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Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
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Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
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Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
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Page 31
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 32
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 33
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 34
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 35
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 36
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 37
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 38
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 39
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 40
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 41
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 42
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 43
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 44
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 45
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 46
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 47
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 48
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 49
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 50
Suggested Citation:"2 A Primer on Earthquake-Induced Soil Liquefaction." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
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Page 51

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2 A Primer on Earthquake-Induced Soil Liquefaction Key Findings and Conclusions  Earthquake-induced soil liquefaction phenomena result from the interaction of soil particles and porewater under the shear stress and shear strain reversals induced by earthquake shaking.  Evaluation of liquefaction hazards requires assessment of the susceptibility of soils to liquefaction and of the potential for liquefaction triggering and its consequences given anticipated ground motions.  Not all soils are susceptible to liquefaction, but saturated, granular soils such as sands, silty sands, low-plasticity silts and gravels have liquefied in past earthquakes.  Liquefaction triggering depends on the level of ground shaking and the density and initial effective stress of the soil; other factors such as soil type and age and the presence of pre-earthquake shear stresses also influence the potential for triggering.  Liquefaction can result in damage to buildings, bridges, lifelines, and other constructed facilities. In many cases, the damage is a consequence of permanent deformation of the liquefied soil.  In most cases, the in situ resistance of a soil to liquefaction currently cannot be predicted accurately using laboratory testing. As a result, procedures developed to predict liquefaction triggering and consequences rely on field observations documented in liquefaction case histories. These include both empirical and numerical analyses, which need to be validated and calibrated using field data before they can be considered reliable. This chapter describes, in general terms for the less technical of this report’s readers, the basic behavior of liquefiable soils under cyclic loading, including the effects of material type, load amplitude and duration, soil density, initial effective stress, initial shear stress, and age of the soil deposit. The susceptibility of various soil types (e.g., sand, gravel, and fine-grained soils) to liquefaction triggering, the engineering measures of soil resistance to triggering, and the primary consequences of triggering are also described. These descriptions are intended to provide only a general overview; many topics in this chapter are expanded on in the chapters that follow. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW 29

30 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES MECHANISM OF LIQUEFACTION Understanding liquefaction triggering and its consequences requires an understanding of soil and fluid mechanics. Liquefiable soils are frictional materials; their resistance to deformation is influenced by how tightly the individual particles are pressed together. The term “effective stress” (Box 2.1) is used to describe the stress associated with these interparticle contact forces. Effective stress is defined as the external total stress on the soil less the internal porewater pressure in the soil. It has long been recognized that soil behavior is governed by the effective stress. Soils under high effective stress are generally stiff and strong, and soils under low effective stress can be soft and weak. Dry soil subjected to monotonic (i.e., unidirectional) shear loading may either decrease or increase in volume depending on its initial density and initial effective confining stress and on the levels of induced shear strain. Initially loose soil typically will tend to contract (i.e., decrease in volume and become denser) as it is sheared. Under the same confining pressure, initially dense soil will first contract but then dilate (i.e., increase in volume and become looser) as it is sheared. On the other hand, when initially loose or dense dry soils are subject to repeatedly reversing (cyclic) shear stress, they tend to decrease in volume, or contract, regardless of whether initially loose or dense. When saturated soils are unable to contract due to water in the soil pores, the water pressure increases. If it reaches the level of the initial effective stress, liquefaction can be triggered. The extent to which a soil tends to contract or dilate during shearing dominates liquefaction behavior. Soils may be saturated (i.e., pore spaces are filled with water) or unsaturated (i.e., pore spaces are filled with both water and air or just air). The degree of saturation (i.e., the fraction of the pore space occupied by water) dominates liquefaction behavior. The degree of saturation must be close to 100% for a soil to liquefy (Okamura and Soga, 2006; Yegian et al., 2007). BOX 2.1 Effective Stress Soils are an assemblage of individual particles (commonly with dimensions between 0.001 and 75 mm) that transmit applied stresses either through contact forces at particle contact points (see Figure 1) or by a fluid (i.e., liquid or gas) pressure within the void space between the particles. It has been shown that normal forces at particle contact points control volume change, stiffness, strength, compressibility, and other important characteristics of the soil (Bishop, 1959; Skempton, 1960), but it is impractical to account for all of those individual contact forces. Instead, an averaged applied normal stress is determined by assuming that the components of the forces acting normal (i.e., perpendicular) to a given plane have a well-defined average over the multiparticle scales for which the theory is to be applied. Then an averaged interparticle normal stress (referred to as effective stress in soil mechanics and by geotechnical engineers) is calculated by subtracting the fluid pressure within the void space from the averaged applied normal stress. Effective stress may be explained as follows. By force equilibrium, the total stress due to the weight of soil particles, water, and any externally applied loads acting normal to any plane must be balanced by the sum of the pore-fluid pressure and the interparticle forces acting on the plane divided by the area of the plane. The effective stress principle states that the effective (interparticle) normal stress governs the engineering behavior of soil. Annunciation of the effective stress principle by Terzaghi (1925) is generally considered to be the birth of modern soil mechanics. In a saturated soil, the pore-fluid pressure and the porewater pressure are the same. Therefore, the effective stress may be written as ’ =  – u, where ’ is the effective normal stress,  is the total normal stress, and u is the porewater pressure (see Figure 2). If the porewater pressure increases while the total PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

A PRIMER ON EARTHQUAKE-INDUCED SOIL LIQUEFACTION 31 stress remains constant, the effective stress decreases. This reduction of effective stress is central to triggering liquefaction. FIGURE 1 Interparticle contact forces in a saturated soil. Soil stiffness and strength are governed by these interparticle forces, but evaluating their magnitude is prohibitively expensive. Therefore, the average normal stress on the plane of interest in the soil is used to represent the interparticle forces. COURTESY: S. Kramer. ’ =  – u FIGURE 2 Illustration of the effective stress principle. The effective stress on the horizontal plane is the average intergranular normal stress on the plane. It is equal to normal stress on the horizontal plane minus the porewater pressure. COURTESY: S. Kramer. Behavior Under Monotonic Shear Loading Monotonic shear loading refers to load conditions under which the shear stress (or principal stress difference or deviator stress) or shear strain in the soil increases without change of direction. Any increase in static load (e.g., as a result of foundation or embankment construction) will induce monotonic shear loading in a soil. A decrease in soil strength (e.g., the strength loss associated with liquefaction) can result in monotonic shearing deformation of a soil subjected to an initial static shear stress (e.g., in sloping ground or beneath a foundation or an embankment). When monotonically sheared, initially loose and dense dry sands deform in fundamentally different manners with respect to their stress-strain and volume change (drained) behavior. Figure 2.1 illustrates the stress-strain and volume change behavior (change in void ratio) of a dry soil in loose and dense states confined under the same effective stress. The loose soil can be seen to contract (decreasing void ratio) and the dense soil to dilate (increasing void ratio) with increasing shear strain. Note that at large shear strain, neither loose nor dense soil continues to change in volume (as shown in Figure 2.1b). At large strains both loose and dry soil reach the same constant void ratio, termed the critical void ratio (ec). The critical void ratio is related uniquely to the effective confining pressure via the critical void ratio (CVR) line (often called the critical state line or steady- state line), as shown in Figure 2.2. The CVR line marks the boundary between loose and dense states; that is, between where the soil demonstrates contractive and PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

32 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES dilative responses under monotonic shear loading. All soils tend to move toward the CVR at large monotonic shear strains. Loose soil (i.e., soil with a void ratio greater than the CVR) is contractive; its void ratio or effective stress must decrease to reach the CVR line. Dense soil (i.e., soil with a void ratio lower than the CVR) is dilative; its void ratio or effective stress must increase to reach the CVR line. If loads are applied to a saturated soil when there is not sufficient time for the water to move in or out of voids (e.g., the load is applied rapidly or the soil has a low permeability), the loading is said to be undrained. Since the water in a saturated soil is nearly incompressible, the tendency for volume change will cause changes in porewater pressure and effective stress. In loose soils, the tendency for contraction leads to increased porewater pressure and reduced effective stress. In dense soils, the tendency for dilation leads to reduced porewater pressure and increased effective stress. Saturated soils with initial states that plot well above the CVR line are highly contractive and, after reaching a peak shearing resistance at low strains, will generate high pore pressures with concurrent large reductions in effective stress. Such soils can often shear to large strains with low shearing resistance. Dense soils with initial states that plot well below the CVR line will be dilative with decreasing porewater pressures and increasing effective stress; these soils mobilize high shearing resistance at large strains when saturated due to decreasing pore pressure and concurrent increase in effective stress. Soils of intermediate density (i.e., having density between what is considered loose and dense) exhibit initially contractive behavior and mobilize a peak shearing resistance at low strains followed by a reduction in shearing resistance. After mobilizing the peak shearing resistance, however, these soils will dilate until a constant shearing resistance is reached. These three conditions are illustrated in Figure 2.3 and are discussed in more detail by Castro (1969). A steady state of deformation, in which a soil shears with constant volume, constant effective stress, constant shearing resistance, and constant strain rate, has been postulated to be a unique function of void ratio and effective stress for a given soil (Castro and Poulos, 1977; Poulos, 1981). Steady-state deformation can be represented graphically on a plot of void ratio versus effective stress as a steady state line (SSL). To simplify the explanation, the SSL can be considered nearly equivalent to the critical state CVR line. The degree to which a soil contracts or densifies, therefore, depends on its state (i.e., its density and effective stress conditions) relative to the SSL. The state parameter ( is a measure of how far the initial state of a soil plots above (or below) the SSL and can be represented mathematically as  = e – ess where e is void ratio and ess is the void ratio under steady-state conditions. The value of  reflects the influence of both mass density and effective stress on soil behavior, and it is an index of a soil’s tendency to change volume. This tendency for volume change is, in turn, related to the tendency for pore pressure and strength to change in a saturated soil when sheared. Soils with positive state parameters plot above the SSL (see Figure 2.4b) and exhibit levels of contractiveness (i.e., the tendency for pore-pressure increase and strength loss when saturated) that increase with increasing state parameter value. Soils with negative state parameters plot below the SSL and tend to be dilative (i.e., have a tendency for pore-pressure decrease and strength gain when saturated) upon monotonic shearing. Under cyclic loading, soils with both PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

A PRIMER ON EARTHQUAKE-INDUCED SOIL LIQUEFACTION 33 positive and negative state parameters will tend to generate increasing pore pressure and can liquefy under sufficiently strong loading. The transition from contractive to dilative behavior observed in soils of intermediate density occurs at the phase transformation point (Ishihara et al., 1975); that is, the point at which the soil transitions from contractive to dilative behavior. The phase transition point represents a point of minimum local shearing resistance. Undrained loading beyond the phase transformation point produces dilation, a decrease in pore pressure, a higher effective confining pressure, and, consequently, higher strength at large strains. The point at which a local minimum shearing resistance is observed at moderate strain levels has been called the quasi-steady state of deformation (Alarcon-Guzman et al., 1988). The shearing resistance at very large strain (i.e., at the point where the shearing resistance no longer changes) is referred to as the ultimate steady state (USS) strength (Yoshimine and Ishihara, 1998) to distinguish it from the quasi-steady state strength. As the void ratio of a soil decreases with increasing effective stress, the relationship between the quasi-steady state line (QSSL) and ultimate steady state line (USSL) of a soil changes with effective stress. Figure 2.5 illustrates the relationship between the quasi-steady state and ultimate steady state at different initial effective stress levels (Yoshimine and Ishihara, 1998). The figure also illustrates the relationship of the isotropic consolidation line (ICL), the line describing the relationship between void ratio and the effective confining pressure for one particular soil prior to shearing the specimen. Because the ICL is typically flatter than the USSL, the state parameter increases, and the soil becomes more contractive and potentially more liquefiable, as the effective confining pressure increases. FIGURE 2.1 (a) Stress-strain and (b) stress-void ratio curves for loose and dense sands at the same effective confining pressure subject to drained loading. Note that the loose and dense soils eventually converge on the same shear resistance and void ratio as shear strain increases. SOURCE: After Kramer, 1996. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

34 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES FIGURE 2.2 Void ratio versus minor principal effective stress for deformation at constant volume (i.e., the critical void ratio, or CVR, line) relationship separating dilative soil states from contractive soil states. SOURCE: Kramer, 1996. FIGURE 2.3 Schematic illustration of the undrained behavior of a sand in initially loose, dense, and intermediate density states in undrained monotonic loading: (a) stress-strain behavior, and (b) effective stress path behavior. Point marked with “x” indicates phase transformation point. Note that the ultimate shear resistance of all three specimens falls on the “failure line” on the effective stress path plot, though at different values of mean effective normal stress. SOURCE: After Kramer, 1996. (a) (b) FIGURE 2.4 (a) Behavior of initially loose and dense specimens under drained and undrained monotonic loading conditions (with graphical illustration of state parameter for loose specimen) and (b) steady state line with contours of equal state parameter as defined in Equation 2.1. SOURCE: Kramer and Stewart, in preparation. COURTSEY: S. Kramer. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

A PRIMER ON EARTHQUAKE-INDUCED SOIL LIQUEFACTION 35 FIGURE 2.5 Stress-strain and stress path behavior for an element of soil consolidated to three different initial states for testing. The paths show the transition of a soil from dilative to contractive as the effective confining stress increases (after Yoshimine and Ishihara, 1998). Note that stress scales are not equal for the three cases on the right-hand side; the ultimate steady strength would be highest for Case C and lowest for Case A. Behavior Under Cyclic Shear Stress Reversal Earthquake loading may be characterized by repeated shear stresses of fluctuating intensity, with the added characteristic that the direction of the applied shear stress reverses. Shear stress reversal is an important characteristic of the earthquake loading, as both loose and dense soils tend to contract at small induced shear strains and therefore generate positive excess porewater pressures when subject to shear stress reversal (Martin et al., 1975). In steep slopes subjected to weak shaking, the combination of high static shear stress and low cyclic shear stress can keep the shear stress from reversing direction under undrained conditions, thereby preventing liquefaction triggering. Cyclic shear stresses may also exceed the shear capacity of soils in sloping ground and trigger a flow slide if the soil is loose of the critical state or contractive. The stress-strain and pore-pressure generation behavior of sands subject to cyclic loading have been investigated through laboratory tests. The stress-strain and stress path behaviors of a clean sand1 subjected to undrained cyclic simple shear loading are described in Box 2.2. With repeated stress reversal loading cycles, all liquefiable soils exhibit increases in excess pore pressure (i.e., porewater pressure in excess of the initial steady-state pressure) and associated effective vertical stress decreases. In liquefaction analyses, the excess pore pressure is often expressed as a pore pressure ratio (ru)—defined as the ratio of the excess pore pressure to the initial vertical effective stress. The pore-pressure ratio is an index of how close a soil is to liquefaction. Prior to cyclic loading, ru = 0.0. As excess porewater pressure is generated, and thus 1 According to accepted engineering standards (ASTM, 2011), “clean” sands refer to those with less than 5% fines content (i.e., grains smaller than 0.075 mm). PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

36 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES ru increases, the soil softens and the cyclic shear strain amplitude increases. The rate at which softening and strain levels increase accelerates as liquefaction is approached. In some cases— e.g., for sites with no initial static shear stress on the horizontal plane and for loose soil sites where cyclic loading induces stress reversal—ru can approach and may actually reach 1.0 (at which point the effective stress is zero). For sites without an initial static shear stress on the horizontal plane, ru approaches and may actually be equal to 1.0 at liquefaction. For sites with an initial static shear stress on the horizontal plane not subject to stress reversal, flow may occur (or initiate) at values of ru less than 1.0. The softening and deformation produced by flow are driven by the static shear stresses. Effect of Initial Effective Stress At a given density, steady-state and state parameter concepts predict that contractiveness increases with increasing effective stress. Increased effective stress in the field is usually accompanied by increased density (i.e., smaller void ratio), however, which tends to reduce contractiveness. The cumulative effect of increased effective stress on contractiveness, therefore, depends on whether the steady state line is steeper or flatter than the consolidation curve. Typically, as shown in Figure 2.5, the steady state line is somewhat steeper than is the consolidation curve. As a result, the cyclic stress ratio required to cause liquefaction decreases with increasing initial effective confining pressure. Figure 2.6 presents the results of cyclic simple shear laboratory tests performed without an initial static shear stress on the horizontal plane on soils of the same density but different effective confining pressures (Vaid and Sivathayalan, 1996). The CSR values in Figure 2.6 are all for the same number of cycles to liquefaction. These curves illustrate the decrease in the cyclic shear stress ratio required to trigger liquefaction in a given number of cycles with increasing vertical effective stress (i.e., increasing depth in the ground). PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

A PRIMER ON EARTHQUAKE-INDUCED SOIL LIQUEFACTION 37 FIGURE 2.6 Curves presenting the cyclic stress ratio required to cause liquefaction (i.e., cyclic resistance ratio, CRR) in simple shear tests after 10 cycles of loading illustrating the effect of increasing initial vertical effective stress (and therefore depth) on liquefaction resistance (after Vaid and Sivathayalan, 1996). BOX 2.2 Undrained Cyclic Laboratory Testing of Liquefiable Soil The behavior of liquefiable soils such as clean sands subjected to cyclic shear loading with stress reversals has been evaluated extensively in the laboratory for more than 40 years. Results of such tests generally are not considered reliable for evaluating the liquefaction potential of soils in the field because it is difficult to recover representative undisturbed samples. The tests have led, however, to better understanding of the mechanics of liquefaction and the factors that affect liquefaction potential. The figure below illustrates the behavior of a soil subjected to harmonic undrained loading in cyclic simple shear without initial static shear stress on the horizontal plane. More detailed discussion of laboratory testing methods can be found in Chapter 5. The following behaviors are illustrated:  Pore pressure increases steadily (and vertical effective stress decreases) each cycle until the effective stress path reaches the point at which the soil transitions from a contractive phase of response to a dilative phase—often referred to as the phase transformation point (at cycle 21 in Figures a.2 and a.4). Phase transformation points have been observed to fall on a unique line, often referred to as the phase transformation line, for a given soil. The stiffness of the soil decreases steadily and mildly up to the phase transformation point (Figure 1).  With continued loading, the vertical effective stress becomes very low as the effective stress path approaches the origin. It then becomes many times greater as the soil dilates and the effective stress path moves up the failure envelope (Figure 2).  The soil stiffness also increases and decreases within individual loading cycles (Figure 1) when the phase transformation line is crossed. The stiffness becomes low when the effective stress is low and then increases significantly as the soil dilates and the effective stress increases. This results in banana-shaped stress-strain loops characterized by concave-up curves as the shear strain reaches its peak within a loading cycle.  The maximum increase or decrease in the shear strain during cyclic loading, the shear strain amplitude, is small until the effective stress reaches a very low value (Figure 3). The shear strain PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

38 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES amplitude then increases quickly from cycle to cycle, even though the effective stress path does not change. The rate at which the stiffness increases with increasing vertical effective stress at the end of each banana-shaped stress-strain loop controls the strain amplitude in the later cycles of the test. These general patterns of behavior inform our understanding of the behavior of soil in the field as liquefaction is approached. They provide the physical basis for constitutive stress-strain modeling of the behavior of liquefiable soils discussed in Chapters 7 and 8. (1) (2) (1) (1) (1) (1) (3) (4) (1) (1) (1) (1) FIGURE Illustration of behavior of typical loose liquefiable soil under cyclic loading. (1) Illustration of the stress-strain behavior of an initially stiff soil (indicated by the steep curve). (2) Illustration of the effective stress path during the test. Excess pore pressure increases with more cycles and the effective vertical stress decreases. (3) Illustration of shear strain development prior to cyclic loading, ru = 0.0. After 22 cycles of loading, the effective stress path reaches the origin (i.e., ru  1.0), at which point the effective stress is nearly zero and liquefaction has been triggered. (4) Effective stress reduction. Numbers indicate cycle number (after Kramer and Stewart, in preparation). COURTESY: S. Kramer. Liquefaction Susceptibility Liquefaction susceptibility refers to the potential for liquefaction to be triggered in a soil. Not all soils are susceptible to liquefaction, and the degree of susceptibility may depend on the soil grain size and plasticity.2 The level of earthquake shaking and the in situ state of the soil 2 The plasticity of soil is the ability of a unit of soil to undergo permanent deformation under stress without cracking. Plasticity is also an index of how changing the water content in a soil changes its behavior. Clayey soils, generally considered more plastic than silty or sandy soils, can be deformed without cracking over a wide range of water content. Tests to quantify plasticity have been standardized (ASTM, 2010). The level of plasticity is defined using a plasticity index. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

A PRIMER ON EARTHQUAKE-INDUCED SOIL LIQUEFACTION 39 (e.g., density, soil fabric) at the time of shaking determine whether a susceptible soil will liquefy in a particular earthquake. Factors that influence liquefaction susceptibility are reviewed briefly and qualitatively herein. Determining susceptibility to liquefaction is discussed more comprehensively in Chapter 4. Liquefaction-susceptible soils generally are found within a narrow range of geologic environments and soil age (Youd, 1991). Fluvial (i.e., river transported), colluvial (i.e., gravity transported, such as at the base of a cliff), aeolian (i.e., wind transported), and other processes that sort and deposit soils by grain size can result in the deposition of loose soils susceptible to liquefaction. Liquefaction has occurred in alluvial-fan, beach, estuarine, and other deposits. Many constructed earth fills (i.e., man-made soil deposits) are susceptible to liquefaction, including, in particular, deposits created by hydraulic filling, which can replicate river and stream processes. Primary compositional factors that influence the liquefaction susceptibility of soils include grain size and, for the finer-grained fraction, the plasticity. Grain size and plasticity are used to identify soils not considered to be liquefiable (e.g., those that are fine grained and of medium to high plasticity). Criteria for establishing whether or not a fine-grained soil is susceptible to liquefaction are also addressed in Chapter 4. FACTORS AFFECTING LIQUEFACTION POTENTIAL AND ITS CONSEQUENCES A number of factors affect the potential for initiation of liquefaction. The next sections include discussions on the effects of load amplitude, soil type, initial shear stress, shear strain amplitude, age, and hydraulic conditions. Effect of Load Amplitude, Duration, and Density The excess pore pressure generated in a monotonic loading test on loose, saturated sand increases with increasing shear stress, so the excess pore pressures under cyclic loading should be expected to increase at a faster rate as the amplitude of the cyclic loading increases. An increase in the duration of strong shaking should also be expected to increase excess pore- pressure generation and the potential for triggering liquefaction. Figure 2.7 shows the relationship between the number of uniform stress cycles required to induce liquefaction for a soil at three different initial relative densities. The cyclic shear stress required to trigger liquefaction (i.e., the CSR defined in Box 2.1), the number of cycles required to trigger liquefaction, or both, for a given CSR can both be seen to increase with increasing density. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

40 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES FIGURE 2.7 Cyclic strength curves used to characterize the resistance of a soil to liquefaction, showing the influence of soil density (expressed as the relative density, Dr) on the variation of the number of uniform stress cycles required to trigger liquefaction with the amplitude of applied loading (after De Alba, P., H.B. Seed, and C.K. Chan. 1976. Sand liquefaction in large-scale simple shear tests. Journal of the Soil Mechanics and Foundations Division 102(GT9):909–927. With permission from ASCE.). Effect of Soil Type on Soil Behavior Under Cyclic Loads The potential for a soil to liquefy during undrained cyclic loading is influenced by the extent to which the soil skeleton tends to contract under such conditions. Loose, granular, and nonplastic soils such as sands, gravels, and nonplastic silts can have high levels of contractiveness. Therefore, porewater pressures within them can build, the soil softens and the cyclic shear strain amplitude increases, and liquefaction will be triggered. Plastic fine-grained soils may exhibit some softening under cyclic loads, but their tendency to contract is less, and excess pore pressure may stabilize at values such that the effective stress does not approach zero (and the soil does not liquefy). Effect of Initial Shear Stress No shear stress exists on horizontal planes at level-ground free-field sites (e.g., zero slope) prior to earthquake shaking. In sloping ground, however, at level sites near slopes (e.g., riverbanks and embankment toes), and at level sites on which structural loads are imposed, there will be an initial shear stress on the horizontal plane prior to earthquake shaking. That initial horizontal shear stress can affect the rate of pore-pressure generation. Experimental studies (Seed and Harder, 1990; Boulanger, 2003b) have shown that pore pressures are generated more quickly in highly contractive (e.g., very loose) soils in the presence of an initial horizontal shear stress than when no initial horizontal shear stress is present. In less contractive or dilative soils (e.g., medium dense to dense), however, the presence of an initial horizontal shear stress tends to suppress pore-pressure generation. Laboratory studies have shown that the three-dimensional stress state and the shear strain level can also affect pore-pressure development (Boulanger, 1990; Kammerer, 2002; Cetin and Bilge, 2015). PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

A PRIMER ON EARTHQUAKE-INDUCED SOIL LIQUEFACTION 41 Effect of Shear Strain Amplitude Different specimen preparation procedures produce soil specimens with different fabrics, and differences in stress-based measures of cyclic shearing resistance observed in tests on specimens reconstituted using different procedures (Ladd, 1974; Mulilus et al., 1977) led investigators to explore the use of cyclic strain amplitude as a measure of loading, or demand, for liquefaction. Data compiled by Dobry and Ladd (1980) suggest that the relationship between pore pressure and cyclic shear strain is relatively insensitive to initial variations in the soil fabric. That finding is consistent with the finding of Silver and Seed (1971): shear-induced volume change is more closely related to cyclic shear strains than to cyclic shear stresses. Figure 2.8 shows the relationship between pore-pressure ratios and shear strain amplitude developed by Dobry and Ladd (1980) from testing of reconstituted specimens of two soils, one of which was prepared using three different specimen preparation methods (see Chapter 5). FIGURE 2.8 Data for specimens prepared by different methods to Dr = 60% and subjected to 10 cycles of uniform strain amplitude, suggesting a unique relationship between cyclic shear strain amplitude and pore-pressure ratio (after Dobry, R., and R.S. Ladd. 1980. Discussion to “Soil liquefaction and cyclic mobility evaluation for level ground during earthquakes,” by H.B. Seed and “Liquefaction potential: Science versus practice,” by R.B. Peck. Journal of the Geotechnical Engineering Division 106(GT6):720– 724. With permission from ASCE). Effect of Age Empirical observations (e.g., Youd and Hoose, 1977; Youd and Perkins, 1978) indicate that earthquake-induced liquefaction is more common in younger (e.g., Holocene age) soil deposits than in older (e.g., Pleistocene age and older) soil deposits. Experimental investigation of age effects is complicated by the difficulty simulating changes on geologic timescales, but combinations of experimental and field data have shown that older soils are generally more resistant to pore-pressure generation than younger soils are (Troncoso et al., 1988; Seed, 1979; Arango et al., 2000; Robertson et al., 2000). Mechanisms of aging effects are not understood completely, but they may involve particle and particle group reorientation, increases in in situ lateral stresses, grain interlocking, chemical precipitation (cementation), and internal stress arching (Mitchell and Solymar, 1984; Mitchell, 1986; 2008; Schmertmann, 1991) operating on PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

42 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES different timescales. It should be noted that aging effects can be “erased” when a soil’s structure or fabric is disturbed or destroyed by events. Thus, the “age” of a soil deposit from a liquefaction standpoint can be reset to zero by the triggering of liquefaction, even if large strains do not develop (Andrus et al., 2009; Hayati and Andrus, 2009; Maurer et al., 2014a). The influence of soil age on liquefaction potential is discussed in more detail in Chapters 3 and 4. Hydraulic Considerations Redistribution and dissipation of excess porewater pressure can strongly influence the behavior of a soil as it approaches and undergoes liquefaction. Excess porewater pressure generally develops in a spatially variable manner, producing hydraulic gradients that cause water to flow from locations of high hydraulic head (i.e., locations of high excess porewater pressure) toward locations of low hydraulic head (locations of lower excess porewater pressures).3 Water from areas of high excess porewater pressure will often flow toward the ground surface. Sand boils, the most common surficial evidence of liquefaction, may form when water and entrained soil particles are ejected at the ground surface from a shallow liquefied soil layer (see Figure 2.9). Excess porewater pressure migration and dissipation can be impeded if a low-permeability soil layer lies above a liquefiable soil layer. In such cases, upward flowing porewater can build up below the base of the low-permeability layer, and the density of the soil just below the low- permeability layer can become very low, as illustrated in Figure 2.10. In extreme cases, a water interlayer may form at the base of the low-permeability layer. Because porewater migration may continue after the ground has stopped shaking, the most critical condition with respect to the stability of a liquefied soil profile may exist after shaking ends. FIGURE 2.9 Schematic illustration of sand boil and water interlayer formation (after Sims and Garvin, 1995). Water and entrained material may migrate from a liquefying layer through non-liquefying layers to the surface. 3 The presence of an ambient artesian groundwater pressure could increase the rates and extents at which excess porewater pressures develop, redistribute, and dissipate. A correlation between such pressure to liquefaction hazards has been made between artesian pressures and liquefaction during the 2010 M 7.1 earthquake in New Zealand (e.g., Cox et al., 2012), although other researchers are less certain about the contributions (e.g., Tayler et al., 2012). PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

A PRIMER ON EARTHQUAKE-INDUCED SOIL LIQUEFACTION 43 FIGURE 2.10 Illustration of void redistribution in which the volume of the liquefiable layer remains constant but density changes within the layer. Note the loosening of the sand layer below the overlying low-permeability clay layer, leading to a decrease in the strength of this layer (modified from NRC, 1985). EARTHQUAKE LOADING Liquefaction is triggered when the loading applied to a liquefaction-susceptible soil exceeds the liquefaction resistance of the soil. Therefore, the evaluation of liquefaction triggering requires establishing consistent measures of earthquake loading and liquefaction resistance. Early investigations of liquefaction resistance were based on laboratory testing to develop cyclic strength curves like those shown in Figure 2.7. Earthquake loading was, and still is, based on representative values of the amplitude of the uniform cyclic shear stress and the number of uniform loading cycles for the design earthquake. This approach requires development of procedures to establish a representative earthquake-induced shear stress and the number of uniform loading cycles as a function of earthquake magnitude, as discussed in the next section. Ground Motion Intensity Measure For most liquefaction analyses, the intensity of the design ground motion is characterized by the peak cyclic shear stress and an earthquake magnitude. The peak cyclic shear stress, in turn, depends on the design peak ground acceleration (PGA). As noted in Box 1.3, 65% of the peak cyclic shear stress ratio is typically used as the value of the equivalent uniform cyclic load. This reduced value of the peak cyclic shear stress was introduced to characterize the earthquake loading to account for the fact that the peak cyclic shear stress occurs only once during the earthquake and that all of the other load cycles during the earthquake are of lower amplitude. With subsequently introduced magnitude scaling factors (MSFs) that account for durational effects of the earthquake shaking, however, the use of the reduced (i.e., 65%) peak cyclic shear stress ratio is no longer needed; nevertheless, it has been retained for historical purposes (i.e., use of a different percentage reduction in peak cyclic shear stress ratio will result in either a shift in the triggering correlation or a commensurate offset in the MSFs such that the cyclic stress ratio adjusted to an M 7.5 will remain unchanged). PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

44 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES Sources of the Design Intensity Measure The earthquake intensity measure used in design (e.g., the peak ground acceleration) is typically established based on a ground motion hazard analysis that considers the potential for strong ground shaking at the site from recognized earthquake sources. Seismic hazard analyses can be performed deterministically or probabilistically. Deterministic analyses consider a particular scenario event, often taken as the largest assessed magnitude earthquake occurring at the shortest source-to-site distance. Probabilistic analyses account for the distributions of magnitude, source-to-site distance, and levels of ground shaking, and they compute the probabilities of exceeding different ground shaking levels based on the probabilities of all combinations of magnitude, distance, and ground shaking level. Project-specific hazard analyses may be performed for certain projects, such as for critical infrastructure or in situations where lives may be at risk. Alternatively, published results of seismic hazard analyses from a recognized authoritative source (e.g., from the U.S. Geological Survey National Seismic Hazard Mapping Program)4 may be used. The selection of an appropriate earthquake magnitude for use with a probabilistic design acceleration is discussed in Chapter 4. SOIL RESISTANCE TO LIQUEFACTION The evaluation of liquefaction resistance has evolved in the last 40 years from laboratory- based methodologies to a field-based framework, largely because obtaining high-quality samples from the field for laboratory testing is difficult. The field-based framework characterizes liquefaction resistance using a “representative” in situ test parameter from a test such as the standard penetration test (SPT; described in Box 2.3), the cone penetration test (CPT; described in Box 2.4), and shear wave velocity (Vs) testing (described in Box 2.5). Methods for evaluating soil resistance to liquefaction are discussed in detail in Chapter 4. 4 See, for example, http://geohazards.usgs.gov/deaggint/2008/documentation.php. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

A PRIMER ON EARTHQUAKE-INDUCED SOIL LIQUEFACTION 45 BOX 2.3 Standard Penetration Test For many years the standard penetration test (SPT) was the most common test for evaluating the in situ consistency of soil, and it may still be very common in many areas. In the SPT, a standardized thick- walled sampling tube (see Figure 1) is placed at the bottom of a borehole and driven into the ground by means of a slide hammer with a standard mass and falling distance (see Figure 2). The number of hammer blows required to push the tube into the bottom of the borehole for three six-inch intervals is recorded. The SPT blow count, N, is the value of the sum of the number of hammer blows to penetrate the second and third six-inch intervals. SPT procedures are codified in ASTM standard test methods D1586-11 and D6066-11 (ASTM International, 2011a,b). The standard allows for a certain amount of discretion in the methodology, however, which results in significant variability in SPT blow count measurements. An advantage of the SPT is that it provides a soil sample that can be used for visual classification, grain-size analysis, Atterberg limits determination, and other soil index properties. Nevertheless, in addition to the uncertainties associated with blow count values, SPT measurements can be limited if taken at widely spaced depth intervals (e.g., 0.75 to 1.5 meters). Features such as thin potentially liquefiable soil layers may be missed. Because of the lack of true standardization of the SPT, energy transmitted from the SPT hammer to the tip of the SPT sampler is highly variable. Another major factor influencing blow counts is the effective overburden pressure in the ground at the depth where the blow count is measured. As a result, stress- corrected blow counts (N1), energy corrected blow counts (N60), and stress and energy-corrected blow counts (N1)60 are often used in engineering practice (Kavazanjian et al., 2011). The various corrections to measured SPT resistances are described in Chapter 4. FIGURE 1 Cross section of the SPT Sampler. Note that the figure shows a sampler to be used with liners to create a uniform inside diameter of 1.375 in. (34.9 mm). SOURCE: Coduto et al., 2011 (adapted from ASTM D1586). PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

46 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES FIGURE 2 Illustration of one setup for a Standard Penetration Test (SPT) employing a hammer, rope, and cathead per ASTM D 1586-11. Note that the figure shows a “donut” hammer with an average efficiency of 40%, requiring an energy correction factor of 0.67 to convert the measured value to N 60, the energy corrected blow count. An additional correction for overburden pressure is required to convert N 60 to (N1)60, the stress and energy corrected blow count often used in SPT correlations. SOURCE: Coduto et al., 2011 (adapted from Kovacs et al., 1981). BOX 2.4 Cone Penetration Test The cone penetration test (CPT) is an in situ test used to characterize the stratigraphy and properties of soils. In the CPT, a standardized conical tipped probe is pushed into the ground at a rate on the order of 20 millimeters per second. The force per unit area on the tip of the probe (referred to as the tip resistance) and the force per unit area on the sleeve behind the tip (referred to as the sleeve resistance) are measured as the probe is advanced into the ground. The CPT is codified in ASTM standard D5778-12 (ASTM International, 2012). The CPT provides a nearly continuous (typically 5-10 cm resolution) record of tip and sleeve resistance with depth that can be interpreted to identify thin layers and small-scale fluctuations often missed during drilling, sampling, and SPT testing. CPT probes can be outfitted with pressure transducers (piezocone or CPTu tests) to measure porewater pressures and with geophones (seismic CPT) to measure shear wave velocities. A recent development is a cone-shaped sampler that can be pushed by a CPT rig that can recover small soil samples for evaluation of soil index properties (e.g., grain size). Cone penetrometers cannot be used in very stiff or dense soils, and they are considered unreliable in soils containing gravel-sized and larger particles due to the difficulties in penetrating them. The CPT is commonly used to investigate liquefaction case histories, but because the CPT does not provide soil samples and the CPT-pushed cone-shaped sampler is not widely in use, some drilling and sampling may still be required as part of a subsurface investigation. Figure 1 shows schematic cross sections of typical electrical cone penetrometers, with two possible locations of the porous filter. The most common CPTu probe is the one shown on the bottom half of the figure, with the pore pressure transducer at the shoulder between the conical tip and the probe sleeve. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

A PRIMER ON EARTHQUAKE-INDUCED SOIL LIQUEFACTION 47 FIGURE 1 Cross-section of typical electrical cone penetrometers. SOURCE: Mayne, 2007. BOX 2.5 Shear Wave Velocity Measuring shear wave velocity (Vs) is another test used to characterize soils in situ. Vs refers to the speed at which a shear wave (one type of wave generated by an earthquake) propagates through the ground. The speed of wave propagation depends on the density of the soil, the directions of wave propagation and particle motion, and the effective stresses in those two directions. Vs, by convention, refers to the shear wave speed at very small amplitudes. Vs is related to the shear modulus of the soil at small strain, Gmax, and the mass density of the soil, , by the equation: 𝑉 𝑆 = √𝐺 𝑚𝑎𝑥 /𝜌 where is equal to the total unit weight of the soil divided by the acceleration of gravity. Vs measurements can be economical and non-invasive (i.e., there may not be a need to penetrate the ground surface to make the measurement). The latter capability can be beneficial if soil profiles contain inclusions (i.e., gravel or cobble inclusions) that can make testing difficult or even prohibit SPTs and CPTs. There are many Vs measurement techniques, including downhole measurements (ASTM International, 2014a), cross-hole measurements (ASTM International, 2014b), suspension logging (Nigbor and Imai, 1994), and non-invasive methods (Stokoe and Santamarina, 2000). Because non-invasive Vs tests do not provide soil samples, however, some drilling and sampling may still be required as part of a subsurface investigation. CONSEQUENCES OF LIQUEFACTION The triggering of liquefaction can lead to various consequences to soil and site properties as well as to physical damage, economic loss, and potential loss of life. Some factors that influence the main consequences of liquefaction are described below. Descriptions of liquefaction consequences and procedures for predicting those consequences are presented in Chapters 6 and 7. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

48 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES Alteration of Ground Motion The response of buildings, bridges, pipelines, and other elements of infrastructure underlain by liquefiable soils will be strongly influenced by how liquefaction affects the characteristics of the ground surface motions. Ground motion frequency change often occurs suddenly when liquefaction is triggered as a result of the rapid reduction in shear stiffness at high pore-pressure ratios. The frequency content change can be so abrupt and obvious that it is easily noticed in ground surface accelerograms. While the onset of liquefaction typically reduces the intensity of the ground surface acceleration (as compared to ground surface acceleration had the site not liquefied), the lower- frequency components of the ground motion may increase subsequent to liquefaction triggering. Even relatively low accelerations at low frequencies can produce very large, long-period displacements following triggering—sometimes referred to as ground oscillations. Cases of damaging ground oscillation have been reported in a number of earthquakes (Youd and Keefer, 1994; Youd, 2003; Holzer and Youd, 2007). Lateral Spreading and Flow Sliding Initial shear stresses under sloping ground conditions can drive permanent lateral deformations of the ground subsequent to liquefaction. These lateral deformations are referred to in practice as lateral spreading and flow sliding. When initial shear stresses are less than the residual strength of the soil (i.e., the shearing resistance of the liquefied soil at large strain), but the seismically induced stresses exceed the residual strength, lateral displacement builds up incrementally during the earthquake and ceases when the cyclic loading stops and results in lateral spreading. When initial static shear stresses are greater than the available strength of the liquefied soil (e.g., in ground with a greater slope or lower soil density), lateral deformation will not only build up during the earthquake but also continue to accumulate after the shaking stops. This mechanism is referred to as a flow slide. While lateral spreading deformations may be limited enough to be accommodated in engineering design, displacements caused by flow sliding typically cannot be accommodated in design and must be addressed through mitigation measures. Both lateral spreading and flow sliding are often accompanied by cracking of the ground, separation between the ground and embedded structures, ejection of soil and water from the ground cracks, and surficial settlement (e.g., formation of a graben) due to the lateral movement. When the level of lateral spreading is small, these effects may be subtle and hard to detect. When earthquake shaking generates nonuniform excess pore pressures, hydraulic gradients may develop that can, in turn, cause increased excess porewater pressures in other areas. This would then result in a decrease in the effective stress and strength. This strength decrease can be important for stability after the shaking has stopped. Figure 2.11 illustrates this phenomenon for an embankment, but it can also occur beneath natural and man-made slopes. Slope failures associated with this type of pore pressure redistribution have occurred from minutes to hours after strong earthquake ground shaking (Seed et al., 1973; Ishihara, 1984). When porewater flow is impeded by low-permeability soils, effective stresses can become extremely low and void ratios can become extremely high. Such void redistribution effects can, in extreme cases, lead to the development of water interlayers beneath the low-permeability zones. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

A PRIMER ON EARTHQUAKE-INDUCED SOIL LIQUEFACTION 49 FIGURE 2.11 Mechanism of delayed embankment failure due to redistribution of excess pore pressure following earthquake shaking: (a) high pore pressure levels are generated below center of the embankment where initial, static shear stresses are low; (b) following shaking, pore pressures increase and effective stresses decrease in areas of high static shear stress (after Kramer and Stewart, in preparation). COURTESY: S. Kramer. Liquefaction-Induced Settlement Volume loss due to the dissipation of excess pore pressure in the liquefied soil during and after an earthquake generally results in ground surface settlement. Settlement can cause damage to structures if the settlement is not uniform beneath the structure. Settlement can also leave a gap beneath the pile-supported structures, cause distress to utilities buried in the soil, and alter drainage grades at a site. The total volumetric strain (volume loss) following liquefaction is not explained entirely by pore-pressure dissipation and the reconsolidation of the soil. This is because cyclic loading of the soil beyond the liquefaction triggering changes the soil structure such that the soil is more compressible than it was in its original state. Procedures for predicting volumetric strain in saturated sands are discussed in Chapter 7. The settlement of structures underlain by liquefiable soil is also affected by local interaction between the structure and the soil, and it can differ significantly from the “free-field” settlement (Unutmaz and Cetin, 2012; Bray and Dashti, 2014; Bray et al., 2014). Settlement of the ground surface can also be associated with lateral soil movement and with the extrusion of soil (ejecta) from beneath the ground surface. Damage to Foundations Liquefaction-induced damage to structures supported on shallow foundations is typically caused by differential vertical and horizontal displacements. This is particularly true for structures supported on isolated spread footings or lightly reinforced mats, as is often the case for residential and light commercial structures (Cubrinovski et al., 2011; Bray et al., 2014). Structures supported on stiffer mat foundations may also be subject to distortion, settlement, and tilting when underlain by liquefiable soils. Embedded structures supported on mats or shallow PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

50 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES foundations may also be subject to damaging lateral loads and displacements associated with lateral spreading and flow sliding. Liquefaction-induced damage to deep foundations is associated typically with lateral spreading and flow sliding. Lateral movement of the ground can induce lateral loads on pile caps and bending moments and shear forces in the deep foundation elements. The magnitude of these loads depends on the stiffness of the structure-foundation system and restraint provided by non- liquefiable soils above and below the liquefied zone (see Figure 2.12). Settlement of liquefied soil during and following an earthquake can lead to negative skin friction (i.e., downdrag loads) on pile foundations as discussed in Chapter 7, although reports of damage to deep foundations due to downdrag from the settlement of liquefied soil are rare. FIGURE 2.12 Effects of lateral spreading of a liquefied soil with a non-liquefied crust on pile displacement, d, and bending moment, M: (a) short embedment and thin crust, (b) deep embedment and thin crust, (c) deep embedment and thick crust. The magnitude of the moment induced on the pile depends on the thickness of the crust, the thickness of the liquefied layer, and the magnitude of the lateral displacement; the worst case scenario is a thick crust laterally displacing above a thin liquefied soil layer, as the thin liquefied layer localizes the bending in the pile within a narrow zone. COURTESY: S. Kramer. Damage to Retaining Structures Bulging or tilting of retaining walls, sliding of the wall, and damage to structures on or behind the wall may result if liquefaction-induced increases in earth pressure and decreases in lateral resistance are not anticipated in design. In the 1995 Hyogo-ken Nanbu (Kobe, Japan) earthquake, many gravity quay wall structures at the Port of Kobe settled, tilted, and slid laterally due to a combination of increased lateral pressure, reduced lateral resistance, and bearing capacity failure beneath the wall structures (see Figure 2.13). These deformations destroyed gantry cranes that ran on rails along the tops of the walls and made the berths unserviceable, resulting in significant economic loss to the port (see Chapter 1). PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

A PRIMER ON EARTHQUAKE-INDUCED SOIL LIQUEFACTION 51 (a) (b) FIGURE 2.13 Damage to container cranes caused by liquefaction-induced lateral spreading of earth- retaining structures in the 1995 Hyogo-ken Nanbu (Kobe, Japan) earthquake. Liquefaction-induced damage rendered most of the berths in the port unserviceable, leading not only to physical damage but also to loss of business that continues to impact the port to this day. SOURCE: NISEE, Pacific 5 Earthquake Engineering (PEER) Center, the University of California, Berkeley. 5 See https://nisee.berkeley.edu/elibrary/Image/K0124. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

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Earthquake-induced soil liquefaction (liquefaction) is a leading cause of earthquake damage worldwide. Liquefaction is often described in the literature as the phenomena of seismic generation of excess porewater pressures and consequent softening of granular soils. Many regions in the United States have been witness to liquefaction and its consequences, not just those in the west that people associate with earthquake hazards.

Past damage and destruction caused by liquefaction underline the importance of accurate assessments of where liquefaction is likely and of what the consequences of liquefaction may be. Such assessments are needed to protect life and safety and to mitigate economic, environmental, and societal impacts of liquefaction in a cost-effective manner. Assessment methods exist, but methods to assess the potential for liquefaction triggering are more mature than are those to predict liquefaction consequences, and the earthquake engineering community wrestles with the differences among the various assessment methods for both liquefaction triggering and consequences.

State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences evaluates these various methods, focusing on those developed within the past 20 years, and recommends strategies to minimize uncertainties in the short term and to develop improved methods to assess liquefaction and its consequences in the long term. This report represents a first attempt within the geotechnical earthquake engineering community to consider, in such a manner, the various methods to assess liquefaction consequences.

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