Response Of Underground Structures Research Articles

Analysis of thermo-mechanical damage around tunnel and deposition boreholes of an underground nuclear waste disposal facility at the Forsmark site (Sweden) by 3D coupled FDEM simulations

This study focuses on rock damage potentially developing in the near-field of a planned underground nuclear waste repository at the Forsmark site (Sweden). In hard, crystalline rocks, mechanical damage in the form of spalling may be induced during construction by overstressing of the excavation periphery. During operation, thermal damage may develop due to additional thermo-elastic stresses forming in response to the increasing rock temperatures induced by the heat-emitting spent nuclear fuel. Prediction of damage occurrence, location, and extent is critical for an effective repository design and long-term safety assessment as it may negatively affect the long-term isolation properties of the host rock. In this paper, the response of underground structures was studied using a novel 3D coupled thermo-mechanical simulator based on the finite-discrete element method (FDEM). It is the first numerical study to date that explicitly captures both mechanical and thermal fracturing processes while using the latest repository design and site-specific geomechanical input data. A sensitivity study is performed to investigate different combinations of rock mechanical properties, in-situ stresses, and deposition tunnel geometry on the host rock behaviour. Rock mass deconfinement is shown to promote the development of tensile damage in the tunnel sidewalls and floor with fracture surfaces growing parallel to the excavation boundaries. The negative effects deriving from the adoption of a relatively narrower tunnel cross-section and from an increase of horizontal in-situ stresses are highlighted. Thermo-mechanical analyses capture the rock mass behaviour following an increase of borehole surface temperature to 100 °C. Numerical results indicate that the temperature evolution is affected by the shape of the underground cavities and their distance from the heated boreholes. The coupled thermal expansion of the rock induces additional stresses which, in turn, promotes further damage. Despite this increase, however, the total amount of induced rock damage at final conditions remains relatively low.

A numerical framework for underground structures in layered ground under inclined P-SV waves using stiffness matrix and domain reduction methods

A numerical framework was proposed for the seismic analysis of underground structures in layered ground under inclined P-SV waves. The free-field responses are first obtained using the stiffness matrix method based on plane-wave assumptions. Then, the domain reduction method was employed to reproduce the wavefield in the numerical model of the soil–structure system. The proposed numerical framework was verified by providing comparisons with analytical solutions for cases involving free-field responses of homogeneous ground, layered ground, and pressure-dependent heterogeneous ground, as well as for an example of a soil–structure interaction simulation. Compared with the viscous and viscous-spring boundary methods adopted in previous studies, the proposed framework exhibits the advantage of incorporating oblique incident waves in a nonlinear heterogeneous ground. Numerical results show that SV-waves are more destructive to underground structures than P-waves, and the responses of underground structures are significantly affected by the incident angles.

An Arc-Consistent Viscous-Spring Artificial Boundary for Numerical Analysis of Seismic Response of Underground Structures

A new arc-consistent viscous-spring artificial boundary (ACVAB) was proposed by changing a traditional flat artificial boundary based on the theory of viscous-spring artificial boundaries. Through examples, the concept underpinning the establishment and specific setting of the boundary in the finite element software were described. Through comparison with other commonly used artificial boundaries in an example for near-field wave analysis using the two-dimensional (2D) half-space model, the reliability of the ACVAB was verified. Furthermore, the ACVAB was used in the numerical analysis of the effects of an earthquake on underground structures. The results were compared with the shaking table test results on underground structures. On this basis, the applicability of the ACVAB to a numerical model of seismic response of underground structures was evaluated. The results show that the boundary is superior to common viscous-spring boundaries in terms of accuracy and stability, and therefore, it can be used to evaluate radiation damping effects of seismic response of underground structures and is easier to use.

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.

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.

Mountain tunnel under earthquake force: A review of possible causes of damages and restoration methods

Abstract Accurate seismic assessment and proper aseismic design of underground structures require a comprehensive understanding of seismic performance and response of underground structures under earthquake force. In order to understand the seismic behavior of tunnels during an earthquake, a wide collection of case histories has been reviewed from the available literature with respect to damage classification, to discuss the possible causes of damage, such as earthquake parameters, structural form and geological conditions. In addition, a case of Tawarayama tunnel subjected to the 2016 Kumamoto earthquake is studied. Discussion on the possible influence factors aims at improving the performance-based aseismic design of tunnels. Finally, restoration design criterion and methods are presented taking Tawarayama tunnel as an example.

Seismic fragility and risk assessment of an unsupported tunnel using incremental dynamic analysis (IDA)

Seismic assessment of underground structures is one of the challenging problems in engineering design. This is because there are usually many sources of uncertainties in rocks and probable earthquake characteristics. Therefore, for decreasing of the uncertainties, seismic response of underground structures should be evaluated by sufficient number of earthquake records which is scarcely possible in common seismic assessment of underground structures. In the present study, a practical risk-based approach was performed for seismic risk assessment of an unsupported tunnel. For this purpose, Incremental Dynamic Analysis (IDA) was used to evaluate the seismic response of a tunnel in south-west railway of Iran and different analyses were conducted using 15 real records of earthquakes which were chosen from the PEER ground motion database. All of the selected records were scaled to different intensity levels (PGA=0.1-1.7 g) and applied to the numerical models. Based on the numerical modeling results, seismic fragility curves of the tunnel under study were derived from the IDAcurves. In the next, seismic risk curve of the tunnel were determined by convolving the hazard and fragility curves. On the basis of the tunnel fragility curves, an earthquake with PGA equal to 0.35 g may lead to severe damage or collapse of the tunnel with only 3% probability and the probability of moderate damage to the tunnel is 12%.

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.

Theoretical and Numerical Analysis on the Seismic Response of Underground Structures in Existence of Isolators

In this paper, an analytical method of the beam with springs and dampers fixed at the ends was proposed based on equivalent single degree of freedom (SDOF) system and secondary Lagrange’s dynamic equations, in order to develop a new effective method to enhance the aseismic capability of underground structures. The dynamic response of elastically supported and damply supported beams subjected to both seismic loading and static axial loading was analyzed by the proposed analytical procedure. The theoretical results were validated by the numerical simulation. In order to further investigate the effects of springs and dampers fixed at the ends of the columns in nonlinear response situation, the 3D nonlinear seismic responses of the Dakai metro station structure with and without the isolators were analyzed by ABAQUS respectively. It is demonstrated that: (1) the proposed analytical procedure can predict the dynamic response of beams with elastic and damper supports subjected to both seismic loading and axial loading. (2) Setting isolators at the supports of the column could enhance the aseismic capability of the structure effectively. (3) The axial static loading induced by the gravity of the soil and structure provide the constraint on the column, and therefore could not be neglected in the structural dynamic analysis.

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.

Stochastic response of underground structures

This investigation uses a linear elastic finite-element method to analyse the stochastic dynamic response of the Bolu tunnel in Turkey when subjected to spatially varying earthquake ground motion. The research conducts a parametric study to discover the critical magnitude of earthquake ground motion that causes tensile cracking in the reinforced concrete tunnel lining. The Drucker–Prager yield criterion determines the forces that cause cracking of reinforced elements of three cross-sections of the tunnel system subjected to spatially varying seismic ground motion. The study applies spatially varying ground-motion models, wave passage, incoherence and site response as well as the uniform ground-motion model to three cross-sections of the tunnel–soil deposit interaction system. The purpose is to determine the stochastic seismic behaviour of the tunnel. Of particular emphasis is the importance of the site response effect that arises from the difference in the local soil conditions at the different support points of the tunnel–soil deposit interaction system. The results of the analyses demonstrate that the number of cracked elements increases as the amplitude of earthquake ground motion increases. In addition, the means of the maximum values obtained from the spatially varying ground-motion case are compared with those of specialised earthquake ground-motion models. The result is proof that variations in local soil conditions have important effects on the stochastic response of tunnels.

Dynamic load analysis of underground structure under effect of blast wave

A semi-analytical method of solving the problem of dynamic stress concentration of arbitrary underground structure under the effect of blast waves was introduced. Using the Fourier transform theory, the shock waves (in the forms of SH-waves) can be converted into frequency bands. After employing complex functions and conformal mapping, the admittance functions of various underground structures were obtained. Then, the problem of the time domain dynamic stress response of underground structure can be easily solved through the Fourier inverse transform. At last, the results and curves of the dynamic stress for the square, triangle and horseshoe cavity were presented.

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.

Dynamic Similitude of Underground Structures

Similitude requirements for predicting the response of dynamically loaded buried structures by means of scale models were developed by dimensional analysis and were checked experimentally. Hollow circular and square aluminum tubes were buried in dry compacted Ottawa sand and loaded dynamically by dropping a cylindrical weight on the soil surface. The strains induced on the inner surfaces of the tubes were measured using SR-4 strain gages. Complete strain-time curves and data indicating the variation of peak strain and rise time with depth of burial are presented. Reasonable correlation is obtained between both the peak strain data and the complete strain-time curves for the several structures tested. In general, the study demonstrates the feasibility and value of the use of scale models for the prediction of the dynamic response of underground structures.