Pipelines are important structural elements that are frequently used today to meet many infrastructures needs such as drainage, natural gas or water transmission. In this context, the usability of such structures, which are important elements of infrastructure systems, especially after disasters such as earthquakes, is of great importance. For this reason, within the scope of this study, a parametric investigation of the seismic behaviors of a natural gas pipeline system under mainshock-aftershock sequences have been carried out, specifically taking into account the soil-natural gas pipeline interaction (SNGPI) in the help of finite element model (FEM) proposed. Before developing the model of SNGPI system proposed using solid element, the fundamental mode frequencies of the pipeline system modeled using the solid element for the verification have been compared with those of obtained from the pipeline system modeled using the beam element and the analytical solutions. After verification of proposed model is demonstrated, SNGPI system has been modeled and its fundamental modes have been compared with mode frequencies of soil stratum obtained from well-known simple analytic solutions. After this stage, the dynamic analyses of natural gas pipeline (NGP) system in the time domain have been carried out using four different soil systems and four different mainshock-aftershock sequences. The results of the nonlinear time-history analyses have been investigated in terms of the stress and the displacement responses. Parametric evaluations show that the greatest displacements and the stresses occurring at the considered nodes of NGP system may be importantly affected from mainshock-aftershock sequences and soil stiffness changes. As the soil stiffness decreases, both the peak stresses and displacements increased significantly. On the other hand, the same responses obtained under mainshock loadings, which have relatively lower peak ground acceleration (PGA)/ peak ground velocity (PGV) ratio compared to aftershock loadings, are generally larger than those obtained under aftershock loadings.

Aftershocks frequently induce further damage to slopes that have already been compromised by mainshocks. Most of the current research concentrates on the case-based studies of structural response to the mainshock-aftershock sequences (MAS), however, the influence of the MAS parameter characteristics has not been adequately considered. In this study, the peak characteristic, spectrum characteristic, cumulative characteristic and polarity effect of the MAS were considered, the correlation between 21 MAS parameters and slope response were analyzed, and the response characteristics of soil slope under the MAS action were comprehensively and systematically revealed. The results show that: (1) Aftershocks can induce significant incremental damage to slopes, with the extent of this damage being contingent upon the severity of damage caused by the mainshock; (2) Among the MAS parameters, the Cumulative Absolute Velocity (CAV(ma)) and Peak Ground Velocity (PGV(ma)) are optimal for assessing the response of soil slopes under MAS conditions. Furthermore, the incremental damage caused by aftershocks can be predicted using the displacement increment ratio (delta(D)); (3) The polarity of the MAS has an impact on the displacement of the slope, following the pattern: MAS along-slope direction > mainshock along-slope direction and aftershock reverse-slope direction > aftershock along-slope direction and mainshock reverse-slope direction > MAS reverse-slope direction; (4) The MAS polarity also affects the correlation between the MAS parameters and the slope displacement response, especially for the aftershock displacement. The research results aim to provide a foundation for the selection of evaluation factors and the analysis of soil slopes stability under the MAS action.

Most existing seismic behavior analyses of underground structures simply consider a single earthquake. Meanwhile, the diaphragm wall, as an enclosure structure, is regarded as a security reserve and is always ignored in current studies. Herein, the characteristics of a diaphragm wall-subway station system with different connection modes under earthquake sequences were investigated using numerical simulation. The damage degree of the structural component was calculated through quantitative analysis of the tensile damage picture. The seismic damage level of the station structure was evaluated to characterize the damage transition effect induced by the aftershock according to the inter-story drift angle. Moreover, an empirical model for predicting the inter-story drift angle with respect to different peak accelerations was proposed. The research results indicate that the effect of the connection mode between the sidewall and the diaphragm wall on the damage evolution and deformation behavior of the station structure is significant. Compared with that of the compound wall structure, the seismic damage to the sidewall of the composite wall structure is much less severe, but the slabs become more vulnerable and suffer more severe damage. The accumulative damage triggered by aftershocks aggravates the extent of structural damage and even leads to damage transition. The conclusions illustrated in this paper contribute to a better understanding of the seismic resistance design of diaphragm wall-subway station systems under earthquake sequences.