Seismic Performance Of Underground Structures Research Articles

Endurance time analysis for seismic performance of underground structures subjected to mainshock–aftershock sequences

The seismic response analysis of subway underground structures only considers the impact of a single mainshock, ignoring the aftershocks, which may cause more serious secondary damage within a short time after strong earthquakes. To investigate the seismic performance of underground structures under sequential earthquakes and improve the understanding of aftershocks, we analyzed the dynamic response of subway station structures under mainshock–aftershock sequences. A typical two-story and three-span subway station was selected as the prototype, and a finite element model of a soil–underground structure interaction was established. Based on the basic concept of endurance time and design response spectrum of relevant seismic standards in China, an endurance time acceleration function was generated, and nine mainshock–aftershock sequences were constructed. Thereafter, the seismic responses of subway stations under the action of mainshock–aftershock sequences, such as lateral deformation characteristics and damage failure law, were discussed and analyzed. The results show that the endurance time method can be used as a new and efficient method to study the seismic performance of underground structures subjected to mainshock–aftershock sequences. The effects of aftershocks on the damage and lateral displacement of underground structures were preliminarily revealed. The structural deformation response of each layer varies greatly due to the different damage states of the mainshock. The maximum story drift θ of the structure increases with the increase of loading seismic intensity. From the perspective of seismic damage and relative deformation, the structural dynamic response, considering the effect of sequential earthquakes, is greater than that of the mainshock alone, which reflects the disadvantage of aftershocks in seismic design. The results provide a reference for seismic analysis and damage assessment of structures under sequential earthquakes.

A new seismic input method for layered soil and its application in dynamic response analysis of underground structure

With the acceleration of urbanization, the seismic problem of underground engineering is getting more and more attention. In actual projects, foundation soils often show a complex layered distribution. In terms of layered ground motion input, this study propose a new layered ground motion input method based on infinite element boundary, which avoids the application of spring and damping elements on the boundary nodes of layered soil, simplifies the ground motion input process and has high accuracy at the same time. To reasonably simulate the nonlinear seismic response of layered soil, the equivalent linear model of soil is nested to Mohr-Coulomb (MC) model to establish the viscoelastic-plastic model. The seismic response analysis procedure of the soil-structure monolithic model considering the nonlinearity of the layered soil is given. On this basis, the soil-structure nonlinear seismic response analysis is carried out to study the effect of burial depth on the seismic performance of underground structures, taking a project as an example. The results show that with the increase of the burial depth of the structure, the top–bottom displacement difference shows a change law of first increasing and then decreasing under the action of earthquake; From the internal force response, the most unfavorable burial depth exists due to the joint influence of the burial depth of the structure and the characteristics of the layered soil; For the same burial depth condition, the dynamic bending moment caused by the seismic load action is larger compared to the shear response.

Seismic performance of underground structures improved by using PET FRP retrofitting central columns

The seismic failure mechanism of underground structures presents that the insufficient lateral deformation capacity of central columns will result in the collapse of the overall structure. Therefore, this study proposed applying Polyethylene Terephthalate Fiber-reinforced Polymer (PET FRP) to enhance the lateral deformation capacity of the central columns of underground structures and then improve the seismic capacity of the overall structures. The numerical models of PET FRP retrofitting reinforced concrete (RC) columns subjected to high axial loads and increasing cyclic deformations were verified by experimental results. With the calibrated models, the inter-story deformation capacity of the underground structures with and without PET FRP retrofitting was contrastively analyzed, and the seismic response of the two underground structures subjected to both horizontal and vertical ground motions was simulated and discussed. It was found that the PET FRP retrofitting structure experienced smaller earthquake threats due to the larger lateral deformation capacity. Furthermore, the repairability of the PET FRP retrofitting underground structures after earthquakes was also investigated. Numerical results presented that applying PET FRP retrofitting central columns of underground structures could improve the seismic performance of overall underground structures.

Quasi-static model test and numerical simulation of a single-story double-span underground subway station

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.

Static pushover test of spring-underground structure system for seismic performance analysis of underground structure

The static test is appropriate for studying the seismic performance of underground structures due to the large size of the model structure and the controllable stress level. By setting elastic elements around the model structure, the static pushover test of the spring-underground structure system could simulate the initial stress state of the underground structure and soil-structure interactions during the pushover process. Taking the Daikai subway station damaged in the 1995 Hanshin earthquake in Japan as the model structure, a series of static pushover tests of underground structures with a geometric scale of 1/5 were performed. The impact of vertical ground motion on the seismic performance of underground structures was studied by conducting two static pushover tests of spring-underground structure system under different vertical loads; the impact of surrounding soil on the seismic performance and failure process of underground structures was studied by conducting static pushover tests with and without springs under the same vertical load. The relationship between the horizontal pushover force and the structural interstory drift, the concrete strain change process of structural weak vertical member, the structural failure phenomenon, and the seismic performance of the test object can be obtained from the tests, which can provide valuable information for experimental research on the seismic performance of underground structures. The test results demonstrate that the vertical ground motion decreases the ductility of underground structure, and the surrounding soil improves the lateral collapse resistance of underground structure.

Influence of vertical ground motion on the seismic response of underground structures and underground-aboveground structure systems in liquefiable ground

Vertical ground motion components are often observed during earthquakes, which could affect the seismic performance of underground structures. This study investigates the influence of vertical ground motion on the seismic response of underground structures and underground-aboveground structure systems (UASS) in liquefiable ground. Both centrifuge shaking table tests and numerical simulations are conducted. The centrifuge shaking table tests provide basic insights into the seismic response of underground structures under simultaneous horizontal and vertical shaking, and also serve to validate the numerical simulation method. Numerical simulations are conducted using solid–fluid coupled FEM with a constitutive model for soil liquefaction analysis. The analysis results show that although the influence of vertical ground motion on the shearing related motions and internal forces of underground structures is limited, its influence on the axial force of the structure is significant, especially for the center column. The inertia effect of the aboveground structure in UASS can amplify the influence of vertical ground motion via structure-soil-structure interaction. For UASS, the horizontal and vertical relative positions of the aboveground structure and the underground structure also affect the influence of vertical ground motion.

Performance of an underground structure seismic mitigation system improved by frictional deformation absorbing braces

Earthquake-induced failure of underground structures attributes to their insufficient horizontal deformation capacity, which is mainly controlled by the horizontal deformation capacity of central columns. Therefore, the developed measures to update the seismic performance of underground structures are either to enhance the deformation capacity of central columns or to decrease seismic loads transmitted to columns. This paper presented an underground structure seismic mitigation system to reduce the earthquake-induced damage of underground structures by decreasing the horizontal deformation imposed on structures. In this system, frictional deformation absorbing braces are set between underground structure and earth retaining structure. The axial deformation of the braces is capable to decrease the earthquake-induced horizontal deformation of the overall structure. Furthermore, axial force of the braces can help the earth retaining structure to resist the earthquake-induced soil pressure. The deformation behaviour of the frictional deformation absorbing brace under axial loads was experimental firstly studied. Experimental results presented that the brace had a stable deformation property and could provide expected braced force. Then seismic response of the underground structure seismic mitigation system was simulated. Numerical results presented that the seismic behaviour of underground structure was sharply improved and the structure experienced very small damage even under strong earthquakes. Finally, the stiffness and strength of braces influencing the seismic performance of underground structure were discussed detailedly. The reasonable stiffness and strength of the braces for the target structure were suggested.

Analysis of Shaking Table Tests of Underground Structures considering the Influence of the Structure-Soil Interface

To investigate the seismic performance of underground structures under the action of the structure-soil interface, in this study, experiments were performed using plexiglass structures (two pieces) and a concrete structure (one piece) as the research objects. The surface of one plexiglass structure was prepainted with a layer of cement mortar as the contact surface between the structure and soil, and the other plexiglass structure was not treated and used for comparison. A rigid model box measuring 2.25 m × 2.25 m × 1.5 m was placed on a 3 m × 3 m shaking table, and the box was filled with the configured model soil and the underground structure prepared in advance. Input transverse uniform excitation was imparted to the whole system. A shaking table model test was performed on the underground structures to analyse the acceleration response, stress strain, and earth pressure changes in the underground structure, and the influence of the contact surface on the seismic dynamics of the underground structure was evaluated. The test results showed that under uniform excitation, the dynamic characteristics of the underground structures were greatly affected by the intensity and depth of the seismic waves. (1) When the soil-structure contact was considered, the stress and strain of the structures increased significantly, and the stress-strain value was significantly greater than the stress-strain value of the soil-structure interface in a fully bonded state. (2) There were inconsistencies between the acceleration peak curve of the plexiglass structure considering the contact effect and the acceleration peak curve of the plexiglass structure without considering the contact effect. The difference between the two lies mainly in the corresponding maximum peak acceleration and the Fourier spectrum amplitude. With respect to the value and frequency composition, regardless of whether the input acceleration intensity was 0.2 g or 0.5 g, the peak acceleration of the organic structure was greater when the contact surface effect was considered than without the contact surface effect. Therefore, the structure-soil interface needs to be considered in actual engineering. The presence of the contact surface improves the safety of the structure and is helpful for seismic design. The results of this study provide a basis for further research on the influence of soil-pipe contact on the seismic response of underground structures.

Earthquake resilience of urban underground structures: State ofthe art

<p indent="0mm">Earthquake-resilient underground structures can reduce earthquake-induced damage, protect life and property, and maintain their functions with minor or no repair after an earthquake. As earthquake-resilient cities are increasingly developed, improving the seismic resilience of underground structures has become a popular topic in research on underground structure earthquake engineering. This paper systematically summarizes research progress on the improved seismic resilience of urban underground structures. The principle of earthquake-resilient underground structures is first introduced. Then, the latest achievements in these structures are summarized from two aspects, namely, improvements in the seismic performance of underground structures during earthquakes and restoration of the function of underground structures after earthquakes. Finally, the research trend of earthquake-resilient underground structures is discussed. Most of the technologies proposed to improve the seismic resilience of underground structures focus on reducing earthquake-induced damage. Research on restoring the function of underground structures after earthquakes is generally insufficient. Future work should focus on restoring the function of underground structures after earthquakes, especially the restoration degree and speed. Moreover, new structural systems and larger-scale model tests, which form the basis of assessments of the seismic behavior of underground structures, should be further developed. The seismic design standards and codes of earthquake-resilient underground structures should be put forward, and the popularization and further applications of earthquake-resilient underground structures should be conducted.

Seismic performance improvement of underground frame structures by changing connection type between sidewalls and slab

Seismic failure mechanism of underground frame structures presents that the collapse of underground structures attributes to the insufficient seismic resistance of central columns. Therefore, researches have been conducted to improve the seismic performance of underground structures by enhancing the loading-carrying capacity of the central columns or decreasing the seismic loads transmitted to central columns. For this purpose, this study proposed a new approach by changing the connection type between sidewalls and slab: setting shock absorbing gap (SAG) between sidewalls and ceiling slab, which aims to decrease seismic loads transmitted to central columns. The proposal and working mechanism of SAG were illustrated detailedly. Afterwards, nonlinear finite element models, in which a developed elastoplastic constitutive model was used to simulate the 3D strength and softening behaviour of surrounding soils, were built for simulating the static and dynamic responses of underground structures. Based on the numerical results, SAG affecting the internal forces of structural components before earthquakes were discussed firstly. Then the seismic efficiency of SAG updating the seismic performance of underground frame structures was analyzed by discussing earthquake-induced damage mode of overall structures, soil pressure on structures, earthquake-induced deformations and internal forces of structural components. Finally, the seismic performance of underground frame structures influenced by the width of SAG and frictional coefficient of the sliding surface was investigated, and advice for reasonably using SAG was suggested.

Centrifuge and numerical modelling of the seismic response of tunnels in two-layered soils

Soil deposits inevitably affect the seismic performance of underground structures, and underground tunnels are commonly constructed on sites composed of layered soil deposits. In this study, dynamic centrifuge tests and numerical modelling are conducted to enhance the fundamental understanding of the kinematic soil-tunnel interaction for tunnels constructed in layered soils that consist of muddy clay and silty clay. The site seismic response and tunnel seismic performance between the centrifuge and numerical modelling are compared. The acceleration time histories, amplification factors of the input motions, and the Fourier spectra between free field and structural field are experimentally and numerically investigated. For comparative purposes, parametric studies of tunnels in a single muddy clay and a single silty clay using verified numerical modelling are carried out. In addition, the dynamic bending moment and axial forces obtained by centrifuge and numerical modelling are compared with the solutions predicted by analytical methods in prior studies.

Influence of buried depth on seismic capacity of underground subway stations through performance-based evaluation

Buried depth is an important physical factor that affects the seismic performance of underground structures yet remains scarcely considered in existing seismic design. This paper aims to determine the influence of buried depth variation on the seismic capacity of underground subway stations in the framework of performance-based seismic design with particular focus on the influence on the inter-storey drift ratio limit. A brief introduction is presented to determine the performance level of subway station structures based on the pushover method. The influence of buried depth on the seismic performance index (inter-storey drift ratio) of subway station structures is investigated, as well as its influence on the shear distribution and failure mode of the key structural component, that is the middle column. It is found that the inter-storey drift ratio limit generally decreases with increasing buried depth and the station ductility worsens. The failure mode of the middle column changes from large eccentric compression to small eccentric compression with increasing buried depth owing to increased axial compression. The results are applied to estimate an optimum buried depth that improves the overall safety of subway station structures.

THE SEISMIC BEHAVIOUR OF TUNNELS IN LIQUEFACTION SOIL: A NUMERICAL STUDY

Underground structures can be severely damaged during strong earthquakes. Potential damage to infrastructure due to uplift increases in liquefied soils. Several models can be used to simulate soil liquefaction. This study presents a new numerical method to estimate the overall response of shallow-buried tunnels and to interpret the liquefaction mechanism in saturated sand around the structure under seismic action. The new method stems from the energy-based concept that can be used to compute the excess pore water pressure () and turn it into external forces that would cause the uplift of the underground structure. This study uses a comparative numerical study of implementing the computer code, FLAC on the seismic performance of underground structures using this energy-based approach procedure to estimate pore pressure built-up and evaluate the underground structures' uplift. Moreover, the proposed pore water pressure model is validated against well-documented laboratory data. Also, several study parameters have been considered to investigate the seismic behavior of tunnels.

Upgrading the seismic performance of underground structures by introducing lead-filled steel tube dampers

Central-column failure can cause large underground structures to collapse during strong earthquakes. The use of lead-filled steel-tube dampers (LFSTDs) in the seismic design of such structures is proposed to mitigate or prevent such an eventuality. Three-dimensional nonlinear dynamic analyses of the Daikai subway station with and without LFSTDs under the action of the Hyogoken–Nambu earthquake has been performed, considering soil-structure interaction. The feasibility of the proposed method was verified from four aspects—force, deformation, damage, and energy dissipation. The effects of variability in seismic inputs, strength ratio, and arrangement mode of dampers on the structural dynamic response characteristics have been discussed. Results reveal that the application of LFSTDs to large underground structures remarkably reduces the force, deformation, and damage associated with the central column under seismic loading. Additionally, a strength ratio of 0.4 is recommended when using LFSTDs, whereas in cases without LFTSTDs, the strength of the central column must be appropriately enhanced.

Study on the Influence of Buried Depth on Seismic Performance of Underground Structure

With the large-scale construction of urban rail transit in China, subways often cross the bottom of surface buildings or are located near them. In order to study the complex catastrophic correlation effects of the underground structure of the subway, the soil and the adjacent existing structures under earthquakes, we performed numerical simulation calculations based on ANSYS to conduct research on the effects of underground structures and adjacent existing structures under earthquakes. The interaction law of nearby existing buildings, the influence mechanism of the neighboring building’s depth factors on the seismic response of the complex system, and the system’s seismic response law were determined.

Effect of vertical ground motions and overburden depth on the seismic responses of large underground structures

Underground structures such as subway stations, which are operated at a high capacity and frequency, are prone to damage by large earthquakes. To study the seismic performance of underground structures, this paper presents a three-dimensional nonlinear finite element simulation of the seismic behavior of subway stations, considering the influence of vertical ground motions and structural overburden depth. The results show that the consideration of vertical seismic motions increases the structural seismic responses; the influence extent depends greatly on the characteristics of the vertical seismic excitation. The station′s overburden depth influences the structural response significantly and alters the structural resonant frequency; the influence of the structural burial depth is a complex problem. Besides, it is deduced that the influence of structural burial depth tends to vanish with an increase in the induced frequency, particularly for deeper stations; more attention should be paid to deeper structures built on soft sites or deeper structures that suffer from long-period seismic shaking; the stiffer tunnel experiences less deformation, and the lateral earth pressure applied on the stiffer tunnel is higher. A few results in this paper are consistent with a damage survey of the Daikai subway station damaged in the 1995 Kobe Earthquake.

Seismic Performance Levels of a Large Underground Subway Station in Different Soil Foundations

ABSTRACT With the soil constitutive model developed for the dynamic large deformation of soft soil, a finite element model was constructed to simulate the soil–underground structure static and dynamic coupling interaction system, in which the dynamic contact between soil and underground structure and the dynamic damage of reinforced concrete were considered. The seismic performance and damage mechanism of the large underground subway station in different soil foundations were investigated and evaluated in detail. It was proven that the interaction mechanism between the soil and underground structure would change with the type of soil foundation and the peak acceleration of input ground motion, which can be evaluated by the interaction coefficient between the soils and underground structure. At the same time, some regulations on the seismic performance of underground structure in China code are already unsuitable for evaluating the seismic performance of a large underground subway station in the soft soil foundation or subjected to a strong earthquake, which should be investigate by the specialized time–history analysis method instead. In this study, five seismic performance levels were firstly defined by the inter-storey drift angle of underground structure and corresponding earthquake damage states.

Large-Scale Shake Table Test on Behavior of Underground Structure with the Curved Portion During an Earthquake

Underground structures are generally considered to have high seismic performance and expected to play an important role as a base for reconstruction even after a destructive earthquake. Rigidity changing points, such as jointed and curved portions of underground structure, where localized deformation and stress is supposed to be generated, are ones of the most critical portions in terms of seismic performance of underground structure. Because the underground structure in a mega-city functions as a network, local damage could lead to fatal dysfunction. Accordingly, rigidity changing points and their surrounding area could significantly influence the resiliency of urban functions, and it is indispensable to evaluate their seismic performance and dynamic responses during earthquakes. The responses of rigidity changing points and their surrounding area to earthquakes have been tried evaluating by using large-scale numerical analyses, there is no case available where the responses have been measured in detail. For this reason, it is difficult to verify the validity of the results of such evaluations.In light of the above, the shake table test was conducted at E-Defense using a coupled specimen of soil and underground structures to obtain detailed data, especially on the localized responses around rigidity changing points during the earthquake. Based on the data obtained, the behavior of the underground structure with a curved portion at the time of an earthquake was analyzed comprehensively. As a result of the analysis on the test data, it is found that there is a strong correlation between the localized deformation of the curved portion of the tunnel and the displacement of the surrounding ground. In addition, it is necessary to conduct a three-dimensional seismic response analysis not only around the rigidity changing point but also in wider area.

도시철도 개착터널의 확률적 내진성능평가

최근 한반도 주변국들이 강진으로 인해 큰 피해를 입었고, 국내에서도 중진이 몇 차례 발생하여 우리나라에서도 지진재해에 대한 관심이 높아졌다. 특히 지진재해대책법의 제정으로 사회기반시설물의 내진대책이 의무화되어 내진성능평가 및 내진보강기술이 절실히 필요하게 되었다. 지중구조물에 대한 내진성능평가는 지반 동특성의 영향을 받게 되는데, 만일 지반의 동특성이 불확실하면 내진성능평가의 결과도 불확실하게 된다. 본 연구에서는 불확실한 지반의 동특성이 지중구조물의 내진성능평가에 미치는 영향을 분석하기 위해 FOSM방법을 이용하는 방법을 제시하였다. 불확실한 지반 동특성을 확률변수로 가정하고 FOSM방법을 통해 불확실성이 전파되어 구조물의 내진성능평가 결과가 확률변수로 계산되도록 하였다. 이 방법을 서울시 도시철도 개착터널의 일부구간에 적용하고, N값의 불확실성을 고려한 결과 설계하중에 대한 구조물의 내진성능 만족여부를 확률로 계산할 수 있었다. In addition to recent strong earthquakes in neighboring countries, mid-size earthquakes in Korea have brought up attention. Since the Earthquake Disaster Measures Act has been established, the needs for researches on the seismic performance evaluation and seismic retrofit techniques have increased in Korea. Dynamic soil properties affect the seismic performance of underground structures. Therefore, uncertain soil properties will cause an uncertain seismic performance of underground structures. The objective of this study is to investigate the effect of uncertain soil properties to the seismic performance evaluation of underground structures using the First-Order Second Moment method. Uncertain soil properties are considered as random variables and uncertainty is propagated to the decision variable of the performance evaluation process. The proposed methodology is applied to an open-cut tunnel of a part of Seoul Metropolitan Subway system where the N-value is considered as a random variable. The example shows that the proposed methodology is simple, and yet practical.

Study on seismic performance of underground structure subjected to fault displacement and the effectiveness of the isolation measure using polymer material

現行の耐震設計では，地震による“揺れ”については多くの研究がされおり，その対策について検討されている．しかし，断層運動による”ずれ”の影響が考慮されている研究はまだ少ない．そこで，本研究では地盤－地中構造物－断層系の詳細な3次元有限要素解析を行い断層変位による地中構造物への影響を評価する．更に地震の揺れに対する対策として利用されている免震層の断層変位に対する適用性についても検討する．