Impact of bidirectional seismic shearing on the volumetric response of sand deposits

Abstract

Numerical simulation results of dry and saturated sand deposits subjected to both unidirectional and bidirectional seismic shearing were carried out and compared to quantify the increase of shear-induced volumetric response during shaking. The engineering demand parameters quantifying such response during the shaking events were considered to be the surface settlement for the dry deposits, and the depth-averaged peak excess pore water pressure and thickness of the liquefied layer for the saturated deposits. The numerical simulations made use of a three-dimensional continuum, coupled, dynamic, finite-difference platform and an anisotropic bounding surface plasticity constitutive model. Results of a series of centrifuge tests on saturated level ground sand deposits subjected to bidirectional shearing were used to validate the model capabilities for capturing the volumetric response of sand deposits. Over 1000 simulations were carried out in this study on homogeneous sand deposits with different densities and subjected to ground motions applied as uni- and bidirectional shearing. The dry models exhibited an 80% increase of surface settlement and in the saturated models depth-averaged peak excess pore water pressure ratios were up to 60% higher. Moreover, for the loose to medium dense 30 m-deep uniform deposits, the liquefied sand layers were in average 5–6 m thicker. These outcomes highlight the need to account for bidirectional seismic shearing when predicting the shear-induced volumetric response of sand deposits and related damaging phenomena such as liquefaction or seismic-induced settlement, among others. Furthermore, the simulation results shown the necessity for defining optimal ground motion intensity measures to characterize and scale ground motions for bidirectional shearing.