ML20151H126
| ML20151H126 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 04/12/1988 |
| From: | Grant W SHANNON & WILSON, INC. |
| To: | |
| Shared Package | |
| ML20151H012 | List:
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| References | |
| WA-601-1, NUDOCS 8808010153 | |
| Download: ML20151H126 (7) | |
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i WA-601-1 UNCERTAINTIES IN LIQUEFACTION HAZARD ANALYSES by W. Paul Grant, Shannon & Wilson, Inc. i INTRODUCTION i !
Major damage and property losses have' occurred during earthquakes as a result of i
! liquefaction or lateral spreading of the subsurface soils. These phenomena occur l l as an indirect result of earthquake ground shaking. Liquefaction is a phenomena l f in which a loose deposit of sand existing below the water table loses its internal I f shear strength when subjected to severe earthquake ground motions. Lateral ;
i spreading is essentially an extension of the liquefaction concept, applied to ,
conditions of a sloping ground surface. Thus lateral spreading is characterized 2
by the horizontal flow of liquefied soil toward an open channel or an open slope. !
, Liquefaction or lateral spreading may affect various systems or facilities. ,
j l Lifeline structures, such as water and sewer lines or gas lines, may be severed as
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a result of liquefaction or lateral spreading. Liquefaction or lateral spreading j i
may cause movements in bridge abutments which could result in bridge decks being j
crushed or falling from their supports. Liquefaction may affect buildings by 4
differential foundation settlement which could lead to distress of the 2
) superstructure. Finally, liquefaction of submarine slopes may result in tsunami-like waves which may damage coastal facilities similar to damage that occurred in >
the towns of Seward and Valdez, Alaska, during the 1964 Good Friday earthquake.
The implications of earthquake-induced ground failures may be far reaching.
Earthquake-induced ground failures which affect bridges may render these j
l Structures inoperative imediately following an earthquake and hinder emergency l l response teams such as fire fighters and ambulance crews. Liquefaction may also create life threatening situations if differential movements within a building ,
foundation results in a collapse of the structure, injuring its occupants.
l Furthermore, the effects of liquefaction or lateral spreading may extend far [
beyond the imediate response to the earthquake and encompass the economic ;
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i j recovery of community, requiring public funds for infrastructure repair and funds ;
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from the private sector to repair damaged buildings and plants. !
As earthquake-induced ground failures may have a significant impact upon '
1 comunities in the Pacific Northwest during a future earthquake, the purp:,,se of !
this paper is tu briefly discuss the current state of engineering practice in >
i evaluating these hazards, delineate areas of uncertainty in the analyses, and !
- recomend areas requiring further research studies. I HAZARD ASSESSMENT 1 .
I l There are three major factors which control the occurrence of liquefaction:
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) e Earthquake severity
! e High groundwater table i
) e Liquefiable soils i
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All three of the above factors must be simultaneously present for liquefaction to
( occur. These factors are typically evaluated by geotechnical engineers performing i f a liquefaction hazard analysis of a building site. Specifically, borings are l (
drilled at the building site for the purposes of (cundation design and j liquefaction analysis. Liquefaction potential is typically evaluated using {
simplified procedures which compare the strength of the soil as determined from the site borings with the strengths of soils at other locations which have t experienced liquefaction during previous earthquakes. Based upon this empirical assessment, the liquefaction potential of the site is evaluated and appropriate
} remedial measures are developed.
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Aside from individual studies at specific sites, various liquefaction j i
microzonation studies have been performed for other cities in the United States
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{ (Power and others, 1982; Roth and Kavaranjian, 1984). The end product of the j
studies is typically an area map indicating potentially hazardous artas of {
liquefaction. +
Research sponsored by the U.S. Geological Survey is currently being j conducted to construct similar maps for the Puget Sound area. The major l j
attraction of such microzonation maps is that they may be used by public agencies i
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WA-601-1 and others to provide a quick assessment of liquefaction potential over a wide
~( city area. While the specific nature and techniques used in these studies may vary among investigators, the results are typically the same in,that areas of fill or most recent alluvial deposits which exist in low-lying areas are assigned the highest liquefaction hazard potential. Areas having low liquefaction potential typically correspond with older and denser sediments that are located above the water table.
Based upon the simplified model of alluvial or fill soils in low-lying areas having the highest liquefaction potential, it would be concluded that major portions of populated areas in the Pacific Northwest would be at risk during an earthquake. Specifically, these would correspond to industrial areas along the Duwamish in Seattle, tide flat areas in Tacoma, the low-lying areas adjacent to the Sound in Olympia, and finally low-lying areas along the Columbia and Willamette Rivers in Portland.
Potential liquefaction within these areas brings up interesting questions affecting public policy. First, should new development be limited in these high
( risk areas? If buiding is not restricted, then should there be special or standardized studies to define the extent of liquefaction for each new building or should there be standardized procedures for mitigating the occurrence of liquefaction at these locations? Finally, should existing structures which have been designed and constructed without special consideration for liquefaction receive special retrofitting to mitigate this hazard?
UNCERTAINTIES OF ANALYTICAL PROCEDURES i i
I While liquefaction potential is routinely analyzed for building sites in the l Pacific Northwest, there are a number of factors or uncertainties which affect the
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results of the analyses or the recommendations provided by the engineering firms. l These factors may be categorized into uncertainties involving earthquake potential in the Pacific Northwest and non-standardized design procedures. ;
i One of the grettest factors affecting liquefaction evaluations conducted for sites in the Pacific Northwest is the uncertainty regarding the largest earthquake which
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could affect the region., Previously it was believed the largest earthquake that ;
could affect the Puget Sound area would be a magnitude 7.5 event (U.S. Geological Survey, 1975) centered i_n the Puget Lowland. The magnitude of this postulated !
j earthquake is generally consistent with historical earthquakes, the largest being !
, the magnitude 7.1 Olympia earthquSac of April 13, 1949 (SW-AA, 1978). However, research by Heaton and Kanamori (1984) has suggested that larger earthquakes, j associated with tectonic subduction off the coast of Washington, could affect western Washington and Oregon. Currently, the U.S.G.S. is sponsoring research to '
f investigate physical evidence for the past occurrence of such a large event I (Atwater,1987), c i
l The potential occurrence of a subduction zone earthquake in the Pacific Northwest j }'
would have a major impact upon liquefaction analyses. The potential occurrence of a subduction zone earthquake in the Northwest would imply that current earthquake f
l design standards are too low and should ce increased, Therefore, it is first i i
necessary to agree upon the hazard potential of a subduction zone in tne Pacific Northwest before any appropriate liquefaction design studies may be accomplished.
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{ ( The second major area involving uncertainties in liquefaction evaluation focuses i upon design standards. Uncertainties or non-standard analytical procedures within i this category would include:
s e Earthquake recurrence interval
! e Liquefaction analysis procedure j e Site assessment e Remedial treatment l e Retrofitting '
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j Aside from the issue of the potential for a subduction zone earthquake in the i
l Pacific Northwest, liquefaction analyses are also affected by the critiera which j
) establishes the design level earthquakes. Currently there are no standard or
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aCCepteo guidelines for establishing an earthquake return interval for
{ liquefaction analyses. The U.S. Navy typically uses an earthquake return i l
i interval of approximately 200 years for evaluating liquefaction poter.tial of their !
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WA-601-1 major facilities.
Guidelines from the Applied Technology Council typically k
recommend a design earthquake having a 500-year recurrence interval for the design of buildings. Therefore, based upon these two different agencies, there is a substantial difference on the definition of a design earthquake. Accordingly, a design earthquake selected for a liquefaction analysis by one agency may indicate unsatisfactory performance whereas an analysis performed using guidelines from another agency would indicate satisfactory site performance.
in addition to discrepancies between agencies in definitior,of a design earthquake, there are also discrepancies in procedures for evaluating liquefaction potential.
Liquefaction potential may be evaluated using empirical procedures which are based upon a correlation of soil properties determined from field testing with sites where liquefaction has occurred in prior earthquakes (Seed and Idriss, 1981) to more analytical procedures involving steady state analysis of soil behavior.
Aside from these two techniques, there are a range of other empirical and analytical procedures which are being used in the engineering field.
Thus, the analytical procedures for evaluation liquefaction potential may result in significantly different assessments of the liquefaction hazard.
i After having analyzed the site for its liquefaction potential, the geotechnical engineer may still exercise some latitude in judgment in evaluating the liquefaction hazard.
As an example, a shcIlow spread footing foundation system would not be appropriate for a building which is located upon near-surface soils which may liquefy.
However, this same foundation may provide satisfactory ,
performance if the zone of liquefaction is limited to a relatively thin layer located well below the base of the foundation. Thus, it is obvious there may be a wide range of opinions on site-specific hazard for soil conditions t etween these extremes.
Another area of non-standarized design procedures involves remedial treatmen addressing itquefaction potential.
Remedial schemes for addressing liquefaction potential could include soil densification by a number of different field techniques to supporting the structure on piling which transfers building load below the zone of liquefaction.
Thus, attendant with each of these remedial 1
procedures the solution. are uncertainties involving the design methodology and the adequ
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UA-601-1 A final item regarding uncertainties and design standards is the issue of retrofitting existing structures which were originally designed without consideration for potential liquefaction. This represents a major policy issue for public agencies that could affect the life safety and economic well being of a connunity. This area is further complicated by the fact that many existing structures have experienced major earthquakes in the Pacific Northwest, such as the magnitude 7.1 Olympia earthquake in 1949 witnout major damage. However, when one considers that the ground accelerations associated with this earthquake, particularly in Seattle, were relatively low (on the order of 0.10 g) and that a large subduction zone earthquake could result in much larger ground accelerations, the argument of past successful performance during prior earthquakes becomes less convincing.
HAZARD REOUCTION from the above discussion it is clear that there are a number of uncertainties that may affect the determination of liquefaction potential and furthermore the public safety and well being of a community. Some of these factors, such as !
l ; further research into evaluating the potential of a subduction zone earthquake in the Pacific Ncrthwest, can be directly addressed by scientific research. Other factors regarding non-standardization of analysis techniques or code procedures are less well defined in scientific terms and overlap into areas involving risk '
and public policy. Many of these items will be addressed in the future years by [
building officials and design agencies on the local and national levels. ,
I l Thus, it is our opinion that the imediate goal to reduce earthquake hazards from !
l liquefaction would be to establish the potential magnitude and recurrence interval i
for a subduction zone earthquake in the Pacific Northnest. Such research should '
l be substantiated with field evidence of the occurrence of such an event in recent geologic times. Another area of technical research would focus upon the j
j development of liquefaction hazard maps for major metropolitan areas in the j
Pacific Northwest. The major emphasis on these studies would be to delineate j
areas of liquefaction on a regional basis. The purpose of these hazard maps would be to aid the public and private sector in land use planning, building ,
] development, and planning for disaster response. Specifically, the maps could be used to locate projects out of high seismic risk areas or to plan for high j ( foundation costs for structures located within these areas.
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O WA-601-1 REFERENCES Atwater, B.F., 1987. Evidence for great holocene earthquakes along the outer coast of Washington state, Science Vol. 236, pp. 942-944 '
Heaton, T.H., ar,J Kanamori, H.,1984, Seismic potential associated with subduction in the northwestern United States, Bulletin of the Seismological Society of America, Vol. 74, No. 3, pp. 933-941.
j Power, M.S. , Dawson, A.W... Burger, V. , and Perman, R.C. ,1982, Evaluation of liquefaction susceptibility in the San Diego, California urban area, Report submitted to tre U.S. Geological Survey under Contract 14-08-001-19110, March.
1 Roth, R.A., and Kavazanjian, E., Jr., 1984, Liquefaction susceptibility mapping for San Francisco, California, Bulletin of the Association of Engineering Geologists. Vol. XXI, No. 4, pp. 459-478.
i Seed, H.B., and Idriss, I.M., 1981. Evaluation of liquefaction potential of sand 3
deposits based upon observations of performance in previous earthquakes, Proceedings, Session on Insitu Testing to evaluate liquefaction susceptibility, ASCE National Convention St. Louis, Missouri, October 26-30, preprint Vol.81-544
, Shannon & Wilson, Inc., and Agbabian Associates (SW-AA), 1978, Geotechnical and strong motion earthquake data from U.S. accelerograph stations, Vol. 4, Report to i the U.S. Nuclear Regulatory Commission.
4 U.S. Geological Survey, 1975, A study of earthquake losses in the Puget Sound :
l Washington area, U.S. Geological Survey Open-file Report /L-375.
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