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• New concept was defined to evaluate the damages of the underground structure (US). • New model was firstly used to predict the seismic performance levels of US. • A quantification system for the seismic damage of shallow buried US was advanced. • IDR was suitable to describe the seismic damage of shallow-buried US. • PGV was found more suitable for the seismic intensity measure of US in soft soil. This paper proposes a new method for evaluating the seismic performance and fragility of large underground structures. A typical underground subway station consisting of a two-story and three-span structure was used as the research subject. Considering the variation of sites and the uncertainty of input ground motions, we developed a finite element model of a nonlinear dynamic soil-underground structure interaction system. Based on the incremental dynamic analysis method, the nonlinear seismic responses of the subway station structure were calculated for 900 cases. Python (software) was used to accelerate the processing of the large number of numerical simulation data. Based on the seismic damage characteristics of an underground structure, this paper proposes a new concept of average seismic damage value to evaluate the damage condition of the underground structure to overcome the inadequacies induced by an expert's intuitive judgment. Meanwhile, the random forest prediction model was adopted to predict the seismic performance levels (SPLs) of the underground structure, and a non-parametric test was performed to verify the correlation between the degree of seismic damage and the inter-story drift ratios (IDR) of the large underground structure. This proved that the IDR could describe the seismic damage of shallow-buried underground structures; we advanced a precise quantification system of the SPL for shallow-buried subway station structures with two-story and three-span using probability statistics. The peak ground velocity (PGV) is more suitable than the peak ground acceleration (PGA) for use as the seismic intensity measure (IM) for large underground structures when the stiffness of the soil foundation becomes soft. In addition, the seismic fragility curves of the shallow-buried subway station structure were constructed and analyzed using the developed quantification system for the SPL. [ABSTRACT FROM AUTHOR]

• The soil–underground structure interaction model was developed and verified simply. • A method for determining seismic performance and fragility of underground structure was established. • A comparative evaluation of the different IMs for underground structures was carried out. Currently, research on the underground structures seismic fragility is still in its infancy state. In this study, we carried out numerical simulation on soil-underground structure system (SUSI) by taking the Daikai metro station damaged in the Kobe earthquake as analysis object. Considering input ground motion characteristics, based on incremental dynamic analysis (IDA method), a statistical numerical modeling method was established to classify underground structure seismic damage status and its seismic performance level. The analysis results showed that the input ground motions had a significant effect on the IDA curves. The rationality and randomness of the selected ground motions were important influencing factors to ensure the underground structures seismic fragility curves. In relation to the damage that occurred to the Daikai station in 1995, the peak ground acceleration (PGA) and peak relative lateral displacement (PRLD) of the site were suitable for estimating the seismic performance of the shallow buried structure constructed. Finally, the seismic fragility curves were compared with the existing research results, and a seismic damage assessment was carried out, which verified the effectiveness of the statistical numerical modeling method developed in this study. [ABSTRACT FROM AUTHOR]

• Designed elastic sliding bearings installed on the top of columns of underground structure. • Seismic response and damage mechanism of an isolated underground structure is simulated. • Some findings were made on the seismic performance of underground structure with sliding bearing. Existing studies on earthquake damage have shown that the center column of a shallow-covered underground structure is more prone to be damaged by a strong earthquake than other parts, as seen by the damage induced during the Kobe earthquake in 1995. In this study, in regard to the seismic damage process of an underground structure, elastic sliding base-isolated bearings are installed on the top of its columns. It is found that the seismic damage to the center columns and the medium plate of the underground structure is effectively alleviated by the sliding bearing. Consequently, the axial stress responses of the center columns are remarkably reduced and completely in a compressive state during a strong earthquake, which can effectively improve the overall seismic performance of the underground station structure. However, the mechanical transfer properties between a column and a longitudinal beam are also significantly changed by the elastic sliding bearings. Thus, the seismic damage to the slabs near the side walls of the underground structure is slightly aggravated following the elastic sliding bearing installation on the top of the columns. [ABSTRACT FROM AUTHOR]

Ground liquefaction is one of the severest threats that ensue subsequent damage to the subway underground structure during and after strong earthquakes. However, few studies on the seismic response of large-scale subway underground structures in liquefiable site, especially for complex subway stations, have been reported. Existing researches that consider the diaphragm wall as safety reserve for the seismic design of subway stations are oblivious of the influence of the diaphragm wall on the deformation and uplift of underground structures. Contradictory to the original intention, the diaphragm wall may have adverse effects on the overall deformation behavior and force redistribution of the structure. Thus comprehensive exploration of the interaction between the diaphragm wall and the subway station in liquefiable site during and after earthquake should be carried out to rationalize the design of the underground structure. Numerical simulations of the seismic response of different soil-structure interaction systems are implemented and discussed in the present study. In the numerical simulation, the constitutive model for saturated sand and the Arbitrary Lagrange-Euler (ALE) method to prevent mesh distortion are applied to constitute the static and dynamic coupling finite-element model. Earthquake responses of the soil and the complicated underground subway station are then investigated. Modeling results are analyzed in respect of earthquake responses yielding the liquefaction distribution of the site, the uplift feature of the unequal-span subway station, the dynamic settlement and vector characteristics of the surrounding soil, the lateral deformation and the earthquake-induced damage of the underground structure. It is found that the uplift of the unequal-span subway station is featured by uneven uplift across the spans. When the diaphragm wall is installed, the tension damage degree of the bottom plate of the subway station are intensified, while the components of cantilever span in the upper layer suffer the less. The resulted new findings from this study shed light on the seismic performance and seismic design method for the unequal-span subway stations at a liquefaction site. [ABSTRACT FROM AUTHOR]

The subway shield tunnel of Sanyang Road in Wuhan, China, is a Yangtze River-crossing tunnel project with a mega diameter of 15.7 m, which crosses several soil layers with sharply different properties. To investigate the seismic characteristics of this special subway tunnel under spatially varying earthquake ground motions (SVEGMs) with high efficiency, the concept of response displacement method was utilized and realized with a refined free-field model and a simplified soil-tunnel model. Two sets of borehole accelerograms, denoting near-field and far-field ground motions, respectively, were selected and taken as the reference to generate fully non-stationary SVEGMs. A lot of responses along the longitudinal direction under multi-support excitation (MSE), including intersegment opening width, inner forces, and acceleration, were observed and compared with those under identical support excitation (ISE). The comparative results showed that the responses of subway shield tunnel under MSE are generally higher than those under ISE. The longitudinal distributions of intersegment opening width and bending moment response of lining are dominantly affected by the site conditions. Excitation methods, spectral characteristic and PGA of input earthquake can also have effect on the longitudinal distribution of tunnel response. The results can contribute to revealing the difference between seismic response characteristics of shield tunnel under MSE and those under ISE and predicting the underlying location of seepage or severe damage of shield tunnel. The analysis procedure established in this paper possesses highly potential for application to the aseismic design of practical shield tunnel project. [ABSTRACT FROM AUTHOR]

• Effective stress characterization is present for the seismically induced seabed-tunnel interactions. • A new source term is defined in the Biot's coupled solid–fluid formulation for the residual EPWP. • Model calibrations are carried out by retrospective simulations of cyclic axial–torsional tests. • Effect of ocean environmental loads on the seismic response of immersed tunnel are interpreted. Unlike the terrigenous site condition, the seabed site is characterized by complex hydrodynamics in the ocean environment (i.e. ocean waves and currents). This paper investigates the significance of the ocean environmental loads on the seismic interactions between the buried immersed tunnel and the surrounding soil undergoing buildup of excess pore water pressure (EPWP) and residual liquefaction. A fully coupled nonlinear effective stress analysis framework is proposed, integrating liquefaction/solidification analysis procedure with consideration of cyclic plasticity, of partial drainage, of finite soil deformation and of soil-structure interaction. A new source term is defined in the Biot's coupled solid–fluid formulation for the residual EPWP, as the phase-resolved plastic volume strain of soil skeleton. The proposed model is calibrated against the benchmarking data of undrained stress-controlled cyclic axial–torsional test. The numerical results reveal that the continuous disturbance of ocean wave and current has major consequences for the seismic soil-structure interactions such as EPWP response, liquefaction pattern of seabed over the contact areas and uplift/settlement of immersed tunnel, especially at the post-liquefaction stage. It is concluded that ignoring the environmental loads caused by ocean wave and current in seismic design practice of immersed tunnels can be misleading. [ABSTRACT FROM AUTHOR]

This paper presents a robust modeling method for fully coupled effective stress analyses of wave–induced liquefaction scenarios around immersed tunnels. The method is based on the Biot consolidation theory, and special emphasis is placed on modeling the high–strain behavior of the liquefiable seabed and the evolving boundary conditions at the tunnel–seabed interface. Motivated by the experimental results, the existing Masing model is extended to the high–strain regime in the cyclic plasticity framework to capture the liquefaction–induced nonlinear behavior of the sand. In particular, a newly–observed shear–volume coupling model is implemented into plastic flow rule to realize volumetric compaction by cyclic shearing and exercise as the source term for residual excess pore pressure accumulation in the Biot's equation. Subsequently, the proposed cyclic plasticity model is implanted into an explicit time matching finite difference analysis platform, permitting a comprehensive simulation of the intensive response of the immersed tunnel in the seabed experiencing the excess pore pressure accumulation and residual liquefaction. Moreover, a model calibration procedure is presented based on the cyclic laboratory sample test data and prescribed loading paths. Finally, the liquefaction–induced uplift of an immersed tunnel under progressive wave action is studied numerically. The obtained results provide the cause of liquefaction and the resulting consequences for tunnel uplift in ocean wave environments. [ABSTRACT FROM AUTHOR]

A series of large-scale shaking table tests were performed to investigate the seismic response of an underground subway structure with irregular sections in adverse soil conditions (soft and liquefiable sites). Emphasis is placed on the influence of the adverse soil conditions on the response of the irregular underground structures and over areas of soil-structure interface, involving buildup of pore pressures and resulting large deformations. The results indicate that the seismic responses of the structure and the soil were sensitive to input motions with richer low-frequency components. The seismic damage of the underground structure was more significant in the soft site as compared with that in the liquefiable site. The liquefiable site worked as a natural seismic isolation system for the underground structure, significantly reducing the damage potential to the model structure. Under the action of a strong earthquake, the horizontal displacement response of the liquefiable site sharply increased compared with that of the soft site. With respect to the structure response, the interlayer displacement of the structure in the soft site was approximately twice that in the liquefiable site. The lateral deformation of the structure was greater in the soft site and the interior structure column was more prone to damage. Correspondently, the peak and the residual strains in the structural components in the soft site were higher than those in the liquefiable site were. The results provided insights into the site effects of adverse soil conditions on the seismic response of underground structures, especially on the seismic damage process and the failure mechanisms of the structure members. [ABSTRACT FROM AUTHOR]