Face Stability Of Shallow Tunnels Research Articles

Analyses on face stability of shallow tunnel considering different constitutive models

Based on the finite element limit analysis method, the stability of the face in case of active failure under three constitutive models, the Mohr-Coulomb model (MC), the modified Cambridge model (MCC) and the Drucker-Prager model (DP), were analyzed. The ultimate support pressure of the face and the influence of factors such as different burial depth ratios (C/D), cohesion (c) and friction angle (φ) in the MC model are also discussed. The results show that the safety factor obtained by the MCC model under the same support pressure is always smaller than that of the MC model, and the difference is the largest when there is no support pressure. As the support pressure increases, it will gradually approach the MC model. When the support pressure is small, the safety factor obtained by the DP model is larger than the MC model, but when the support pressure is large, it is smaller than the MC model, and the final difference tends to be stable. It is necessary to select an appropriate constitutive model according to different rock masses in practical engineering. The self-stabilizing performance of the face is not affected by C/D, and the ultimate support pressure will increase with the increase of C/D, decrease linearly with the increase of cohesion, and decrease with the increase of friction angle. When the friction angle is small, the ultimate support pressure is greatly affected by C/D, and when the friction angle is large, it is hardly affected by C/D.

Two-Dimensional Numerical Model for Stability Analysis of Tunnel Face Based on Particle Flow Code

In this paper, numerical simulations of face stability of shallow tunnels are carried out by using the particle discrete element codes PFC2D from the microscopic view. Progressive instability of the tunnel face is achieved through the withdrawal of the baffle used to simulate the tunnel face. Under different retreating displacement of the tunnel face, the evolution laws of support pressure on the tunnel face, the ground deformation, ground surface settlement and contact force chain in front of the tunnel face are studied. The results show that with the withdrawal of the tunnel face, the support pressure on the tunnel face can be divided into four stages, namely, the rapid decline stage, the minimum stage, the slow rising stage, and the horizontal stability stage. Moreover, based on the simulation results of the particle contact force chain, discriminated methods of failure zones are proposed. The research results obtained from this paper will provide theoretical support for the reasonable value of support pressure of a tunnel face in practical engineering.

Simulation of subway tunnel construction process based on fi-nite element analysis

Simulation of subway tunnel construction process based on fi-nite element analysis

Application of constitutive model in simulation analysis of tunnel construction

Application of constitutive model in simulation analysis of tunnel construction

Analysis of Instability Mode and Limit Support Pressure of Shallow Tunnel Face in Sands

The stability analysis of the tunnel face is not only essential for guaranteeing the safe construction of urban shallow tunnels, but also directly affecting the influence degree of tunnel construction on nearby structures. The primary concerns in the stability analysis of the tunnel face are the instability mode of surrounding rocks and the limit support pressure on the tunnel face. In this paper, face stability of shallow tunnels in sands was conducted using a symmetrical model test. The ground surface settlement, support pressure on the tunnel face and progressive instability modes of sands at tunnel face are measured by using an LVDT (Linear Variable Differential Transformer) displacement sensor, high-precision pressure sensor and high-definition digital camera, respectively. The test results indicate that the shear failure band appears in sands in front of the tunnel face and develops from the tunnel invert to the tunnel crown. The upper sands undergo stress redistribution, and the pressure arch appears with initial form of “ellipsoid”, then of the “pyramid”. Moreover, the support pressure on the tunnel face experiences four stages, namely, rapid decline stage, the minimum stage, slowly raises stage and stable stage during tunnel excavation. The research results of this paper will provide theoretical support for the reasonable value of the support pressure on the tunnel face in practical engineering.

Kinematic Stability of Tunnel Face in Non-uniform Soils

In the paper, the kinematic analysis is carried out to evaluate the face stability of shallow tunnel in the framework of plasticity theory. In view of the non-uniformity of soil strength parameters, a discretization technique is introduced to generate possible failure mechanisms which respect the associative flow rule and requirement of the non-uniformity of soil strength parameters. As an earthquake situation is considered, a pseudo-dynamic method is adopted to portray the distribution of seismic acceleration. Some comparisons are made to the conventional analysis, including discretization-based pseudo-static analysis and log-spiral-based pseudo-static analysis, to prove the validity of the programming code and accuracy of the proposed method. The soil strength parameters are considered varying in the vertical direction. The effect of distribution of strength parameters is discussed, and distribution form is initially assumed to be linear profile and, then, to be other nonlinear functional profile, including power profile, constant profile and quadratic profile. The results derived from further parametric study show that although the influence of horizontal seismic coefficient is more significant than the vertical one, the vertical effect cannot be ignored in most circumstances, and some factors affect the stability cyclically. In addition, in the linear profile of soil strength parameters, the influence of gradient on supporting pressure is rather small comparing to the average. The influence of non-uniformity of soil strength parameters and seismic parameters on the shape and collapsed area of the failure mechanism is successively studied in this paper. All the optimized solutions are presented in the figures and tables for practical application.

Kinematic Stability of Tunnel Face in Non-uniform Soils

In the paper, the kinematic analysis is carried out to evaluate the face stability of shallow tunnel in the framework of plasticity theory. In view of the non-uniformity of soil strength parameters, a discretization technique is introduced to generate possible failure mechanisms which respect the associative flow rule and requirement of the non-uniformity of soil strength parameters. As an earthquake situation is considered, a pseudo-dynamic method is adopted to portray the distribution of seismic acceleration. Some comparisons are made to the conventional analysis, including discretization-based pseudo-static analysis and log-spiral-based pseudo-static analysis, to prove the validity of the programming code and accuracy of the proposed method. The soil strength parameters are considered varying in the vertical direction. The effect of distribution of strength parameters is discussed, and distribution form is initially assumed to be linear profile and, then, to be other nonlinear functional profile, including power profile, constant profile and quadratic profile. The results derived from further parametric study show that although the influence of horizontal seismic coefficient is more significant than the vertical one, the vertical effect cannot be ignored in most circumstances, and some factors affect the stability cyclically. In addition, in the linear profile of soil strength parameters, the influence of gradient on supporting pressure is rather small comparing to the average. The influence of non-uniformity of soil strength parameters and seismic parameters on the shape and collapsed area of the failure mechanism is successively studied in this paper. All the optimized solutions are presented in the figures and tables for practical application.

Three-dimensional analysis of excavation face stability of shallow tunnels

Several studies have been focused on minimizing construction risks of underground works. However, additional studies regarding excavation face stability, which has great influence on global performance of shallow tunnels, will offer more security criteria to engineers. The aim of this work is to compare the results from an existing analytical and a numerical method in assessing the behavior of the excavation face subjected to different overburden depth and ground conditions. Typical parameters of shallow soils were considered for all simulations. The analytical method was applied to assess the excavation face pressure for different tunnel covers. Numerical methods were applied using ABAQUS 3D® finite elements software, which considered a continuous, homogeneous medium in a 6-meter-wide tunnel excavated at different depths, and assessed the need for support and stabilization pressure on the face. Furthermore, the influence of a well on the excavation line was also considered for such assessment. As result, excavation face was defined as stable by the analytical method; nevertheless, it was obtained a very low safety factor. On the other hand, results from the numerical method showed the need to apply pressure on the excavation face. Those limit support pressure values at tunnel face showed a direct correlation with the increase of cover-to-tunnel diameter ratio. Finally, the numerical method showed a more realistic result when compared with the analytical method.

Face stability of shallow tunnels in partially saturated soil: centrifuge testing and numerical analysis

In this paper the face stability of shallow tunnels in partially saturated soil is investigated by means of centrifuge testing and numerical analysis. A loess soil is chosen for this study and its properties are characterised by laboratory testing. Centrifuge model tests are carried out at different soil densities and with different depths of water table. The numerical analyses are performed with an advanced constitutive model for partially saturated soil. The constitutive model is implemented in a finite-element code using the Bishop's effective stress. The model parameters are calibrated based on drained triaxial compression tests on partially saturated soil. The wetting collapse of the tunnel face is simulated by imposing a zero pore pressure condition at the ground surface. The numerical results are compared with the centrifuge tests.

Centrifuge model test on the face stability of shallow tunnel

This paper is an investigation of face stability on a small-scale tunnel model in a geotechnical centrifuge. By making use of symmetry, half of the tunnel cross section was considered. The support at excavation face was provided by a piston, which was adjusted during flight. Some aspects on the collapse at tunnel face are investigated for different overburden pressures: failure mechanism, surface settlement, stress acting at tunnel face, and the required face support counteracting the earth pressure. Ground deformation was observed through a transparent wall and measured by digital image correlation. The results from centrifuge model tests were compared with theoretical models.

Experimental investigation of the face stability of shallow tunnels in sand

Various models have been proposed for the prediction of the necessary support pressure at the face of a shallow tunnel. To assess their quality, the collapse of a tunnel face was modelled with small-scale model tests at single gravity. The development of the failure mechanism and the support force at the face in dry sand were investigated. The observed displacement patterns show a negligible influence of overburden on the extent and evolution of the failure zone. The latter is significantly influenced, though, by the initial density of the sand: in dense sand a chimney-wedge-type collapse mechanism developed, which propagated towards the soil surface. Initially, loose sand did not show any discrete collapse mechanism. The necessary support force was neither influenced by the overburden nor the initial density. A comparison with quantitative predictions by several theoretical models showed that the measured necessary support pressure is overestimated by most of the models. Those by Vermeer/Ruse and Léca/Dormieux showed the best agreement to the measurements.