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More and more underground infrastructures have been built in special soil in recent years, especially in the loess area of western China. The loess, as a special soft soil, can have particular influence on the seismic characteristics of underground structures. Therefore, the seismic performance and safety evaluation of underground structures in loess area should be concerned. A series of large-scale shaking table tests were performed to investigate the damage mechanisms of a frame-type subway station structure in a loess site. The test results showed that the model soil softening and the stiffness degradation of the soil-structure system were more serious under stronger input motions. The acceleration response of the soil-structure interaction system in a loess site was more like that of a free-field when input PGA is lower. However, the soil-structure interaction phenomenon was more remarkable both in horizontal and vertical direction when higher PGA was input. The dominant frequency of the input motion had an inverse effect on the lateral soil deformation. The direct-force action mode of the dynamic earth pressure acting on underground structures presents a trumpet-type distribution. Meanwhile, the soil-structure interaction exhibited complex mechanical behavior, and slipping or gapping in test cases with higher PGA may occur. The strain measured in the center pillar and middle slab of the subway station was larger than that in other components. The results provide an insight into how strong ground motion might induce the influence and a possible way to describe quantitatively the damage of subway structure in loess ground. •A series of shaking table tests of subway model structure in a loess soil were conducted. • The dynamic characteristics of the loess soil under different ground motions were analyzed. • The seismic behavior of the subway station structure was obtained and analyzed through the test. [ABSTRACT FROM AUTHOR]

Abstract This paper investigates the effect of concrete diaphragm wall on the seismic response of underground structures using numerical simulation. The seismic response of a two-story subway station supported by concrete diaphragm walls located in Nanjing, southeast China was analyzed. A finite element model was developed to simulate the soil-diaphragm wall-underground structure interaction during earthquakes. This model simulated the nonlinear dynamic behavior of soils using a visco-plastic model and concrete damage using a plastic-damage constitutive model. The effect of the diaphragm wall was explored related to the burial depth of the subway station. The simulation results showed that during a strong earthquake, the diaphragm wall reduced the lateral deformation of the lower story of the structure. However, the effect was very limited to the upper story of the structure. In contrast, during a small earthquake, the diaphragm wall increased the lateral deformation of the upper story of a deeply buried structure. The concrete diaphragm wall also increased the settlement of a deeply buried structure. The earthquake damage to the underground structure increased with the increase of the burial depth. The seismic tensile damage to the diaphragm wall was found adjacent to the invert of the structure. The damage to the diaphragm wall increased the uplift potential of a shallowly buried structure during earthquakes. Findings from this paper provide guidance to the seismic design of large underground structures buried in similar grounds. Highlights • FE model is developed to simulate dynamic soil-diaphragm wall-structure interaction. • The effect of concrete diaphragm wall (CDW) is related to burial depth of structures. • The CDW may increase earthquake induced deformation of underground structures. • The CDW may aggregate damage to underground structures in certain parts. • The CDW is vulnerable to earthquake damage adjacent to the invert of structures. [ABSTRACT FROM AUTHOR]

The model test method is crucial for studying the seismic performance of underground structures. Presently, model tests for the seismic response of underground structures are primarily performed with 1-g and N-g centrifuge shaking table. The 1-g shaking table model test is limited by the stress similarity in the soil-structure interaction system and cannot reproduce the seismic damage process and extent of the underground structure. The small table size of the N-g centrifuge model test makes it difficult to reflect the spatial effects of the seismic response of large underground structures. Seeking a large-scale model test method that can directly simulate the seismic damage process is essential for investigating the seismic performance of underground structures. Herein, based on the seismic response characteristics of underground structures, a quasi-static pushover test device applicable to underground structures was designed, and the corresponding tests and numerical model calculations were executed using the Daikai subway station as a prototype. Based on the macroscopic phenomena of structural damage development of the subway station in the test, the damage mechanism of the station structure was analyzed and compared with the numerical model. The strain amplitudes, displacements, and digital scatter strain color maps were analyzed to verify the reliability of the cyclic pushover loading test simulating the seismic performance level and damage process of the underground structure. Simultaneously, the rationality of the proposed quantification system for classifying the seismic performance level of single-story underground structures was verified. • A large-scale quasi-static pushover model test considering the soil-underground structure interaction was proposed. • The reliability and feasibility of the new model test were verified by numerical simulation. • The seismic performance level and amplitude of the drift ratio can be accurately reflected by the model test. [ABSTRACT FROM AUTHOR]

Considering the complexity of the soil-structure dynamic interaction mechanism, underground structures are restrained and influenced by the surrounding soil in earthquakes, and their seismic damage under strong earthquakes is different from that of above-ground structures. The seismic response of underground structures is influenced by several coupling factors, including structural form, ground motion parameters, and soil properties, etc. In addition, the burial depth of the underground structure is also one of the critical parameters affecting its seismic response. In this study, the computational model of soil-structure dynamic interaction at different burial depths was established by using the finite element software ABAQUS with a single-story, double-span Daikai subway station as the prototype. The influence laws of different burial depth conditions on the seismic response characteristics such as inter-story drift ratio, internal force of critical sections, and seismic damage of subway stations are systematically studied. Meanwhile, this paper compares the correlation and applicability of PGA (peak ground acceleration), PSSRD (peak site soil relative displacement), and SSSRD (structural side soil relative displacement) with the seismic response of underground structures under different burial depths by applying the principle of vulnerability analysis. The results show that PGA and SSSRD are relatively better IMs (intensity measure) in shallowly buried structures, while the relative superiority of PSSRD gradually increases with increasing burial depth, and PSSRD is a more reasonable IM in deeply buried underground structures. • There is a most unfavorable buried depth for the seismic performance of underground structures. • The maximum inter-story drift ratio is a good indicator to quantify the seismic damage state of underground structures. • The seismic intensity measures are different for the underground structures with different buried depth. [ABSTRACT FROM AUTHOR]

Sand liquefaction is one of the leading causes of severe earthquake damage. In this study, shaking table tests were carried out to investigate the dynamic interaction between the saturated sand and the irregular subway stations. In this test, a method for the preparation of a liquefiable foundation based on a flexible sensing chain combined with water sedimentation was proposed. Typical test results were interpreted, including the development and spatial distribution of pore pressure, surface subsidence, acceleration, deformation, and dynamic earth pressure. Results show that the spatial distribution of excess pore water pressure is changed by the irregular underground structure, and the liquefiable soil below the bottom plate of the structure can be divided into I, II, and III areas based on the degree of liquefication. The surface subsidence is significantly related to Arias intensity and is concentrated within the range of 5 %–95 % of the intensity. The structural floating is synchronized with the liquefaction of the saturated sand, and the uneven floating and inclination of the underground structure are remarkable, anti-flotation design of underground structures should take into account differences in section forms. During the liquefaction of saturated sand, the foundation soil has conspicuous lateral deformation, especially in the deep soil layer. The shape of the dynamic earth pressure increments turns from triangular to linear, when the foundation soil changes from non-liquefied to liquefied. Compared with the limiting values of the elastic inter-story drift ratio, the maximum inter-story drift ratio of the structure is smaller. The tensile strain responses on the columns, particularly in the lower layer, are more prominent than those at the slabs and side walls. • Shaking table model test is designed on the saturated sand-underground structure interaction system. • Dynamic interaction between irregular-section underground structures and saturated sand is researched, including the different liquefaction states for saturated sand. • A method for the preparation of a liquefiable foundation based on a flexible sensing chain combined with water sedimentation was proposed. • The liquefiable soil below the bottom plate of the structure can be divided into three areas based on the degree of liquefaction. • Differences in cross-section forms should be considered in the anti-floating design of underground structures. [ABSTRACT FROM AUTHOR]

With the increasing utilization of fiber-reinforced polymer (FRP) bars in concrete structures, the issue of retrofitting damaged FRP-reinforced concrete (FRP-RC) structures will emerge. However, limited research is available on the seismic performance of damaged FRP-RC structures after retrofitting. In this study, cyclic tests were conducted to evaluate the efficiency of precast ultra-high performance concrete (UHPC) plates in retrofitting the seismic performance of GFRP-RC columns with continuous or lap-spliced GFRP bars. Three GFRP-RC columns, among which one column with lap-spliced details was strengthened with precast UHPC plates, and the other two columns were retrofitted with precast UHPC plates combined with NSM after simulated seismic damage testing. To compare the retrofitting efficiency of this method for GFRP-RC and steel-RC columns, a previous study on steel-RC columns retrofitted with UHPC was incorporated. For the pre-damaged GFRP-RC columns after retrofitting with precast UHPC plates combined with NSM, the peak load experienced an increase of 76 %–79 %, and the ductility was complete restored. The failure modes of strengthened or retrofitted GFRP-RC columns differ from those of strengthened or retrofitted steel-RC columns. Further, the flexural strength of GFRP-RC columns and beams after being strengthened or retrofitted by UHPC can be predicted well based on the cross-section analysis model. • Cyclic behavior of pre-damaged GFRP-RC columns retrofitted with UHPC was investigated. • The seismic performance of GFRP-RC columns was significantly improved after retrofitting with precast UHPC plates. • The design method accurately predicts the flexural strength of GFRP-RC columns and beams strengthened with UHPC. [ABSTRACT FROM AUTHOR]

A finite element method (FEM) for simulating seismic wave fields of three-dimensional (3D) local irregular topography is proposed in this article. The author begins by developing a program to solve the seismic response of 3D layered foundations using the frequency-domain stiffness matrix method. This enables the user to easily obtain the input of ground motion for the finite element model, as well as conduct finite element simulations of the seismic wave field of incident P-, SH-, SV-, and Rayleigh waves coming in from any direction. This method solves the problem that previous 3D finite element modeling methods couldn't handle SV waves when the angle of incidence is greater than the critical angle. Furthermore, it effortlessly accounts for the dispersion properties inherent in layered foundations. A series of 3D seismic wave field simulation examples are employed to validate the method's accuracy. In the end, this method is subsequently applied to further investigate the seismic response of frame structures in sedimentary valleys, and the significance of soil nonlinearity and topographic effects on the seismic response of structures is presented. • A two-step FEM is proposed to study the seismic response of irregular terrain. • It is capable of simulating seismic waves coming in at any angle. • It works for body and surface waves and can consider wave dispersion. • It also applies to soil-structure interactions and nonlinearities. • Good accuracy of the method is presented, and topographic effects are verified. [ABSTRACT FROM AUTHOR]

A novel and efficient method for simulation of spatially correlated earthquake ground motions (SCEGMs), named as improved Hilbert spectral representation method, was presented. By using Hilbert–Huang transform (HHT), the known record can be decomposed into a series of intrinsic mode functions (IMFs) and a residual function. Targeting on each IMF, simulated IMFs for every field point were derived through a spatial correlation model. The frequency dependence of the spatial correlation structure was simplified such that only the correlation at the predominant frequency of each target IMF was considered. SCEGMs can then be obtained by superposition of simulated IMFs and the original residual function. Taking the Suai shield tunnel project in China as reference, the intersegment opening width response of shield tunnel under multiple support excitation (MSE) and identical support excitation (ISE) were investigated and compared. First, numerous accelerograms, including horizontal and vertical components, were generated based on selected real records and artificial acceleration time histories on bedrock to realize MSE. The generalized response displacement method (GRDM) in combination with a large-scale FEM model was then utilized to proceed with the study. Results showed that the MSE can lead to a higher opening width of tunnel in general. The distribution of longitudinal opening width under MSE is also different from that under ISE. The sharp variation of soil layer stiffness along the longitudinal direction of tunnel may lead to extremely large intersegment opening width or severe damage. [ABSTRACT FROM AUTHOR]