Stability Analysis Of Tunnel Research Articles

Stability analysis and long-term behaviour of deep tunnels in clay formations

Abstract Demonstrating the feasibility of constructing tunnels in deep clay formations is an important goal of the Belgian RD&D programme on the geological disposal of radioactive waste. In 2002 a major achievement was reached when the HADES Underground Research Laboratory (URL) in Boom Clay was extended with the construction of the Connecting Gallery. This demonstrated that it is feasible to construct galleries in poorly indurated clays using industrial techniques. To monitor the mechanical behaviour of the gallery and assess its stability, strain gauges were embedded in the segmental gallery lining and prisms were installed on the segments. These sensors provide valuable information that will support the design of future galleries. This paper presents 20 years of monitoring data in the Connecting Gallery and a first analysis of these data in terms of Boom Clay behaviour. In addition, the key findings are compared with those of a similar analysis performed by Andra (the French Radioactive Waste Agency) at the Meuse/Haute-Marne URL. The latter URL is excavated in the Callovo-Oxfordian claystone. The comparison identifies general trends and highlights similarities between the behaviour of tunnels in poorly indurated clay (Boom Clay) and in claystone.

Stability Analysis of Box Tunnel Working Face under Non-Uniform Support Pressure

For rectangular section tunnel construction, soil bin pressure fluctuates under biaxial emergence conditions. Support pressure is difficult to uniformly distribute, and it can easily cause instability of the tunneling face. The Silo-wedge model is used to perform horizontal strip splitting of parts of the soil at risk for instability under unsupported conditions. In the ultimate equilibrium analysis, the strength discounting method is introduced, a new expression for the safety factor is derived by combining the horizontal strip splitting equilibrium equations, and the safety factor is solved numerically. When the opening range is greater than 0.75 and the effective internal friction angle of the soil is less than 25°, the safety coefficient is less than 1, and destabilization damage is likely to occur. The field application of a box tunnel project in Suzhou demonstrates that the research results have good engineering applicability to the issue.

Stability analysis of tunnel under coal seam goaf: Numerical and physical modeling

The goaf formed by coal seam mining would dramatically reduce the strength and stiffness of the ground in and around the goaf, which is not conducive to tunnel excavation near the mined-out area. By establishing the Finite differential method and Discrete element method coupling numerical analysis method and conducting similar model test, the influence of dip angle, thickness and distance of coal seam goaf on the stability of unsupported tunnel excavation is studied. The results show that when the tunnel under the goaf is excavated, the circumferential stress increment increases first and then decreases along the radial direction of the tunnel, the radial stress increment gradually decreases to zero along the radial direction of the tunnel, the displacement is approximately distributed in a trough shape, and the maximum displacement is at the top of the tunnel. The area of the stress loosening zone (SLZ) is negatively correlated with the dip angle and the distance of the goaf, and positively correlated with the thickness. The SLZ near the goaf is larger, and the peak value of the asymmetry is about 1.35. When the thickness of the mined-out area is 1.8–2.1 m, the SLZ and displacement around the tunnel increase sharply and then become stable. When the dip angle of the mined-out area is greater than 30° or the distance exceeds 1.3 times tunnel diameter (D), the asymmetry converges. The research results of this paper are of great importance to the design and construction of tunnel support and the formulation of excavation schemes.

Dynamic stability analysis of metro tunnel in layered weathered sandstone

In this study a coupled-eulerian–lagrangian (CEL) approach has been applied to assess the internal blast loading conditions, for the tunnel built in three layers of sandstone rock. The three weathered stages of sandstone have been considered in this study i.e., slightly, medium and also highly weathered sandstone in three different layers. The sandstone rock weathering increases as we move towards the ground surface from deep subsurface. The elastoplastic finite element model with varied overburden depth with dimensions of 60 m (L × B × H) each to incorporate different parametric cases. An explosive is assumed at the center of the tunnel, with the capacity of hundred kg of trinitrotoluene (TNT), the explosive is assumed to be hanging in the air inside the tunnel opening with equal distance from all sides. The TNT sphere and air that is inside the rock tunnel has been modelled through CEL technique to simulate actual in-situ condition. Through Mohr-Coulomb, Concrete Damage Plasticity, and Johnson-Cook constructive material model the elastoplastic behaviour of different materials like rock, steel bars and concrete has been simulated respectively. The cage of steel bars has been embedded through interaction constraint in concrete liner to produce reinforced concrete liner. In the initial stages of simulation with 5 m of overburden depth the tunnel has been placed in the upper layer of sandstone. For further analysis, the position of the tunnel has been changed for overburden depth of fifteen, twenty-five and thirty-five meter. In accordance with the result of the simulation, for rock in terms of acceleration, velocity, and dis-placement, the depth of the overburden and the displacement of the crown are inversely related. No damage in terms of compression is seen in this simulation, but in all instances, a small damage owing to tensile failure has been noted.

Estimation of in-situ rock strength from borehole geophysical logs in Australian coal mine sites

This study aims to improve the conventional empirical rock strength estimation method that is widely adopted in the Australian coal mining industry and provide more accurate rock uniaxial compression strength (UCS) predictions based on artificial neural network (ANN) models. 274 laboratory-measured UCS data (sample dimension: 60 mm (diameter) × 150 mm (height)) were collected from two longwall coal mines in Australia, along with the four geophysical logs (sonic, gamma, neutron, and porosity logs) and rock density of the tested core specimens. A two-layer ANN model was developed from these data based on the Levenberg–Marquardt algorithm as a base model. As compared to conventional sonic-UCS fitting equations, the mean percentage error, root mean squared error and maximum absolute error of the proposed ANN model were reduced from 34.27%, 15.23 MPa and 70.66 MPa to 20.67%, 11.02 MPa and 47.66 MPa, respectively. Further investigations were conducted by splitting the dataset based on rock lithology and mine locations, and separate ANN models were developed based on the divided subsets. Both lithology-specific and site-specific models exhibited improved estimation accuracy in comparison to the base model, indicating lithology and local geological conditions could considerably affect the estimation accuracy. Among the three types of models, the lithology-specific models yielded the most accurate predictions over the 274 data (MPE: 17.55%; RMSE: 8.84 MPa), followed by the site-specific models and the base model in descending order. Overall, all three models significantly outperformed the current empirical methods adopted in the studied mine sites. The outcomes of this study could provide more accurate UCS predictions for further geotechnical analysis in underground constructions, such as tunnel stability analysis and estimating in-situ stresses based on borehole breakout.

A novel hydro‐mechanical coupled multiphysics model for slurry penetration in sand with application to face stability analysis of shield tunnel

AbstractThe slurry penetration in tunnels driven by the slurry TBM in the highly permeable water‐bearing ground is a complicated multiphysics process, which significantly affects the stability of the excavation face but has not been fully modelled. This study establishes a novel hydro‐mechanical coupled multiphysics modelling approach for face stability analysis. Different from existing methods, the proposed one is able to quantify simultaneously the spatial and temporal variation of slurry concentration, viscosity, stagnation gradient, soil porosity and pore pressure, as well as the soil skeleton deformation induced by the transferred slurry pressure in two or three‐dimension. The spatial and temporal pressure transfer from slurry suspension to soil skeleton during slurry penetration can be obtained. The coupled formulations are implemented into a finite element package for initial and boundary value problems at the engineering scale and enhanced by the Petrov‐Galerkin weighted residual stabilisation technique. After being validated through the experimental slurry infiltration tests, it is applied to a field case analysis to predict the excess pore pressure in front of the tunnel face, which shows good agreement with the field measurement. The efficiency of the applied slurry pressure and the pressure transfer mechanism are thus verified. Comparative calculations about the influence of the pressure transfer model are presented to further improve the understanding of the pressure transfer mechanism. The proposed approach is also verified applicable to estimate the optimum slurry pressure.

Numerical Stability Analysis of Large Section Tunnels Using the Double-Side Heading Method: A Case Study of Xiamen Haicang Evacuate-Channel

Large section tunnels have huge excavation spans, complex excavation procedures, mutual influence among various procedures, multiple disturbances of the surrounding rock, and discontinuous connection among linings under different procedures, which bring great difficulties to their construction. The Caijianwei Mountain No. 2 tunnel of the Xiamen Haicang Evacuate-channel project in China belongs to a super large section tunnel. The tunnel’s maximum excavation section span reaches 30.52 m, and the excavation area reaches 421.73 m2. Its excavation sequence, the timing of the second primary lining support, temporary support disassembly, and secondary lining support of the double-side heading method were analyzed using numerical methods. The excavation sequence of the double side-heading method from the right pilot tunnel to the left pilot tunnel and then to the middle pilot tunnel can avoid a large horizontal displacement at the arch waist and control the deformation of the lining. The excavation sequence from the middle pilot tunnel to the two side pilot tunnels was conducive to the bearing performance of the concrete at the arch crown. The excavation sequence from the two side pilot tunnels to the middle pilot tunnel can better protect the concrete safety at the arch waist. The timing of the second primary lining support, temporary support removal, and secondary lining support have different effects on the deformation and stress of the surrounding rock and support structures. In the construction process, the timing of the second primary lining support, temporary support disassembly, and secondary lining support should be determined according to the actual deformation and stress of the site section. The research results can provide a reference for the construction design of the double-side heading method for large section tunnels.

Sensitivity Analysis of Factors Affecting the Stability of Deep Buried Tunnel

The excavation of deep tunnels has significant spatial and temporal effects on the stress distribution of the surrounding rock. Accurately describing the distribution characteristics of the surrounding rock stress state is a key issue in analyzing the failure mechanism of the surrounding rock. Based on the numerical analysis, a study on the influencing factors of the surrounding rock stress state during the excavation of deep tunnels was conducted. The results show that (i) the surrounding rock was divided into stress mutation, disturbed, and stables zones using the quantitative index: the peak and stable value of the stress triaxiality, which can reflect the influence of the principal stress orientation; (ii) the evolution law of the stress path of the surrounding rocks in different areas was obtained, and the detailed loading method of laboratory test is proposed, which could consider the influence of the principal stress magnitude and orientation during tunnel excavation; and (iii) five variable indices were used to conduct sensitivity analysis on the influencing factors of the surrounding rock stress state. Specifically, the direction of in situ stress and lateral pressure coefficient considerably influences the disturbance range, and the depth the tunnel is buried affects the deformation and failure characteristics of the surrounding rock. This paper provides a modified and quantified test method for tunnel stability analysis.

A meso-scale numerical modelling method for the stability analysis of tunnels in sandy cobble stratum

This paper presents a meso-scale numerical modelling method to investigate the stability of tunnels in sandy cobble stratum. Using Monte Carlo simulation, Python routine is implemented to randomly generate and filter the location information of blocks including center coordinates, sizes and orientation angles, then extract and import them into CAD modelling software in batches to mesh. The problem of overlapping detection for elliptical and ellipsoidal blocks is solved based on affine transformation, achieving higher efficiency and applicability than the existing methods. Then, using the proposed numerical modelling method, convergence-confinement analysis of circular tunnels with different cover depths is performed in plane strain condition. Meanwhile, face stability analysis of shield tunnels is carried out in three-dimensional condition. Results show that sandy cobble stratum may not be in accordance with the scale-independent soil-rock mixture. Blocks with small sizes are geotechnically significant and contribute much to the overall strength. When the cover depth ratio reaches a certain level, limit support pressure remains basically unchanged due to the strong arching effect above tunnel crown, while permissible support pressure increases approximately linearly to control deformation. Compared with the multi-scale model, hardening soil model considering the overall strength is suitable for the stability analysis of tunnels in sandy cobble stratum at the macro-scale with high precision, while the macro homogeneous model ignoring the presence of blocks provides a conservative (i.e. uneconomical) prediction for the design of tunnel stability.

A 3D rotational silo-torus model for face stability analysis of circular tunnels

To avoid tunnel collapse during construction, a critical support pressure needs to be determined and applied on the tunnel face. This paper presents the development of a new 3D limit equilibrium model including geometries of a lower torus and an upper silo. The torus geometry was derived using a newly developed spatial discretization technique and subdivided by a wedge slice scheme. The critical support pressure was determined iteratively using the equations given in this paper. The model adopted different lateral stress coefficients for the silo and the torus geometry under different arching levels. The interface behaviour was also considered between the sliding torus and the surrounding soils. Validating with centrifuge tests, analytical and numerical tests demonstrated the accuracy of the proposed 3D model which is based on a realistic side rotational mechanism. The shear force distribution on the rotational surface was presented at the state of limit equilibrium. Finally, the effects of geotechnical and geometrical parameters (c’, φ’ and Tm) were discussed on the face stability of circular tunnels.

Three-dimensional face stability analysis of rock tunnels excavated in Hoek-Brown media with a novel multi-cone mechanism

In order to give a more accurate prediction of the required face pressure, this work revisits the three-dimensional (3D) face stability problems of rock tunnels excavated in Hoek-Brown media. The Piecewise Linear Method (PLM) is introduced to account for the nonlinearity of the rock strength rather than the simple linear substitution for the strength envelope. A novel multi-cone rotational mechanism consisting of n truncated curvilinear cones is generated respecting the plastic normality flow rule. Two typical linearized measures: Generalized Tangential Technique (GTT) and Equivalent Mohr-Coulomb parameters Method (EMM) are also included in this study for comparison and analysis. In terms of the energy-work balance equation, the required face pressure is formulated as an optimization problem depending on 2n variables and is resolved by a recently-proposed Marine Predators Algorithm (MPA). Parametric study implies that the solution of two-dimensional (2D) analysis is very conservative, despite being safe in engineering, it is uneconomical compared with 3D analysis. In 3D analysis, the required face pressure predicted by PLM is significantly improved by 9 % compared with that of GTT, and the results calculated from EMM show a good consistency with PLM and GTT for heavily fractured rock mass, while for high-quality rock masses, the diversity between the results of EMM and other two methods is distinct. To further extend the application of such modified mechanism, design charts of the normalized face pressure are calculated for a quick assessment of face stability in rock masses with different quality.

Face stability analysis of circular tunnels in layered rock masses using the upper bound theorem

An analysis of tunnel face stability generally assumes a single homogeneous rock mass. However, most rock tunnel projects are excavated in stratified rock masses. This paper presents a two-dimensional (2D) analytical model for estimating the face stability of a rock tunnel in the presence of rock mass stratification. The model uses the kinematical limit analysis approach combined with the block calculation technique. A virtual support force is applied to the tunnel face, and then solved using an optimization method based on the upper limit theorem of limit analysis and the nonlinear Hoek–Brown yield criterion. Several design charts are provided to analyze the effects of rock layer thickness on tunnel face stability, tunnel diameter, the arrangement sequence of weak and strong rock layers, and the variation in rock layer parameters at different positions. The results indicate that the thickness of the rock layer, tunnel diameter, and arrangement sequence of weak and strong rock layers significantly affect the tunnel face stability. Variations in the parameters of the lower layer of the tunnel face have a greater effect on tunnel stability than those of the upper layer.

Probabilistic stability analysis of qanat tunnels in c-φ soil considering soil spatial variability

Some newly built high-speed railways inevitably pass through the qanat region, which may potentially cause the failure of the qanat tunnels upon embankment and traffic loadings. Thus, it is of great importance to evaluate the stability of the qanat tunnels under surcharge for the design of railways. In the context of the Tehran–Isfahan railway construction in Iran, the soil properties obtained from a series of site investigation and laboratory tests show an important spatial variability. To better consider the spatial variability of soil strength parameters, a series of random adaptive finite element limit analysis (RAFELA) is performed in this study to investigate the stability of the qanat tunnels in the cohesive-frictional soils. The emphasis is placed on the effects of the coefficient of variation and vertical spatial correlation length of the strength parameters (i.e. soil cohesion c and effective fiction angle φ) and the qanat tunnel depth (H/D). Results show that considering the variabilities of both c and φ is more unfavourable to the qanat stability compared to the cases with only one strength parameter (c or φ) variability being considered. Detailed stability evaluation charts are obtained to assess the probability of failure of the qanat tunnels under different conditions.

Discrete Element Simulation of Fracture Evolution around a Borehole for Gas Extraction in Coal Seams.

The stability analysis of underground caverns, tunnels, and boreholes is of great significance to underground engineering. In order to study the stress change of the coal around a hole and the evolution law of fractures during the coal seam drilling process, the discrete element simulation of the coal seam drilling process was carried out by using the particle flow code (PFC2D). The results show that during the drilling process, under the influence of confining pressure and drilling disturbance, the coal stress field around the hole and the development of fractures around the hole have the characteristics of zoning and dynamic evolution. In the axial direction of the borehole, it is divided into front and rear areas, and in the vertical axial direction, it is divided into the drilling disturbance zone and the confining pressure main control zone. During the drilling process, the direction of the maximum principal stress in the front zone gradually changes from the vertical hole axis to the direction parallel to the hole axis, and tension fractures are mainly developed along the drilling direction. In the rear zone, the principal stress direction tends to be stable and the principal stress value undergoes dynamic changes, and a large number of vertical hole axis tension fractures are developed. The drilling disturbance zone appears near the hole wall and has an important influence on the stability of the hole wall, while the confining pressure main control zone determines the antireflection effect around the hole and the influence radius of the hole. This work helps the understanding of the damage range and failure characteristics of the surrounding rock during the drilling process and has great significance for the guidance of drilling design.

Tunnel stability analysis of coral reef limestone stratum in ocean engineering

The islands and coasts in the South China Sea have developed coral reefs, which are the basis for the development of ocean resources and the construction of ocean engineering. However, the biogenetic coral reef limestone in the sea has the physical properties of much porosity, low strength and low density, which is obviously different from the traditional geological rock mass during the underground engineering construction. So in this paper, surrounding rock stability of coral reef limestone stratum in tunnel excavation is studied by physical model tests and numerical simulation. Firstly, according to the similarity theory, quartz sand, barite powder, high strength gypsum, calcareous sand and cement are selected to configure the similar materials of coral reef limestone by the orthogonal ratio test and physical mechanics test. Secondly, the triaxial geomechanical model tests of tunnel excavation in coral reef limestone stratum are carried out. The influence of tunnel shape, support method and rock strength on the tunnel stability is studied by analyzing the variation laws of displacement and stress of the surrounding rock. Finally, the numerical calculation of tunnel excavation in coral reef limestone stratum is conducted and the results of physical model tests are verified. The influence of tunnel depth on the surrounding rock stability is further studied. The results show that: (i) Compared with the horseshoe shaped tunnel, the displacement of the circular tunnel at the haunch and springer is significantly increased, and the stress concentration phenomenon occurs. (ii) The stress release rate and displacement of surrounding rock decrease significantly when the anchor support is added or the strength of reef limestone increases. (iii) The stress release rate and displacement of surrounding rock increase with the tunnel depth, so the support strength of surrounding rock should be increased.

Three-Dimensional Upper Bound Limit Analysis of Tunnel Stability with an Extended Collapse Mechanism

A three-dimensional collapse mechanism that can consider a combined collapse of the tunnel roof and the side walls is proposed in this work. The three-dimensional upper bound support pressure is formulated with the power balance principal in the upper bound theorem. The nonlinear Mohr-Coulomb failure criterion is used to replace the commonly used linear MohrCoulomb failure criterion. The method has been validated by a series of examples, in which the three-dimensional collapse mechanism and support pressures are in a good agreement with the numerical results and solutions found in the literatures. Furthermore, sensitivity analyses of the geotechnical and geometrical parameters on the support pressure are conducted and the collapsing range is measured. The results show that a higher value of nonlinear failure coefficient, tensile strength, initial cohesion and tangential internal friction angle can increase tunnel stability, while tunnel stability is threatened by a higher value of burial depth, unit weight, tunnel width and height. The predicted collapse range increases noticeably with the increase of the nonlinear coefficient. This study is of great significance for predicting the three-dimensional safety support pressure and collapse mechanism of tunnel.

Stability Analysis and Construction Parameter Optimization of Tunnels in the Fractured Zone of Faults.

In order to improve the construction method of highway tunnel fault, improve the excavation level, improve the construction efficiency, reduce the project cost, and shorten the construction period, so as to find a specific road, this paper puts forward the research method of tunnel stability analysis and construction parameter optimization in the fault fracture zone. First, this paper analyzes the specific geographical and geological environment of the construction site. Second, this paper compares and analyzes the effects of the drilling and blasting method and full-face tunnel boring machine (TBM) in construction and further analyzes the surrounding rock deformation, over excavation, and under excavation, as well as the range of loose circles when the surrounding rock is stable. Then, this paper discusses the minimum smooth blasting parameters under these conditions. Finally, the actual blasting effect of tunnel construction is tested and the optimization algorithm model of tunnel fault drilling and blasting parameters is established. The results show that the proposed optimization model of drilling and blasting construction parameters for highway tunnel faults based on the Support Vector Regression (SVR) algorithm combined with a genetic algorithm (GA) has a short calculation time and high parameter optimization accuracy. It is very feasible to optimize the construction parameters of the fault drilling and blasting method, which can greatly improve the construction efficiency, carry out the detailed simulation, reduce the cost, and increase safety. In summary, it has a certain reference significance for the optimization of highway tunnel construction and future research by drilling and blasting method under complex geological conditions in my country.

Sparse Polynomial Chaotic Expansion for Uncertainty Analysis of Tunnel Stability

Uncertainty is an intrinsic property of rock engineering because of the complicated geology conditions, rock failure mechanism ambiguity, and the nonlinear mechanical behavior of surrounding rock mass. We developed a novel framework to handle the uncertainty by combing the Sparse polynomial chaotic expansion (SPCE), numerical model, and reliability method. The SPCE model was used to map the complex relationship between the response of the surrounding rock mass and its uncertainty. The first-order reliability method (FORM) evaluated the reliability index and failure probability. Based on the SPCE model and FORM, a simple global optimization algorithm (SHGO) seeks design points and corresponding reliability indexes. A circular tunnel verified the developed framework with a close-form solution. The reliability index, design point, and failure probability were in excellent agreement with the FORM and Monte Carlo simulation. This indicated that the SPCE model could be used as a surrogate model for the analytical solution to approximate the tunnel response (including deformation and size of the plastic zone). Then, the developed framework was employed in a horseshoe tunnel by combing with the numerical model. The results further proved that the developed framework is feasible and effective for handling uncertainty in rock engineering. Furthermore, the developed framework is effective, efficient, and accurate for reliability analysis and provides a helpful tool to approximate the response of rock structure to avoid the time-consuming numerical model in practical rock engineering.

Probabilistic Stability Analysis of Deep Rock Tunnel Excavated by Mechanized Tunneling Considering Anisotropic Initial Stresses

Due to the complex geological conditions, the construction of underground tunnels in the rocks exhibiting creep behaviors is a great challenge and the structures’ long-term stability is of main importance. The excavation by tunnel boring machines (TBM) of a tunnel that is supported by a double flexible/concrete liner is considered a potential technical solution. However, the efficiency of such a support system on the stability at long-term of a tunnel must be verified in the real condition by considering both the effects of uncertainties of the time-dependent rock behavior and anisotropy of initial stress state. In this paper, the subset simulation and the Sobol global sensitivity analysis are chosen to estimate the failure probability of the tunnel supports and quantify the importance of different random parameters, respectively. The results reveal that considering the anisotropy of initial stresses increases the failure probability, especially in the concrete support elements. In addition, the parameters of initial stresses are the ones of main importance for both liners according to the Sobol indices. Therefore, the anisotropy of initial stresses and the related uncertainties should be considered for reliable tunnel designs.

Face stability analysis of large shield-driven tunnel in rock-soil interface composite formations

A new 3D log-spiral model (LS-M model) is proposed to determine the minimal support pressure on the tunnel face of a large shield-driven tunnel in rock-soil interface (RSI) composite formations. In the proposed LS-M model, we define the RSI angle ω and use a new approach to calculate the equivalent tunnel face area, which provides a collapse zone with more realistic geometry than the traditional wedge model. And it has acceptable accuracy with simpler implementation than limit equilibrium analysis. Comparing with previous studies and 3D numerical analysis, it indicates that: (i) the LS-M results agree well with others in full-soil formations on the variation patterns of minimum support pressure and stability coefficients Nc and Nγ; (ii) the critical RSI angle ωcr, which is predominantly influenced by soil cohesion, increases with the soil property values; (iii) the limit support pressure starts to increase with ω only when ω > ωcr; (iv) the peak support pressure occurs at lower C/D with a lower ω; (v) ω can only affect stability coefficients Nc and Nγ when ω and the friction angle are relatively small, while Ns is substantially influenced by RSI angle ω.