2.6_Chapter 2.6 - Geology/SoilsSection 2.6
Geology/Soils
Section 2.6 – Geology/Soils
Espola Road Improvement Project Final EIR 2.6-1
May 2013
2.6 GEOLOGY/SOILS
This section describes existing geologic conditions in the Project study area and vicinity,
identifies associated regulatory requirements, and evaluates potential impacts and mitigation
measures related to implementation of the proposed Project. TerraCosta Consulting Group, Inc.
(TCG) prepared a Geotechnical Design Report for the proposed Project (TCG 2004). This
analysis is summarized below along with other applicable data, with the complete report
included as Appendix E of this EIR.
2.6.1 Affected Environment
Geologic Setting
Regional Geology/Topography
The Project study area is within the Peninsular Ranges Geomorphic Province, a region
characterized by northwest-southeast trending structural blocks and intervening fault zones.
Typical rock types within the Peninsular Ranges include a variety of granitic rocks associated
with the Cretaceous (between approximately 65 and 135 million years old) Southern California
Batholith (i.e., a large intrusive body). In western San Diego County, granitic rocks are often
intruded into older metamorphic units, and are overlain by a sequence of younger marine and
non-marine sedimentary strata. These sedimentary rocks are associated primarily with a number
of sea level transgression-regression (i.e., advance and retreat) cycles occurring over
approximately the last 55 million years.
Topographically, the Peninsular Ranges Province is composed of generally parallel ranges of
steep-sloping hills and mountains separated by alluvial valleys. More recent uplift and erosion
of the western San Diego County geologic column described above has resulted in the
characteristic canyon and mesa topography present today, as well as the deposition of surficial
materials including Quaternary (approximately two million years or less in age) alluvium and
topsoil. The City of Poway and the Project study area encompass portions of two alluvial valleys
and adjacent rocky foothills, as described below.
Site Geology/Topography
Approximately the southern half of the Project study area is located within an alluvial valley
associated with Rattlesnake and Poway creeks. The central portion of the study area
encompasses rocky foothills exposing mostly granitic rocks, while the northern end of the
alignment is within the southern portion of Green Valley. Surface exposures in the study area
and vicinity include recent construction fill and topsoil deposits, Quaternary alluvium and
slopewash, undifferentiated Cretaceous granitic rocks, and the Jurassic (between approximately
135 and 190 million years old) Santiago Peak Volcanics (California Geological Survey [CGS],
formerly the California Division of Mines and Geology [CDMG] 1975). These materials are
described below in order of increasing age, with alluvial and geologic units shown on
Figure 2.6-1, General Geology.
Section 2.6 – Geology/Soils
Espola Road Improvement Project Final EIR 2.6-2
May 2013
Topography within the Project study area is characterized by relatively level alluvial valleys in
the northernmost and southern portions of the alignment, and moderately to steeply sloping
rocky foothills in the central area (Figure 2.6-1). Study area elevations range from a low of
approximately 580 feet above mean sea level (AMSL) near Twin Peaks Road at the southern end
of the alignment, to 820 feet AMSL at Titan Way to the north.
Stratigraphy
Construction Fill (not mapped on Figure 2.6-1)
Construction fill consisting of silty sand to sandy clay materials is present in portions of the
study area, with these deposits related to existing development, including roadways. While no
known information is available regarding the conformance of local fills with engineering and
industry standards (e.g., criteria such as fill moisture content and compaction), field studies
conducted by TCG indicate that the fill deposits are probably derived from local sources and are
suitable for use as engineered fill provided they are properly reworked and moisture conditioned
during placement (Appendix E).
Topsoil (not mapped on Figure 2.6-1)
Topsoil mapping within the study area and vicinity has been conducted by the U.S. Natural
Resources Conservation Service (NRCS, formerly the Soil Conservation Service [SCS] 1973).
Four distinct soil series represented by nine different soil types are mapped within the study area,
with a summary description of soil characteristics provided in Table 2.6-1, Description of
On-site Soil Properties. Much of the Project study area has been previously graded and/or paved
through activities such as road construction. It is likely that native soils in these areas have been
largely removed or altered (e.g., by mixing with fill), although intact topsoils are assumed to be
present in relatively undisturbed sites (e.g., the east side of the study area just north of High
Valley Road [approximate Station 51+00]).
Table 2.6-1
DESCRIPTION OF ON-SITE SOIL PROPERTIES
Soil Series Physical Characteristics/
Mapped Location
Expansion
(shrink-
swell)
Potential
Reactivity Erosion
Potential
Fallbrook
Sandy Loam,
5 to 9 percent
slopes
Well-drained, sandy loam derived
from granitic rock. These soils
occur in generally level portions of
the southern and central study area.
Moderate
Moderately
acidic to neutral
(pH 6.0-7.3)
over profile
Low to
Moderate
Fallbrook
Sandy Loam,
9 to 15 percent
slopes, eroded
Well-drained, sandy loam derived
from granitic rock and occurring on
steeper slopes in much of the
northern and central study area.
Moderate
Moderately
acidic to neutral
(pH 6.0-7.3)
over profile
Moderate
Section 2.6 – Geology/Soils
Espola Road Improvement Project Final EIR 2.6-3
May 2013
Table 2.6-1 (cont.)
DESCRIPTION OF ON-SITE SOIL PROPERTIES
Soil Series Physical Characteristics/
Mapped Location
Expansion
(shrink-
swell)
Potential
Reactivity Erosion
Potential
Fallbrook
Rocky Sandy
Loam, 9 to
30 percent
slopes
Well-drained, rocky sandy loam
derived from granitic rock and
including surface boulders (10 to
25 percent) and outcrops
(10 percent). These soils occur on
steep slopes in the northeastern
study area.
Moderate
Moderately
acidic to neutral
(pH 6.0-7.3)
over profile
Moderate
to High
Placentia
Sandy Loam,
2 to 9 percent
slopes
Moderately well-drained sandy
loam with a sandy clay subsoil.
These soils are derived from
alluvium and occur in gently to
moderately sloping portions of the
northernmost study area.
High
Moderately
acidic to
moderately
alkaline (pH 5.6
to 8.4) over
profile
Low to
Moderate
Placentia
Sandy Loam,
Thick Surface,
0 to 2 percent
slopes
Moderately well-drained sandy
loam with a sandy clay subsoil.
These soils are derived from
alluvium and occur in level portions
of the southernmost study area.
High
Moderately
acidic to
moderately
alkaline (pH 5.6
to 8.4) over
profile
Low
Placentia
Sandy Loam,
Thick Surface,
2 to 9 percent
slopes
Moderately well-drained sandy
loam with a sandy clay subsoil.
These soils are derived from
alluvium and occur in gently to
moderately sloping portions of the
southern study area.
High
Moderately
acidic to
moderately
alkaline (pH 5.6
to 8.4) over
profile
Low to
Moderate
Ramona Sandy
Loam, 0 to 2
percent slopes
Well-drained sandy loam with a
sandy clay subsoil. These soils are
derived from alluvium and occur in
level portions of the southern and
central study area.
Moderate
Moderately
acidic to neutral
(pH 5.6-7.3)
over profile
Low
Visalia Sandy
Loam, 0 to 2
percent slopes
Moderately well-drained sandy
loam derived from alluvium. These
soils occur in level portions of the
southernmost study area.
Low
Slightly acidic
(pH 6.1 to 6.5)
over profile
Low
Visalia Sandy
Loam, 9 to 15
percent slopes
Moderately well-drained sandy
loam derived from alluvium. These
soils occur in moderately sloping
portions of the northwestern study
area.
Low
Slightly acidic
(pH 6.1 to 6.5)
over profile
Moderate
Source: SCS 1973
Section 2.6 – Geology/Soils
Espola Road Improvement Project Final EIR 2.6-4
May 2013
Quaternary Alluvium and Slopewash (Qal/Qsw)
Alluvial deposits are present in the valley/drainage areas located at the northern and southern
ends of the study area. These alluvial materials consist primarily of unconsolidated silt, sand,
gravel and cobble-size grains derived from local bedrock, with estimated thicknesses of between
approximately 5 and 15 feet (Appendix E). Slopewash (or colluvium) occurs along the base of
most local slopes, and is often inter-layered with alluvial deposits. Colluvial deposits consist of
loose, sandy clay with some gravel, and typically exhibit a more angular texture than alluvium.
Local colluvium is estimated to range up to 10 feet in thickness (Appendix E).
Undifferentiated Cretaceous Granitic Rock (Kg)
Undifferentiated granitic rock underlies the entire study area and vicinity, and is generally
exposed at the surface within the study area between approximately Station 31+50 (just south of
Durhullen Drive) and the northern terminus near Titan Way. Bedrock encountered within
approximately three to six feet of the surface along the entire alignment during Project
geotechnical investigation was generally weathered, although variability in the degree and depth
of weathering was observed. Specifically, south of Station 54+00 (approximately 300 feet north
of High Valley Road), somewhat less weathered bedrock was noted below the described three- to
six-foot surface zone, while generally unweathered bedrock was observed below the surface
zone north of Station 54+00 (Appendix E).
In addition, seismic wave patterns observed during geotechnical investigation indicate that
corestones (i.e., rounded boulders formed in place) are present near the boundary of weathered
and non-weathered bedrock (Appendix E).
Jurassic Santiago Peak Volcanics (Jsp)
The Santiago Peak Volcanics consist mainly of fractured metavolcanic rocks, although
metasedimentary and other (e.g., pyroclastic) members are also typically present. Exposures of
the Santiago Peak Volcanics are present approximately 450 feet east of the study area and may
underlie the Project alignment at depth. This unit was not observed during Project geotechnical
investigation, however, and is not expected to be encountered during Project development.
Groundwater
Shallow groundwater was encountered at a depth of approximately 13 feet below the surface in
one exploratory boring conducted within the study area during Project geotechnical
investigation. This boring site (B-12) is located near Rattlesnake Creek, approximately 500 feet
south of Twin Peaks Road. Groundwater was not observed in any of the other 11 borings
excavated within the study area, with these borings extending to depths of between five and
21 feet. The geotechnical report notes, however, that perched groundwater conditions may
develop within the study area during the rainy season. Perched groundwater generally consists
of one or more small, unconfined aquifers supported by shallow impermeable or semi-permeable
strata, with perched aquifers typically variable in volume and extent with seasonal precipitation
and/or irrigation levels.
Section 2.6 – Geology/Soils
Espola Road Improvement Project Final EIR 2.6-5
May 2013
Structure/Seismicity
As previously noted, the study area lies within the Peninsular Ranges Geomorphic Province,
with major physiographic features consisting of structural blocks separated by faults. Subsurface
materials within the study area are predominantly massive
The study area is located in a broad, seismically active region of southern California, and is subject
to significant hazards from moderate-to-large earthquakes. The intersection between the North
American and Pacific tectonic plates in southern California is spread across a wide area that is
bounded on the east by the San Andreas Fault in Imperial Valley, and on the west by a complex
fault zone located approximately 60 miles offshore. The intervening area includes a series of
generally related fault structures, including the Elsinore, San Jacinto, and Rose Canyon fault zones
(Figure 2.6-2, Regional Fault Map). No known faults are located within or adjacent to the study
area, with the closest active fault structures associated with the Rose Canyon Fault Zone,
approximately 16 miles to the west.1 Portions of the Rose Canyon Fault Zone are identified as
Earthquake Fault Zones by the CGS, with this designation intended to “[r]egulate development
near active faults so as to mitigate the hazard of surface fault rupture” (CGS 2007, refer to the
related discussion below under Regulatory Framework). No other designated fault rupture hazard
zones are mapped within the study area and vicinity (CGS 2007, City 1991a).
Other major faults (i.e., in addition to the Rose Canyon Fault Zone) within an approximate
60-mile radius of the study area were identified during a computer search conducted as part of
the Project geotechnical investigation (Appendix E). The results of this search, along with
related seismicity data, are shown on Table 2.6-2, Fault Summary – Deterministic Site
Parameters. Specifically, computer analysis of peak horizontal ground acceleration within the
study area determined a maximum estimated value of approximately 0.13g (where g equals the
acceleration due to gravity) in association with a maximum credible earthquake of magnitude 6.9
along the Rose Canyon Fault Zone. A maximum credible earthquake is defined as the maximum
earthquake considered capable of occurring under the presently known tectonic framework.
Table 2.6-2
FAULT SUMMARY – DETERMINISTIC SITE PARAMETERS
Fault Name
Approximate
Distance
(Miles)
Estimated Maximum Earthquake Event
Maximum
Earthquake
Magnitude
Peak Site
Acceleration (g)
Rose Canyon 16.0 6.9 0.128
Elsinore-Julian 22.0 7.1 0.098
Newport-Inglewood 26.3 6.9 0.065
1 Active faults are defined as those exhibiting historic seismicity or displacement of Holocene (approximately
11,000 years or less in age) materials, while potentially active faults have no historic seismicity and displace
Pleistocene (between approximately 11,000 and 2 million years old) but not Holocene strata.
Section 2.6 – Geology/Soils
Espola Road Improvement Project Final EIR 2.6-6
May 2013
Table 2.6-2 (cont.)
FAULT SUMMARY – DETERMINISTIC SITE PARAMETERS
Fault Name
Approximate
Distance
(Miles)
Estimated Maximum Earthquake Event
Maximum
Earthquake
Magnitude
Peak Site
Acceleration (g)
Elsinore-Temecula 27.1 6.8 0.057
Coronado Bank 28.9 7.4 0.085
Earthquake Valley 29.0 6.5 0.040
Elsinore-Coyote Mountain 38.4 6.8 0.034
San Jacinto-Anza 44.2 7.2 0.039
San Jacinto-Coyote Creek 44.2 6.8 0.034
Elsinore-Glen Ivy 49.2 6.8 0.023
San Jacinto-Borrego 50.3 6.6 0.019
San Jacinto-San Jacinto Valley 52.5 6.9 0.023
Palos Verdes 56.9 7.1 0.025
Source: TCG 2004
Regulatory Framework
The proposed Project is subject to a number of regulatory requirements and industry standards
related to potential geologic and soil hazards. These guidelines typically involve measures to
evaluate risk and mitigate potential hazards through design and construction techniques.
Specific guidelines encompassing geologic and soil criteria that may be applicable to the design
and construction of the proposed Project include applicable criteria from the following: (1) the
California Seismic Hazards Mapping Act (Public Resources Code; Division 2, Chapter 7.8,
§2690 et seq.); (2) the City of Poway Municipal Code Section 16.48.030 and General Plan
(1991a); (3) the International Code Council (ICC) International Building Code (IBC; ICC 2006);
(4) the California Building Code (CBC, California Code of Regulations [CCR], Title 24, Part 2);
(5) the Greenbook Committee of Standard Specifications for Public Works Projects (Greenbook;
2003); and (6) the National Pollutant Discharge Elimination System (NPDES) General
Construction Activity and General Groundwater Extraction permits. Summary descriptions of
these guidelines are provided below, with associated requirements included in the analysis of
Project impacts as applicable. Discussion of the identified NPDES standards is provided in
Section 3.1.3 of this EIR (Hydrology and Water Quality), due to the relationship between these
requirements and water quality concerns.
California Seismic Hazards Mapping Act
The California Seismic Hazards Mapping Act provides a statewide seismic hazard mapping and
technical advisory program to assist local agencies in protecting public health and safety relative
to seismic hazards. The Act provides direction and funding for the State Geologist to compile
seismic hazard maps and to make those maps available to local governments. The Act, along
with related standards in the Seismic Hazards Mapping Regulations (CCR Title 14, Division 2,
I:\Gis\B\BAH-01Espola Rd\Map\EIR\Fig2-6-2_Faults.indd -EV Regional Fault Map
ESPOLA ROAD IMPROVEMENT PROJECT
Figure 2.6-2
Section 2.6 – Geology/Soils
Espola Road Improvement Project Final EIR 2.6-7
May 2013
Chapter 8, Article 10, §3270 et seq.), also directs local governments to require the completion
and review of appropriate geotechnical studies prior to approving development projects.
City of Poway Standards
The City of Poway Municipal Code Section 16.48.030 (Soils and/or Engineering Geology
Investigation Reports) identifies requirements for determining when geotechnical investigations
are required to be conducted, as well as the appropriate content and application of such studies.
Specific criteria are identified for preliminary soils, engineering geology and seismicity studies,
with all three of these elements included in the Project Geotechnical Design Report
(Appendix E).
The City General Plan identifies a number of general goals and policies related to soil and
geologic issues, including use of appropriate grading techniques/specifications, implementation
of erosion/sedimentation controls, proper roadway design and identification/mitigation of
geologic and seismic hazards (e.g., through requiring appropriate technical studies, as noted
above).
International Building Code and Greenbook Standards
The IBC and Greenbook standards are produced through joint efforts by industry groups such as
the American Public Works Association (APWA) and ICC to provide standard specifications for
engineering and construction activities, including measures to address geologic and soil issues.
Specifically, these measures encompass issues such as seismic loading parameters
(e.g., classifying seismic zones and faults), engineered fill specifications (e.g., compaction and
moisture content), expansive soil characteristics and pavement design. The referenced
guidelines, while not comprising formal regulatory requirements per se, are widely accepted by
regulatory authorities and are routinely included in related standards such as municipal grading
codes. The IBC and Greenbook guidelines are regularly updated to reflect current industry
standards and practices, including criteria such as ASTM International (ASTM, formerly known
as the American Society for Testing and Materials).
California Building Code Standards
As previously noted, the CBC encompasses a number of requirements related to geologic issues,
including seismic safety (Chapter 23); foundation and retaining wall design (Chapter 29); site
demolition and excavation (Chapter 33); and grading, drainage and erosion control (Chapter 70).
The CBC is based on the previously described IBC, with appropriate amendments and
modifications to reflect site-specific conditions in California.
Section 2.6 – Geology/Soils
Espola Road Improvement Project Final EIR 2.6-8
May 2013
2.6.2 Thresholds of Significance
The Project would result in significant impacts to geology/soils if it would:
Expose people or property to substantial risk related to seismic hazards including ground
rupture, ground acceleration, liquefaction and seismic settlement, landsliding and
tsunamis/seiches/earthquake-induced flooding.
Expose people or property to substantial risk from unstable geologic and related
conditions including compressible soils/differential settlement, manufactured slope
instability, and expansive or corrosive soils.
Substantially increase on- or off-site erosion/sedimentation levels.
Be subject to substantial constraints on Project construction or operation due to geologic,
soil or related conditions within the study area such as shallow groundwater/drainage,
shallow bedrock or oversize material.
2.6.3 Impacts
The Project Geotechnical Design Report (Appendix E) concludes that “[t]here are no significant
geotechnically related issues that would adversely influence project design…or…specifications.”
A number of potential effects related to geology/soils may occur, however, during both short-term
(construction) and long-term (operational) Project phases. Specifically, identified geotechnical
concerns include potential hazards associated with: (1) seismic-related effects such as ground
rupture, ground acceleration, liquefaction/seismic settlement, landsliding, and tsunamis/seiches and
earthquake-induced flooding (with potential tsunami/seiche and earthquake-induced flooding
issues addressed in Section 3.1.3 of this EIR, Hydrology and Water Quality); and (2) non-seismic
hazards including erosion/sedimentation, compressible soils/differential settlement, manufactured
slope instability/retaining walls, expansive or corrosive soils, shallow groundwater/drainage, and
shallow bedrock/oversize material (with erosion/sedimentation impacts also evaluated in this
section.
The Project Geotechnical Design Report identifies a number of general and issue-specific
recommendations to address these concerns, including measures such as review of Project plans
and applicable field activities by the Project geotechnical engineer, and conformance with
appropriate regulatory requirements and industry standards. Potential seismic and non-seismic
hazards are described below by issue, with associated geotechnical recommendations and
regulatory/industry considerations provided where appropriate.
Seismic Hazards
Ground Rupture
Ground rupture and related effects such as lurching (i.e., the rolling motion of surface materials
associated with passing seismic waves) can adversely affect surface and subsurface structures.
No significant impacts related to seismically induced ground rupture (and related effects) are
anticipated from implementation of the proposed Project. This conclusion is based primarily on
the fact that no known active or potentially active faults likely to be subject to such phenomena
Section 2.6 – Geology/Soils
Espola Road Improvement Project Final EIR 2.6-9
May 2013
are located within or adjacent to the study area. While the potential for such effects cannot be
totally discounted (e.g., there could be unknown faults within the study area), it is characterized
as “negligible to low” in the Project Geotechnical Design Report (Appendix E).
Ground Acceleration (Ground Shaking)
As described above under Affected Environment and shown on Table 2.6-2, the estimated
maximum peak ground acceleration levels within the study area are approximately 0.13g in
association with a maximum credible earthquake event along proximal segments of the Rose
Canyon Fault Zone. This level of ground acceleration potentially could result in significant
impacts to Project facilities, including pavement, foundations, retaining walls, utilities, and
drainage structures. The Project Geotechnical Design Report includes a number of
recommendations to address seismic loading criteria, including conformance with applicable
elements of the IBC/CBC (e.g., appropriate seismic zone, soil profile and fault type parameters),
and review of Project plans and construction activities by the Project geotechnical engineer.
Additional regulatory requirements and industry guidelines that may be applicable to seismic
design considerations include the previously referenced City of Poway Municipal Code/General
Plan criteria and Greenbook standards (refer to the above discussion under Regulatory
Framework). Specific measures that may be used to accommodate seismic loading pursuant to
the above requirements include: (1) proper site preparation (e.g., clearing and grubbing);
(2) removal/replacement or treatment (e.g., recompaction) of unsuitable materials
(e.g., compressible soils); (3) use of approved fill with appropriate composition, depth,
application methodology, moisture content and compaction parameters (pursuant to ASTM
standards); (4) use of appropriate design and location criteria for pavement, foundations,
retaining walls, footings and utilities; (5) provision of adequate site drainage techniques to avoid
saturation of surficial materials (e.g., positive grading and use of subdrains); and (6) use of
properly prepared and reinforced concrete and masonry.
An additional potential impact related to ground acceleration involves open trenches/excavations
that may be entered by construction employees and/or equipment. Such open excavations
potentially could be subject to caving under the described ground acceleration conditions, with
associated potential impacts related to worker safety and equipment damage. The Project
Geotechnical Design Report provides specific recommendations regarding the incorporation of
applicable Occupational Safety and Health Administration (OSHA) and the California Division
of Occupational Safety and Health (CAL/OSHA) regulations, including trench inspection and the
use of appropriate shoring pursuant to 29 CRF Part 1926, Occupational Health Standards-
Excavations (Appendix E).
Implementation of, and conformance with, recommendations in the Project Geotechnical Design
Report, as well as appropriate regulatory/industry standards and technical reviews, would avoid
or reduce identified potential ground acceleration impacts to below identified significance
thresholds.
Section 2.6 – Geology/Soils
Espola Road Improvement Project Final EIR 2.6-10
May 2013
Liquefaction and Seismic Settlement
Liquefaction is the phenomenon whereby soils lose shear strength and exhibit fluid-like flow
behavior. Loose, granular (i.e., cohesionless) soils with relative densities of less than
approximately 70 percent are most susceptible to these effects, with liquefaction generally
restricted to saturated or near-saturated soils. Liquefaction most typically results from seismic
ground acceleration, with related settlement and loss of support potentially resulting in
significant impacts to surface and subsurface facilities. Based on observation of surface and
subsurface conditions, the Project Geotechnical Design Report concludes: (1) potential
liquefaction within the study area is limited primarily to the alluvial areas along the southern
limits of the Project alignment; (2) the primary Project facility potentially subject to liquefaction
effects would be the paved roadway; and (3) the main impact to pavement from liquefaction
would be settlement, with estimated liquefaction-induced settlements of two inches or less. The
report goes on to state that “[l]iquefaction induced ground settlements would have limited impact
to the proposed roadway pavement.” In addition, the Project design would incorporate
regulatory guidelines to address potential liquefaction hazards, if such conditions are
encountered during field activities. Specifically, this would include standard industry measures
from IBC/CBC, Greenbook or other applicable sources (as described above under Regulatory
Framework), including removal of liquefiable soils and replacement with engineered fill. As a
result of these considerations, potential liquefaction impacts would be below identified
significance thresholds.
Settlement of other (i.e., non-liquefiable) subsurface deposits also can result from seismic
activity. The Project Geotechnical Design Report concludes, however, that subsurface soils
within the study area “[a]re not susceptible to significant amounts of seismic-induced
settlement…” and that “[t]hese settlements will likely be on the order of one-quarter inch or
less.” Based on these conclusions, potential seismic settlement impacts would be below
identified significance thresholds.
Landsliding
The occurrence of landslides and other types of slope failures (e.g., rockfalls) are influenced by a
number of factors, including slope grade, geologic and soil characteristics, moisture levels and
vegetation cover. Landsliding can be triggered by one or more specific (or combinations of)
events, including seismic activity, gravity, fires and precipitation. No existing landslide or slope
instability hazards were identified in the Project Geotechnical Design Report, which concludes
that “[t]he risk associated with seismic-induced slope instability is negligible.” Based on this
conclusion, potential seismically induced landsliding impacts would be below identified
significance thresholds.
Non-seismic Hazards
Erosion/Sedimentation
The study area encompasses a number of topsoil or other surficial materials with moderate to
high erosion potential as previously noted (refer to Table 2.6-1). Proposed excavation, grading
Section 2.6 – Geology/Soils
Espola Road Improvement Project Final EIR 2.6-11
May 2013
and construction activities potentially could result in erosion and downstream sediment transport
(i.e., sedimentation). Discussion of potential erosion and sedimentation impacts is provided in
Section 3.1.3, Hydrology and Water Quality, of this EIR due to the relationship between this
issue and water quality concerns.
Compressible Soils/Differential Settlement
One or more of the surficial deposits identified within the Project study area may be susceptible
to post-construction compression and differential settlement (i.e., varying degrees of settlement
over short distances). The Project Geotechnical Design Report (Appendix E) concludes,
however, that post-construction differential settlement is anticipated to be less than 0.5 inch,
provided that other recommendations regarding footing and subgrade design are properly
implemented. Based on these considerations, potential impacts related to compressible
soils/differential settlement would be below identified significance thresholds.
Manufactured Slope Instability/Retaining Walls
The Project design includes a number of manufactured (cut and fill) slopes and retaining walls.
These types of structures can be subject to instability effects from causes including gravity
movement, excessive moisture or hydrostatic pressure (retaining walls), improper
design/construction, and lack of vegetation cover. Resulting conditions such as soil slump or
creep, soil saturation and wall failure can adversely affect both the structures themselves and
downslope facilities.
The Project Geotechnical Design Report (Appendix E) identifies the following recommendations
related to manufactured slopes and retaining walls: (1) all fill slopes should be constructed of
engineered fill in accordance with Project specifications regarding benching, site preparation and
compaction; (2) all non-reinforced earth fill slopes should have a maximum grade of 2:1
(horizontal to vertical), and all reinforced earth slopes should be constructed at a maximum grade
of 1:1 (unless other gradients are authorized by the Project engineering geologist); (3) all cut
slopes with a hinge height of 10 feet or less should have a maximum grade of 1:1, and all cut
slopes with hinge heights exceeding 10 feet should have a maximum grade of 1:1 for the first
(bottom) 10 feet and a maximum grade of 1.5:1 for the remaining slope height; (4) all top of
slope drainage should be prevented from flowing down slope faces, and appropriate erosion
control measures and landscaping should be installed on all applicable slopes; (5) footings
constructed at the tops of slopes should be set back a distance of one-third the slope height, with
a maximum setback of 40 feet; and (6) retaining wall design should incorporate appropriate
standard specifications related to backfill, drainage and earth pressures. Implementation of these
(and other appropriate) recommendations would reduce potential impacts related to
manufactured slopes and retaining walls below identified significance thresholds.
Expansive Soils
Expansive (or shrink-swell) behavior is attributable to the water-holding capacity of clay
minerals and can adversely affect the integrity of facilities such as footings or pavement. A
number of mapped native soils within the study area exhibit moderate to high expansion
Section 2.6 – Geology/Soils
Espola Road Improvement Project Final EIR 2.6-12
May 2013
potential due to clayey subsoils (refer to Table 2.6-1). As previously described, much of the area
proposed for development has been previously developed or disturbed, with associated native
soils expected to have been largely replaced or altered during this activity. Based on these
observations, the potential for occurrence of expansive materials in the described portions of the
study area is considered generally low.
Expansive materials are likely to be present in areas with little or no previous disturbance,
however, and could result in significant impacts to proposed facilities such as pavement, utilities
or footings/foundations. If expansive soils are encountered, the proposed construction and
facility design would be required to conform to site-specific recommendations of the Project
geotechnical engineer, as well as regulatory guidelines including applicable City of Poway,
IBC/CBC, Greenbook, and/or other standards. Specific measures that could be used to address
such potential effects pursuant to the above requirements include a number of standard remedial
options such as: (1) removal and replacement of unsuitable materials with fill exhibiting a IBC
Expansion Index of less than 50; (2) restricting the placement of expansive materials to provide
appropriate minimum depths/distances from finish grade/manufactured slope faces; and (3) use
of IBC (or other appropriate) compaction and moisture content criteria to treat expansive soils
(e.g., mixing with low-expansion fill). Implementation of these types of measures and
conformance with the noted technical recommendations/industry standards would reduce
potential impacts related to expansive soils below identified significance thresholds.
Corrosive Soils
A number of mapped native soils within the study area exhibit potentially corrosive acidic and/or
alkaline pH levels, and could (along with proposed fill materials) also encompass corrosive
properties related to properties such as resistivity (i.e., electrical resistance), soluble chlorides, and
soluble sulfates. Long-term exposure to corrosive soils could result in deterioration and eventual
failure of underground concrete and metal structures, including foundations or utility lines.
Based on the conclusions provided in the Project Geotechnical Design Report and the relative
uncertainty regarding the presence and extent of corrosive materials within the study area,
associated long-term impacts are considered potentially significant and mitigation is identified
below.
Shallow Groundwater/Drainage
As described under Affected Environment, shallow groundwater was observed in one boring
during study area geotechnical investigation, and perched groundwater conditions could be
subject to stability impacts related to shallow groundwater/site drainage. Retaining walls, for
example, could be adversely affected if backfill is not adequately drained and excessive
hydrostatic pressure develops.
The Project Geotechnical Design Report (Appendix E) includes recommendations to provide
adequate drainage facilities in appropriate areas, including backfill drainage systems
(e.g., subdrains) for retaining walls and pipelines, restriction of drainage down manufactured
slopes (e.g., with brow ditches or terrace drains), and use of gravel or crushed rock in culvert
Section 2.6 – Geology/Soils
Espola Road Improvement Project Final EIR 2.6-13
May 2013
subgrades. Implementation of these types of measures and conformance with the noted
geotechnical recommendations would reduce potential impacts related to shallow
groundwater/site drainage below identified significance thresholds.
Shallow Bedrock/Oversize Material
As described under Affected Environment, shallow granitic bedrock is exposed in or underlies
the entire study area, and exhibits variable degrees of weathering. In addition, corestones may be
present along portions of the weathered/non-weathered bedrock interface. Based on observations
during the Project geotechnical investigation, the following conclusions are provided:
From the southern study area boundary to Station 22+00 (just south of Mountain Road),
proposed excavations generally are expected to encounter only surficial materials
(e.g., fill and alluvium), with no shallow bedrock anticipated.
From Station 22+00 to 54+00 (approximately 300 feet north of High Valley Road),
proposed excavations are generally expected to encounter surficial materials underlain by
weathered bedrock that should be rippable with standard construction equipment.
From Station 54+00 to the northern study area boundary, proposed excavations are
generally expected to encounter a thin veneer of surficial materials underlain by both
weathered and non-rippable bedrock (and potentially corestones).
Subsurface conditions potentially could generate oversize material north of Station 22+00. The
use of oversize materials in engineered fill can produce effects such as differential compaction
and settlement (as previously defined), with related issues including potentially significant
impacts to (for example) overlying pavement or footings.
The Project Geotechnical Design Report concludes that excavated materials generally can be
used for fill, provided oversize material is properly placed. A number of standard industry
handling and placement criteria for oversize materials could be employed in such circumstances,
including: (1) limiting the placement of materials between approximately one and four feet in
maximum dimension to areas a minimum of 15 horizontal feet from slope faces, 5 feet below
finish grade, or 3 feet below the deepest utility, whichever is deeper; (2) limiting the placement
of materials greater than four feet in maximum dimension to locations approved by the Project
geotechnical engineer; and (3) disposing of oversize materials that cannot be placed in
conformance with the above requirements in an approved off-site location. Implementation of
the described geotechnical recommendations and conformance with applicable regulatory/
industry guidelines would avoid or reduce potential impacts related to oversize material below
identified significance thresholds.
2.6.4 Mitigation Measures
Potentially significant geology/soils impacts identified above were limited to long-term effects
from the possible occurrence of corrosive soils. All other identified potential geotechnical
impacts would be appropriately addressed through required regulatory compliance and/or
proposed Project design measures following confirmation of roadway conditions, drainage, etc.
during final construction drawings review.
Section 2.6 – Geology/Soils
Espola Road Improvement Project Final EIR 2.6-14
May 2013
The following mitigation measure, in concert with implementation of all applicable
recommendations from the Project Geotechnical Design Report (and related field observations)
and conformance with identified industry/regulatory standards, would reduce identified impacts
related to corrosive soils below a level of significance.
An investigation of potential corrosion hazards within the Project study area shall be conducted
by a qualified corrosion engineer prior to Project implementation. The results of this analysis
shall be incorporated into final Project design, as appropriate, to mitigate potential corrosion
impacts, and may include (but not be limited to) measures such as: (1) excavation (or
overexcavation) and treatment, and/or removal and replacement (i.e., with non-corrosive
engineered fill) of corrosive materials; (2) use of non-corrosive and/or corrosion-resistant
building materials in appropriate locations; and (3) installation of cathodic protection devices.