Technical Memorandum
To:
Building Permit Applicants and Consulting Engineers
Subject: Seismic Liquefaction Hazard Evaluation Guidelines for Unincorporated Alameda
County
From:
Daniel Woldesenbet, P.E., Ph.D., County Engineer
Date:
January 9, 2006
1.0 INTRODUCTION
This memorandum presents Alameda County’s requirements for the completion of seismic
liquefaction hazard evaluations in accordance with the State of California’s Seismic Hazard
Mapping Act (SHMA). The requirements apply to projects (as defined in Section 3.0 below) in
the unincorporated areas of Alameda County, which are mapped by the State as liquefaction
Seismic Hazard Zones. The affected areas are delineated on the official Seismic Hazard Zone
maps prepared by the California Geologic Survey. The County requirements for seismic
liquefaction hazard evaluation are consistent with the SHMA and published state guidelines
(CDMG 1997). Reference is also taken from certain recommendations in the Southern
California Earthquake Center document (SCEC 2002).
Seismic slope stability hazards are addressed in the SHMA and in the Seismic Hazard Zone
maps, but are not within the scope of this document.
2.0 BACKGROUND
In response to the 1989 Loma Prieta Earthquake, the California legislature enacted the SMHA in
1990. The SHMA directed the State Geologist to prepare maps of Seismic Hazard Zones to
identify areas of the state suspected of having seismically-induced liquefaction or slope stability
hazards. In order to obtain a preliminary map or a building permit, any project to be constructed
within a mapped Seismic Hazard Zone must have a study that evaluates the site-specific seismic
hazard and its potential impact to the project, and that recommends mitigation measures as
appropriate.
The Seismic Hazard Zone maps have been prepared on United States Geological Survey (USGS)
topographic quadrangle base maps. Mapping is currently in progress for the San Francisco Bay
Area, and official maps are now available for San Francisco County and parts of Santa Clara and
Alameda Counties. The first maps to include portions of unincorporated Alameda County and
other areas under the jurisdiction of Alameda County were officially released in 2003. The rest
of the maps covering the County will be released as they are completed.
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This document presents a discussion of the evaluation requirements for seismic liquefaction
hazards, including methods of investigation, analysis and mitigation. It also addresses
alternatives to a full liquefaction study that may, in certain cases, be acceptable to the County for
some smaller projects such as new single family residences or second story additions to those
residences.
The text of the SHMA, the Seismic Hazard Zone maps and their accompanying reports, and
guidelines for implementing the studies required by the SHMA, are available at
http://gmw.consrv.ca.gov/shmp/index.htm.
3.0 SEISMIC HAZARD MAPPING ACT
3.1
Legislative Mandate
When the SHMA was adopted, it was the stated intent of the California Legislature “to provide
for a statewide seismic hazard mapping and technical advisory program to assist cities and
counties in fulfilling their responsibilities for protecting the public health and safety from the
effects of strong ground shaking, liquefaction, landslides, or other ground failure and other
seismic hazards caused by earthquakes.”
The State Geologist (i.e., the California Geological Survey [CGS], then known as the California
Division of Mines and Geology [CDMG]) was directed to compile maps identifying Seismic
Hazard Zones. The maps were to be submitted in preliminary form to a State review board and
all affected cities, counties, and state agencies for review and comment. Following a review
period, the State Geologist was to revise the maps, as appropriate, and then provide copies of the
official maps to each state agency, city, or county, including the county recorder, having
jurisdiction over lands containing an area of seismic hazard.
Once any official map is published, the SHMA requires that all cities and counties must
“…require, prior to the approval of a project located in a seismic hazard zone, a
geotechnical report defining and delineating any seismic hazard. If the city or county
finds that no undue hazard of this kind exists, based on information resulting from studies
conducted on sites in the immediate vicinity of the project and of similar soil composition
to the project site, the geotechnical report may be waived…. Each city and county shall
submit one copy of each approved geotechnical report, including the mitigation
measures, if any, which are to be taken, to the State Geologist within 30 days of its
approval of the report.
“In meeting the requirements of [the SHMA], cities and counties shall consider the
policies and criteria established pursuant to [the SHMA]. If a project's approval is not in
accordance with the policies and criteria, the city or county shall explain the reasons for
the differences in writing to the State Geologist, within 30 days of the project's approval.”
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The above paragraphs apply to any designated “project,” which is defined by the SHMA as
having the same meaning as in Alquist-Priolo Earthquake Fault Zoning Act (commencing with
Section 2621) 1 , except as follows:
“A single-family dwelling otherwise qualifying as a project may be exempted by the city
or county having jurisdiction of the project.” [Italics added]
Also,
“ ‘Project’ does not include alterations or additions to any structure within a seismic
hazard zone which do not exceed either 50 percent of the value of the structure or 50
percent of the existing floor area of the structure.”
However,
“Nothing in [the SHMA] is intended to prevent cities and counties from establishing
policies and criteria which are more strict than those established by the board.”
Alameda County believes that, where known or suspected geologic hazards exist, all occupied
structures 2 deserve the protections afforded by a geotechnical/geologic investigation. Therefore,
within the unincorporated areas of Alameda County, the requirements of the SHMA are
considered applicable to single-family residences and a seismic hazards study must be
performed. However, the County also recognizes that a full liquefaction investigation and
evaluation may be expensive for a relatively small project like a single family residence or a
second story addition to a single family residence, and for this reason this document discusses
alternative approaches that may be accepted by the County for certain projects.
1
Section 2621.6 reads as follows:
(a) As used in this chapter, "project" means either of the following:
(1) Any subdivision of land which is subject to the Subdivision Map Act (Division 2 (commencing with
Section 66410) of Title 7 of the Government Code), and which contemplates the eventual construction of
structures for human occupancy.
(2) Structures for human occupancy, with the exception of either of the following:
(A) Single-family wood-frame or steel-frame dwellings to be built on parcels of land for which geologic
reports have been approved pursuant to paragraph (1).
(B) A single-family wood-frame or steel-frame dwelling not exceeding two stories when that dwelling is
not part of a development of four or more dwellings.
(b) For the purposes of this chapter, a mobile home whose body width exceeds eight feet shall be considered to be a
single-family wood-frame dwelling not exceeding two stories.
2
See the Alameda County Grading Ordinance, Title 15, Section 15.36.320-C (known or suspected
geologic hazards), and the Building Code, Title 15, Section 15.08.400B
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3.2
Purpose of the Maps
The Seismic Hazard Zone maps are intended to assist cities and counties in fulfilling their
responsibilities for protecting the public from the effects of earthquake-triggered ground failure
as required by the SHMA, but the CGS cautions that the maps are not to be considered sufficient
to serve as a substitute for the geologic and geotechnical site investigations required under the
SHMA.
Each map has an accompanying “Seismic Hazard Zone Report” prepared by the CGS. These
reports present a discussion of the key analyses and data that went into the preparation of the
final Seismic Hazard Zone map, which is available on line at
http://gmw.consrv.ca.gov/shmp/index.htm. Each report also includes several maps overlain on
the same topographic quadrangle sheet. These present information regarding levels of ground
shaking and depth to groundwater and will be discussed further below.
Information regarding the general approach and recommended methods for preparing the map, is
presented in the CDMG’s Special Publication 118, Recommended Criteria for Delineating
Seismic Hazard Zone in California, which is also available on line at
http://gmw.consrv.ca.gov/shmp/index.htm.
Other liquefaction maps that include Alameda County are also available for information, but are
not directly related to the SHMA:
1. Preliminary Maps Of Quaternary Deposits And Liquefaction Susceptibility, Nine-County
San Francisco Bay Region, California: A Digital Database, Geology By Keith L.
Knudsen, Janet M. Sowers, Robert C. Witter, Carl M. Wentworth, and Edward J. Helley;
Digital Database By Carl M. Wentworth, Robert S. Nicholson, Heather M. Wright, and
Katherine H. Brown, available at http://geopubs.wr.usgs.gov/open-file/of00-444/.
2. Liquefaction Susceptibility in the Bay Area, by the Associate of Bay Area Governments
(ABAB), available at http://www.abag.ca.gov/bayarea/eqmaps/liquefac/liquefac.html
3.3
General Requirements
The County requires that liquefaction evaluations for sites within a Seismic Hazard Zone be
completed by a qualified geotechnical engineer or certified engineering geologist. The
evaluations should generally conform to the following existing guidelines:
1. Special Publication 117: Guidelines for Evaluating and Mitigating Seismic Hazards in
California (SP117). This document is published by the CDMG (now the CGS) and
provides general guidelines for liquefaction screening and investigations.
2. Recommended Procedures for Implementation of DMG Special Publication 117:
Guidelines for Analyzing and Mitigating Liquefaction Hazards in California. This
document is published by the Southern California Earthquake Center (SCEC), and
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provides detailed recommendations for the procedures that should normally be followed
in a liquefaction investigation.
The County recognizes that the state of the practice in liquefaction evaluation is still evolving
and that evaluation methods will change with time as research continues. It is the intent of the
County to update this document from time to time as significant advances are made in the
standard practice. However, professionals are advised to keep abreast of these developments and
modify their practice accordingly.
Both SP117 and the SCEC documents assume that qualified Professional Engineers or Certified
Engineering Geologists using appropriate professional judgment will carry out investigations.
Non-critical or rote implementation of the recommended procedures must not be used apart from
or in lieu of the exercise of professional judgment.
3.4
Other Considerations
The documents cited in the preceding section will be considered the normal guidelines that are
expected to be used in normal situations for projects within Liquefaction Seismic Hazard Zones.
When anything other than the recommended procedures are implemented, an explanation and
justification for the change should be included in the final Seismic Hazard report.
Geotechnical engineers and engineering geologists completing liquefaction evaluations should
note the following:
1. The normal recommended method for drilling to evaluate liquefaction potential is
mud rotary wash. Alternatively, hollow stem drilling may be implemented if
particular care is taken to lessen the likelihood of sand loosening or blowing into the
drill stem due to the hydrostatic pressure head in the sand being higher than the water
in the stem. This may generally be accomplished by maintaining the stem full of
water once groundwater has been encountered.
2. Note that the references in the SCEC liquefaction document cite Bartlett and Youd
(1995) for evaluation of lateral spreading; Youd et al. (2002) have published an
updated version of the appropriate equations, and these updated equations should be
used.
3. For analysis of liquefaction triggering at a site, it is recommended that the magnitudeweighted pseudo-peak acceleration (or “liquefaction PGA”) should be used. These
values are presented in the Liquefaction Opportunity maps included in each Seismic
Hazard Zone report. This approach is preferred because it is simpler to perform, is
easier to review, and is more rigorously correct than the more cumbersome approach
commonly used of determining a probabilistic peak ground acceleration (PGA), deaggregating to select a mode event to be used in the analysis, and applying a
magnitude correction factor to this event, as explained in the SCEC liquefaction
document.
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4.0 SMALL PROJECTS
As described above, Alameda County believes that owners and occupants of single family
residences and proposed second story additions to those residences deserve to have the same
protections from seismic hazards that have been mandated for larger Projects. The County also
recognizes that the costs for a full liquefaction evaluation may be costly relative to the scale of
the total project. For a new single family residence or a second story addition the following
minimal investigative scope, if recommended by a qualified geotechnical engineer or certified
engineering geologist, may be accepted in lieu of that described in SP117 and the SCEC
document.
•
As a minimum, one boring should be advanced to a depth of at least 20 feet. The
boring should be drilled as an auger boring at least until groundwater is encountered.
This enables a measurement of the groundwater level at the time of drilling. Once
groundwater is encountered, the drilling method may be switched to rotary wash, or
continued with a hollow stem auger. If the hollow stem drilling method is used after
groundwater is encountered, the stem should be filled with water during drilling to
provide a positive hydraulic pressure at the tip of the auger to resist loosening or
blowing out of sands. If an existing boring is located within reasonably close
proximity to the project and meets these criteria, the project geotechnical engineer
may use this boring if it is considered representative of conditions at the site.
The project proponents may, with the recommendation of the project geotechnical engineer and
based on subsurface information from at least one boring as described in the preceding
paragraph, propose to mitigate liquefaction using one of the following methods:
1. Shallow Spread Footings with Grade Beams
• Found all footings a minimum of 24 inches below grade
• Tie all isolated footings together with grade beams
• Continuous footings should have minimum of two No. 5 steel bars at top and
bottom
• Any slab-on-grade floor should be at least 5 inches thick and reinforced with No.
4 bars at 24 inches on center
• Any slab-on-grade floor should be doweled to footings with No. 4 bars at 24
inches on center
2. Use a reinforced concrete structural slab with a minimum thickness of 12 inches.
3. Use a post-tensioned slab foundation with a minimum thickness of 8 inches.
4. Use drilled pier foundation with all piers tied together with grade beams. (Driven
piles may be used if it is considered that vibrations during driving will not cause
damage to adjacent properties.)
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5. Beneath and for 5 feet beyond the footprint of the project, replace soils to a depth of 5
feet with properly engineered fill that is reinforced by inclusion of a minimum of two
layers of geogrids or woven geotextile fabric, as recommended by the project
geotechnical engineer.
6. Use an alternative method recommended by the geotechnical engineer that is equal to
or exceeds the mitigation methods listed above.
It is emphasized that the above alternatives are presented as minimum mitigation measures that
will normally be considered for acceptance by the County. The County does not make any
assurance regarding the seismic performance of any specific structure supported by one of these
alternatives at any specific site. The appropriate mitigation approach should be selected based
on evaluations by the project geotechnical engineer, engineering geologist, and/or structural
engineer in discussion with the project proponent.
5.0 ITEMS TO BE ADDRESSED IN SEISMIC HAZARD REPORT
General requirements for liquefaction evaluation reports are presented in SP117 and the SCEC
liquefaction document.
5.1 Items to be Addressed in Seismic Hazard Report
If one of the above small project alternatives is allowed and is selected in lieu of a complete
liquefaction investigation, the project consultants must still prepare a Seismic Hazard Report. At
a minimum the following items should addressed in the small project Seismic Hazard Report:
1. Discuss expected groundwater levels in the vicinity, including any available
information but including at least the Seismic Hazard Zone Report and any
information from the County database.
2. Discuss the potential for lateral spreading toward open faces, such as creek banks or
cut slopes. This discussion should include at least a mention of the historical
liquefaction sites found in the appropriate Seismic Hazard Zone Report.
3. Provide at least a qualitative discussion of the potential for seismically-induced
settlement.
4. Provide at least a qualitative discussion of the anticipated performance of the
structure in a design level earthquake due to shaking, spreading, settlement, and loss
of bearing capacity, as appropriate. This may in part be supported based on the
Ishihara criteria presented at the end of the SCEC liquefaction document. (This
discussion may be prepared by the Project structural designer.)
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5.2 Filing of Seismic Hazard Reports
Three copies of each Seismic Hazard Report must be submitted to the County for the following
uses:
1. First copy: to be kept in the County files for future reference.
2. Second copy: to be submitted to the County’s reviewer. Reviews will be based in part
on the criteria presented in this document and in the referenced documents.
3. Third copy: upon acceptance of the report by the County this copy will be submitted by
the County to the CGS, as required by law.
6.0 LIQUEFACTION HAZARD DATA SHEET
The County requests that each Seismic Hazard Report be accompanied by a completed
Liquefaction Hazard Data Sheet. A blank Liquefaction Hazard Data Sheet is included with this
document. In the future an electronic copy (in Microsoft Word Format) will be available from
the County’s web page.
7.0 REFERENCES
Alameda County. Building Code.
Association of Bay Area Governments, undated. Liquefaction Susceptibility in the Bay Area.
California Division of Mines and Geology, 1997. Special Publication 117: Guidelines for
Evaluating and Mitigating Seismic Hazards in California (SP117).
California Division of Mines and Geology, 1998. Maps of Known Active Fault Near-source
Zones in California and Adjacent Portions of Nevada.
California Division of Mines and Geology, 1999. Special Publication 118, Recommended
Criteria for Delineating Seismic Hazard Zones in California.
California Geological Survey, 2003a. Seismic Hazard Zone Report for the Hayward 7.5-Minute
Quadrangle, Alameda County, California.
California Geological Survey, 2003b. Seismic Hazard Zone Report for the Newark 7.5-Minute
Quadrangle, Alameda County, California.
California Geological Survey, 2003c. Seismic Hazard Zones [Map] for the Hayward 7.5-Minute
Quadrangle, Alameda County, California.
California Geological Survey, 2003d. Seismic Hazard Zones [Map] for the Newark 7.5-Minute
Quadrangle, Alameda County, California.
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CDMG: see California Division of Mines and Geology.
CGS: see California Geological Survey. (Formerly known as the California Division of Mines
and Geology.)
Jennings, Charles W., 1994. Fault Activity Map of California and Adjacent Areas with Locations
and Ages of Recent Volcanic Eruptions, California Division of Mines and Geology, Map
No. 6.
NCEER, 1997. Proceedings of the NCEER Workshop on Liquefaction Resistance of Soils,
Technical Report NCEER-97-0022, December.
SCEC: see Southern California Earthquake Center.
Southern California Earthquake Center, 1999. Recommended Procedures for Implementation of
DMG Special Publication 117: Guidelines for Analyzing and Mitigating Liquefaction
Hazards in California.
Southern California Earthquake Center, 2002. Recommended Procedures for Implementation of
DMG Special Publication 117: Guidelines for Analyzing and Mitigating Landslide
Hazards in California.
Youd, T. Leslie, Corbett M. Hansen, and Steven F. Bartlett, 2002. “Revised Multilinear
Regression Equations for Prediction of Lateral Spread Displacement,” in Journal of
Geotechnical and Geoenvironmental Engineering, Vol. 128, No. 12. December.
Youd, T. Leslie, I.M. Idriss, Ronald D. Andrus, Ignacio Arango, Gonzalo Castro, John T.
Christian, Richardo Dobry, W.D. Liam Finn, Leslie F. Harder Jr., Mary Ellen Hynes,
Kenji Ishihara, Joseph P. Koester, Sam S.C. Liao, William F. Marcuson III, Geoffrey R.
Martin, James K. Mitchell, Yoshiharu Moriwaki, Maurice S. Power, Peter K. Robertson,
Raymond B. Seed, and Kenneth H. Stokoe II, 2001. “Liquefaction Resistance of Soils:
Summary Report from the 1996 NCEER and 1998 NCEER/NSF Workshops on
Evaluation of Liquefaction Resistance of Soils,” in Journal of Geotechnical and
Geoenvironmental Engineering, Vol. 127, No. 10. October.
U.S. Geological Survey, 2000. Preliminary Maps Of Quaternary Deposits And Liquefaction
Susceptibility, Nine-County San Francisco Bay Region, California: A Digital Database,
Geology By Keith L. Knudsen, Janet M. Sowers, Robert C. Witter, Carl M. Wentworth,
and Edward J. Helley; Digital Database By Carl M. Wentworth, Robert S. Nicholson,
Heather M. Wright, and Katherine H. Brown. Open-File Report 00-444.
USGS: See U.S. Geological Survey.
Attachment: Liquefaction Hazard Data Sheet
Liquefaction Hazard Evaluation Requirements
January 9, 2006