Effects of top-soil properties on seismic deformations of underground structures

Underground structures have become an indispensable part of modern urban life, such as tunnels, metro stations and pipelines. As the design and construction of these structures evolve, their behavior during earthquakes has become an increasingly significant consideration. Contrary to above-ground structures, the earthquake response of underground structures is not controlled by the internal forces of the structure but by the ground with whom it is in interaction. In addition, the response of the structure is influenced by the structural parameters and the expected earthquake characteristics. In particular, the depth of burial is one of the main parameters affecting the seismic response of the structure. Experience shows that the behavior during an earthquake is relatively better with increasing depth. However, the effect of surface soil characteristics, defined as soil along the burial depth, on underground structure behavior has been relatively understudied. The purpose of this study is to investigate the effect of changes in the strength and plasticity of the surface soil on the behavior of underground structures under earthquake excitation. For this purpose, one-dimensional soil response analyses were performed with different surface soil scenarios to determine the free field deformations, and the soil structure interaction response behavior in these scenarios was investigated by two dimensional pseudo-static analyses. In addition, to determine the dominant effects of surface soil properties on the behavior, the changes in shear wave velocity, shear strength and plasticity index, which are important parameters of the complex structures of cohesive soils, were focused on. The results of the investigations are a description of the variation of the racking coefficient, which is defined as the ratio of the differential deformation of the soil in free field conditions to the differential deformation experienced by the structure along its height, with different surface soil properties and the soil property that plays the dominant role in this variation.