5.3 GEOLOGY AND SOILS - Santa Clarita, California
Transcript of 5.3 GEOLOGY AND SOILS - Santa Clarita, California
Impact Sciences, Inc. 5.3-1 Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR0122.027 March 2010
5.3 GEOLOGY AND SOILS
SUMMARY
The impact analysis presented in this section evaluates project impacts related to geologic hazards. The geotechnical
analyses prepared for the project concluded that the project site is suitable for the proposed uses from a geotechnical
perspective. Mitigation measures are recommended which would reduce project impacts associated with expansive
and corrosive soils and dust and soil erosion to a less than significant level.
INTRODUCTION
This section summarizes the geotechnical study that was prepared for the proposed project. Refer to
Appendix 5.3 for the following geotechnical report:
� Allan E. Seward Engineering Geology, Inc., Geologic and Geotechnical Report, Environmental ImpactReport (EIR)-Level Review of Site Conditions for the At-Grade Railroad Crossing for Lyons Avenue Extension.2010.
This report summarize findings regarding existing geology, existing surficial deposits, potentially
significant geologic and surficial impacts, and recommended mitigation measures for these impacts. The
focus of the geotechnical report was for the Lyons Avenue at-grade rail crossing component. This
component would involve new construction and grading of this area of the proposed project sites. The
existing 13th Street at-grade rail crossing has already been constructed and developed. With
implementation of the proposed project this component would have changes that would prohibit
crossing, however, these changes would not require new earthwork which would require geotechnical
analysis.
EXISTING CONDITIONS
Regional Geology
The City of Santa Clarita is located in the Transverse Range Geomorphic Province of California, which is
characterized by east-west trending mountains and faults. Sedimentary basins within the Transverse
Range Geomorphic Province include the Ventura, Soledad, and Ridge Basins, and the San Fernando
Valley. The Ventura, Soledad, and Ridge Basins are the result of the interplay of the San Andreas Fault
and the Transverse Range fault systems. Seismic activity along the San Andreas Fault is in response to
differential movement between the Pacific geologic plate (west of the fault) and the North American
geologic plate (east of the fault). Transverse Range faults generally reflect crustal (reverse) faults.
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The project site is located within the central part of the Transverse Ranges geomorphic province of
Southern California, in the eastern portion of the Ventura Basin. The Ventura Basin has been tectonically
down-warped in the geologic past to produce a large-scale synclinal structure in which a thick sequence
of Cenozoic sediments has accumulated. At shallow depths, the subject site is underlain by
sub-horizontal alluvium deposited in the flood plain southwest of Newhall Creek. These deposits are
underlain by the Plio-Pleistocene, nonmarine Saugus Formation at depth. No faults or folds have been
identified at the site on any published geologic map of the area.1 A geologic overview map can be seen in
Figure 5.3-1, Geologic Overview Map .
Bedding within the alluvial deposits is nearly horizontal. The geologic structure of the underlying Saugus
Formation bedrock strikes roughly east to west and dips gently to the north.
Geomorphology
The site topography is dominated by paved surfaces and two sets of existing railroad tracks
approximately 75 feet west of Newhall Creek. Ground surface elevations range from about 1,258 feet at
the northernmost portion of the site to about 1,267 feet at a high point along the existing set of railroad
tracks at the easternmost portion of the site.
Geologic Units
The project site is underlain entirely by Quaternary alluvium at shallow depths. Artificial fill and railroad
ballast have been placed below the railroad tracks. Pavement and aggregate base have been placed
beneath existing roadways. Details of these units are provided below and can be seen in Figure 5.3-2,
Project Site Geology.
Quaternary Alluvium
Quaternary alluvium (Qal) underlies the subject road improvement areas and surficial fills. Based on data
obtained by Allan E. Seward Engineering Geology, Inc., (AESEGI) for the proposed adjacent Old Town
Newhall Library, this alluvium generally consists of interbedded layers of loose to dense, poorly graded
sand, silty sand, and gravely sand. Interbedded layers of sandy silts and clays are also present. These
materials are generally medium dense to dense and uncemented in the upper 30 to 40 feet and locally
loose in the upper 10 feet.
1 Allan E. Seward Engineering Geology, Inc.,Geologic and Geotechnical Report, (2010) 4.
Project SiteProject Site
Geologic Overview Map
FIGURE 5.3-1
112-027•01/10
SOURCE: Allan E. Seward Engineering Geology, Inc., Geological and Geotechnical Report – January 2006
APPROXIMATE SCALE IN FEET
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MAIN STREETMAIN STREET
Project Site GeologyFIGURE 5.3-2
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SOURCE: Allan E. Seward Engineering Geology, Inc., Geological and Geotechnical Report – January 2010
APPROXIMATE SCALE IN FEET
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Project Site Boundary
Legend:
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Artificial Fill
Artificial fill (af) was apparently placed below the existing railroad tracks to elevate the tracks above the
Newhall Creek flood plain. The engineering characteristics of this material are currently unknown.
Railroad Ballast
Railroad ballast (RB) consisting of crushed natural rock was placed to support and elevate the two sets of
railroad tracks at the site.
Groundwater
Review of historic groundwater data from the Seismic Hazard Map for the Newhall Quadrangle, Robson
(1972) and Los Angeles Flood Control District (LACFCD) water well records indicates that historic-high
groundwater at the proposed project site is between 75 and 100 ft below ground surface (bgs). The
locations of nearby water wells, as seen on Figure 5.3-3, Water Well Location Map, show that the nearest
water well is located approximately 750 feet to the north of the project site. The AESEGI review of historic
water well records also determined that historic ground water levels ranged from 33 feet bgs to 103 feet
bgs, seeAppendix 5.3.
Temporary, perched ground water conditions may exist below Newhall Creek following periods of
significant rainfall and runoff. During subsurface explorations for the adjacent Old Town Newhall
Library site, exploratory borings were drilled to a maximum depth of 50 feet. These explorations did not
encounter groundwater.
Seismicity
The project site is located in the seismically active Southern California region. Earthquake-related hazards
typically include ground rupture, ground shaking, and ground failure. Faults identified as active or
potentially active in published geologic literature are not known to be present within or adjacent to the
subject site. However, the project site is situated in the seismically active Transverse Ranges and can be
expected to experience strong ground shaking from earthquakes generated on active regional faults, as
evidenced by the strong ground shaking generated by the January 17, 1994, Northridge earthquake
(magnitude 6.7).
Ground Rupture and Shaking
Review of the Alquist-Priolo Earthquake Fault Zone Map for the Newhall Quadrangle, the Seismic Safety
Element of the Los Angeles County General Plan, and published geologic maps indicates that no active or
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potentially active faults traverse the subject site.2 Review of the site topography and aerial photographs
listed did not reveal any lineaments or other indicators suggestive of faulting at the site. The nearest
known active fault is the San Gabriel Fault, which is 3.7 kilometers (km), or 2.22 miles,3 from the site at its
nearest point, as seen in Figure 5.3-4, Fault and Earthquake Epicenter Location Map and in Table 5.3-1,
Summary of Nearby Faults described below.
Table 5.3-1, summarizes potential earthquake sources near the site, including estimates of maximum
seismic magnitude that are considered geologically feasible for these sources, per the State of California.4
Table 5.3-1Summary of Nearby Faults
Closest Distance to Project Site (km)
Fault Name Surface TraceSurface Projectionof Rupture Area
MaximumMagnitude Slip Rate (mm/yr)
San Gabriel 3.7 3.7 7.0 1.0
Holser 4.3 3.1 6.5 0.4
Northridge (E. OakRidge)
5.1 5.1 6.9 1.5
Santa Susana 7.1 0.0 6.6 5.0
Sierra Madre (SanFernando)
9.9 4.7 6.7 2.0
Verdugo 16.7 15.8 6.7 0.5
Oak Ridge (on shore) 17.8 17.8 6.9 4.0
San Cayetano 22.2 22.2 6.8 6.0
Sierra Madre 24.5 21.4 7.0 3.0
Simi-Santa Rosa 26.7 26.7 6.7 1.0
San Andreas 33.1 33.1 7.8 34.0
mm/yr = millimeters per year; km = kilometersSource:Allan E. Seward Engineering Geology, Inc., Geologic and Geotechnical Report, 2010. See Appendix 5.3.
2 Allan E. Seward Engineering Geology, Inc.,Geologic and Geotechnical Report, (2010) 6.3 1.0 kilometer is equal to 0.6 mile.4 Determined through use of the US Geological Survey program, “Seismic Hazard Curves and Uniform Hazard
Response Spectra.”
Project Site
Water Well Location Map
FIGURE 5.3-3
112-027•01/10
SOURCE: Allan E. Seward Engineering Geology, Inc., Geological and Geotechnical Report – January 2006
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Fault and Earthquake Epicenter Location MapFIGURE 5.3-4
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SOURCE: Allan E. Seward Engineering Geology, Inc., Geological and Geotechnical Report – January 2010
APPROXIMATE SCALE IN MILES
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Project Site
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The California Department of Mines and Geology defines active faults as those that have had surface
displacement within Holocene time (about the last 11,000 years). Potentially active faults are ones that
have had surface displacement during the last 1.6 million years; inactive faults have not had surface
displacement within the last 1.6 million years.
Seismically induced ground acceleration is the shaking motion that is produced by an earthquake.
Probabilistic modeling is done to predict future ground accelerations. Review of the Seismic Hazard
Evaluation Report for the Newhall Quadrangle indicates that the peak horizontal ground acceleration
(PHGA) with a 10 percent chance of exceedance in 50 years for alluvial conditions at the project site is
about 0.8 gravity (g). The potential PHGA at the project site equal to two-thirds of the peak acceleration
with a 2 percent chance of exceedance in 50 years is 0.54 g (as defined in the 2007 California Building
Code [CBC]).
Ground Failure
”Ground failure” is a general term that refers to secondary, permanent ground deformation caused by
strong earthquake shaking, including liquefaction of saturated granular deposits or fine-grained soils
with low plasticity, lateral spreading, ground lurching, seismic settlement (dynamic densification) of
loose, poorly consolidated materials, differential materials response, slope failure, sympathetic movement
on weak bedding planes or non-causative faults, and shattered ridge effects.
“Liquefaction” is defined as the transformation of a granular material from a solid state to a liquefied
state as a consequence of increased pore water pressure. During ground shaking, the alluvial grains are
packed into a tighter configuration. Pore water is squeezed from between the grains, increasing the pore
pressure. As the pore pressures increases, the load bearing strength of the material decreases. There are
areas within the City of Santa Clarita that overlie unconsolidated alluvium with a high groundwater
table. These areas are primarily found near the Santa Clarita River and its tributaries.
The project area is underlain primarily by alluvium, which is found throughout the canyon bottoms and
river areas of the City. Alluvium is silt deposited by creeks and rivers. The major hazard associated with
alluvium is liquefaction. Alluvium tends to be unconsolidated, meaning there is space between the
“grains” of soil. If alluvium is sufficiently saturated, and an earthquake occurs, the soil can take on more
liquid characteristics, damaging structures. According to the Seismic Hazard Map for the Newhall
Quadrangle, the project site is not located in a zone in which investigation of liquefaction potential is
required. The depth to historic high ground water at the site is greater than 50 feet. The potential for
liquefaction and associated seismic settlements and lateral spreading is therefore considered very low.
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Based on review of the seismic hazard map for the Newhall Quadrangle, the subject site is not located in
a zone in which investigation of potential for earthquake-induced landslides is required. Potential at the
site for slope failures and shattered ridge effects is considered nonexistent due to the flat nature of the
site. Potential at the site for differential materials response and slippage along weak, inclined bedding
planes is considered to be negligible because the site is underlain by consistent, flat-lying alluvial
deposits.
Slope Stability
Earthquake-induced slope failures include activation and reactivation of landslides, rock falls, debris
falls, and surface failures. No slopes greater than 10 feet in height exist at the project site or are proposed
on the current site plan. A 4-foot-high slope exists on the southwestern margin of elevated alignment for
the railroad tracks, which will be eliminated at Lyons Avenue to construct the proposed project. The bank
of Newhall Creek located approximately 75 feet northeast of the railroad tracks is approximately 10 feet
high.
Soil Compressibility/Consolidation
Based on explorations for the adjacent Newhall Library site, the alluvium in this area is typically medium
dense to depths of 30 to 40 feet, with local loose zones in the upper 10 feet. A maximum of 5 feet of fill is
currently proposed above this material. The density of the artificial fill placed below the railroad tracks is
currently undefined.
Dam Inundation
No dams currently exist in the Newhall Creek Drainage and the site is not in a dam inundation area per
the Flood and Inundation Hazard Map (Plate 6) of the Los Angeles County Safety Element of the General
Plan.
REGULATORY FRAMEWORK
Federal Regulations
No specific federal regulations were identified that impact the geology and soils considerations. State and
local regulations (e.g., building codes) reflect national and international building codes; this is discussed
below.
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State Regulations
Alquist-Priolo Earthquake Fault Zoning Act
The Alquist-Priolo Special Studies Zones Act5 seeks to mitigate the hazard of fault rupture by prohibiting
the location of structures for human occupancy across the trace of an active fault. The main purpose of
the act is to prevent the construction of buildings used for human occupancy on the surface trace of active
faults. The law requires the State Geologist to establish regulatory zones (known as Earthquake Fault
Zones or Special Studies Zone) around the surface traces of active faults and to issue appropriate maps.
The act was renamed in 1994 to the Alquist-Priolo Earthquake Fault Zoning (APEFZ) Act. The nearest
known active fault is the San Gabriel Fault, which is 3.7 km from the site at its nearest point.
The APEFZ Act mandates that cities and counties (lead agencies) require that within an Earthquake Fault
Zone (EFZ) geologic investigations be performed to demonstrate that potential development sites are not
threatened by surface fault displacements from future earthquakes. To aid the various jurisdictions that
function as lead agencies for project approvals in California, the California Geological Survey (CGS) must
delineate Earthquake Fault Zones on standard U.S. Geological Survey topographic maps (1-inch-equals-
2000-feet scale) along faults that are "sufficiently active and well defined" as defined in the APEFZ Act.
Quoting from the implementation guide, Special Publication 42,6
Zone boundaries on early maps were positioned about 660 feet (200 meters) away from the faulttraces to accommodate imprecise locations of the faults and possible existence of active branches.The policy since 1977 is to position the EFZ boundary about 500 feet (150 meters) away frommajor active faults and about 200 to 300 feet (60 to 90 meters) away from well-defined, minorfaults. Exceptions to this policy exist where faults are locally complex or where faults are notvertical.
Lead agencies are responsible to regulate most development projects within the Earthquake Fault Zones
as described in the APEFZ Act, but may enact more stringent regulations. Certain smaller residential
developments can be exempt.
5 Details regarding the Alquist-Priolo Earthquake Fault Zone Act can be found at http://www.consrv.ca.gov/CGS/rghm/ap/index.htm. Accessed in January 2010.
6 Details regarding the Alquist-Priolo Earthquake Fault Zone Act can be found at http://www.consrv.ca.gov/CGS/rghm/ap/index.htm. Accessed in January 2010.
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Seismic Hazards Mapping Act
The Seismic Hazards Mapping Act7 (SHMA) addresses the primary earthquake hazard, strong
groundshaking, as well as the secondary hazards of liquefaction, earthquake-induced landslides, and, in
some areas, zones of amplified shaking. As with the APEFZ Act, the CGS is the primary state agency
charged with implementing SHMA, and CGS provides local jurisdictions with seismic hazard zone maps
that identify areas susceptible to liquefaction, earthquake-induced landslides, and amplified shaking.
Site-specific hazard investigations are required by the SHMA when a development project is located
within one of the Seismic Hazard Mapping Zones (SHMZ) defined as a zone of required investigation.
Lead agencies with the authority to approve projects shall ensure that:
The geotechnical report shall be prepared by a registered civil engineer or certified engineeringgeologist, having competence in the field of seismic hazard evaluation and mitigation. Thegeotechnical report shall contain site-specific evaluations of the seismic hazard affecting theproject, and shall identify portions of the project site containing seismic hazards. The report shallalso identify any known off-site seismic hazards that could adversely affect the site in the event ofan earthquake.
And
Prior to approving the project, the lead agency shall independently review the geotechnical reportto determine the adequacy of the hazard evaluation and proposed mitigation measures and todetermine the requirements of Section 3724(a), above, are satisfied. Such reviews shall beconducted by a certified engineering geologist or registered civil engineer, having competence inthe field of seismic hazard evaluation and mitigation.
CGS Special Publication (SP) 1178 and companion volumes for implementation of the SP 117 process (one
volume for liquefaction and one volume for earthquake-induced landslides)9 provide detailed guidance
for lead agencies to review SHMA reports. The overall goal is to protect the public by minimizing
property damage and the loss of life.
7 Details regarding the Seismic Hazards Mapping Act can be found at http://gmw.consrv.ca.gov/shmp/SHMPpgminfo.htm.
8 California Geological Survey, Special Publication (SP) 117: Guidelines for Evaluating and Mitigating Seismic Hazardsin California; http://gmw.consrv.ca.gov/shmp/webdocs/sp117.pdf.
9 Recommended Procedures for Implementation of SP 117; http://www.scec.org/resources/catalog/LiquefactionproceduresJun99.pdf and http://www.scec.org/resources/catalog/LandslideProceduresJune02.pdf.
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California Building Code
The 2007 CBC10 includes additions to the previous building code that make it more stringent, in
particular with regard to seismic and earthquake conditions for critical structures such as essential
facilities, public schools and hospitals. The CBC, which is included in Title 24 of the California
Administrative Code, is a compilation of three types of building standards derived from three different
sources:
� Those adopted by state agencies without change from building standards contained in nationalmodel codes (e.g., the International Building Code [IBC])
� Those adopted and adapted from the national model code standards to meet California conditions(e.g., most of California is Seismic Design Categories D and E)
� Those authorized by the California legislature, which constitute extensive additions not covered bythe model codes that have been adopted to address particular California concerns (e.g., thespecification of Certified Engineering Geologist rather than engineering geologist)
International and national model code standards adopted into Title 24 apply to all occupancies in
California except for modifications adopted by state agencies and local governing bodies. Facilities and
structures such as power plants, freeways, emergency management centers, and dams are regulated
under criteria developed by various California and federal agencies.
Local
City of Santa Clarita Unified Development Code
All grading and excavation must comply with Chapters 17.20 to 17.30 (Division 3) of the City of Santa
Clarita Unified Development Code (UDC). Rules and regulations contained within these chapters
provide for the control of excavation, grading, and earthwork construction, including fills or
embankment activities. During the grading permit application process, the City Engineer may require
engineering geological and soil reports, as well as seismic hazard zone studies be prepared for proposed
developments. The engineering geological report would require an adequate description of the geology
of the site, along with conclusions and recommendations regarding the effect of geologic condition of any
proposed development. Soil reports would be required to characterize the existing soil resources on a site,
and provide recommendations for grading and design criteria. Development in seismic hazard zone will
require studies that evaluate the potential for seismically induced liquefaction, soil instability, and
earthquake induced landslides to occur on a site.
10 California Building Code, Title 24, Part 2. http://www.bsc.ca.gov/apprvd_chngs/appStan.htm.
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The City of Santa Clarita enforces structural requirements of the building code, the Alquist-Priolo Special
Studies Zones, and sound engineering and geotechnical practices in evaluating structural stability of
proposed new development.
PROJECT IMPACTS
Significance Threshold Criteria
According to Appendix G of the State CEQA Guidelines, the project would normally have a significant
effect on the environment if it would
� 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 EarthquakeFault Zoning Map issued by the State Geologist for the area or based on other substantialevidence of a known fault (refer to Division of Mines and Geology Special Publication 42);
� strong seismic ground shaking;
� seismic-related ground failure, including liquefaction;
� 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 theproject, and potentially result in on- or off-site landslide, lateral spreading, subsidence, liquefaction orcollapse;
� be located on expansive soil, as defined in Table 18-1-B of the Uniform Building Code (1994), creatingsubstantial risks to life or property; or
� have soils incapable of adequately supporting the use of septic tanks or alternative wastewaterdisposal systems where sewers are not available for the disposal of wastewater.
The proposed project is a public improvement project that will not utilize wastewater facilities or septictanks. Consequently, no further analysis is required for this threshold.
The City of Santa Clarita Local CEQA Guidelines (Resolution 05-38) adopted on April 26, 2005, also serve as
the basis for identifying thresholds to determine the significance of the environmental effects of a project
on this resource area and have been included for analysis.
� There will be a significant impact if the project were to include movement or grading of earthexceeding 100,000 cubic yards.
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The proposed project would grade approximately 6,000 cubic yards of fill. As this quantity is less than100,000 cubic yards, no further analysis is required for this threshold.
Impact Analysis
The following geotechnical aspects of the proposed project were evaluated in this impact analysis.
� The grades at the northeast end of Lyons Avenue and adjacent portions of Railroad Avenue will needto be raised in order to tie into the existing grade at the railroad crossing. Retaining walls may beused to accommodate the changes from the proposed grades to adjacent properties proposed toremain at existing grades.
Threshold 5.3-1 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 recentAlquist-Priolo Earthquake Fault Zoning Map issued by the State Geologistfor the area or based on other substantial evidence of a known fault (referto Division of Mines and Geology Special Publication 42); strong seismicground shaking; seismic-related ground failure, including liquefaction; orlandslides
Rupture is primarily of concern where a project site overlies or is immediately adjacent to a known fault.
No known faults are located within the project area with the nearest known fault 3.7 km from the project
site. Therefore, the impacts related to the rupture of a known earthquake fault would be less than
significant.
The proposed project is located in Southern California, a geologically and tectonically active region,
where large magnitude, potentially destructive earthquakes are common. Therefore, ground motions
from moderate or large magnitude earthquakes could affect the project site during the design life of the
project.
The nearest fault is the San Gabriel Fault located 3.7 km northeast of the project area. Other faults in the
vicinity include the Holser and Santa Susana. The most likely significant event in the area could occur
along the San Andreas Fault, located 16 miles northeast of the City.
As discussed above in Seismicity, the PHGA with a 10 percent chance of exceedance in 50 years for
alluvial conditions at the project site is about 0.8 g. The potential PHGA at the project site equal to
two-thirds of the peak acceleration with a 2 percent chance of exceedance in 50 years is 0.54 g (as defined
in the 2007 CBC).
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The current standards for construction provided in the CBC are designed to safeguard against major
failures and loss of life, but are not intended to prevent damage, maintain function or provide for easy
repair. Conformance to code standards does not constitute any kind of guarantee or assurance that
significant structural damage will not occur in the event of a maximum level of earthquake ground
motion. However, it is reasonable to expect that a well-planned and constructed structure will not
collapse in a major earthquake and that protection of life will be reasonably provided, but not with
complete assurance. Therefore, potential impacts would be less than significant as the project is an
infrastructure project and contains no habitable structures and would be constructed to the most current
CBC standards.
Ground failure is a general term for seismically induced, secondary, permanent ground deformation
caused by strong ground motion. This includes liquefaction, lateral spreading, ground lurching, seismic
settlement of poorly consolidated materials (dynamic densification), differential materials response,
sympathetic movement on weak bedding planes or non-causative faults, slope failures, and shattered
ridge effects.
The project area is underlain primarily by alluvium, which is found throughout the canyon bottoms and
river areas of the City. Alluvium is silt deposited by creeks and rivers. The major hazard associated with
alluvium is liquefaction. Alluvium tends to be unconsolidated, meaning there is space between the
“grains” of soil. If alluvium is sufficiently saturated, and an earthquake occurs, the soil can take on more
liquid characteristics, damaging structures. However, as identified on the City of Santa Clarita, North
Newhall Seismic Hazard Zone Map (2005) and the USGS, State of California Seismic Hazards Zones,
Newhall Quadrangle, the project site is not designated to contain areas for potential liquefaction. As there
was no encounter of groundwater at 50 feet bgs, potential impacts from seismically induced liquefaction
would be less than significant.
Based on review of the seismic hazard map for the Newhall Quadrangle, the subject site is not located in
a zone in which investigation of potential for earthquake-induced landslides is required. Potential at the
site for slope failures and shattered ridge effects is considered nonexistent due to the flat nature of the
site. Potential at the site for differential materials response and slippage along weak, inclined bedding
planes is considered to be negligible because the site is underlain by consistent, flat-lying alluvial
deposits. Consequently, seismically induced landslides on the project site would have a less than
significant impact.
As seen in Figure 5.3-3, no water wells or oil wells have been drilled at the project site. Therefore, the
potential for the proposed project to compromise the integrity of wells is non-existent. As a result,
potential impacts would be less than significant.
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No dams currently exist in the Newhall Creek Drainage and the site is not in a dam inundation area per
the Flood and Inundation Hazard Map (Plate 6) of the Los Angeles County Safety Element of the General
Plan. The potential for dam inundation is therefore considered nonexistent and impacts would be less
than significant.
Mitigation Measures
None required.
Residual Impacts
Impacts would be less than significant.
Threshold 5.3-2 Result in substantial soil erosion or the loss of topsoil
During construction of the proposed project, the soils on-site may become exposed, and thus subject to
erosion. However, the project is required to comply with existing regulations that reduce erosion
potential. The proposed project will comply with South Coast Air Quality Management District
(SCAQMD) Rule 403, which would reduce the potential for wind erosion. Similarly, water erosion during
construction would be substantially reduced by complying with the National Pollution Discharge
Elimination System (NPDES). As further detailed in the Hydrology and Water Quality section, NPDES
requires the construction of the project to incorporate Best Management Practices (BMPs) to reduce
erosion and prevent eroded soils from washing off site.
According to the Geologic/Geotechnical Report prepared by Allan E. Seward Engineering Geology, Inc.,
the project site consists of quaternary alluvium, artificial fill, and railroad ballast. Potential operational
impacts due to erosion would be less than significant as the proposed improvements would cover the
area with asphalt or concrete.
Mitigation Measures
No mitigation measures are required.
Residual Impacts
Impacts would be less than significant.
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Threshold 5.3-3 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
Based on explorations for the adjacent Newhall Library site, the alluvium in this area is typically medium
dense to depths of 30 to 40 feet, with local loose zones in the upper 10 feet. A maximum of 5 feet of fill is
currently proposed above this material. The density of the artificial fill placed below the railroad tracks is
currently undefined. As a result, potential impacts may be significant.
Mitigation Measures
The following mitigation measures shall be implemented.
MM5.3-1 Prior to issuance of a grading permit, additional hydro-compression or consolidation
testing shall be conducted to aid in evaluation of settlement within identified geologic
units during future geotechnical investigations for Grading Plans. Possible mitigation of
settlement of project soils would include removal and recompaction of loose or soft
material or by increasing the thickness of the pavement section.
MM 5.3-2 During construction activities, excavated topsoil shall be salvaged, stockpiled, and
subsequently placed over fill areas to assist in revegetation and to minimize erosion and
loss of topsoil. The use of any excavated soils must be deemed appropriate by the
contracted Geotechnical Consultant for use as backfill material.
Residual Impacts
Subject to implementation of mitigation impacts would be less than significant.
Threshold 5.3-4 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
The site alluvial materials are generally granular and are not typically expansive in nature. The expansion
index of the site materials should be verified with laboratory testing at the Grading Plan stage. If
expansive materials are encountered, options to mitigate potential adverse affects include modification of
the base and pavement section. Non-expansive soil can be placed behind retaining walls to mitigate
potential distress.
Potential removal and recompaction of shallow, loose soils may be required at the site. The native soils
are expected to shrink in volume when placed as compacted fill.
5.3 Geology and Soils
Impact Sciences, Inc. 5.3-19 Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR0122.027 March 2010
AESEGI’s past experience with similar soils on nearby sites suggests that the on-site soils likely have a
low concentration of sulfate and chloride, and low acidity. This indicates a low potential for corrosion of
concrete and, therefore, it would be anticipated that Type I or II Portland Cement will be satisfactory for
use at the site. The resistivity of similar soils near the site tested by AESEGI indicates that they are
typically moderately corrosive to ferrous metals. Therefore, there may be potential corrosive impacts to
the proposed project.
Mitigation Measures
The following mitigation measures shall be implemented.
MM5.3-3 Expansive materials at the site shall be evaluated by the Project Geotechnical Engineer
during the grading plan stage of development. Expansion potential of site soils can be
mitigated by controlling the water content and density of fill soils, by specifying
embedment and reinforcement of structures, and by removing the expansive materials
and replacing them with compacted material with low expansion potential. Other
potential mitigation would include modification of the base and pavement section.
MM5.3-4 Soils shall be evaluated by the Project Geotechnical Engineer prior to issuance of grading
permits for corrosive characteristics of the site soils. If corrosive soils are encountered,
options to mitigate potential corrosive soils include protective wraps and coatings for
buried metal pipes and special types of cement that are resistant to corrosion.
MM 5.3-5 Specifications for retaining walls shall be provided on grading plans.
MM 5.3-6 Any oversized material that may be encountered during construction shall not be
incorporated into potential compacted fill for the at-grade crossing. Specifications and
guidelines for handling and disposal should be addressed by the project Geotechnical
Engineer prior to the issuance of grading permits.
MM 5.3-7 The percent shrinkage of this material shall be verified prior to the issuance of grading
permits so that the potential volume of import fills can be accurately anticipated. The
shrinkage characteristics of the existing fills shall also be evaluated if removals are
required.
Residual Impacts
Subject to implementation of mitigation measures, impacts would be less than significant.
5.3 Geology and Soils
Impact Sciences, Inc. 5.3-20 Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR0122.027 March 2010
MITIGATIONMEASURESALREADY INCORPORATED INTO THE PROJECT
Recommendations provided by the geotechnical engineer identified in Geologic and Geotechnical Report;
EIR-Level Review of Site Conditions, prepared by Allan E. Seward Engineering Geology, Inc., and dated
January 7, 2010, shall be incorporated as standard conditions of approval for the proposed project.
CUMULATIVE IMPACTS
Geotechnical impacts are site specific in nature and each development site is subject to, at minimum,
uniform site development and construction standards relative to seismic and other geologic conditions
that are prevalent within the locality and/or region. Because the development of each site would have to
be consistent with City of Santa Clarita requirements for projects in the City and the Unified
Development Code as they pertain to protection against known geologic hazards, impacts of cumulative
development would be less than significant given known geologic considerations.
CUMULATIVE MITIGATIONMEASURES
No significant cumulative geotechnical impacts would occur; therefore, no cumulative mitigation
measures are recommended.
UNAVOIDABLE SIGNIFICANT IMPACTS
With implementation of the above-identified mitigation measures, project-specific impacts associated
with geology and soils would be reduced to less than significant. Therefore, no unavoidable significant
project-specific impacts are anticipated.