the 2007 California Building
Code.
Mitigation Measure IV.F-2.3 Seismic Design Criteria
The project applicant shall obtain building permits during all phases of the Precise and Master Plan
through the City of South San Francisco Building Division. Final Design Review of planned buildings
and structures shall be completed by a licensed structural engineer for adherence to the seismic design
criteria for planned commercial and industrial sites in the East of 101 Area of the City of South San
Francisco to reduce impacts from strong seismic ground shaking. Buildings shall be designed in
accordance with the East of 101 Area Plan Geotechnical Safety Element polices, which state that
buildings shall be designed to resist earthquakes so that they not be subject to catastrophic collapse under
foreseeable seismic events, and will allow egress of occupants in the event of damage following a strong
earthquake. As new development is proposed over the project site from the Precise Plan and subsequent
phases of the Master Plan, each development shall require Final Design Review of planned buildings and
structures completed by a licensed structural engineer for each phase of the project based.
Impact IV.F-3: The proposed project would not expose people or structures to potential substantial
adverse effects, including the risk of loss, injury or death involving potential seismic ground failure,
including liquefaction, densification, and differential settlement.
The project site is located on competent bedrock of the Franciscan Complex and according to ABAG
hazard maps the site is not within a liquefaction hazard zone. The Geotechnical Investigation of the
project site by Treadwell and Rollo concluded that the dense to very dense sandy site soils are not
less than significant
susceptible to liquefaction. This would be considered a impact and no mitigation
measures are required.
Impact IV.F-4: The proposed project would be subject to seismic-related ground failure, including
liquefaction and landslides or be located on a geologic unit or soil that is unstable and subject to
landslide.
No landslides are mapped across the property. The project site has a naturally gentle slope, which has
been graded to a nearly level pad for the currently existing development. Due to this grading there is an
approximately 1.5:1 (horizontal: vertical) cut slope along the southeastern border of the project site. There
are also approximately 2:1 (h: v) slopes (likely fill) along Gateway Boulevard. More cuts may be
necessary, requiring construction of retaining walls, which could fail if improperly designed. The impact
of landslides is potentially significant. Implementation of Mitigation Measure IV.F-4.1 would ensure
proper design of retaining walls and foundations, thereby reducing the impact of landsliding to a level of
less than significant
.
Gateway Business Park Master Plan IV.F. Geology/Soils
Draft Environmental Impact Report Page IV.F-15
City of South San Francisco October 2009
Mitigation Measure IV.F-4.1 Landsliding
The project applicant shall ensure all phases of the Precise and Master Plan that proper foundation
engineering and retaining wall design shall be performed under the direction and guidance of the
geotechnical engineer of record and in accordance with the recommendations of the Geotechnical
Investigation. Geotechnical Investigations for each phase of the Precise and Master Plan shall be
reviewed and approved by the City’s Geotechnical Consultant and by the City Engineer for compliance
with the recommendations of the Geotechnical Investigation. As new development is proposed over the
project site from the Precise Plan and subsequent phases of the Master Plan, each development shall
require proper foundation engineering and retaining wall design in accordance with the recommendations
of the Geotechnical Investigation and reviewed and approved by the City’s Geotechnical Consultant and
by the City Engineer for each phase of the project based.
Impact IV.F-5: The proposed project would result in soil erosion.
All phases of the project would involve mass grading in a sensitive area near the San Francisco Bay.
During construction, grading would disturb soil and displace any topsoil that could potentially impact
vicinity drainages, and would eventually impact Colma Creek and the Bay. This would be a potentially
significant impact during and following site construction activities. The project applicant will ensure that
dust, erosion, and pollution control measures including soil stabilization techniques and other best
management practices will be followed during construction activities to reduce the potential for loose
soils impacting nearby drainages. Implementation of these practices as described in Mitigation Measures
IV.F-5.1 and 5.2 would ensure that soils disturbed during construction would not be mobilized by either
less than
storm- or construction-related runoff and therefore reduce the impact of soil erosion to a level of
significant
.
Mitigation Measure IV.F-5.1 Soil Erosion
The project applicant shall complete an Erosion Control Plan to be submitted to the City in conjunction
with the Grading Permit Application for the Precise Plan and subsequent phases of the Master Plan. The
Plan shall include winterization, dust, erosion and pollution control measures conforming to the ABAG
Manual of Standards for Erosion and Sediment Control Measures, with sediment basin design
calculations. The Erosion Control Plan shall describe the "best management practices" (BMPs) to be used
during and after construction to control pollution resulting from both storm and construction water runoff.
The Plan shall include locations of vehicle and equipment staging, portable restrooms, mobilization areas,
and planned access routes.
Recommended soil stabilization techniques include placement of straw wattles, silt fences, berms, and
gravel construction entrance areas or other control to prevent tracking sediment onto city streets and into
storm drains.
Public works staff or representatives shall visit the site during grading and construction to ensure
compliance with the grading ordinance and plans, and note any violations, which shall be corrected
immediately.
Gateway Business Park Master Plan IV.F. Geology/Soils
Draft Environmental Impact Report Page IV.F-16
City of South San Francisco October 2009
Mitigation Measure IV.F-5.2 Soil Erosion
In accordance with the Clean Water Act and the State Water Resources Control Board (SWRCB), the
project applicant shall file a Storm Water Pollution Prevention Plan (SWPPP) prior to the start of
construction of the Precise Plan and all subsequent phases of the Master Plan. The SWPPP shall include
specific best management practices to reduce soil erosion. This is required to obtain coverage under the
General Permit for Discharges of Storm Water Associated with Construction Activity (Construction
General Permit, 99-08-DWQ).
Impact IV.F-6: The proposed project would be located on expansive soils.
The geotechnical investigation performed by Treadwell and Rollo did not identify expansive material in
the sand and sand with clay native site soils. However, some of the near surface fill materials consist of
sandy clay that may have expansive properties. This impact would be mitigated through adherence to
foundation, pavement and slabs on grade design recommendations put forth in the Geotechnical Reports
prepared for each phase of the project. Recommendations include: over excavation of materials two feet
below foundations and replacement with engineered fill compacted to 95 percent relative to maximum dry
density under building footprints; floor slabs underneath garages 1 and 2 shall be underlain by 6 inches of
Class II aggregate base compacted to 95 percent relative to maximum dry density; and the upper 6 inches
of soil under pavement areas shall be compacted to 95 percent relative to maximum dry density.
Incorporation of the measures as specified in the Geotechnical Report would reduce the impact of
less than significant
expansive soils to a level of .
Impact IV.F-7: The proposed project is not located in an area where soils are incapable of adequately
supporting the use of septic tanks or alternate waste water disposal systems where sewers are not
available for the disposal of waste water.
The project would be connected to the existing sanitary sewer system and alternate waste water disposal
no impact
systems will be unnecessary. Therefore, there is and no mitigation measures are required.
CUMULATIVE IMPACTS
Geotechnical impacts related to future development in the East of 101 Area of the City of South San
Francisco would involve hazards associated with site-specific soil conditions, erosion, and ground-
shaking during earthquakes. The impacts on each site would be specific to that site and its users and
would not be common or contribute to (or be shared with, in an additive sense) the impacts on other sites.
In addition, development on each site would be subject to uniform site development and construction
standards that are designed to protect public safety. Therefore, cumulative geology and soils impacts
less than significant
would be .
LEVEL OF SIGNIFICANCE AFTER MITIGATION
Implementation of Mitigation Measures IV.F-2.1 through IV.F-2.3, IV.F-4.1, IV.F-5.1, and IV.F-5.2
identified in this section would adequately mitigate all potential impacts related to geology and soils.
less than significant
These impacts would also be reduced to a level.
Gateway Business Park Master Plan IV.F. Geology/Soils
Draft Environmental Impact Report Page IV.F-17
City of South San Francisco October 2009
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