Understanding Excess Pore Water Dissipation in Soil Liquefaction Mitigation

Abstract

AbstractSoil liquefaction has conventionally been studied in cyclic laboratory tests as a pure undrained condition, assuming that excess pore water pressure is unable to dissipate during rapid shearing. As such, dissipation of excess pore water pressure generated during shearing is often not modelled in classic cyclic simple shear and triaxial tests. In contrast, dissipation of excess pore pressure does occur in the field following major earthquakes in the form of severe ground settlement and sand boils where fine granular soils were found. In this theme lecture, two aspects of soil liquefaction are discussed. First, a study on the interaction of excess pore water pressure generation and dissipation on liquefiable clean sand in the cyclic triaxial test setup is conducted. Results show that pore water dissipation at merely a fraction of the permeability of the soil can have a significant effect to the soil’s susceptibility to liquefaction. This calls for future cyclic laboratory tests to adopt similar near-perfect undrained condition to better reflect a more realistic representation of soil liquefaction in the field. Second, a critical cyclic shear stress ratio is obtained, where excess pore pressure dissipation dominates excess pore pressure generation, beyond which the soil is no longer susceptible to liquefaction. Complementing this ratio is the identification of a critical void ratio in which the excess pore pressures do not build up despite considerable shearing amplitude, implying the soil is no longer susceptible to liquefaction at that degree of densification. Such information would serve as a guidance to mitigate soil liquefaction in soil densification operations.KeywordsSoil liquefactionPore pressureGenerationDissipationCyclic shear stress ratioVoid ratio