Influence of bidirectional shaking on pore water pressure generation for liquefaction triggering evaluation

Abstract

Over the past years, prior research has shown that bidirectional horizontal shaking can increase seismic settlements in dry sand, the buildup of excess pore water pressures in saturated granular soils, and the liquefied depth. However, the state of practice for either simplified liquefaction triggering procedures or numerical/experimental seismic simulation is typically for one-dimensional (1D) conditions, without considering the impact of bidirectional horizontal shaking on soil deposits with stratigraphic variabilities. In this paper, three-dimensional (3D), fully-coupled, nonlinear finite element analyses are used to evaluate how bidirectional horizontal shaking affects the seismic response of layered liquefiable soil deposits. The two horizontal components of each selected motion were rotated to find the maximum rotated (RotD100) peak ground acceleration (PGA). The two horizontal components and the RotD100 component were subjected to the base of each 3D soil column model, to represent the site response analysis under bidirectional (BD) and 1D shaking, respectively. The numerical results showed that the BD shaking and pore water pressure migration increased the excess pore water pressure (EPWP) buildup in soils, particularly for medium-dense and dense soils. This led to an increase in liquefied depth when the soil deposits were under moderate-intensity shaking, as compared to the model under 1D shaking or evaluated by the simplified liquefaction triggering procedure. For the case considered, the BD shaking amplified the maximum excess pore water pressure ratio by about 1.3 times on average (with a standard deviation of 0.3) compared to that under 1D shaking, particularly for PGA ranging from 0.002~0.3g. However, the impacts of BD shaking on the EPWP generation became minor when PGA was greater than 0.3_g. These results indicated that the impact of bidirectional shaking on the seismic response of porous media must be considered with extreme care when assessing the risk of liquefaction.