APPENDIX E
Glossary

The definitions below of terms specific to liquefaction reflect typical usage in the geotechnical earthquake engineering. Some of the definitions include variants used in related fields.

Aging: Time-dependent changes in engineering properties of a soil. The agents of change may include consolidation, secondary compression, weathering, and cementation. Where resistance to liquefaction increases with time, aging may be reversed by liquefaction or another disturbance that effectively resets the geotechnical clock.

Amplification or deamplification of seismic waves: Modification of the amplitude and phase of seismic waves. Caused regionally by structures in Earth’s crust (e.g., amplification in basins that are filled with soft sediments) and locally by near-surface deposits (e.g., deamplification by a liquefied horizon).

Arias intensity (I_a): The integral of the square of the acceleration in an accelerogram over time, multiplied by π and divided by 2g, usually expressed in m/sec or cm/sec.

Becker hammer: Device used to measure penetration resistance in gravelly soils in situ.

Clay: Soil consisting mainly of particles finer than 0.002 mm to 0.005 mm, typically with some plasticity. The properties are determined in large measure by the type and amount of specific clay minerals present.

Cone penetration test (CPT): An in situ soil testing procedure in which a standardized rod with a conical tip is pushed into the soil at a constant rate. The resistance at the tip (q_c) and along a frictional sleeve (f_c) are measured continuously as the probe advances.

Consolidation: Reduction of soil skeleton volume and increase in density as water flows out of the pores and excess porewater pressure is dissipated.

Critical depth: The depth in a soil profile where liquefaction is presumed most likely to occur.

Critical layer: The layer in a soil profile where liquefaction is presumed most likely to occur.

Critical void ratio (e_c): The void ratio at which soils are neither dilative nor contractive at large shear strains.

Cross-hole wave velocity measurement: A testing procedure to measure primary or secondary wave velocities (p- and s-wave velocities, respectively) in which both the source and the receivers for a stress wave are located at the same depth in the soil, usually in two or more bore holes.

Cyclic resistance ratio (CRR): The capacity of a soil at a particular depth and state to resist liquefaction triggering. It is evaluated by processing field data from standard penetration tests (SPT), cone penetration tests (CPT), shear-wave velocity (V_s) measurements, or other tests. These measurements are usually calibrated, through case histories, to the minimum cyclic stress ratios (CSRs) at which surface manifestations of liquefaction were produced. Thus, the CRR is an estimate of the value of the CSR that can be sustained at a particular site.

Cyclic shear strain (γ_cyc): Shear strain induced in soil due to cyclic loading.

Cyclic stress ratio (CSR): The seismic demand induced at a particular depth in the soil, usually expressed as the average earthquake-induced cyclic shear stress (τ_cyc) divided by the initial vertical effective stress (σ^′_vo).

Damping: Dissipation of energy associated with the deformations in a dynamically loaded material or system. The critical damping ratio, commonly shortened to damping ratio (D or β), normalizes the damping in a material or system to the damping necessary to prevent oscillatory motion in free vibration. A damping curve relates the damping ratio to the amplitude of shear strain induced in the soil.

Depositional environment: The setting—such as a lake, a river channel, an alluvial fan, or an aeolian dune—in which sediments are laid down. The conditions can yield distinctive combinations of particle size and layering (sedimentary facies) that influence susceptibility to liquefaction (Bennett and Tinsley, 1995).

Downdrag: See Negative skin friction.

Down-hole wave velocity measurement: A testing procedure to measure primary or secondary wave velocities (p- and s-wave velocities, respectively) in which the source of a stress wave is located at the surface and the receiver is located at depth in the soil, usually in a borehole.

Drained conditions: Conditions in which the hydraulic conductivity (permeability) is so large or loading is so slow that any excess porewater pressures dissipate and do not contribute to the response of the soil.

Effective stress (σ^′): The normal stress in a soil element from which the porewater pressure has been subtracted. It represents the portion of the total stress that is transmitted by contacts between grains in the soil skeleton, but it is not equal to the contact stress between individual grains.

Effective stress path: The path of the effective stresses during loading or unloading of a soil sample as plotted in some specified stress space.

Effective stress principle: A basic tenet of soil mechanics holding that the volume change and strength behavior of a soil are governed by the effective stress rather than the total stress.

Excess pore pressure: Porewater pressure in excess of the value for hydrostatic or steady-state flow conditions. The excess pore-pressure ratio (r_u) is the excess porewater pressure divided by the initial vertical effective stress.

Fines content: The percentage by weight of the solid soil material consisting of particles finer than fine sand—usually taken as less than 0.075 mm.

Flow failure: Gravity-driven mass transport of soils in a fluid-like manner over distances commonly measured in tens or hundreds of meters.

Ground failure: Permanent differential ground movement capable of damaging infrastructure.

Ground oscillation: Cyclic movement of the ground due to earthquake shaking, usually felt and observed at the surface.

Holocene: An interval of geologic time spanning the past 11,700 sidereal years (http://quaternary.stratigraphy.org/definitions).

Hydraulic fill: Landfill emplaced by transporting sediment through pipes as a watery suspension and depositing the slurry at the site. The resulting deposit is commonly loose, sorted by grain size, and highly susceptible to liquefaction.

Intensity: A qualitative measure of the severity of earthquake shaking at a particular place. Determined from observations of the earthquake’s effects on humans, buildings, and the Earth’s surface. Compare with Magnitude, below.

Intensity measure: A quantitative measure of ground motion characteristics, such as peak ground acceleration or Arias intensity.

Isotropic consolidation line (ICL): Plot of effective volumetric stress and strain for a test in which the applied stress is isotropic.

Lateral spread: Distributed lateral extensional movements on gentle slopes or toward a free face. The extension is often marked by cracks at the ground surface. The movements are presumed to involve the entire soil mass above a zone of liquefaction. In some cases this zone may be a discrete horizon, while in others it may be distributed through thicker sedimentary deposits.

Liquefaction: The phenomena of seismic generation of excess porewater pressures and consequent softening and loss of strength of saturated granular soils, typically manifested by fluid-like behavior. The material is typically sand, less commonly silt or gravel.

Liquefaction feature: Geological evidence for liquefaction. May refer to features at the ground surface (sand boils, lateral spreads), below ground (dikes, which cut steeply across bedding; and

sills, which are commonly horizontal), or both. Sedimentologists ascribe most so-called liquefaction features to fluidization, in which water expelled from a liquefied soil entrains particles in its path and moves from regions of high to low hydraulic head. In liquefaction assessment this distinction has been made chiefly in the study of paleoliquefaction.

Liquefaction potential: A measure of the tendency of a particular soil to liquefy due to a given level of earthquake shaking.

Liquefaction susceptibility: In the mapping of liquefaction potential, as defined by Youd and Perkins (1978), the ability of a soil to liquefy irrespective of the level of earthquake shaking.

Magnitude (M): A quantitative measure of the relative size of an earthquake, irrespective of the observer’s location, often referred to as the moment magnitude (M_w). The moment magnitude scale does not saturate and, therefore, is a better measure for larger earthquakes than are other magnitude scales, which include the local, or “Richter,” magnitude, M_L. Compare with Intensity, above.

Magnitude scaling factor (MSF): A factor that adjusts the cyclic stress ratio (CSR) for durational effects, which correlate to earthquake magnitude. The adjustment is necessary because the standard liquefaction curves are based on earthquakes of M 7.5; in general, a larger earthquake lasts longer and has more cycles of loading, while a smaller earthquake has fewer cycles.

Modulus of elasticity (E): Elastic modulus relating longitudinal stress to longitudinal strain in the absence of transverse restraint. Also called Young’s modulus.

Modulus reduction curve: A curve relating the reduction of the soil stiffness, as measured by the shear modulus to the amplitude of shear strain.

Multichannel analysis of surface waves (MASW): A procedure in which the velocities of shear waves are determined from surface waves (Rayleigh waves) that are generated at a point on the ground surface and received at three or more stations located on the surface at various distances from the source.

Negative skin friction: Frictional forces acting on a pile generated by the surrounding soil moving down relative to the pile rather than supporting it. Also called downdrag.

One-dimensional site response analysis: Mathematical analysis in which the earthquake is assumed to be composed of vertically propagating shear waves.

Overburden correction (K_σ): A multiplicative factor used to correct the cyclic resistance ratio for the effects of initial confining pressure different from 1 atmosphere or 100 kPa.

Paleoliquefaction: Liquefaction induced by a prehistoric (or pre-instrumental) earthquake. Typically inferred from sand boils, dikes, and (or) sills (see Liquefaction feature).

Peak ground acceleration (PGA): The maximum horizontal acceleration amplitude in a recorded earthquake ground motion.

Performance-based design: A process in which the performance of a facility being designed is evaluated over the entire range of possible loadings, rather than for one or more discrete ground motion return periods or events. The approach is intended to allow rational decisions regarding appropriate design levels, including decisions related to the added value of increasing the seismic resistance. Financial, construction, and operational issues are considered over the life cycle of the facility. The approach usually involves probabilistic descriptions of earthquake ground motions and the facility’s response to them.

Phase transformation point or line: Point on a stress plot (such as a p-q plot) at which a soil shifts between contractive and dilative behavior, or a line connecting such points obtained from tests under different initial stress states.

Piezocone: A form of the cone penetration test (CPT) in which the cone is instrumented to enable measurement of porewater pressures.

Pleistocene: An interval of geologic time between 11,700 years ago and 2.58 million years ago (http://quaternary.stratigraphy.org/definitions).

Poisson’s ratio (ν): Ratio between horizontal strain and vertical strain in a longitudinally loaded elastic sample without transverse restraint.

Porewater pressure (u): The pressure in the fluids filling the pores of a soil. Sometimes called the pore pressure.

Post-liquefaction settlement: Settlement that occurs following soil liquefaction, usually due to dissipation of excess porewater pressure.

Primary wave: A stress wave in the body of a geologic deposit in which the particle motion is in parallel with the direction of wave propagation. Also called a p-wave or pressure wave.

Principle of effective stress: See Effective stress principle.

Probabilistic Liquefaction Hazard Analysis (PLHA): A probabilistic estimation of the exposure to liquefaction at a site.

Probabilistic Seismic Hazard Analysis (PSHA): An estimation of a site’s or a region’s exposure to seismic motion in which the output and input parameters are described probabilistically.

P-wave velocity (v_p or V_p): The propagation velocity of a p-wave.

Relative density (D_R): A measure of the density of a granular soil between its loosest and densest states, as determined from standardized laboratory tests. It can be expressed in terms of minimum and maximum void ratios instead of densities. It is defined as (e_max – e) / (e_max – e_min), where e denotes void ratio.

Residual strength: Shear strength in a soil that has liquefied, and which may be undergoing shear strains orders of magnitude greater than those at which the liquefaction was triggered.

Response spectrum: A plot of the maximum responses of similarly damped single degree of freedom oscillators subjected to the same earthquake base motion. The maximum responses are displayed as a function of the natural period or frequency of the oscillators.

Sand: A soil with particles in the size range 0.075-4.75 mm (engineering) or 0.062-2.0 mm (geology); or a granular soil dominated by such particles.

Sand boil: Conical deposit of sand resulting from eruption of sand-bearing water. The term is sometimes reserved for eruptions that are driven by seepage beneath a levee, in which case a sand blow is associated with earthquake-induced liquefaction.

Saturation: In geotechnical engineering, a condition in which the voids are completely filled with water. When only part of the void space is filled, the degree of saturation is the ratio between the volume of water in the voids and the total volume of the voids expressed as a percentage.

Seismic compression: Earthquake-induced reduction of volume in a partially saturated soil.

Seismic cone: A form of the cone penetration test (CPT) in which a sensor is mounted to detect incoming stress waves.

Shear modulus (G): Elastic modulus (stiffness) relating shear stress to shear strain.

Shear strain amplitude: The maximum increase or decrease in the shear strain during cyclic loading.

Shear wave: A stress wave in the body of a rock or soil in which the particle motion is perpendicular to the direction of propagation. Also called a secondary wave and an s-wave.

Shear wave velocity (V_s): The propagation velocity of a shear wave.

Silt: A granular soil with particle sizes in the range 0.002-0.005 to 0.075 mm (engineering) or 0.004 to 0.062 mm (geology).

Simple shear test: A laboratory test in which a sample is loaded with horizontal shear independently of the vertical and horizontal loading.

Soil: Engineering term for unconsolidated earth material above bedrock. Not limited to weathered horizons in which plants may be rooted.

Soil behavior type index (I_c): A parameter derived from the normalized tip and frictional resistance in the cone penetration test and used to infer soil type.

Spectral acceleration (s_a) or spectral velocity (s_v): The value of the acceleration or velocity at a particular frequency in a response spectrum.

Standard penetration test (SPT): An in situ soil testing and sampling procedure in which a standard, thick-walled, split-spoon sampler is driven into the soil at the bottom of a borehole by a 140 lb (623 Newton) hammer dropping a distance of 30 in (0.76 m). The results are reported in terms of the number of blows needed to drive the sampler 30 cm into the ground (N). The results are corrected, as needed, by multiplying N by correction factors for borehole diameter (C_B); deviation from 60% of the free-fall energy reaching the sampler (C_E); overburden stress different from 1 atmosphere (1 ton/sq. ft. or 100 kPa) (C_N); drill rod lengths less than 10 to 30 m (C_R); and the absence of liners in a sampler (C_S). The fully corrected result is denoted (N₁)₆₀.

State parameter (ψ): The difference between the void ratio of a soil and its void ratio at the steady-state condition for a given effective confining stress.

Static shear stress correction (K_α): A factor multiplied by the cyclic resistance ratio (CRR) to account for the effects of initial shear stress on horizontal planes.

Static shear stress ratio (α): The ratio of the static shear stress to the initial normal effective vertical stress.

Strain controlled: Describing a test in which the loading is applied in terms of strains or displacements.

Stress controlled: Describing a test in which the loading is applied in terms of forces or stresses.

Stress reduction coefficient (r_d): The shear stress at depth z for a flexible soil column divided by the shear stress at depth z for a rigid soil column given the same peak ground acceleration at the surface.

Stress wave: A pattern of particle movement in a geologic medium induced by sudden displacement or loading such as by an earthquake. Also called a seismic wave.

Surface waves: Stress waves whose patterns are modulated by the presence of a free surface. The most common are Rayleigh waves in which the particles move in vertical retrograde elliptical paths in the direction of wave propagation and are the basis of the multichannel analysis of surface waves (MASW) test. Other types of surface waves include Love waves in which the particles displace parallel to Earth’s surface in a direction perpendicular to the propagation direction.

Threshold shear strain: The cyclic shear strain at which residual excess porewater pressures are generated.

Triaxial test: A laboratory procedure in which a cylindrical sample is loaded with independent control of the vertical and horizontal loading.

Triggering: The initiation of liquefaction with no consideration about the deformation potential or instability of the soil.

Undrained: Refers to a loading pattern applied to a saturated soil that occurs so rapidly there is not time for porewater pressures to dissipate or for pore fluids to flow (i.e., volume remains constant).

Unit weight (γ): Weight of a sample of material divided by its volume. Subscripts are used to indicate the unit weight of water (γ_w) and the total unit weight of soil (γ_t).

Up-hole wave velocity measurement: A testing procedure in which the source of a stress wave is located at depth in the soil and the receiver is located at the surface.

Void ratio (e): The ratio between the volume of voids in a soil and the volume of solids.

Young’s modulus (E): Elastic modulus relating longitudinal stress to longitudinal strain in the absence of transverse restraint. Also called the modulus of elasticity.

NOTATION AND SYMBOLS

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