ENGINEERING GEOLOGY/Liquefaction 525 Liquefaction J F Bird, Imperial College London, London, UK R W Boulanger and I M Idriss, University of California, Davis, CA, USA ß 2005, Elsevier Ltd All Rights Reserved Introduction Liquefaction-induced ground failure has caused widespread damage and devastation; a recent example is the closure of the Port of Kobe, Japan’s busiest port, following the 17 January 1995 earthquake, largely due to the liquefaction-related failure of reclaimed land (Figure 1) The potential consequences of liquefaction are far reaching, ranging from settlement or tilt of individual building foundations to the spread of fire when the water supply system is damaged by permanent ground deformations Liquefaction is the loss of shear strength of a saturated cohesionless soil due to increased pore water pressures and the corresponding reduction in effective stress during cyclic loading Liquefaction and its associated ground deformation are very complex phenomena, and the term ‘liquefaction’ has been used to describe a wide range of soil behaviour This article focuses on the susceptibility, triggering, and consequences of earthquake-induced liquefaction; liquefaction caused by non-earthquake-related loading is not included Although the emphasis here is mainly the liquefaction of saturated cohesionless deposits such as sands or silty sands, it is important to consider that strength loss in fine-grained soils under earthquake loading can also pose a significant hazard The assessment of liquefaction hazard is an essential part of any engineering project in seismic regions and is needed to make informed decisions with respect to mitigation options, foundation design, and emergency response and recovery plans based on what is considered to be an acceptable level of risk and as it approaches zero, the shear resistance of the soil will also approach zero This loss of effective stress and shear resistance is known as liquefaction The stress–strain behaviour of liquefying sand depends strongly on its relative density When loose sand liquefies, the gravitational static shear stresses may exceed the shear resistance of the soil and rapid deformation with very large shear strains can commence; this is referred to as flow deformation The soil behaviour is termed ‘contractive’, and the shear resistance exhibited by the liquefied soil during flow deformation is termed the ‘residual strength’ When moderately dense granular soils are cyclically sheared, pore pressures may similarly rise and liquefaction can be triggered However, rather than undergoing flow deformation, the soil particle structure may try to expand as it reaches a certain level of shear strain, resulting in what is termed ‘dilative’ behaviour For undrained conditions, this leads to a reduction in the pore water pressures and a corresponding increase in effective stress and shear resistance A shear stress reversal, however, such as will occur many times during earthquake shaking, may cause the soil particle structure to be incrementally contractive and the state of zero effective stress may be temporarily reached once more This continued cycle of zero effective stress and strength regain is termed ‘cyclic mobility’ The cumulative deformations can be significant, particularly if the duration of shaking is long, but dilative soils not exhibit very large flow deformations in the way that contractive soils (Figure 2) The Principles of Liquefaction As a saturated cohesionless soil is cyclically loaded, its particle structure can tend to collapse to a denser arrangement If the soil’s permeability and the site stratigraphy are such that drainage cannot occur immediately, then, as the collapse occurs, stresses will be transferred from the soil grain contacts to the pore water, leading to an increase in pore water pressure In simple terms, when the pore water pressure increases, the effective stress (total stress minus pore water pressure) on the particle structure will reduce, Figure Liquefaction of reclaimed fills at the Port of Kobe in 1995 caused complete suspension of operations The lateral dis placement of the quay walls in this picture pulled apart the crane legs, causing collapse Photograph by Leslie F Harder, Jr