Transcript of 01896 DEIR 4.4 Geology and Soils 1
Microsoft Word - 01896 DEIR 4.4 Geology and Soils 1.docGEOLOGY AND
SOILS 4.4
The geology and soils analysis is based on the Preliminary
Geotechnical Review prepared by Petra Geotechnical, Inc. (see
Appendix E) and discusses potential impacts due to rupture of a
known earthquake fault, strong seismic ground shaking,
liquefaction, landslides, and unstable geologic units or soil. As
analyzed in the Initial Study, the proposed project will not result
in substantial erosion or loss of topsoil, be located on expansive
soils, or include the use of septic tanks or alternative waste
water disposal systems. Therefore, this DEIR does not further
analyze these topics. No comments regarding geology and soils were
submitted during circulation of the Notice of Preparation
(NOP).
Existing Conditions
TOPOGRAPHY The southern portion of the site is relatively flat with
a slight gradient to the north. Elevations begin to rise just south
of the MacNeil Mansion. Several small retaining walls within the
lower garden area are used to gain elevation. The site continues to
gain elevation north of the MacNeil Mansion with additional
retaining walls. Slopes ascend in the northern portion of the site
and continue onto the adjacent properties as graded fill slopes.
Elevations range from 680 feet above sea level (asl) in the
southeast corner of the project site to 760 feet asl along the
northern property boundary.
REGIONAL GEOLOGY The project site is located along the northern
edge of the San Gabriel Valley, which is an alluvial filled valley
bounded by the Sierra Madre Fault Zone and San Gabriel Mountains to
the north, the Puente Hills on the south, the Covina and Indian
Hills on the east, and by the Raymond Basin on the west. The
project area is located in the foreland slopes at the foot of the
San Gabriel Mountains. This foreland area is characterized by
shallow to moderately steep slope flanks that are underlain by fan
deposits that have been shed from the proto-mountains, which in
turn mantle sedimentary bedrock. Surficial deposits including
alluvial soils in the canyon bottoms, slopewash/mass wasted
materials on the ridge flanks, weathered soils and artificial fill
also mantle portions of the foreland area on and adjacent to the
project site.
SUBSURFACE CONDITIONS The Preliminary Geotechnical Review prepared
by Petra Geotechnical, Inc. included a comprehensive evaluation of
the seismic setting and focused on interpreted fault lineaments in
the southern portion of the project site. The study included a
review of existing published and unpublished reports and studies
conducted on the adjacent properties. On-site subsurface
investigations included seven bucket auger borings ranging from 11
to 36 feet deep, two Cone Penetrometer soundings to depths of 25
and 36 feet deep and the excavation and logging of an approximately
130 foot long, 15 to 20 foot deep trench. The trench was excavated
across an interpreted fault lineament. Subsurface investigations
found that the project site is underlain by artificial fill
(af/Caf), alluvium (Qal), and bedrock of the Topanga Formation
(Tt). Artifical fill (af/Caf) is non-engineered fill material (af).
Artificial fill was placed on the project site as part of original
site development. These materials were derived from near site
sources and consist of a varied admixture of sand, silt, and clay.
The fill materials, where encountered, were found to be dry to
moist and loose to medium dense and varied in depth from a few feet
to 5+ feet. Compacted/engineered fill (Caf) has been placed in the
southeast portion of the site and along the north, east and south
property boundaries. These materials were placed as part of the
grading construction of the Rosedale Development and consist
primarily of silty sand and sand that is moist and dense. The
placement of these fill sections was observed and tested by the
geotechnical consultant for the Rosedale project. Review of grading
reports indicates that these materials were compacted and moisture
conditioned to generally accepted industry standards. Alluvium
(Qal)/older Alluvium (Qol) are alluvial soils that were deposited
in tributary canyons by streams and/or surface flow. These
materials have been mapped in the southern, western and eastern
portions of the project site. Based on the boring logs and grading
exposures, alluvium consists of loose to medium dense, dry to
saturated sand and silty sands with occasional gravel and cobble
sized material. Alluvial thicknesses vary from a few feet to as
much as 35+ feet in the
Geology and Soils 4.4
4.4-2 Dhammakaya International Meditation Center
southern portion of the property. Older alluvium was observed in
the recently excavated trench. These materials are similar to the
younger alluvial soils but are dense and well consolidated. The
Topanga Formation (Tt) underlies the site and is mantled by
alluvium and fill deposits. Based on the trench exposure, this
bedrock unit consists of thick bedded sandstone layers with thin
clay and siltstone interbeds. The sandstone is weathered in the
upper 1 to 2 feet and moderately to well indurated.
GEOLOGIC HAZARDS Various types of ground failure can occur as a
result of earthquakes shaking and can cause substantial damage to
the built environment. Ground failure types include settlement,
collapse, subsidence, liquefaction, and slope failure.
SETTLEMENT Differential seismic settlement occurs when seismic
ground shaking causes one type of soil or rock to settle more than
another type. Settlement is more likely to occur in areas of
alluvium. Settling can damage structures and infrastructure by
unevenly depressing soils underlying building foundations. COLLAPSE
Collapsible soils consist of loose, dry, low-density materials that
collapse and densify with the addition of water or excessive
loading. Such soils are typical in areas of young alluvial fans,
debris flow sediments, and aeolian (wind-blown) deposits. Collapse
occurs when subsurface soils are excessively saturated at levels
deeper than those reached by an average rainfall and the clay bonds
holding the soil grains together are weakened. Collapse can result
in cracked foundations, floors, and walls.
SUBSIDENCE Land subsidence is a gradual settling or sudden sinking
of the Earth’s surface due to subsurface movement of earth
materials. More than 80 percent of the identified subsidence in the
United States is caused through overdrafting of groundwater.
Drainage of organic soils, undergrounding mining, natural
compaction, and thawing of permafrost can also cause subsidence.
Similar to collapse and settlement, subsidence causes large areas
of land to sink, thereby potentially damaging foundations, walls,
and floors.
LATERAL SPREAD Lateral spreading is the downslope movement of
surface sediment due to liquefaction in a subsurface layer. The
downslope movement is due to gravity and earthquake shaking
combined. Such movement can occur on slope gradients of as little
as one degree. Lateral spreading typically damages pipelines,
utilities, bridges, and structures.
LIQUEFACTION Liquefaction is a phenomenon that occurs when soil
undergoes transformation from a solid state to a liquefied
condition due to the effects of increased pore-water pressure. This
typically occurs where susceptible soils (particularly the medium
sand to silt range) are located over a high (less than 50 feet from
the surface) groundwater table. Affected soils lose all strength
during liquefaction and foundation failure can occur.
Regulatory Setting
SEISMIC HAZARDS MAPPING ACT The California Geological Survey (CGS)
provides guidance with regard to seismic hazards under the Seismic
Hazards Mapping Act. Seismic hazard zones are identified and mapped
by the CGS to assist local governments in land use planning. The
intent of the Act is to protect the public from the effects of
strong groundshaking, liquefaction, landslides, ground failure, or
other hazards caused by earthquakes. In addition, CGS Special
Publication 117, Guidelines for Evaluating and Mitigating Seismic
Hazards in California, provides guidance for the evaluation and
mitigation of earthquake-related hazards for projects within
designated zones of required investigations.
Geology and Soils 4.4
Draft Environmental Impact Report 4.4-3
CALIFORNIA BUILDING CODE The California Building Standards Law
states that every local agency enforcing building regulations must
adopt the provisions of the California Building Code (CBC) within
180 days of its publication; however, each jurisdiction can require
more stringent regulations issued as amendments to the CBC. The
publication date of the CBC is established by the California
Building Standards Commission and the code is known as Title 24 of
the California Code of Regulations. In the past, the CBC was
modeled on the Uniform Building Code (UBC); however, beginning with
the 2007 version, the CBC is now modeled after the International
Building Code (IBC). It should be emphasized that the building
codes provide minimum requirements to prevent major structural
failure and loss of life. The City of Azusa adopted the 2013 CBC
through Chapter 14-1 (Codes Adopted) of the Municipal Code. The
2013 CBC bases its seismic design criteria on maximum considered
ground motion through maps prepared by the USGS for the National
Seismic Hazard Mapping Program (see Section 1613). Pursuant to the
CBC, soils reports are required to be submitted prior to issuance
of grading permits.
CITY OF AZUSA GENERAL PLAN The Azusa General Plan includes the
following goals and policies related to geological hazards: Goal 1
Ensure the continued functioning of essential (critical, sensitive,
and high occupancy) facilities following a
disaster; help prevent loss of life from the failure of critical
and sensitive facilities in an earthquake; and help prevent major
problems for post-disaster response, such as difficult or hazardous
evacuations or rescues, numerous injuries, and major cleanup or
decontamination of hazardous materials.
Goal 2 Minimize to the greatest extent feasible the loss of life,
serious injuries, and major social and economic
disruption caused by the collapse of, or severe damage to,
vulnerable structures (e.g., buildings, bridges, water storage
facilities, key railroad components) resulting from an
earthquake.
Goal 5 Encourage the preparation of a plan to facilitate the rapid
and effective recovery of the city following an
earthquake. Identify alternative financing sources for the repair
and reconstruction of disaster related damage.
Thresholds of Significance Significant impacts related to geology
and soils would result from implementation of the proposed project
if the project would:
A. Expose people or structures to potential substantial adverse
effects, including the risk of loss, injury, or death
involving:
i. 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;
ii. Strong seismic ground shaking; iii. Seismic-related ground
failure, including liquefaction; iv. Landslides
B. 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
Geology and Soils 4.4
4.4-4 Dhammakaya International Meditation Center
Environmental Impacts Impact 4.4.A. – i – Impacts to life and
property resulting from the rupture of a known earthquake fault as
delineated on the most recent Alquist-Priolo Earthquake Fault
Zoning Map will be less than significant with mitigation
incorporated. As stated within the Preliminary Geotechnical Review
prepared by Petra Geotechnical, Inc., the project site is located
within a zone requiring investigation as defined by the
Alquist-Priolo Earthquake Fault Zoning Act based on the Earthquake
Fault Zones map for the Azusa 7.5-minute quadrangle (released on
November 6, 2014)1 (See Exhibit 4.4- 1,Earthquake Fault Zones &
Seismic Hazard Zones). The Alquist-Priolo Earthquake Fault Zoning
Act defines an active fault as one that “has had surface
displacement within Holocene time (about the last 11,000 years).”
The main objective of the Alquist Priolo Act is to prevent the
construction of dwellings on top of active faults that could
displace the ground surface resulting in loss of life and property.
The Act sets forth standards of investigation and analysis that
must be met in site studies. As a result, Petra Geotechnical, Inc.
completed a comprehensive Fault Investigation and Evaluation
(Appendix C of the Preliminary Geotechnical Review) for the project
site including conclusions concerning the structural setting of the
site, surrounding environs, and the possible existence of fault
traces. The study included a review of earlier investigations, thus
providing a regional context for the fault investigation and a
review of published articles covering the regional geology. The
subsurface evaluation consisted of seven bucket auger borings
ranging from 11 to 36 feet deep, two Cone Penetrometer soundings to
depths of 25 and 36 feet deep, and excavation and logging of a
130-foot long, 23-foot deep fault trench within the southern
portion of the project site (See Exhibit 4.4-2, Fault Trench
Profile). Alquist-Priolo-level fault investigations are not
intended to assess the intensity and hence effects of ground
motions on the proposed structures. The investigation is solely
intended to avoid placement of habitable structures on active
faults as defined by the State of California. Based on the results
of the Fault Investigation and Evaluation, Petra Geotechnical, Inc.
determined that the project site is within a tectonically active
area characterized by the active and well located Sierra Madre
fault and the less active and less well located Duarte fault (See
Exhibit 4.4-1). As demonstrated by earlier investigations by
consultants such as Earth Consultants International, Inc., Richard
Mills and Associates, and Pacific Soils Engineering, faults located
on the project site are not active per Alquist-Priolo requirements;
that is, they have been documented not to displace Holocene
sediments. According to the Fault Investigation and Evaluation, a
review of stereo-paired aerial photographs identified two potential
lineaments crossing the project site; one mapped in the
northwestern portion of the project site in the vicinity of the
Organizational Housing buildings and one mapped in the southern
portion of the site, through the proposed Meditation Hall. Those
two locations were the focus of the fault study on the project
site. Extensive geologic study surrounding the project site has
focused on unnamed fault “C” (UNC) and unnamed fault “D” (UND).
Unnamed fault “C” is located north of the project site, just south
of Sierra Madre Avenue and not within a projection that could
affect the project site. Unnamed fault “D” trends across a portion
of the Rosedale property west of the project site as documented in
investigations conducted by Lawson Geotechnical Consultants. The
projection of UND in a northeast to northerly direction clips the
far northwestern corner of the project site. While Earth
Consultants International, Inc. recently concluded in 2014, based
on the literature and examination of Lawson Geotechnical
Consultants trenches, that the fault is not active according to
Alquist-Priolo definitions, review of UND remains on-going.
Regardless of the status of UND, the proposed improvements, most
notably the proposed Organizational Housing buildings, are located
over 100 feet from the projection of UND, which is sufficient
separation regardless whether or not UND is ultimately determined
active. Since the proposed improvements in the northwest portion of
the project site near the projection of UND consist of a parking
lot and landscaping, which are not subject to further
Alquist-Priolo review, no significant impacts would occur from
faulting in the northern portion of the project site.
Geology and Soils 4.4
Draft Environmental Impact Report 4.4-5
A photo lineament suggested a possible east – west trending fault
crossing the southern portion of the project site, which could
potentially impact the proposed Meditation Hall. Petra excavated
and logged a 130-foot long, 23-foot deep, fault trench in the
location of the Meditation Hall. The fault trench began at the edge
of the Xavier Building and trended south toward East Monrovia
Place. The trench was logged by Petra Geotechnical, and the results
are included in Appendix D to the DEIR. The trench was examined by
the City’s geotechnical consultant, RMA, and by the State
Geologist. All reviewers concluded that no faulting was observed in
the trench. Therefore, no significant impacts associated with
faulting would impact the southern portion of the project site and
the construction of the proposed Meditation Hall. While no impacts
associated with active faulting are anticipated, given the location
of the project site within a region of high seismicity, Mitigation
Measure 4.4-A-1 is included to further minimize potential impacts.
Mitigation Measure 4.4-A-1 requires that a consulting geologist be
present during grading activities to observe excavations in order
to document the absence or presence of active faults and to make
recommendations regarding mitigation if active faults are found.
Impact 4.4.A. – ii – Impacts to life and property resulting from
seismically induced strong ground shaking will be less than
significant with implementation of the City’s existing standards.
The project site is subject to strong seismic ground shaking, as
are virtually all properties in Southern California. The project
site is located along the northern edge of the San Gabriel Valley,
which is an alluvial filled valley bounded by the Sierra Madre
Fault Zone and San Gabriel Mountains on the north, the Puente Hills
on the south, the Covina and Indian Hills on the east, and by the
Raymond Basin on the west. According to the Preliminary
Geotechnical Review prepared by Petra Geotechnical, Inc., the type
and magnitude of seismic hazards that may affect the project site
are dependent on both the distance to causative faults and the
intensity and duration of the seismic event. Although the
probability of primary surface rupture is low, ground shaking
hazards posed by earthquakes occurring along regional active faults
do exist and should be taken into account in the design and
construction of the proposed facilities within the project site.
The proposed additions to the project site are subject to the
seismic design criteria of the California Building Code (CBC). The
City of Azusa adopted the 2013 CBC and included it in its Municipal
Code (Section 14-51) on January 1, 2014 (Ordinance No. 13-03, § 3).
The 2013 California Building Code contains seismic safety
provisions with the aim of preventing building collapse during a
design earthquake, so that occupants would be able to evacuate
after the earthquake. A design earthquake is one with a two percent
chance of exceedance in 50 years, or an average return period of
2,475 years. Adherence to these requirements will reduce the
potential of building collapse during an earthquake, thereby
minimizing injury and loss of life. Although structures may be
damaged during earthquakes, adherence to seismic design
requirements will minimize damage to property within the structure
because the structure is designed not to collapse. The CBC is
intended to provide minimum requirements to prevent major
structural failure and loss of life. In addition, the proposed
project is subject to the seismic design criteria recommendations
within the Preliminary Geotechnical Report prepared by Petra
Geotechnical, Inc. Adherence to the recommendations within the
geotechnical report and existing regulations will reduce the risk
of loss, injury, and death; impacts due to strong ground shaking
will be less than significant. Impact 4.4.A – iii-iv – Impacts to
life and property resulting from seismic-related ground failure,
including liquefaction and landslides will be less than
significant. A landslide is defined as the movement of a mass of
rock, debris, or earth down a slope. There are five categories of
landslide movement types: slide, flow, fall, topple, and spread.
According to the Landslide Inventory Map of the Azusa 7.5-minute
quadrangle, no landslide activity has been documented on the
project site.2 Impacts related to landslides will be less than
significant. As discussed herein, liquefaction is a phenomenon that
occurs when soil undergoes transformation from a solid state to a
liquefied condition due to the effects of increased pore-water
pressure. This typically occurs where susceptible soils
(particularly the medium sand to silt range) are located over a
high (less than 50 feet from the surface) groundwater table.
Affected soils lose all strength during liquefaction and foundation
failure can occur. According to the Seismic
Geology and Soils 4.4
4.4-6 Dhammakaya International Meditation Center
Hazards Zone Map for the Azusa 7.5-minute quadrangle, the southern
and northwestern portions of the project site lie within an area
that has been mapped as being potentially susceptible to
earthquake-induced liquefaction.3 According to the Preliminary
Geotechnical Review prepared by Petra Geotechnical, Inc.,
groundwater was encountered at the bedrock contact in several of
the borings drilled in the southern portion of the project site.
This occurrence is likely the result of surface waters which have
percolated down to the relatively impermeable bedrock surface. A
groundwater surface at a depth of approximately 20 feet was
established within the alluvial sediments to the south of the
project site. Saturated alluvial soils were encountered at depths
of 15 feet below preexisting grades during the grading for Rosedale
Avenue adjacent to the northwest property line. Compressible
materials must be removed below the proposed buildings and either
bedrock or dense alluvial soils should be exposed. In the northern
portion of the site in the vicinity of the Organizational Housing,
removals on the order of 5 feet are anticipated. In the southern
portion of the site, in the vicinity of the Meditation Hall, low
density artificial fill and alluvial soils were encountered to
depths of 15 feet and greater below surface. These materials
exhibit low density and are therefore considered unsuitable to
support the proposed structures and will be excavated and
re-compacted as part of remedial grading operations. All earthwork
and grading will be performed in accordance with all applicable
requirements of the grading and excavation codes of the City of
Azusa and is subject to CBC standards including requirements for
excavation, fill, footings, retaining walls, and pier and pile
foundations. Typical design features required by the CBC are ground
improvement or foundational design. Ground improvement includes
removal and recompaction of low-density soils and removal of excess
groundwater. Given that the site is relatively flat, and that all
loose compressible soils will be removed during remedial grading
activities in compliance with CBC requirements, liquefaction is not
considered a potential hazard with respect to the project site.
Impacts related to life and property resulting from seismic-related
ground failure, including liquefaction and landslides will be less
than significant Impact 4.4.B – Impacts related to a geologic unit
or soil that is unstable or that would become unstable as a result
of the project will be less than significant with implementation of
the City’s existing standards and regulations. Landslides and
Liquefaction. Impacts related to landslides and liquefaction are
discussed in impacts 4.4.A.iii-iv herein. Impacts related to
landslides and liquefaction will be less than significant. Lateral
Spreading. Lateral spreading of the ground surface during a seismic
activity usually occurs along the weak shear zones within a
liquefiable soil layer and has been observed to generally take
place toward a free face (i.e. retaining wall, slope, or channel)
and to lesser extent on ground surfaces with a very gentle slope.
Due to the absence of any channel within or near the project site
and the required excavation and re-compaction of the artificial
fill materials and loose natural soils (alluvium) underlying
portions of the project site, the potential for lateral spread
occurring on the project site is considered to be negligible. The
project is required to be constructed in accordance with the CBC.
The CBC includes a requirement that any City-approved
recommendations contained in the soils report be made conditions of
the building permit. All earthwork and grading will be performed in
accordance with all applicable requirements of the grading and
excavation codes of the City of Azusa and in compliance with all
applicable provisions of the 2013 CBC. Compliance with existing CBC
regulations would limit hazard impacts arising from unstable soils
to less than significant levels. Collapse and Subsidence.
Subsurface investigations found that the project site is underlain
by artificial fill (af/Caf), alluvium (Qal), and bedrock of the
Topanga Formation (Tt). As mentioned herein, artifical fill
(af/Caf) is non-engineered fill material that was placed on the
project site as part of original site development. These materials
were derived from near site sources and consist of a varied
admixture of sand, silt, and clay. The fill materials, where
encountered, were found to be dry to moist and loose to medium
dense and varied in depth from a few feet to 5+ feet.
Geology and Soils 4.4
Draft Environmental Impact Report 4.4-7
Compacted/engineered fill (Caf) has been placed in the southeast
portion of the site and along the north, east and south property
boundaries. These materials were placed as part of the grading
construction of the Rosedale Development and consist primarily of
silty sand and sand that is moist and dense. The placement of these
fill sections was observed and tested by the geotechnical
consultant for the Rosedale project. Review of grading reports
indicates that these materials were compacted and moisture
conditioned to generally accepted industry standards. Alluvium
(Qal)/older Alluvium (Qol) are alluvial soils that were deposited
in tributary canyons by streams and/or surface flow have been
mapped in the southern, western and eastern portions of the project
site. Based on the boring logs and grading exposures, alluvium
consists of loose to medium dense, dry to saturated sand and silty
sands with occasional gravel and cobble sized material. Alluvial
thicknesses vary from a few feet to as much as 35+ feet in the
southern portion of the property. Older alluvium was observed in
the recently excavated trench. These materials are similar to the
younger alluvial soils but are dense and well consolidated. The
artificial fill materials and alluvium exhibit low density and are
therefore considered unsuitable to support the proposed structures
and will be excavated and re-compacted as part of remedial grading
operations. All earthwork and grading will be performed in
accordance with all applicable requirements of the grading and
excavation codes of the City of Azusa and in compliance with all
applicable provisions of the 2013 CBC. Given that all loose
compressible soils will be removed during remedial grading
activities in compliance with CBC requirements, collapse and
subsidence are not considered potential hazards with respect to the
project site. Impacts will be less than significant.
Mitigation Measures 4.4-A-1 During site grading activities, the
consulting geologist must observe excavations to document the
absence or presence of active faults; and to make recommendations
regarding mitigation of active faults if found.
Level of Significance with Mitigation Incorporation Impact 4.4.A.i
will be less than significant with the incorporation of Mitigation
Measures 4.4.A-1. Impacts 4.4.A.iii-iv and Impact 4.4.B will be
less than significant without the need for mitigation
incorporation.
References 1 California Department of Conservation. California
Geological Survey. Earthquake Zones of Required Investigation.
Earthquake
Fault Zones. Azusa Quadrangle. Released November 6, 2014. 2
California Department of Conservation. California Geological
Survey. Landslide Inventory Map of the Azusa Quadrangle. J.A.
Treiman and C.J. Wills. December 2007. 3 California Department of
Conservation. California Geological Survey. Earthquake Zones of
Required Investigation. Seismic Hazard
Zones. Azusa Quadrangle. Released March 25, 1999.
Draft Environmental Impact Report 4.4-8
Geology and Soils 4.4
Geology and Soils 4.4