Stability Analysis Of Tunnel Research Articles

Shallow tunnel face stability considering pore water pressure in non-homogeneous and anisotropic soils

An improved approach for the face stability analysis of shallow tunnels in non-homogeneous and anisotropic soil is developed in this study. Based on the discretization technique, an advanced passive failure mechanism is proposed to determine the critical support pressure of the tunnel face in the framework of the upper bound theorem. To better estimate the tunnel face stability, the effects of the pore water pressure and nonuniform ground surcharge on the critical support pressure of the tunnel face are also incorporated in the non-homogeneous and anisotropic soil properties. Then the optimal supporting forces obtained from the proposed failure mechanism are validated by those from the conventional limit analysis model in homogeneous soil and finite element limit analysis in homogeneous and non-homogeneous soils.

Fine Exploration and Control of Subway Crossing Karst Area

A large number of subway projects need to cross all kinds of disaster sources during the construction process. When a disaster source is unknown and uncertain, it is difficult for tunnel stability analysis to conform to the actual situation, which is likely to cause serious geological disasters. Firstly, the accurate location of the source of the disaster is realized via the geophysical method, and the orientation of the target is determined. Secondly, real imaging of the geological disaster source is realized using fine three-dimensional scanning equipment. Finally, the coupling law of the seepage field, displacement field, and stress field of the tunnel surrounding rock are analyzed. The stability of the tunnel is analyzed, and the reasonable karst treatment method is put forward.

The use of Eulerian finite element method in face stability analysis of shield tunnels in undrained clay

This article reports the application of the Eulerian FE method in face stability analysis of shield tunnels in undrained clay. Numerical simulation was first conducted under the total stress framework to verify the accuracy of the approach. Then it was extended to effective stress analysis combined with a critical state model, taking complex behaviours (small strain non-linearity) of soil into consideration. The predictions under the two analysis frameworks were compared in detail, which indicated the small strain formulation of the effective stress constitutive model does not influence the solution for the collapse load but may enhance the predictions of deformation.

Face stability analysis of a shallow square tunnel in non-homogeneous soils

An improved approach for the face stability analysis of shallow square tunnels in non-homogeneous soils is investigated in this study. Based on the discrete technique, an improved 3D failure mechanism is proposed to investigate the face stability of square tunnels by incorporating the soil arching effect in the framework of the limit analysis theory. To better assess the stability issue, the non-homogeneous frictional angle φ is also considered when calculating the critical support pressures. The results obtained from this study are validated by those from the 3D finite element limit analysis and an advanced wedge-prism model based on an existing research on the limit equilibrium method.

Probabilistic Characterization of Rock Mass from Limited Laboratory Tests and Field Data: Associated Reliability Analysis and Its Interpretation

Probabilistic methods are the most efficient methods to account for different types of uncertainties encountered in the estimated rock properties required for the stability analysis of rock slopes and tunnels. These methods require estimation of various parameters of probability distributions like mean, standard deviation (SD) and distributions types of rock properties, which requires large amount of data from laboratory and field investigations. However, in rock mechanics, the data available on rock properties for a project are often limited since the extents of projects are usually large and the test data are minimal due to cost constraints. Due to the unavailability of adequate test data, parameters (mean and SD) of probability distributions of rock properties themselves contain uncertainties. Since traditional reliability analysis uses these uncertain parameters (mean and SD) of probability distributions of rock properties, they may give incorrect estimation of the reliability of rock slope stability. This paper presents a method to overcome this limitation of traditional reliability analysis and outlines a new approach of rock mass characterization for the cases with limited data. This approach uses Sobol’s global sensitivity analysis and bootstrap method coupled with augmented radial basis function based response surface. This method is capable of handling the uncertainties in the parameters (mean and SD) of probability distributions of rock properties and can include their effect in the stability estimates of rock slopes. The proposed method is more practical and efficient, since it considers uncertainty in the statistical parameters of most commonly and easily available rock properties, i.e. uniaxial compressive strength and Geological Strength Index. Further, computational effort involved in the reliability analysis of rock slopes of large dimensions is comparatively smaller in this method. Present study also demonstrates this method through reliability analysis of a large rock slope of an open pit gold mine in Karnataka region of India. Results are compared with the results from traditional reliability analysis to highlight the advantages of the proposed method. It is observed that uncertainties in probability distribution type and its parameters (mean and SD) of rock properties have considerable effect on the estimated reliability index of the rock slope and hence traditional reliability methods based on the parameters of probability distributions estimated using limited data can make incorrect estimation of rock slope stability. Further, stability of the rock slope determined from proposed approach based on bootstrap method is represented by confidence interval of reliability index instead of a fixed value of reliability index as in traditional methods, providing more realistic estimates of rock slope stability.

Tunnel face stability analysis using Kinematical Element Method

This paper introduces a new approach to investigate the tunnel face stability. In the proposed approach a 3D KEM (Kinematical Element Method) model is developed which consists of two rigid blocks, tetrahedron wedge block (lower part) and triangular prism block (upper part). In order to incorporate the influence of the 3D soil arching in silo for predicting the minimum support pressure, it is presumed a 3D active earth pressure acting on the vertical prism’s slip surfaces from the adjoining soil. The results given by the KEM model are compared with the results of numerical simulations, analytical approaches, and physical model tests available in literature. Similar to Terzaghi’s superposition method commonly used in bearing capacity analysis, the minimum support pressure is expressed by summation of cohesion (c), surcharge load (q) and unit weight of the soil (γ) multiplied by non-dimensional bearing capacity coefficients Nc,Nq and Nγ. Simple formulas have been developed to compute the non-dimensional coefficients. The Nγ,Nc, and Nq formulas are presented as function of internal friction angle (φ), and cover depth to tunnel diameter ratio (C/D). The proposed KEM model for the tunnel face stability gives good results in term of minimum support pressure compared to physical model test results, various analytical and numerical solutions for the tunnel face stability analysis of shallow tunnels.

Study of Shear Capacity of Jointed Rock Mass with Prestressed Anchor Bolt

Rock mass failure caused by jointed plane is the common disaster in geotechnical engineering. Research on shearing property of jointed rock mass with prestressed anchor bolt and its mechanical property is an aspect for the stability analysis of rock slope and tunnel. This paper studies the stress and deformation of bolt and jointed plane, as well as anchored body and rock mass during shearing by indoor model testing and theoretical analysis. The shearing strength and mechanical property of anchored jointed plane have been systematically studied, and the additional force caused by deformation of prestressed anchor bolt and jointed rock mass during the shearing has been proposed and verified. The anchorage interaction between rock mass and bolt is revealed.

Study on Face stability analysis of the aeolian sand tunnel

In view of the particularity of the geological environment of the surrounding rock of the aeolian sand, the tunnel will be destabilized due to its poor self-stability during tunnel excavation, which is worthy of attention. In this paper, the three-dimensional discrete element method is used to establish the discrete element numerical model of the excavation surface of the aeolian sand tunnel. The macroscopic mechanical response of the face is obtained and analysed, and the process of instability of the excavation surface is revealed from the macroscopic and microscopic mechanics. Based on this, the reasonable and safe support force of the excavation face is determined. Compared with the existing research, this paper uses the discrete element method to influence the dynamic construction of aeolian sand stratum on the stability of the face, which is closer to the engineering practice. The research results can provide reference for ensuring the stability of the tunnel face in the aeolian sand stratum.

Stability Analysis of Shallow and Bias Loess Tunnel Based on Finite Difference Method

Based on the Mohr-Coulomb elastic-plastic model and the practical engineering background of Mopanshan tunnel, this paper applies the finite-difference software FLAC3D to simulate and analyse the whole process of loess tunnel construction. Then, it analyses the stability of the surrounding rock and sup-port structure after partial excavation of the loess tunnel under the shallow burying and unsymmetrical load-ing condition. The study showed that in the absence of support, the shear failure occurred to the top/upper pilot tunnel of the tunnel face, the failure zone under tensile stress happened to the shallow soil of the earth surface, and the soil of tunnel face appeared to be damaged. Finally, according to the analysis results, a rea-sonable construction method suitable for the shallow and bias loess tunnel is determined.

Efficient 3D probabilistic stability analysis of rock tunnels using a Lattice Model and cloud computing

In this paper, the Classic Lattice Spring Model is developed in order to be applied to probabilistic stability analyses of rock tunnels in low in situ stress environments, where the falling of blocks is the main failure mechanism. For this objective, the method is combined with the Synthetic Rock Mass technique and Barton-Bandis joint constitutive model, giving it a series of advantages: simulations with deformable blocks; possible formation of tension cracks; and an appropriate constitutive model for the mechanical behavior of joints. The use of cloud computing technology is proposed for efficient probabilistic analyses using the Monte Carlo simulation. A case study is presented based on an unsupported section of a shallow rock tunnel excavated in the state of Espírito Santo, Brazil. A 3D probabilistic stability analysis of the tunnel is performed with the proposed methodology. From the results, a positional probability map is elaborated, which indicates the likelihood of block failure around the excavation cross-section. The unstable zones indicated by the map are then compared to the failed region of the real tunnel so as to demonstrate the competence of the methodology.

Face stability analysis of shallow circular tunnels driven by a pressurized shield in purely cohesive soils under undrained conditions

Based on the slip line and limit analysis theories, this paper presents a new 3D collapse mechanism to analyze the face stability of a circular shield-driven tunnel in purely cohesive soil. The new collapse mechanism is a 4-variable multizone mechanism that depends on cover-to-diameter ratios (C/D). The proposed mechanism suggests that failure zones at the tunnel face are primarily formed by shear failure. A comparison of this approach with numerical methods and other kinematic approaches shows that the proposed failure mechanism significantly improves the existing upper-bound solutions for the face stability of circular tunnels in purely cohesive soils.

Stability Analysis and Case Study of Shallow Tunnel Using Pipe Roof Support

Pipe roof system is widely used in the shallow tunnel as the support system. Although the diameter of pipe roof is diversity, the pipe roof system of small diameter pipe roof and small spacing steel grating is widely adopted in China. The stability mechanism of that pipe roof system is to form shed-scaffold system. In that shed-scaffold system, the function of pipe roof is to form micro-soil-arch in overlying soils between the pipes and to transmit the excavation loads. The steel gratings are the main bearing structure. To form the shed-scaffold system effect, three stabilities, stability of soils between pipes, stability of tunnel working face and stability of tunnel foundation, should be satisfied simultaneously. On that basis, the collapse accidents of Tianhengshan highway tunnel and Haide highway tunnel in China are analyzed. It shows that the stability of tunnel foundation would be overlooked easily and causes even overall collapse. At last, the measures to improve the stability are proposed.

Numerical modelling of the underground roadways in coal mines– uncertainties caused by use of empirical-based downgrading methods and in situ stresses

Numerical modelling techniques are not new for mining industry and civil engineering projects anymore. These techniques have been widely used for rock engineering problems such as stability analysis and support design of roadways and tunnels, caving and subsidence prediction, and stability analysis of rock slopes. Despite the significant advancement in the computational mechanics and availability of high speed computing hardware, the input data and constitutive models remain the main source of errors affecting the reliability of numerical simulations. The problem with the input data has been deepened more by introducing empirical-based methods such as GSI classification to downgrade the rock properties from laboratory scale to field scale. The deformability modulus and strength parameters are the main outputs of these downgrading techniques. Numerical modelling users simply apply these downgrading methods and run the model without considering the real mechanics behind the stress induced failure and deformation around the underground excavations. While to the contrary to the commonly used downgrading methods that produce a constant modulus for rock at all depths, the rock modulus is stress dependent and varies with depth. In addition to this, the mechanism of stress induced displacement is not similar to the deformation of a continuum model simulated with equivalent rock properties. Apart from the mechanical characteristics of rocks, the magnitude and orientation of in-situ stresses are two other important parameters that have significant impacts on stress induced rock fracturing. The impacts of these two parameters have also been neglected in many practical cases. This paper discuss this old fashioned topic in more details with presenting the known facts and mechanics which numerical modelling users ignore them due to the unquestioning acceptance of downgrading methods. It also covers the influence of the stress magnitude and orientation on stress induced rock fracturing.

Study on Failure Mode of Shallow Buried Multi-arch Tunnel

The failure mode of surrounding rock of shallow buried multi-arch tunnel was an important content of stability analysis of multi-arch tunnel. Through systematic theoretical study, three-dimensional geo-mechanics model test and field monitoring sample comprehensive means, based on the Hoek-Brown criterion, the numerical analytical solution of the failure mode of surrounding rock of shallow buried continuous arch tunnel was established by the plastic limit analysis, and the three-dimensional geo-mechanical model of the failure mode of the surrounding rock of shallow buried continuous arch tunnel under 3 kinds of excavation methods was studied. Through the field test, the failure mode of shallow buried multi arch tunnel was obtained, and the design theory of shallow buried multi-arch tunnel was improved.

A node-based Smoothed Finite Element Method for Stability Analysis of Dual Square Tunnels in Cohesive-frictional Soils

This paper presents an upper bound limit analysis procedure using the node-based smoothed finite element method (NS-FEM) and second order cone programming (SOCP) to evaluate the stability of dual square tunnels in cohesive-frictional soils subjected to surcharge loading. The displacement field of the tunnel problems is approximated by using NS-FEM triangular elements (NS-FEM-T3). Next, commercial software Mosek is employed to deal with optimization problems, which are formulated as second order cone. Collapse loads and failure mechanisms of dual square tunnels were performed by solving the optimization problems with a series of size-to-depth ratios and soil properties. For dual square tunnels, the distance between centers of two parallel tunnels is the major parameter used to determine the stability. In this study, surcharge loading is applied to the ground surface and drained conditions are considered. Numerical results are verified with those available to demonstrate the accuracy of the proposed method.

Tunnel Stability and Deformation Analysis under Excavation Load

Based on the finite element model of the tunnel influenced by approaching excavation of foundation pit, influence laws of excavation depth and distance on the tunnel stability are studied. The results show that the largest settlement occurs in the left lining, and the maximum uplift occurred in the right lining. The displacement values of the various monitoring section of tunnel structure on the left side are more closer, and the displacement values near the tunnel bus present obvious negative peak displacement. When the excavation distance is increasing, the settlement displacement of the tunnel increases obviously. Local uplift effect caused by unloading of soil gradually disappeared. With the increasing of the approaching distance, the bottom of the foundation pit bears all or part of the passive earth pressure, which makes the active area earth pressure and passive earth pressure area produce greater deformation.

Face stability analysis of large-diameter slurry shield-driven tunnels with linearly increasing undrained strength

The kinematic approach of limit analysis is explored in three-dimensional face stability of large-diameter tunnels in soils with linearly increasing undrained strength with depth. Due to the slurry density by a shield machine, the support pressure is not uniformly distributed on the face. Such a non-uniform distribution cannot be ignored, especially for the large-diameter shield tunnels. This paper includes it into the stability analysis of the tunnel face subjected to local and global failures. The failure mechanism with a spherical cap is adopted to obtain the least upper-bound solutions of local stability of the tunnel face. To evaluate its global stability, a continuous velocity field with a toric envelope is employed and yields the critical pressures on tunnel face against collapse and blow-out. The calculated results are compared with the solutions derived from other well-established methods for verification of the presented approach. An approximation formula based on the derived upper-bound solutions, is given to directly calculate the necessary collapse and blow-out pressures, which can be used for preliminary design in practice. An example is given to illustrate its convenient use.

Analytical and Reliability Study of the Tunnel with Rockbolts in Rock Masses

Rockbolts are a critical reinforcement ways which widely used in tunnel engineering. In this paper, an analytical solution of circular tunnel with rockbolts was proposed based on homogenization method, and then the stability of a circular tunnel was investigated by considering the uncertainty based on the proposed analytical solution. Elastoplastic analytical solution for unsupported circular tunnel was presented briefly in hydrostatic stress field with a linear Mohr-Coulomb yield criterion and a non-associated flow rule. An analytical solution of circular tunnel with rockbolts was proposed through considering rock mass and rockbolts as a new homogeneous, isotropic, parameters strengthened equivalent composite material. A numerical example is used to verify the proposed analytical solution. The results show that the proposed solution can effectively characterize the mechanical behavior of rock mass and rockbolts in tunnel. Then, the proposed solution is adopted to calculate reliability index and failure probability of tunnel. The results show that the proposed method can also be effectively used to perform the stability and reliability analysis of tunnel and rockbolts have an important effect on plastic zone size and displacement of tunnel.

Eulerian finite element model for stability analysis of circular tunnels in undrained clay

Abstract Traditional Lagrangian finite element (FE) method may encounter numerical problems due to severe element distortion in the analysis of tunnel stability, which requires the simulation of large deformation and post-failure behaviour of soil. To overcome these limitations, this paper presents an Eulerian FE model, developed to evaluate the stability of circular tunnels in undrained clay. An application example was first presented in detail to illustrate both the approaches and principles for the simulation. Then parametric analyses of tunnel stability using the Eulerian FE method were performed to verify the validity of the developed model. The computed limit support pressures showed good agreements with the solutions from the finite element limit analysis, demonstrating that it is applicable in the stability analysis of a circular tunnel. Meanwhile, Lagrangian FE analysis was also performed for a comparison of performance with the developed Eulerian FE model. The computed results by Lagrangian FE models were shown significantly overestimating the stability of the circular tunnels, which was found to be caused by the decrease of tunnel diameter during the analysis. The Eulerian FE method developed in this paper can inherently avoid this problem, highlighting its advantage in the stability analysis of circular tunnels.

Stability analysis of support systems using a coupled FEM-DFN model (Case study: a diversion tunnel at Lorestan dam site, Iran)

Various structural discontinuities, which form a discrete fracture network, play a significant role in the failure conditions and stability of the rock masses around underground excavations. Several continuum numerical methods have been used to study the stability of underground excavations in jointed rock masses but only few of them can take into account the influence of the pre-existing natural fractures. In this work, the pre-existing fractures are explicitly modeled as a Discrete Fracture Network (DFN) model, which is fully coupled with the FEM modeling for stability analysis of support systems in a diversion tunnel at the Rudbar Lorestan dam site. Hence, at first, using the surveyed data in the diversion tunnel and an estimation of the suitable probability distribution function on geometric characteristics of the existing joint sets in this region, the 3D DFN model was simulated using the stochastic discrete fracture networks generator program, DFN-FRAC3D. In the second step, a coupled 2D Finite Element Method and the prepared stochastic model were used for analysis of existent (based on technical reports) recommended support systems. The objective here is to grasp the role of the fracture networks on the results of the tunnel stability analysis using FEM modeling and also to compare the results with those obtained through stability analysis without considering the effect of fractures.