On the seismic response of shallow-buried rectangular structures

Compared to bridges, underground structures are inappropriately regarded as less crucial components of road infrastructure in view of their supposedly low seismic vulnerability. The literature indicates, however, that shallow-buried rectangular structures, such as box culverts or rectangular tunnels, can be affected by shaking failure. To avoid the complexity of a fully non-linear soil-structure interaction analysis, a number of simplified methods have been proposed in recent years, which have gained popularity among designers. The aim of this paper is to investigate the applicability limits of such simplified analyses. The study compares the results obtained using simplified approaches with those emerging from non-linear static soil-structure interaction analyses, accounting for the following effects: the frictional behavior of the soil-structure interface, the geometry of the box structure, the overburden depth, the maximum PGA, and the increasing soil stiffness with increasing depth. The outcomes of the analysis indicate that shallow-buried rectangular structures are strongly affected by non-linear frictional effects at the soil-structure interface. The soil-structure interaction under seismic condition is shown to change smoothly from the condition of deep burial to the condition of "null overburden depth". For a given aspect ratio, stiff, shallow-buried rectangular structures prove to be affected more deeply by sliding at the soil-structure interface than flexible structures and, for low aspect ratios, these structures may undergo a rigid rotation (rocking) that may even involve a partialization of the base foundation. For a reliable evaluation of member forces from racking distortions, rocking must be carefully taken into account.