Mitigation of liquefaction-induced uplift of underground structures

Abstract

Buoyancy is one of the most serious manifestations of underground structure’s instability during strong ground motions. Soil liquefaction along with significant reduction of its shear modulus beneath these structures are the main driving factors of this phenomenon. In this article, the seismic behavior of underground access structures embedded in sandy deposits was analyzed using the finite differences (FD) program (FLAC) with an emphasis on structural failures under real earthquakes. A new liquefaction model “energy-based approach” which simulates material cyclic behavior and estimates pore water pressure build-up, was incorporated into the numerical code as a constitutive model of the soil. Also, linear structural elements were used to model the underground structure. Several mitigation methods have been modelled against the structure flotation. As well as a new combined mitigation method and its impacts on the structural performance have been detailed herein. Numerical results showed that gravel drains would effectively dissipate the excess pore water pressure beneath the structure while increasing the burial depth of the structure and adding an impermeable layer under it would increase the vertical effective stress and therefore detracts the ability of excess pore water pressure to push the structure upward. It is found that compiling a gravel drains surround the structure with an impermeable layer beneath the structure would effectively reduce the structural uplift more than any other stand-alone method.