Effect of Jointed Rock Mass on Seismic Response of Metro Station Tunnel-Group Structures

A jointed rock mass (JRM) is the usual case in practical engineering, which has significant effects on its mechanical performance. To clarify the difference in the seismic responses of underground structures in JRM sites or homogeneous rock mass (HRM) sites, two models were prepared to take shaking table tests in a structural laboratory. The HRM site was prepared following the similitude relations of material; meanwhile, underground structures of a metro station were embedded during the casting of the models. The JRM site and structure were made with the same material but produced random joints after the natural drying process. Different frequencies of harmonics were used to excite along the two models in the transverse or the longitudinal direction, respectively. The dynamic effect was evaluated by time-frequency and frequency-domain analyses. The test results compared with the HRM model indicated that the JRM model had a 22% reduction in the transverse fundamental frequency, but the dynamic response of the ground surface was enhanced due to the effect of the joints. Under harmonic excitations of the same intensity, the JRM model produced a greater energy response to the station structure and reduced the acceleration response of the platform in the high-frequency region. Meanwhile, the JRM model produced a peak tensile strain at the connections of the main and subsidiary structures that was 31% larger than that of the HRM model, and the range of tensile strains observed at the platform connecting the horizontal passage was 1.5 times larger than that of the HRM model.