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.

5.3 Geology and Soils


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

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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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80 40 0 80

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Project Site Boundary

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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



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

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2000 1000 0 2000

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Fault and Earthquake Epicenter Location MapFIGURE 5.3-4

112-027•01/10


APPROXIMATE SCALE IN MILES

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Project Site



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.



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.



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.



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.



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.



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.



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).



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.



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.



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.



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.



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.

Documents

5.3 GEOLOGY AND SOILS - Santa Clarita, California