HomeMy WebLinkAbout02160029_Sec05-05 Geology and Soils Project Impacts
West Ming Specific Plan - Draft EIR Geology and Soils
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5.5 - Geology and Soils
5.5.1 - Introduction
This section evaluates the potential impacts on the proposed project from the existing geology and
soils conditions on the project site. Information in this section is based on the following documents.
• Geotechnical Feasibility Study / Geological Hazard Study, Soils Engineering Inc., February 5,
2004. The complete report is contained in Appendix F of the Technical Appendices of the
Draft EIR.
• Addendum #1 to Geotechnical Feasibility Study and Geological Hazard Study for the West
Ming Specific Plan (Sections 11, 13, 14 & portions of 10 &15) In Bakersfield, California. The
complete report is contained in Appendix F of the Technical Appendices of the Draft EIR.
• Metropolitan Bakersfield General Plan, December 2002. This document is available for review
at the City of Bakersfield Planning Department at 1715 Chester Avenue in Bakersfield, CA
93301.
5.5.2 - Environmental Setting
Soils and Geology
The majority of the project site is relatively flat and there is no evidence of historic landslides. There
are no bedrock outcrops present within 1 mile of the site. Some lower areas in the western portion of
the project site are utilized as recharge areas. The elevation change across the site is approximately
10 to 15 feet with a slight downhill slope to the southwest. A portion of the Kern River Canal and
recharge basins also traverse the northern portion of the site. The project site rests on a considerable
thickness of alluvium identified as Recent Quaternary Fan Deposits (Qf).
Surface soils within the project site have been classified according to the Unified Soils Classification
System based on 43 test borings drilled at the project site to a maximum depth of 52 feet below
ground surface. Soils at the Project site consist of interbedded silty sand, clayey sand, sandy clay,
poorly graded sand, and well-graded sand overlying bedrock. The surface soils are in a loose to
medium-dense condition and are typical of the south west area of Bakersfield. Soils encountered in
the top 5 feet on site indicate that highly expansive soil will not be encountered on the project site.
Faulting and Seismicity
The project site is not within an Alquist-Priolo Earthquake Fault Zone. No active faults are located
within the site boundaries or adjacent properties. A subsurface oil field defined fault is located within
the southeast portion of the project site and is considered inactive. Active faults which have the most
potential to produce ground shaking at the Project site are listed in Table 5.5-1 below with the closest
fault being: Kern Front Fault (15.4 kilometers (9.6 miles) northwest). Estimated peak ground
accelerations at the project site are based mainly on fault distance and magnitude. Therefore, faults
which are closest to the site or have large maximum credible magnitudes or both are included in
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Table 5.5-1 below. The White Wolf and San Andreas Faults have produced the majority of the
seismic activity in the area of the project site, and the largest estimated historical site acceleration at
the project site is 0.235g from a 7.7 magnitude earthquake on the White Wolf Fault in July 1952. The
closest earthquake to the site was approximately 19 kilometers (12 miles) away at a magnitude of 5.2
in May 1993.
It is not likely that ground rupture would occur at this site since it is not located within 500 feet (0.095
mile) of a known active fault trace. No Seismic Source Type A faults are within 5 kilometers (3.1
miles) of the project site which are capable of producing large magnitude events of greater than or
equal to earthquakes of 7.0 magnitude and a slip rate of 5 millimeters (0.2 inches) or less per year.
Likewise, no Seismic Source Type B earthquake faults are located within 10 kilometers of the site
which would produce earthquake events greater than or equal to 7.0 magnitude and a slip rate of less
than 5 millimeters annually, or an earthquake magnitude of less than 7.0 and a slip rate greater than 2
millimeters (0.08 inches) annually, or a magnitude greater than or equal to 6.5 and a slip rate less than
2 millimeters annually. The nearest Seismic Source Type B fault is the White Wolf Fault located
approximately 26.7 kilometers (16.6 miles) away. The nearest Seismic Source Type A fault is the
San Andreas Fault which is approximately 47.4 kilometers (29.5 miles) away. The Project site is in
Seismic Zone 4, pursuant to the California Building Code, based on its location to the other know
active faults.
Table 5.5-1 shows that the estimated maximum peak ground acceleration at the site would be 0.24g
from a 6.3 magnitude earthquake on the Kern Front Fault. A 7.2 magnitude earthquake on the White
Wolf Fault would result in an estimated ground motion of 0.233g at the site. A maximum probable
earthquake of magnitude 7.8 on the San Andreas Fault would create a peak site ground acceleration of
0.168g at the site.
Table 5.5-1: Active Faults Within the Project Area
Fault
Approximate
Distance
(Kilometers/Miles)
Maximum
Earthquake
Magnitude (Mw)1
Maximum Peak
Ground Acceleration
Gravity (g)
Estimated
Site Intensity
(MM)2
Seismic
Source
Type
Kern Front 15.4/9.6 6.3 0.246 IX C
White Wolf 28.4/17.6 7.2 0.233 IX B
Pleito Thrust 41.9/26 7.2 0.152 VIII B
San Andreas (1857 Rupture
& Carrizo)
47.4/29.5 7.2 to 7.8 0.110 to 0.168 VII to VIII A
Big Pine 57.6/35.8 6.7 0.059 VI B
Garlock (West) 59.2/36.8 7.1 0.078 VII A
San Gabriel 72.4/45 7.0 0.056 VI B
San Juan 76.2/47.4 7.0 0.053 VI B
1 Richter Scale Magnitude
2 Mercale Scale Intensity
Source: Soils Engineering, Incorporated, 2004
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Liquefaction
According to the Kern County Water Maps (Kern County Water Agency 2003), the depth to the
unconfined aquifer in the fall of 2000 was approximately 80 feet below ground surface. Shallow
groundwater was not encountered in the top 51 feet below ground surface in the 43 test borings
conducted in 2004 by SEI. Limited perched water can be present in areas of heavy irrigation, septic
systems and recharge areas. Based on the soil types present and lack of shallow groundwater, the
liquefaction potential for most of the site is considered low. Due to the presence of water in the
northwestern portion of the site in 2005, SEI conducted , a liquefaction analysis on soil samples
collected from soil boring B12-A, which was taken from the northwest corner of the project site.
Groundwater was encountered at a depth of 15 feet below ground surface at that location, and
subsurface soils were classified as sandy silt. SEI determined that the soils in the northwestern
portion of the site are potentially liquefiable between 15 feet and 17.5 feet below ground surface. SEI
indicated that normal groundwater levels are greater than 50 below ground surface, however,
groundwater was present at higher elevations due to the substantial amount of surface water in the
Kern River and in the recharge ponds in that location.
Settlement
A settlement analysis was prepared by SEI in April 2005. The analysis states that the total dry sand
settlement calculated for the top 15 feet of soil is 0.441 and settlement was 0.34 inches for the
saturated zone between 15 feet and 40 feet for a total settlement of 0.781 inches. According to SEI,
0.781 inches of settlement is considered very minor. In addition, the amount of settlement in the
potentially liquefiable zone between 15 and 17.5 feet was 0.06.
5.5.3 - Thresholds of Significance
According to Appendix G of the State CEQA Guidelines, a 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
Earthquake Fault Zoning Map issued by the State Geologist for the area or based on
other substantial evidence 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;
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• Be located on a geologic unit or soil that is unstable, or that would become unstable as a result
of the project, and potentially result in on- or off-site landslide, lateral spreading, subsidence,
liquefaction or collapse;
• Be located on expansive soil, as defined in Table 18-1-B of the Uniform Building Code (1994),
creating substantial risks to life or property; or
• Have soils incapable of adequately supporting the use of septic tanks or alternative wastewater
disposal systems where sewers are not available for the disposal of waste water.
5.5.4 - Project Impacts and Mitigation Measures
The proposed project would have the potential to expose people and structures to seismic ground
shaking, ground failure, soil erosion, and unstable soils, Following is a discussion of the project
impacts that correspond to the thresholds previously identified in Section 5.5.3.
Impact 5.5.A: The proposed project could expose people or structures to potential substantial
adverse effects, including the risk of loss, injury, or death involving: Rupture of a
known earthquake fault, as delineated on the most recent Alquist-Priolo Earthquake
Fault Zoning Map issued by the State Geologist for the area or based on other
substantial evidence of a known fault; Strong seismic ground shaking; seismic-
related ground failure including liquefaction; and landslides.
The project will change the use of the project site from agricultural, vacant, and oil production uses to
urban uses. During periods of construction, there will be temporary increases in human activity on
the project site from the presence of construction crews. Over the long-term, there will be a
permanent increase in the level of human activity on the project site. The increased levels of human
activity of the project site will increase the potential exposure of persons living and working on the
project site to seismic events including risk of loss, injury, and death related to earthquakes and
related hazards which are described as follows:
Fault Rupture
Although the project site is within the vicinity of several active faults, it is not located within an
Alquist Priolo Earthquake Zone, and no Seismic Source Type A or B earthquake faults are within
proximity to the project site which could produce potentially significant impacts on site. The project
site is located within Seismic Zone 4 of the 2001 California Building Code, and the California
Building Code contains criteria for building design related to Seismic Zone 4 which can be
incorporated into project design and will reduce impacts to onsite structures related to earthquake
events. These criteria include setbacks from the high pressure lines currently located on the project
site. The nearest Seismic Source Type A fault is the San Andreas Fault, located 29.5 miles from the
site, and would be considered potentially significant if located within 15 miles of the project site.
Likewise, the nearest Seismic Source Type B fault is the White Wolf Fault, located 17.6 miles from
the project site, and would be considered potentially significant if located within 10 miles of the
project site. Furthermore, the nearest Alquist-Priolo Earthquake fault zone is approximately 15.4
kilometers (9.6 miles) northeast and will not significantly affect the use of the project site. Therefore,
Project Impacts
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the adverse effects from a rupture of a known active earthquake fault at this site are less than
significant.
Seismic Ground Shaking
In the event of an earthquake on one of the nearby faults (see Table 5.5-1), it is likely that the project
site will experience ground shaking and exposing people and structures associated with the project to
ground shaking, as well as the existing high pressure petroleum and gas lines. The Design-basis
Earthquake ground motion for the project site is estimated at 0.27g based on the proximity and
potential earthquake magnitude of the closest known active fault. An estimated ground motion of
0.235g occurred at the site from a 7.7 magnitude earthquake on the White Wolf Fault in July 1952;
and, the White Wolf Fault has produced most of the historical earthquakes in the vicinity of the
project site. Based on the estimated ground motion, the site is in a Seismic Zone 4 and will require
structural design incorporated in building plans for individual projects meeting the requirements of
the California Building Code related to Seismic Zone 4. With these requirements, the site preparation
and structural foundations will be built to withstand 0.27g at the project site from an earthquake on
the nearest faults. Improvement plans will be reviewed by the City of Bakersfield during site plan
review for individual projects built within the Specific Plan to ensure that the recommendations of the
Geotechnical Engineer (SEI 2004) are included in project design. Likewise structural setbacks from
the high pressure lines present on the project site will be enforced during the City of Bakersfield site
plan review process. This will ensure that the structures will withstand most earthquake events and
the existing high-pressure lines located on the project site will not impact the proposed uses.
Therefore, seismic ground shaking on the project site is considered less than significant.
Seismically-Induced Ground Failure and Landslides
There is a low potential for rock fall and landslides to impact the site in the event of a major
earthquake (SEI 2004). Based on the predicted maximum horizontal accelerations at the project site
and the soil types identified by SEI (2004), minor subsurface settlement may occur on site during a
major earthquake, and this is considered less than significant.
Ground failure from liquefaction could occur in the northwestern area of the project site where the
recharge areas are currently located. This is due to the granular (non-cohesive) consistency of the
soils and the presence of a potentially high groundwater table. The results of the liquefaction analysis
for this area (SEI 2005) indicates that the soils in the northwestern area are potentially liquefiable
between 15 feet and 17.5 feet below ground surface and are non-liquefiable below a depth of 17.5
below ground surface. Therefore, implementation of the project in the northwestern portion of the
site could be significantly affected by liquefaction. The remainder of the site would not be subject to
liquefaction impacts due to the depth of groundwater of more than 50 feet below ground surface.
Seismically-Induced Flooding
The potential for earthquake-induced flooding at the site appears to be very low (SEI 2004). In
addition, the project includes the extension of the existing levee along Ming Avenue to eliminate the
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potential for flooding to occur on the project site during a 100-year flood event. The implementation
of the proposed levee will reduce the potential for seismically-induced flooding to less than
significant.
Mitigation Measures
5.5.A.1 Concurrently with submittal of a tentative subdivision map application or other
development plan in the northwestern corner of the project site, whichever occurs
first, the project applicant shall conduct additional liquefaction studies in the
northwestern corner of the project site (i.e., in the vicinity of the existing recharge
ponds) during recharge periods to fully evaluate liquefaction impacts on specific
development projects in this area of the site. Based on the findings of these studies,
site specific designs shall be incorporated in the grading and building plans to reduce
onsite liquefaction impacts. The scope of the liquefaction studies, findings, and
recommendations to reduce liquefaction shall be reviewed and require approval by
the City of Bakersfield Public Works and Building Departments prior to grading and
building plan approvals.
Level of Significance After Mitigation
Less than significant.
Impact 5.5.B: The proposed project could result in substantial soil erosion or the loss of topsoil.
During the construction phase of the project, activities such as grading and construction will disrupt
surface vegetation and soils and will expose these disturbed areas to erosion by wind and water. The
proposed project will be developed in phases and portions of the project site will have disturbed areas
while other portions will be under current agricultural and oil activities or will have already been
converted to the land uses proposed in the West Ming Specific Plan. Erosion during construction
activities could be significant.
In the long-term and after construction activities have been completed on the project site, the ground
surface will have impermeable surfaces as well as permeable surfaces. The impermeable surfaces
will include roadways, driveways, parking lots, pavement treatment surrounding homes, as well as the
home and building sites. The permeable surfaces will include landscape areas as well as the
detention/retention basins. Landscaping will stabilize the permeable areas while the
detention/retention basins are designed to retain soils. Overall, development of the project would not
result in conditions where substantial surface soils would be exposed to wind and water erosion.
Therefore, the project site would experience less than significant long-term impacts from water and
wind erosion and loss of top soil.
Mitigation Measures
5.5.B.1 Prior to grading plan approval, an erosion control plan for construction activities that
describe the best management practices (BMPs) that will be used to reduce the
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potential for soil erosion and loss of top soil. The erosion control plan shall be
submitted to the City of Bakersfield Public Works Department for review and
approval. The BMPs could include soil stabilizers and silt fencing as well as other
measures.
Level of Significance After Mitigation
Less than significant.
Impact 5.5.C: The proposed project is not 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.
There is no evidence of landslides on the project site, and the project site is not located in an unstable
geologic unit or on soil that is considered unstable. A settlement analysis was prepared by SEI in
April 2005. The analysis states that the total dry sand settlement calculated for the top 15 feet of soil
is 0.441 and settlement was 0.34 inches for the saturated zone between 15 feet and 40 feet for a total
settlement of 0.781 inches. According to SEI, 0.781 inches of settlement is considered very minor
and less than significant. In addition, the amount of settlement in the potentially liquefiable zone
between 15 and 17.5 feet was 0.06 which is also considered less than significant.
The potential for hydro-collapse was also evaluated by SEI. According to SEI’s findings, the
potential for hydro-collapse is low when land has be irrigated. Because the project site has been
farmed since the 1970s, the potential for hydro-collapse is low and considered less than significant.
Mitigation Measures
No mitigation measures are required.
Level of Significance After Mitigation
Less than significant.
Impact 5.5.D: The proposed project is not located on expansive soil, as defined in Table 18-1-B of
the Uniform Building Code (1994) and would not creating substantial risks to life or
property.
Based on the type of soils encountered in the top 5 feet of soil at the 43 exploratory borings conducted
on the project site, SEI determined that it is likely that no significant areas of highly expansive soils
will be encountered. Therefore, implementation of the proposed project would result in a less than
significant impacts related to expansive soils.
Mitigation Measures
No mitigation measures are required.
Level of Significance After Mitigation
Less than significant.
Project Impacts
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Impact 5.5.E: The project does not include the use of septic tanks or alternative wastewater
disposal systems; therefore, the proposed project would not be affected by the soil
capability of adequately supporting the use of septic tanks or alternative
wastewater disposal systems.
Septic tanks or alternative wastewater disposal systems are not proposed. The project will include
lateral connections to City of Bakersfield sewer mainlines. Therefore, no impacts would occur.
Mitigation Measures
No mitigation measures are required.
Level of Significance After Mitigation
No impacts.