Working mechanism of cutoff walls in reducing uplift of large underground structures induced by soil liquefaction

The uplift of large underground structures in saturated liquefiable soils under strong earthquake loadings may induce severe damages to the structures. Various mitigation procedures have been proposed to alleviate such damage, among which installation of cutoff walls next to underground structures was found to be effective. However, the working mechanism of cutoff walls in alleviating uplift of underground structures and the corresponding design parameters are still not clear. The liquefaction induced uplift behaviour of a subway tunnel in saturated sandy deposit over a layer of non-liquefiable soils and the working mechanism of cutoff walls for uplift mitigation purpose were investigated using the fully coupled dynamic finite element code DIANA Swandyne-II. A generalized plasticity model capable of simulating both cyclic liquefaction and pressure dependency of soils was used to model the sandy deposit. It is found that the small effective unit weight of underground structures, the development of excess pore pressure and the flow of liquefied soils were the sufficient and necessary conditions for underground structures to uplift during earthquakes. Cutoff walls could restrain the flow or deformation of liquefied soils and inhibit the uplift of underground structures but they could not necessarily prevent the liquefaction of the enclosed soils. After earthquake loadings, underground structures might settle due to the consolidation of soils and cutoff walls could also reduce the magnitude of settlement. The design parameters of cutoff walls, including the acting lateral pressure, the position, the stiffness and the permeability of cutoff walls, were also analyzed, the findings of which, together with the unveiled working mechanism, would be relevant for the design of cutoff walls for uplift mitigation purpose.