This paper proposes a carbon fiber reinforced polymer (CFRP) retrofitting scheme for improving the seismic performance of atrium-style metro stations (AMS). Past experimental studies have confirmed that the weakest of the AMS during strong earthquakes is located at the upper-story beam ends. However, there is thus far no candidate for a reference approach to retrofitting and strengthening the AMS. This study addresses this gap by applying CFRP retrofitting to both ends of the upper-story beam. The main objective is to assess the effectiveness of the proposed retrofitting scheme. First, a three-dimensional finite element model is developed to simulate dynamic soil-AMS interaction. The validity of the numerical method is assessed via a comparison with measured data from reduced-scale model tests. Second, a numerical model of the AMS retrofitted with CFRP is built using validated methods. Finally, dynamic time-history analyses of the AMS with and without CFRP retrofitting are conducted, and their dynamic responses, including inter-story drift, dynamic strain, and tensile damage, in conjunction with the lateral displacement of the surrounding ground, are compared. Comparison of the results for the non-retrofitted and retrofitted structures shows that CFRP retrofitting significantly reduces both the principal strains and tensile damage factors at the upper-story beam ends while slightly increasing those values at the mid-span of the beam; additionally, it does not change the structural lateral deformation. Therefore, it can be concluded that CFRP retrofitting could effectively improve the seismic performance of the AMS without changing its lateral stiffness.

This paper investigates the seismic performance of complex comprehensive interchange underground structures. As an example, the seismic performance of Nanjing underground bifurcated tunnels constructed in soft soil is analyzed. A three-dimensional (3D) numerical finite element analysis model is established considering soiltunnel interaction to analyze the seismic response characteristics and failure mechanism of the bifurcated tunnels. The results show that the tunnel damage degree is closely associated with the deformation response, and that the inter-story drift ratio of the lower layer is significantly greater than that of the upper layer. The pushover analysis method is used to obtain the seismic performance curve of the tunnel, and inter-story drift ratio limits are defined. The IDA procedure is then used to conduct multiple non-linear dynamic time history analyses based on the 3D finite element model, and IDA curve clusters are drawn with peak ground acceleration (PGA) as the earthquake intensity measures and the peak inter-story drift ratio as the damage measures of tunnels. The corresponding IDA percentiles lines are calculated, and the seismic fragility curves of the bifurcated tunnels are established. The post-earthquake functional failure probability of the bifurcated tunnels is obtained under different seismic fortification performance level.

In this paper, we study the seismic behavior of pile group bridges in inclined liquefiable soil and reveal the failure mechanism by conducting large-scale shaking table tests. Two bridge models were supported by two foundations: a group pile in an inclined liquefiable site and a rigid foundation. Typical results of the model test under a strong event (Tabas 0.3 g) are illustrated, and the effects of soil-group pile-bridge interaction are explored. The inertial and kinematic effects of the pile-pier curvature are evaluated, and the seismic failure mechanisms of the pile-bridge system are revealed. The results demonstrated that the near-pile shallow soil exhibited significant shear dilation response during the occurrence of strong earthquakes, which induced acceleration spikes for both soil and structure. The interaction state was soil pushing the pile and pile pushing the soil during the first and the subsequent strong earthquake, respectively, due to the bridge P-Delta effect. The liquefaction-induced lateral spreading increased the kinematic effect and reduced the inertial effect on the pile head curvature. In addition, the inertial effect on the pile curvature decreased gradually from the shallow layer to the middle of liquefiable soil, while the kinematic effect increased gradually. The results also demonstrated that the rigid foundation assumption overestimated the acceleration demand of the bridge during strong earthquakes; however, it seriously underestimated the lateral displacement. Finally, the lateral spreading shifted the vulnerable position of the pile group-pier system from the pier bottom to the pile head and bottom, and the leading piles sustained more damage than the trailing piles.

So far, little attention has been paid to the investigation on the seismic failure mechanisms of flexible concrete pile groups embedded in the layered soft soil profiles considering the material non-linearities of soil and concrete piles. The purpose of this study is to investigate seismic failure mechanism models of flexible concrete piles with varied groups in silt layered loose sand profiles under horizontal strong ground motions. Three-dimensional finite element models of the pile-soil interaction systems, which include nonlinearities of soil and concrete piles as well as coupling interactions between the piles and soil, were created for Models I, II, and III of the soil domains, encompassing 1x1, 2x2, and 3x3 flexible pile groups with diameters of 0.80 m and 1.0 m. Model I consists of a homogenous sand layer and a bedrock, Models II and III are composed of a five-layered domain with homogeneous sand and silt soil layers of different thicknesses. The linear elastic perfectly plastic constitutive model with a Mohr-Coulomb failure criterion is considered to represent the behavior of the soil layers, and the Concrete Damage Plasticity (CDP) model is used for the nonlinear behavior of the concrete piles. The interactions between the soil and the pile surfaces are modeled by defining tangential and normal contact behaviors. The models were analyzed for the scaled acceleration records of the 1999 Duzce and Kocaeli earthquakes, considering peak ground accelerations of 0.25 g, 0.50 g, and 0.75 g. The numerical results indicated that failure mechanisms of flexible concrete groups occur near the silt layers, and the silt layers have led to a significant increase in the spread area of the damaged zone and the number of damaged elements.

During the 1995 Kobe earthquake, the Daikai Station suffered a severe collapse, drawing more attention to the earthquake damage response of underground structures. However, the performance of underground structures under the mainshock-aftershock sequences has yet to receive much attention. In this paper, the dynamic response and damage development process of the subway station under the mainshock-aftershock sequence were studied and explored through a series of centrifuge shaking table tests. The Selection-Adjustment-Generation method of artificial main aftershock sequence construction was introduced. Steel grits were mixed into the overlying soil to simulate the vertical inertia force. It is found that the Selection-Adjustment-Generation method is easily implemented, maintaining the local time-frequency characteristics of the original seismic waves as much as possible. The failure mechanism of the subway station subjected to mainshock-aftershock sequences can be summarized into two main aspects. One is the reduction of the horizontal deformation capacity of the columns due to the vertical inertial force induced by the overlying soil under the vertical seismic load. Sudden brittle failure will happen to the columns with higher axial pressure under horizontal earthquakes, which weakens their vertical support capacity. The other is the damage accumulation under mainshock-aftershock sequences. The station in the dry sand site is more susceptible to damage for the amplification effect. Both ends of the columns of the subway station experience crack with a strain exceeding 500 mu epsilon during the mainshock. During the second aftershock, the bottom of the outer columns was crushed. In the liquefiable site, the liquefied layer weakens the upward propagation of horizontal seismic load and protects the structure to a certain extent. This series of tests illustrate the damage development process of underground structures under the main-aftershock and provides valuable experimental data for further study of the earthquake damage response of underground structures.