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The response of sands under partially drained states with emphasis on liquefaction

University of British Columbia

The occurrence of liquefaction and instability in saturated sands has been investigated under small departures from the undrained condition. Volumetric deformations are inevitable in field problems on account of the spatial variations in excess pore pressures generated during the earthquake shaking and their subsequent dissipation after, considered together with the relatively high permeability of sands. The departures from the undrained mode were simulated in the laboratory by loading sand under the strain path control, that simulated, not only zero, but small finite amount of controlled volumetric deformations. It is shown that the undrained state of loading normally assumed during earthquake shaking may not represent the most damaging scenario with regard to instability and liquefaction for certain initial density and stress states. At a given initial stress state, the denser sand may become susceptible to liquefaction and prone to instability if rendered partially drained as opposed to when it is completely undrained. For a given density, this potential for liquefaction and instability increases, both with increase in confining stress and static shear. The zone of contractive deformation in stress space where strains associated with strain softening on triggering of liquefaction occur gets enlarged substantially when small expansive volumetric deformations occur. The loose sand, that is liquefiable when undrained, may sustain this vulnerability, although to a lesser degree, even when compressive volumetric strains occur. Criteria for the occurrence of liquefaction and instability to occur under partially drained states have been developed, based on the known behavior of sand in undrained and fully drained conventional (constant confining stress) shear. The effects of the initial state variables - density, confining stress and static shear on partially drained response are assessed both in the triaxial compression and extension modes of deformations, taking cognizance of the inherent anisotropic nature of water deposited sands. For a given initial state, the sand, if liquefiable when undrained, continues to be so over a substantial range of even compressive volumetric strains. Finally, the similarities and differences between the phenomenon of strain softening associated with liquefaction problems and the conditions for instability under constant shear stress are pointed out. The association of strain increment directions with stress and stress increment directions are also explored under partially drained states in the region of effective stress space that does not involve strain softening.

Thesis/Dissertation

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2009-07-27

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http://hdl.handle.net/2429/11336

Civil Engineering

University of British Columbia

UBCV

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