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Undrained response of saturated sands with emphasis on liquefaction and cyclic mobility Chern, Jin-Ching


An experimental investigation of the undrained monotonic and cyclic loading behaviour of a saturated angular sand and a rounded sand under triaxial conditions is presented. These studies are aimed at obtaining a unified approach to the undrained behaviour of sand spanning from strain softening (termed liquefaction or limited liquefaction) to strain hardening response and linking the cyclic loading behaviour to the monotonic loading behaviour. It is also aimed at investigating the differences In undrained loading behaviour of sand with different particle angularity. Under monotonic loading, the strain softening response is initiated and terminated at two distinct values of effective stress ratio termed critical effective stress ratio state (CSR) and phase transformation state (PT), regardless of the relative density and consolidation stress conditions. For strain hardening response, the start of dilation also occurs at the same effective stress ratio of PT for strain softening response. It is shown that the unique steady state line concept for liquefaction is also valid for limited liquefaction. The PT states for strain hardening response, however, form a series of lines, which are function of Initial void ratio, merging into the unique steady state line as the consolidation stresses increase. A 3-D effective stress state behavioural model is developed, which enables prediction of the anticipated undrained loading behaviour (strain softening or strain hardening) from the knowledge of the initial state of the sand. It is shown that a complete specifications of initial state of sand, i.e., void ratio, confining pressure and static shear, is required to predict the type of undrained response, especially for angular sand. Under cyclic loading, if liquefaction develops, the CSR, effective stress ratio at PT state and steady state line are the same as those observed under monotonic loading. If cyclic mobility develops, the effective stress ratio at PT state is also the same as that observed under monotonic loading. Thus, the 3-D effective stress state diagram provides a link between monotonic and cyclic loading behaviour, and is used to develop the criteria for the occurrence of liquefaction and cyclic mobility. The influences of void ratio and confining pressure on the cyclic loading behaviour are similar to those for the monotonic loading behaviour. However, the influence of static shear on cyclic loading behaviour can be completely different depending on whether liquefaction or cyclic mobility is developed. The undrained loading behaviour of rounded sand is similar to that of the angular sand. However, for the range of consolidation stresses of interest, the initial relative density alone provides a good single parameter characterizing the initial state of the sand, and hence its anticipated response.

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