State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences (2021) / Chapter Skim
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1 Introduction
1 Introduction
Pages 13-26
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From page 13... ...
However, liquefaction assessment can be challenging. The factors that govern liquefaction are often difficult to measure or predict, and the results of liquefaction analyses in engineering practice are sometimes contradictory: while one approach might indicate low liquefaction potential, another might warrant ground improvement costing millions of dollars.
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From page 14... ...
SOURCE: Nisee/Peer, University of California, Berkeley. LIQUEFACTION HAZARDS Large regions of the conterminous United States are susceptible to earthquake shaking strong enough to cause liquefaction in water-saturated soils.
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From page 15... ...
leads to a transfer of the load that had been carried by the granular structure to the porewater filling the voids between individual soil grains, resulting in an increase in the porewater pressure. When the porewater pressure rises, the contact forces between the grains are reduced, the soil is more easily deformed, and, in the limiting case, the soil particles may lose contact completely with each other and go into suspension (see Figure 1)
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From page 16... ...
Damaging liquefaction has occurred during many subsequent earthquakes as well. In addition to the damage to the Lower San Fernando Dam described above, liquefaction induced by the M 6.6 San Fernando earthquake in 1971 caused lateral deformations of up to 1.5 meters on gently sloping ground over an area approximately 1,200 meters long and 270 meters wide (Youd, 1971; Holzer and Bennett, 2007)
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From page 17... ...
New Hampshire Newbury earthquake and sand dikes interpreted as evidence for preceding earthquakes in the past 4,000 years Oregon and Sand intrusions along the Columbia River, ascribed to the 1700 Obermeier and Dickenson Washington Cascadia earthquake (2000) ; Takada and Atwater (2004)
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From page 18... ...
STATE OF PRACTICE FOR LIQUEFACTION ASSESSMENT Across the United States, the state of practice for evaluating whether liquefaction can be expected and what its consequences may be varies greatly. Factors influencing the state of practice include the technical sophistication of the practitioners, their clients, and associated regulatory agencies; the size of a project and the severity of consequences given the occurrence of liquefaction; the type of project (e.g., dam, bridge, building, port and harbor facilities, or nuclear power plant)
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From page 19... ...
For example, the interrelationships between earthquake magnitude, distance, occurrence, and extent of liquefaction can be demonstrated by comparison of the liquefaction induced by an M 7.0 event centered approximately 40 km from Christchurch and the M 6.1 event that occurred directly beneath the city. The larger event released approximately 22 times more energy than did the smaller event, but liquefaction damage in Christchurch from the larger event was considerably less.
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From page 20... ...
Higher levels of sophistication may also be typical of liquefaction assessments for projects with major consequences given failure. Design guidance documents and documents that describe best practices for liquefaction assessment have been prepared for some specific classes of projects (e.g., for embankment dams [USBR, 2011]
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From page 21... ...
held two workshops on earthquake-induced soil liquefaction. The report issued following those workshops (Youd et al., 2001)
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From page 22... ...
. The "simplification" is to use the following equation, derived from a simple application of Newton's second law, to obtain a representative value of the CSR rather than to conduct a detailed site response analysis: 𝑃𝐺𝐴 𝜎 𝐶𝑆𝑅 = 0.65 × × × 𝑟 𝑔 𝜎 where PGA is the horizontal component of the peak ground acceleration at the site, g is the acceleration of gravity, rd is a factor that accounts for the nonrigid response of the soil column, σv is the initial total vertical stress in the ground, and σ'v is the initial vertical effective stress in the ground.
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From page 23... ...
Because many assessment approaches applied today are often modifications of past approaches, and because much if not most of the data employed in more contemporary approaches were collected prior to 1996, the committee both describes early methods and databases to determine the uncertainties associated with them and also looks at contemporary assessment approaches to determine how those uncertainties may have been mitigated, exacerbated, or compounded. The report addresses evaluation of earthquake-induced liquefaction only of naturally occurring soils and man-made fills composed of such soils.
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From page 24... ...
BOX 1.4 Statement of Task An ad hoc committee of the National Academies of Sciences, Engineering, and Medicine will solicit input from the technical community and critically examine the technical issues regarding liquefaction hazard evaluation and consequence assessment. The study will assess and evaluate • Sufficiency, quality, and uncertainties associated with laboratory and in situ field tests, case histories, and physical model tests to develop and assess methods for determining liquefaction triggering, and the resulting loss of soil strength and its consequences; • Methods to conduct and analyze laboratory and physical model testing and to collect and analyze field case history data to determine the triggering of liquefaction, and post-liquefaction soil behavior (e.g., strength loss, dilation, and hardening)
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From page 25... ...
Consideration of uncertainties becomes even more important as earthquake engineering practice more generally moves toward performance-based analysis and design.5 This report is organized as follows: • Chapter 2 provides a description of the phenomena associated with earthquake-induced soil liquefaction and the factors influencing them; • Chapter 3 discusses the sufficiency of the case history data on liquefaction and associated phenomena, including field case histories, and provides a critical assessment of those data; • Chapter 4 describes and assesses the simplified stress-based approach to predict the triggering (initiation) of liquefaction; • Chapter 5 assesses alternative approaches to liquefaction triggering assessment such as strain-based, energy-based, and computational mechanics-based approaches; • Chapter 6 describes the assessment of the post-liquefaction shear strength of soils; • Chapter 7 discusses empirical and semiempirical methods to evaluate liquefaction consequences; • Chapter 8 is a discussion of how computational mechanics can be used to predict liquefaction triggering and consequences; • Chapter 9 is a discussion of performance-based engineering methods for probabilistic evaluation of liquefaction susceptibility, triggering, and consequences; and • Chapter 10 provides a synthesis of committee recommendations for advancing the state of practice and state of the art for assessment of earthquake-induced soil liquefaction.
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From page 26... ...
26 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES Summaries of the key findings and conclusions derived from each chapter can be found in a box at the beginning of the chapter, beginning with Chapter 2. A more detailed explanation of each of the bulleted items in the key findings boxes is found within the text of the chapter.
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Key Terms
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