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The Longxi tunnel was one of the most damaged tunnels during the 2008 Wenchuan earthquake in China. What makes the case interesting is that the tunnel crosses a fault zone. Damage from small to heavy cracking was observed both at the portal and inside the tunnel, while sections close to the fault completely collapsed. A full three-dimensional dynamic finite element model of the tunnel and rock system is used to assess the seismic damage observed in the tunnel and to evaluate the influence of the longitudinal and vertical motions on the seismic response. A comparison between the numerical predictions and the damage reported shows a good agreement. The results indicate that the longitudinal earthquake motion has a significant effect on the response of the tunnel structure and should be considered for the structural design of tunnels in seismic zones. In contrast, the vertical seismic motions can generally be neglected. [ABSTRACT FROM AUTHOR]

• Shaking table tests with scale ratio of 1/10 are conducted for a transition tunnel. • Technical details of the test design and its implementation are fully addressed. • Meaningful test data including dynamic rock pressure on tunnel are obtained. • Change of tunnel section has significant influences on tunnel transverse responses. • Inner lining increases rock-tunnel pressures of TBM due to a larger stiffness. A transition tunnel is generally built to connect a TBM tunnel and a drill-and-blast tunnel, given the different cross sections associated with each construction method. The transition tunnel may be sensitive to earthquake damage due to the fact that the tunnel cross-section and stiffness change along the tunnel axis. To study the seismic performance of the transition tunnel structure, a series of shaking table tests are carried out. The transition tunnel model is divided into four regions to consider the change of cross-section. Key technical details of the test setup are presented, with a focus on: layout of the model, cross-sections and stiffness of the four regions, model materials, modeling procedure and sensors' layout. Based on test results, the acceleration response of the tunnel liner and the dynamic pressure between surrounding rock and tunnel liner are discussed. In addition, the peak acceleration, Arias Intensity, and Fourier spectrum are presented for each of the four tunnel regions, to discuss the influence of the change of the cross-sections and stiffness on the seismic response of the tunnel regions. [ABSTRACT FROM AUTHOR]

Sharp transitions in structure stiffness and/or ground properties have a significant influence on the seismic response of tunnels. These issues are not well understood yet, or at least not well considered during design. An analytical solution is derived to investigate the seismic response of long tunnels, built in non-homogeneous ground, subjected to sinusoidal shear motions. It is assumed that the tunnel is excavated in two different soil deposits that have a sharp contact, and there is a transition zone through the contact. It is also assumed that the tunnels can be represented as beams on an elastic medium. Continuity at the contact between the different contact sections of the tunnel is imposed to solve the governing equations of equilibrium. In addition, wave passage effects along the tunnel are considered by including a phase angle in the far-field displacements. Explicit formulations are obtained for tunnel deflection, bending moments and shear forces. The solution is verified by providing comparisons between its results and those from the Finite Element program ABAQUS. A parametric analysis is presented where the effects of the stiffness of the structure, the shear velocity of the soil and the length of the transition zone are investigated. [ABSTRACT FROM AUTHOR]

Abstract: The occurrence of geologic planes is a major feature of rock mass. In underground excavation, the intersection of geologic planes and excavation surfaces generates blocks. A new methodology for block identification is presented. This methodology employs finite element (FE) modeling technique to identify blocks so it is different from the traditional topology-based block identification algorithms. As complex spatial excavation surfaces are precisely simulated by FE model, this methodology can identify blocks considering complex profile and layout of underground caverns. By considering the finiteness of geologic planes, convex and concave blocks can be both identified. As is defined on FE model, the identified block system can be visualized directly by employing FE graphic system. As a result, there is no need to specially develop graphic system for block visualization. A case study is done on a large scale hydropower underground cavern complex. The block identification results prove the effectiveness of the proposed methodology. The results are further compared with the observation data derived from in situ investigation. The comparison indicates that the proposed block identification methodology is reliable. Therefore, it provides a new approach for block analysis. [Copyright &y& Elsevier]

Abstract: When an underground pipe works in high water pressure, cracks often occur in the concrete lining and cause the inner water flowing out to the rock mass. For simulating the hydraulic–mechanical interaction in the process of cracking, a coupling method which based on the three-dimension elasto-plastic finite element method (FEM) is proposed in this paper. The coupling principle can be described as follows: the change of stress field leads to the change of permeability coefficient and the redistribution of the seepage field; the new seepage field will produce new seepage force and lead to the change of stress field. The crack expansion and its influence on the seepage and stress fields are studied in paper. As an engineering example, the coupling model is applied in the structural analysis of the lower bifurcated pipes and the surge chamber of Xiaowan Hydropower Station in China, and the computation results show the coupling method is reasonable. [Copyright &y& Elsevier]

Highlights • Post fire slab-column/wall underground structure is assessed by pushover cyclic tests. • Columns are key members and a catenary mechanism forms after columns shear failure. • Fire of 5 MW intensity causes substantial damage to structure, lacking of capacity. Abstract After a fire, underground concrete structures, specifically in subway stations, are often repaired and reused rather than demolished. The mechanical performances of a structure after being exposed to fire require intensive evaluation before a station is reopened. This paper describes the mechanical behavior of a post-fire subway station structure by means of a structural test. A large-scale slab-column/wall structure was exposed to fire. After one of the columns damaged by fire was repaired, a combination of constant vertical load and cyclic lateral load was applied on the repaired structure. A cyclic pushover test was carried out to discover the mechanical behavior of the post-fire structure under soil pressures and service loads. Damage was accumulated as cyclic loadings increased until the structure failed due to shear crushing of the repaired column. The hysteretic loops of the entire structure were thin, indicating that fire exposure decreased the energy dissipation. Results show that the fire caused substantial damage to the structure, resulting in reduced load bearing capacity, deformation capacity, and seismic resistance. [ABSTRACT FROM AUTHOR]

• A 3D dynamic model is proposed to describe the mechanical properties of faults. • The model can describe the normal stress dependent behavior that occurs during shear. • The effects of a fault on the seismic behaviors of underground caverns are assessed. Faults generally have detrimental effects on the stability of underground caverns. During earthquakes these destructive effects are magnified. In this paper, a fully three-dimensional (3D) dynamic finite element model is developed to assess the effects of a fault on the seismic behaviors of underground caverns. A nonlinear thin-layer element model is developed to describe the complex mechanical properties of the fault. The developed model is first verified by two traditional shear experiments. Then the effects of fault parameters (e.g. the intersection angle, fault thickness, fault shear strength), in situ stress states and the intensity of earthquake on seismic response of underground cavern are analyzed. The predicted results show that when the fault strike is parallel to the longitudinal axis of the cavern and the intersection angle between the fault and the vertical axis in the cavern cross section is approximately 40°, the cavern is in the most unstable state. In addition, the cavern is likely to suffer a collapse when critical peak ground acceleration (PGA) is around 0.3 g under the cases studied in this work. [ABSTRACT FROM AUTHOR]