Dynamic Response Of Underground Structures Research Articles

A three-dimensional numerical simulation method for seismic nonlinear response of underground structure in liquefiable site and analysis of structural damage

Considering the effect of dynamic interaction between liquefiable soil and underground structure, simulating the nonlinear seismic response of underground structures through simple, convenient, and reliable 3D numerical model is still a great challenge at present. A three-dimensional numerical simulation method for dynamic response of underground structure in liquefiable site is proposed based on the centrifuge shaking table test designed for a subway station buried in liquefiable interlayer site carried out in the previous period. The numerical model takes into account the characteristics of sand prone to large deformation after liquefaction and the nonlinear characteristics of the contact between saturated soil and structure. The typical numerical calculation results are compared with the experimental results to verify the correctness of the numerical model, and the extended analysis of the structural damage is carried out, which visually shows the damage distribution of structure under different functional states after earthquake. The analysis of structural damage shows that the junction position of middle column and longitudinal beam is more prone to tensile damage under the horizontal earthquake action; while after considering the horizontal-vertical seismic effect, the middle column will produce additional compressive damage due to the vertical inertia force, and the end of middle column should be paid attention to in seismic design.

Experimental and Numerical Study on Dynamic Response of Underground Structure in Coral Sand Under Earthquakes

ABSTRACT The seismic performance of the liquefiable coral sand sites with underground structures is unclear. To explore the particular dynamic response of coral sand-underground structure, the shaking table tests in conjunction with numerical calculations were carried out. The soil-structure separation, frequency spectrum variation and seismic fortification of underground structures in coral sand were discussed. The results show that the soil liquefaction increases the energy dissipation of the coral sand-underground structure system. However, attention should be paid to the influence of the coral sand nonlinear development near the groundwater level on the seismic stability of the underground structure during strong earthquakes.

Plane shock wave propagation in partially saturated soil

Considered that the gas content of partially saturated soil would decrease under a shock load, which would cause volumetric plastic deformation, Lyakhov's constitutive model is improved. The P-α state equation is used to describe the volume compression relationship of partially saturated soil. Numerical and experimental results are compared to validate the material model. The peak stress and duration of shock wave are crucial to study the dynamic response of underground structures. Considered the saturation of partially saturated soils under natural conditions increases with depth and both peak stress and duration change when shock waves propagate in inhomogeneous media, several partially saturated soil models with linearly decreasing air content in the depth direction are developed. The propagation law of shock waves with various peak stresses and durations in partially saturated soils is studied. The results show that the peak stress caused by the incident load in partially saturated soil first decreases and then increases under incident loads with a shorter duration, and it increases monotonically under incident loads with a longer duration. The rate of change in the incident peak stress is proportional to the air content at the top of the partially saturated soil. The peak free-field stress reaches its maximum, which is 2.2 to 5.5 times the peak stress of the incident load, in fully saturated soil. Long-duration incident loads such as nuclear explosions are more threatening to underground structures than short-duration incident loads. Underground structures buried in fully saturated soil or below fully saturated soil with higher air contents above are more at risk than underground structures buried in partially saturated soil with smaller air contents.

Seismic Intensity Measures and Fragility Analysis for Subway Stations Subjected to Near-fault Ground Motions with Velocity Pulses

ABSTRACT The purpose of this study is to explore the optimum seismic IMs for the probabilistic seismic demand model of subway stations subjected to near-fault seismic excitations with velocity pulses, and establish the seismic fragility curves based on the optimum seismic IMs. A two-storey three-span subway station is used herein as a representative case study for the cut-and-cover rectangular underground structure. Because the dynamic response of underground structure is closely related to the properties of the surrounding soil/rock, four typical classes of engineering sites covering Classes I to IV are selected in accordance with the Chinese code for seismic design of urban rail transit structures. Nonlinear time history analyses considering soil–structure interaction (SSI) are conducted on the two-dimensional numerical model of the subway station embedded in four different engineering sites. An ensemble of 121 near-fault seismic excitations with velocity pulses is used in seismic dynamic analyses so as to define the optimum IMs. Each seismic excitation is converted into the free field load at artificial boundaries of discretized numerical model of SSI system. The efficacy of 21 commonly used ground motion IMs for predicting seismic response of shallowly buried subway stations is discussed in this study. Four different criteria characterizing the adequacy of IMs, including efficiency, practicality, proficiency and sufficiency, are used in identification of the optimum IMs, on account of the statistical regression results of the selected IMs and engineering demand parameters of the subway station structure measured by maximum inter-story displacement ratio. It is concluded that for engineering sites of Classes I and II, the sustained maximum acceleration followed by peak ground acceleration are the optimum IMs, and for engineering sites of Classes III and IV, the velocity spectrum intensity is the optimum IM. Finally, based on the optimized seismic IMs, the fragility curves of subway station embedded in different site classes are developed in this study.

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.

Structural dynamic response of underground structure with vertical ground motion input

Since many cases of structural damage in past earthquakes have been attributed to strong vertical ground motion, our understanding of vertical seismic load effects and their influence on seismic performance of subway station structure is limited. In this study, the Daikai subway station is taken as a typical example. A two-dimensional finite element model of both soil and structure was established using finite element software ABAQUS. Two input approaches of ground motion are considered, including the horizontal component alone and the vertical and horizontal motions simultaneously. Four groups of ground motion records are selected according to the site type of this station and scaled to the strong intensity which can make the station damage. Results show that the vertical seismic load increases the axial force of the column component apparently, while horizontal seismic load has little effect on axial compression ratio.

Dynamic Response of Underground Structure Subjected to Collapse-touchdown Impact Loading

As the urbanization and urban planning in China make progress, many buildings are facing demolition. These buildings to be demolished are often located close to various underground structures built in many cities. Therefore, the impact loading due to the touchdown of these buildings will inevitably affect the underground structures. In this paper, field tests were used to analyze the response of underground structure subjected to the impact loading due to the collapse touchdown of superstructure. The dynamic response of underground pipeline to the collapse of a single-span viaduct was analyzed. The results showed that the vibration responses of both the soil and structure are mainly vertical, which generally attenuate rapidly with the distance away from the collapse touchdown area, and also the impact effect has little influence on small pipes.

Anti-plane dynamic response of a non-circular tunnel with imperfect interface in anisotropic rock mass

In anisotropic rock mass, the dynamic response of a non-circular tunnel with imperfect interface is theoretically presented under an incidence of an anti-plane SH wave. For the anti-plane problem in anisotropic medium, three elastic constants can describe the characteristics of medium anisotropy according to the generalized Hooke’s Law. The wave function expansion method and the complex function are applied to express the wave fields with three unknown coefficients. By introducing the spring elastic model, the imperfect boundary condition around the tunnel lining can be satisfied to determine the coefficients. In numerical examples, effects of rock anisotropy and imperfect interface around the tunnel lining are discussed in detail, accompanied with incident angle and wave frequency. Analytical solutions show that both rock anisotropy and interface have considerable influence on the dynamic response of underground structures, which should be paid attention to for the design and construction of deep underground tunnels.

Effect of Underground Stress Waves with Varied Wavelengths on Dynamic Responses of Tunnels

Underground dynamic loads are usually caused by underground railways, drilling, blasting and so on. Compared with the dynamic loads on the ground, in the condition of underground dynamic loads, there are some differences in the propagation characteristics of stress waves and the effect on dynamic responses of underground structures. In this paper, the underground dynamic loads expressed as harmonic waves were assumed to be applied at some distance from the top of a tunnel. Based on the finite difference software FLAC3D, the harmonic waves of varied wavelength–diameter ratios were considered in the analysis, in order to obtain the regularities of dynamic responses of the tunnel. Accelerations, velocities, maximum principle stresses and minimum principle stresses for the tunnel under different wavelength–diameter ratios were calculated in the numerical simulations. The variations of the peak accelerations, peak velocities, peak maximum principle stresses and peak minimum principle stresses with wavelength–diameter ratios were obtained. The calculation results can help provide a reference for dynamic protection of underground structures.

Significance of Soil Compaction on Blast Resistant Behavior of Underground Structures: A Parametric Study

Dynamic response of underground structures has always been a topic of concern for designers and researchers. The behavior of these complicated systems under blast loading is affected by various factors and parametric studies are required to investigate their significance. The importance of soil density around the underground structure through which, the waves of explosion of a penetrator bomb is transferred, has been studied in this paper by using finite difference method (FDM). According to the results, soils with higher degrees of compactioncan absorb explosion energy more significantly. Therefore, the displacements and stresses of underground structure lining in denser soils are moderately lower. Thebending moment of the lining should be given attention, as regards being a critical design parameter.

Analytical solution of transient dynamic response of spherical cavity in viscoelastic and saturated soils

Based on the Biot's theory of wave propagation in porous media, this paper studies the transient dynamic response of spherical cavity in viscoelastic and saturated soils. The analytical solution in transformed domain is obtained by the method of Laplace transformation, and numerical results are solved through inversed Laplace transformation. The displacements, stresses and pore water pressure developed in the viscoelastic soils are analyzed and compared with those from elastic model, and some new observations are discussed and interpreted from the perspective of material's properties. The findings resulting from the current study are helpful to analyzing the transient dynamic responses of underground structures in the engineering practice.

Influence of Overlying Soil on Dynamic Response of Underground Structure

The article studies the influence of overlying soil thickness on dynamic response of underground stuctures by adopting the method of numerical simulation analysis. It carries on numerical analyses of dynamic response to the underground structures which overlying soil thickness is 0m、1.4m、2.8m、7.0m、14m、21m、28m. The results show that with the increase of overlying soil thickness, the horizontal displacement difference of bottom and top of side walls increases gradually. When the overlying soil thickness exceeds the height of underground structures, the displacement difference basically presents the rule of linear increase, while the displacement difference of the surrounding soil in the same depth do not change with the overlying soil thickness.

Dynamic Response of Underground Structure under Vertical Earthquake Action

The research of stability of underground structures under earthquake action is one of the hot topics in Geotechnical Engineering researches .Through numerical simulation the article studies the dynamic response rules of the underground structure under vertical earthquake action. The result shows that under vertical earthquake action the vertical stress on the roof and bottom plate of underground structure and the same depth in the earth both increases, and the vertical stress on the roof and bottom plate of underground structure is much higher than that on the same depth in the earth (above two times); under vertical earthquake action the underground structure experiences greater vertical stress from time to time, and produces vertical compressive deformation, and sometimes it completely separates from the above soil layer.

Analysis of the Dynamic Response of Underground Structures under Internal Explosion

Analytic solution for the dynamic response of underground structure is generally based on the theory of cylindrical shell. The underground structure constructed to resist internal blast is not suitable to be treated as cylindrical shell structure and is difficult to get analytic solution. In this paper, finite element code LS-DYNA is employed to calculate the dynamic response of underground blast-resistant structures exposed to internal blast. Two structures with different thickness buried in four types of surrounding rocks are calculated, and the influence of the surrounding rocks to the dynamic response of underground blast-resistant structures is analyzed.

Dynamic response of utility tunnel during the passage of Rayleigh waves

The modeling methodologies and calculation of dynamic response of underground structure under Rayleigh waves is investigated in this paper. First the free field responses under Rayleigh waves are analyzed and the numerical results agree well with the theoretical results. Then, the approximate Rayleigh waves are put forward based on the preliminary research, and Rayleigh wave field is obtained through fast Fourier transform technique. Taking a utility tunnel as an example, its dynamic responses under Rayleigh waves is calculated by ABAQUS. The results demonstrate that bending deformation is the main component of structural deformation and the deformation at the top of the structure is about twice as much as that at bottom of the structure. The effect of soil-structure interface and the buried depth of underground structure are also investigated via parameter analysis. For the shallow buried underground structures, Rayleigh waves can be the key factor to control the responses and damage of the structure.

Insights into ground shock in jointed rocks and the response of structures there-in

Ground shock in rock masses is created by dynamic events such as rock-bursts, coal bumps or man-made explosions. This article examines two aspects of the phenomenology: the propagation of ground shock, and its effects on underground structures. Numerous experiments, at all scales, as well as numerical modeling clearly show that geologic discontinuities have a large influence on ground shock propagation through their actions as wave-guides or as decoupling features. Three-dimensional discrete element (DE) numerical models provide additional insights into the dynamic response of underground structures as influenced by joint orientation, joint spacing, characteristics of the loading pulse (duration, peak velocity, peak displacement), and repeated loadings. It is also shown that in hard rocks the deformability of rock blocks has a negligible influence on tunnel dynamic structural response compared to that of the joint sets or bedding. When viewed together with pictures of underground failures, the 3-D DE models are shown to capture quite well the kinematics of jointed rocks, and their power is illustrated through a very large simulation of an underground facility under dynamic loading.

Dynamic Response Of Underground Structures In Soils With Variable Mechanical Properties

In this work, we develop a formulation based on the Boundary Element Method (BEM) for evaluating the dynamic response of underground structures excited by seismically-induced. steady-state, horizontally polarized shear (SH) waves. The surrounding geological medium 1s viewed as an elastic continuum exhibiting large randomness in its mechanical properties. Suitable Green's functions are proposed and subsequently used within the context the BEM formulation. More specifically, the methodology developed herein employs a series expansion for the proposed Green's functions. where the basis functions are orthogonal polynomials of a random argument. These functions are incorporated in a BEM formulation, which is then used for the solution of problems of engineering interest. The presented approach departs from earlier BEM derivations based on perturbations, which were only balid for small amounts of randomness in the elastic continuum.

Dynamic Responses of Underground Structures to Incident P, SV and Rayleigh Waves

Dynamic Responses of Underground Structures to Incident P, SV and Rayleigh Waves

Dynamic response of underground structures to pressure waves: The strip approximation

A simplified method is proposed to treat the dynamic response of rigid embedded structures in either a full or a half-space subjected to plane P-waves. The soil-structure interface is discretized into a finite number of strips, resulting in an analysis which involves only plane surfaces and plane waves. Interaction forces on any strip are determined easily, and the contributions from all the strips are superposed to obtain the resultant forces and torque on the structure. Equations of motion then yield the steady state response of the structure. The Green function for the structure is obtained by Fourier synthesis, and the transient response to any other excitation is then obtained by using the Green function and the Duhamel integral. The entire analysis can be conveniently programmed.

Dynamic response of underground structures by time domain SBEM and SFEM

The dynamic interaction problems of three-dimensional linear elastic structures with arbitrary shaped section embedded in a homogeneous, isotropic and linear elastic half space under dynamic disturbances are numerically solved. The numerical method employed is a combination of the time domain semi-analytical boundary element method (SBEM) used for the semi-infinite soil medium and the semi-analytical finite element method (SFEM) used for the three-dimensional structure. The two methods are combined through equilibrium and compatibility conditions at the soil-structure interface. Displacements, velocities, accelerations and interaction forces at the interface between underground structure and soil medium produced by the diffraction of wave by an underground structure for every time step are obtained. In dynamic soil-structure interaction problems, it is advantageous to combine the SBEM and the SFEM in an effort to produce an optimum numerical hybrid scheme which is characterized by the main advantages of the two methods. The effects of the thickness, the ratio of length and diameter of underground structure and the soil medium on dynamic responses are discussed.