Evaluation of field sand liquefaction including partial drainage under low and high overburden using a generalized bounding surface model

Abstract

The article presents simulations of the seismic liquefaction response of dense and loose clean Ottawa sand under low and high overburden in the centrifuge, using Program FLAC3D and the P2Psand constitutive model. P2Psand was initially calibrated in the paper with cyclic stress-controlled triaxial tests and then modified with information from two centrifuge experiments. The calibrated model was used to simulate four centrifuge tests covering relative densities from 45% to 80% and overburden pressures from about 100 kPa (∼1 atm) to 600 kPa (∼6 atm). The four numerical computations were fully coupled effective stress simulations that allowed for pore water pressure buildup and dissipation at every time step. The calibration yielded very good matches between numerical and experimental results in all four centrifuge experiments, and calibrated P2Psand input parameters are suggested for practitioners in similar clean sands. The simulations confirmed the increased diffusivity of the sand layer under high overburden obtained before from the centrifuge results. The reason is that P2Psand assumes that the sand bulk modulus is proportional to the square root of the mean effective stress, consistent with the similar conclusion derived from the centrifuge data by the authors. The calibrated P2Psand model was also used to perform “no flow” simulations of the same four centrifuge experiments, in which fluid flow was not allowed during or after shaking. No flow simulations are used sometimes in practice to reduce numerical effort, on the assumption that liquefaction in the field is mostly undrained. It was found that this assumption may produce useful engineering results for a low overburden of 1 atm, but it may become increasingly incorrect and too conservative at higher overburden. The reason is that for certain field conditions, fluid flow becomes more significant during shaking under high overburden due to increased sand diffusivity.