Void redistribution-induced shear localization and deformation in slopes

Earthquake shaking of soil deposits generally leads to a decrease in the average void ratio and a long term strengthening of soil deposits. In soil deposits of non-uniform permeability, however, dissipation of excess pore water pressure from one zone may result in void ratio increases in another zone (void redistribution). The strength loss due to loosening of soils in certain zones in a slope or embankment may result in large deformations or failures that may have severe social and economic consequences (e.g. deformation of the Lower San Fernando Dam in 1971).; Seventeen centrifuge models were used to develop a clear understanding of the mechanism and the important parameters by which void redistribution may lead to shear localization. A theoretical framework was developed for assessing the likelihood of slope failure due to void redistribution in a layered infinite slope. A new method of analyzing pore pressure distributions measured by dense arrays of pore pressure transducers in the large centrifuge models showed patterns of consolidation and dilation consistent with this framework.; Low permeability layers may restrict drainage from underlying liquefiable soil and hence densification at the bottom of the liquefiable layer is accompanied by loosening (dilation) near the interface with the overlying low permeability layer. The capacity of the dilating portion of a layer to absorb water prior to failure (dilation capacity) is proportional to its thickness. It is found that the thickness of the dilating layer increases with the slope angle and relative density (dilation angle). The dilating layer is typically much thicker than ten or twenty grain diameters, the thickness of the eventual shear band. Some factors that increase the likelihood of failure due to void redistribution include a higher permeability contrast and shaking intensity, duration and history (i.e. accumulated loosening).; In the past, the shear strength of a liquefiable soil has been estimated from pre-earthquake conditions (void ratio or SPT). Because the shear strength of a soil changes during void redistribution, this method can be inaccurate. Concepts presented in this thesis may be used to develop new ways to minimize the likelihood of failure due to void redistribution.