Tunnel Face Stability Research Articles

Analysis of working face stability of longitudinally inclined shield driven tunnels in frictional soils

The shield machine has the advantage of reaching the predetermined depth more quickly with an inclination angle α, compared to conventional horizontal excavation. This paper aims to investigate the working face stability of shield tunnel driven with an inclination angle. An improved model is proposed by incorporating α into the traditional silo-wedge model. In this model, the contribution of the pressure of the cuttings in the excavation chamber on the working face stability is considered for both cases of α > 0 and α < 0. The optimization result is derived by applying limit equilibrium theory. A series of analyses reveal that the proposed model is predominantly influenced by the frictional angle and cover depth than by cohesion. The working face pressure increases linearly with the increase of α. In order to access the validity of the proposed model, the working face pressure and failure zone calculated by the proposed model are verified with numerical solutions and existing theoretical solutions. The current solution presents a more conservative result than other solutions in the case of α > 0, while predicts a smaller yet more reliable in the case of α < 0. Finally, two previous 1-g model tests with α = 15° and α = -15° are chosen to verify the proposed solution, where demonstrate a great consistence in limit working face pressure and the corresponding failure pattern.

Analysis of face stability for shallow shield tunnels in sand

The stability of the tunnel face is the key problem in shield tunnel construction. This paper focuses on the face stability of a shallow tunnel in sand. Numerical simulation and theoretical analysis are combined to study the limit support pressure and failure zone. Firstly, numerical simulation is employed to study the collapse of the tunnel face, obtaining the limit support pressure and collapse zone. A new failure model suitable for shallow tunnels is constructed based on these numerical simulations. Then, an analytic solution for the limit support pressure is derived using limit analysis upper bound theory. The accuracy and applicability of this proposed model are verified by comparing it with numerical results and classical analytical models. Through this research, it is found that the proposed model provides a more accurate description of situations where soil arches cannot be formed for shallow tunnels in sand, leading to higher accuracy in calculating the limit support pressure. The influence of various factors on stability of the tunnel face is analyzed, revealing mechanisms of tunnel face collapse.

Probabilistic analysis of tunnel face seismic stability in layered rock masses using Polynomial Chaos Kriging metamodel

Face stability is an essential issue in tunnel design and construction. Layered rock masses are typical and ubiquitous; uncertainties in rock properties always exist. In view of this, a comprehensive method, which combines the Upper bound Limit analysis of Tunnel face stability, the Polynomial Chaos Kriging, the Monte-Carlo Simulation and Analysis of Covariance method (ULT-PCK-MA), is proposed to investigate the seismic stability of tunnel faces. A two-dimensional analytical model of ULT is developed to evaluate the virtual support force based on the upper bound limit analysis. An efficient probabilistic analysis method PCK-MA based on the adaptive Polynomial Chaos Kriging metamodel is then implemented to investigate the parameter uncertainty effects. Ten input parameters, including geological strength indices, uniaxial compressive strengths and constants for three rock formations, and the horizontal seismic coefficients, are treated as random variables. The effects of these parameter uncertainties on the failure probability and sensitivity indices are discussed. In addition, the effects of weak layer position, the middle layer thickness and quality, the tunnel diameter, the parameters correlation, and the seismic loadings are investigated, respectively. The results show that the layer distributions significantly influence the tunnel face probabilistic stability, particularly when the weak rock is present in the bottom layer. The efficiency of the proposed ULT-PCK-MA is validated, which is expected to facilitate the engineering design and construction.

Seepage effect on progressive failure of shield tunnel face in granular soils by coupled continuum-discrete method

Seepage induced by the difference of hydraulic head between the ground and the shield’s chamber will adversely affect the stability of tunnel face when tunnels are excavated through water-rich stratum. In order to investigate the influence of seepage on the stability of tunnel face, a 3D continuum-discrete method is proposed to simulate the failure of tunnel face. The large deformation behavior of soil is determined by the discrete element method and the fluid flow in soil phase is described by the Darcy fluid model and continuity equation. A series of simulations are carried out to obtain the failure mechanism and critical support pressure of tunnel face. Then the results under dry, undrained and seepage conditions are analyzed and the seepage effect on the stability of tunnel face with different water depth is investigated. Besides, a comparison between the simulation results and solutions in previous researches is conducted. Additionally, the micro-mechanical analysis under dry and seepage conditions are presented. The results show that the coupled method can well evaluate the stability of tunnel face considering seepage effect and provider useful guidance for practical tunnel designs.

A particle-scale insight into the face stability of shallow EPB shield tunnels in dry cobble-rich soil

The aim of this paper is to provide a particle-scale insight into the face stability of shallow (C/D = 1.0; C = tunnel buried depth and D = tunnel diameter) earth pressure balanced (EPB) shield tunnels in cobble-rich soil, considering both the dynamic excavation process and particle size gradation. The work was performed by three dimensional discrete element method (DEM). Results show that particle size greatly influences the movement of cobble particles. The coarse particles serve as the skeleton of the ground and provide the stability. The erosion of fine particles has little influence on tunnel face stability, while the erosion of middle particles weakens tunnel face stability. The limit support pressure pf increases with increasing cutterhead rotating speed, and the normalized limit support pressure pf/γD (γ = soil unit weight, D = tunnel diameter) obtained in this paper is larger than existing researches. The abrupt increase of middle particles in the muck composition can be recognized as the forewarning information against face failure.

Investigation of the failure mechanism and theoretical model of bolt-reinforced shallow tunnel faces with different bolt lengths

Using fiberglass bolts to reinforce a tunnel face is a practical auxiliary technology for ensuring tunnel face stability in soft ground. The reinforcing effect and the economics of this technology are significantly affected by bolt length. However, to date, the failure mechanism of bolt-reinforced tunnel faces with different bolt lengths has rarely been investigated. To reveal the failure mechanism of bolt-reinforced shallow tunnel faces, in this study, the stability of bolt-reinforced tunnel faces with different bolt lengths was investigated by using laboratory tests and numerical simulations, and a simplified theoretical model for practical engineering was proposed. The face support pressure and failure pattern for different bolt lengths during the face collapse process were obtained, and the influence of bolt length on face stability was clearly revealed. More specifically, the results show that face stability increases with increasing bolt length, and the reinforcing effect of face bolts is governed by the shear failure at the soil-grout interface first in the stable zone of the tunnel face and then in the failure zone. Once the bolt length in the stable zone is larger than that in the failure zone, face stability will not be improved with increasing bolt length; thus, this bolt length is referred to as the optimal bolt length Lopt. The Lopt value is slightly larger than the initial failure range (in the unreinforced condition) and can be approximately calculated by Lopt = (1 − 0.0133φ)D (φ is the friction angle of the soil, and D is the tunnel diameter) in practical engineering. Finally, a simplified theoretical model was established to analyse the stability of reinforced tunnel faces, and the results are in good agreement with both laboratory tests and numerical simulations. The proposed model can be used as an efficient tool for the design of face bolts.

Stability analysis of a 3D shallow tunnel face considering the inclined excavation and inclined ground surface

In tunnel engineering, the phenomena of inclined excavation and inclined ground surface for mountain tunnels, urban tunnels and cross-river (sea) tunnels are common due to the constraints of landform, design and construction. Inclined excavation and inclined ground surface have significant influences on the stability of shallow tunnel faces. However, their comprehensive influences on the active and passive failure modes of a shallow tunnel face need to be further studied. An improved three-dimensional failure model of a shallow tunnel face with multi-blocks is constructed in this study while considering their comprehensive influences, and the reliability of active and passive failure models is verified through comparative analysis. In addition, a series of detailed parametric analyses and key failure modes are carried out. The tunnel inclined angle (δ1) and inclined angle (δ2) of the ground surface have significant influences on the shallow tunnel face stability for active and passive failure modes. The percentages of change in the ground surface failure region are 47.81% for active failure and 5.82% for passive failure when δ1 varies from −10° to 10°, and 24.44% and 18.57%, respectively, when δ2 varies from −10° to 10°. In practical engineering, δ1 and δ2 must be controlled to within a certain range because a large δ1 can easily induce active failure, and a large δ2 can easily induce passive failure of a 3D shallow tunnel face.

Stability of tunnel face in unsaturated sand possessing apparent cohesion: A micro-macro analytical approach

Although the stability of tunnel face in the dry and saturated sandy ground is widely studied, the unsaturated sandy ground which possesses apparent cohesion is more common in engineering. For remedying this deficiency, the theoretical association between apparent cohesion and the saturation degree is firstly established in microscopic prospective. Then, the formation mechanism of the self-stabilized arch and the limit support pressure (LSP) of the tunnel face are derived by incorporating apparent cohesion into the macroscopic limit equilibrium analysis of the multi-arches model. Subsequently, the validities of the proposed approach in estimating apparent cohesion, loosening zone height and LSP are well confirmed (the average error rates of LSP are within 12 %) via comparisons with direct shear tests, model tests and other existing methods. Finally, as revealed by the parametric discussion, under the effect of apparent cohesion, LSP is negatively correlated with compactness, internal friction angle, and contact angle while decreases firstly (to a minimum value of 0.09γD ∼ 0.15γD) and then increase with the rise of saturation degree. Besides, the LSP has a parabolic distribution along the depth with its peak value emerges between 0.3D and 0.45D.

Revisiting the face stability of circular tunnels driven in strength nonlinearity soils

The face stability of shield-driven tunnels in soil materials with distinct strength nonlinearity is re-evaluated, based on the kinematic limit analysis theorem. The nonlinear Power-Law failure criterion is piecewise approximated by a multi-tangent technique to improve the conventional single straight-line substitution. A new multi-cone rotational mechanism formed by n curvilinear cones is proposed to accommodate the nonlinearity of the strength envelope considering the associative flow rule. The analytical solution of the critical support pressure on tunnel faces is derived from the energy equilibrium equation. Calculation results show that the multi-tangent method can improve the calculated values by up to 20% compared to the generalized tangent method. According to the back-calculated stress vectors, it is interestingly found that the stress level is not the critical factor influencing the tunnel face stability, while the shear strength parameters are the direct factor. Although the upper-bound analysis is a kinematic method without involving stress analysis, the parametric form of the strength criterion gives a chance to infer the possible stress distribution on the rupture surface and explain the variational trend of the result based on the distribution of back-calculated stress vectors.

Limit Equilibrium Models for Tunnel Face Stability in Composite Soft-Hard Strata

The tunnel face stability in composite strata, commonly referred to as the soft upper and hard lower condition, is a critical challenge in tunnel construction. The soft–hard ratio (SA) strongly influences the limit support pressure as well as the failure mechanism experienced by a tunnel face. This study focused on the Xingye Tunnel project in the Xiangzhou District of Zhuhai City. By conducting numerical simulations, the impact of different SAs on the limit support pressure was investigated. Furthermore, a limit equilibrium model was established on the basis of the analysis of the results of numerical simulation. The findings were then compared and analyzed alongside those of relevant theoretical models. In the event of tunnel face instability of composite strata, the deformation tends to be concentrated mainly in the soft soil layer, with less noticeable deformation in the hard rock layer. The investigation of different SAs revealed a linear decrease in the limit support pressure ratio of the tunnel face in composite strata as SA decreases. The self-stability of the tunnel face was observed when SA ≤ 0.125. Moreover, the limit support pressure ratio predicted by the truncated log-spiral model (TLSM) exhibited a higher degree of agreement with the results of numerical simulation than those of other relevant models. The superiority of TLSM was mainly demonstrated in the range of SA = 0.25 to 1.0.

Inverse reliability analysis and design for tunnel face stability considering soil spatial variability

The traditional deterministic analysis for tunnel face stability neglects the uncertainties of geotechnical parameters, while the simplified reliability analysis which models the potential uncertainties by means of random variables usually fails to account for soil spatial variability. To overcome these limitations, this study proposes an efficient framework for conducting reliability analysis and reliability-based design (RBD) of tunnel face stability in spatially variable soil strata. The three-dimensional (3D) rotational failure mechanism of the tunnel face is extended to account for the soil spatial variability, and a probabilistic framework is established by coupling the extended mechanism with the improved Hasofer-Lind-Rackwits-Fiessler recursive algorithm (iHLRF) as well as its inverse analysis formulation. The proposed framework allows for rapid and precise reliability analysis and RBD of tunnel face stability. To demonstrate the feasibility and efficacy of the proposed framework, an illustrative case of tunnelling in frictional soils is presented, where the soil's cohesion and friction angle are modelled as two anisotropic cross-correlated lognormal random fields. The results show that the proposed method can accurately estimate the failure probability (or reliability index) regarding the tunnel face stability and can efficiently determine the required supporting pressure for a target reliability index with soil spatial variability being taken into account. Furthermore, this study reveals the impact of various factors on the support pressure, including coefficient of variation, cross-correlation between cohesion and friction angle, as well as autocorrelation distance of spatially variable soil strata. The results also demonstrate the feasibility of using the forward and/or inverse first-order reliability method (FORM) in high-dimensional stochastic problems. It is hoped that this study may provide a practical and reliable framework for determining the stability of tunnels in complex soil strata.

Three-dimensional seismic face stability of shield tunnels in undrained clay

Tunnel face stability has received increasing research interest over the past few decades. However, computing time-efficient and safe three-dimensional solutions under seismic loading is still an unsolved problem, while case studies indicate that seismic loading can be one critical destabilizing factor affecting tunnel stability. The primary objective of this work is to fill this gap in knowledge by providing compromising and computationally efficient solutions, along with their respective lower and upper bounds, to compute face stability under seismic conditions. The analyses employ the finite element limit analysis method to evaluate the limit support pressure in undrained clay, considering horizontal pseudo-static seismic forces pointing outwards from the face. Moreover, the analyses employ both constant and linearly increasing shear strengths with depth. The results are summarized as dimensionless stability charts and tables to facilitate their interpretation and future use for tunnel design. A new design equation has been developed to evaluate the stability of the tunnel face considering the effect of seismic forces. Additionally, the effects of different parameters on the shape of the failure mechanism have been investigated by analysing the distribution of shear dissipation.

Probabilistic analysis of tunnel face stability in spatially variable soil

the limit analysis incorporated with the random field theorem is an effective approach for the probabilistic analysis of tunnel face stability. However, the existing incorporation procedure is not sufficiently efficient and accurate. A modified random field model, which is directly superimposed onto the rotational failure mechanism, is proposed to address this issue. In comparison with previous methods, the proposed approach avoids the generation of redundant random field elements and evades the inaccurate matching procedure of the elements, thereby enabling efficient and accurate estimation of the failure probability of a tunnel face. Moreover, cross-correlated random fields with the rotated anisotropy are readily generated with the proposed approach. Parametric sensitivity analysis is performed with Monte Carlo simulations to investigate the effects of the coefficient of variation, the scale of fluctuation, cross-correlation coefficient, and rotated anisotropy on the failure probability of a tunnel face. It is found that: (1) the failure probability is more sensitive to the scale of fluctuation in the vertical direction than that in the horizontal direction; (2) neglecting the cross-correlation of soil shear strength parameters leads to a slightly conservative design for tunnel face stability; (3) the influence of rotated anisotropy on the failure probability of a tunnel face is limited unless the soil exhibits high levels of uncertainty in its friction angle. Application of the proposed approach to multi-layered soil demonstrates its superiority in the probabilistic analysis of tunnel face stability in spatially variable soil.

Theoretical modeling of the pre-cutting system performance for the tunnel face stability in very weak rock masses

By forming continuous pre-lining around the tunnel's perimeter, pre-cutting is a recognized tunneling technique that enables full-face excavation of a tunnel even in unfavorable ground conditions. In this study, a theoretical method is put out to simulate the performance of this kind of pre-support in squeezing conditions. The loading distribution on the pre-cutting shells is determined for this purpose using the LDP of an unsupported tunnel, and the non-linear stiffness of the pre-cutting shell is then determined using the shell theory. The convergence confinement method is used to calculate how much the pre-supported tunnel isdeformed. The non-uniform loading distribution also results in the tangential and longitudinal bending moments on the pre-cutting shell. To show the effectiveness of pre-cutting shells in extremesqueezingconditions, several examples are solved. As a result, a few graphs for the preliminarydesign of a tunnel using this technique are given. The graphs show the overall tunnel displacement for various pre-cutting shells under various geological conditions. The outcomes demonstrate good agreement between FALC 3D resultsand the ones by thesuggested analytical model. A preliminary design can be made using the assessment of the proposed methodonthe loads on the pre-cutting system.

Face stability analysis of a shield tunnel excavated along inclined strata

The horizontally layered or even inclined strata are often encountered in practical shield tunneling. The influence of inclined strata on face stability of shield tunnels is not fully investigated by the existing studies. This paper adopts both theoretical analysis and numerical simulation to carry out research on face stability in inclined strata. The spatial discretization technique is adopted to construct a three-dimensional (3D) kinematic failure mechanism considering intersection between inclined soil interface and tunnel face. An analytical solution for critical support pressure is obtained. Besides, the critical support pressure and 3D kinematic mechanism are compared with numerical results to verify accuracy and effectiveness of analytical model. The influences of dip and position of inclined strata on face stability are thoroughly studied. The proposed failure mechanism can serve as a reference for face stability analysis in inclined strata.

Influence of stone content on face stability for tunnels in sandy cobble strata

As a result of the complex geological conditions of sand-cobble stratum, various types of problems may arise during construction of tunnels crossing sand-cobble stratum, the most prominent of which is the stability problem of the face. In order to study the stability of the tunnel face in sand-cobble stratum, first, a continuous medium model of sandstone soil is established, and numerical simulation is carried out on the three-axis compression test of sandstone soil to analyze its micro-mechanical characteristics. Second, a calculation model of simulated sandstone strata excavation is established by FLAC3D finite difference software. Then, the safety factor K calculated by the strength reduction method is introduced as the evaluation index of the face stability, and the face stability of sand-cobble tunnels with a series of stone content, tunnel burial depth and tunnel diameter are analyzed. The results show that the macro stress-strain curve of sand-cobble soil in the three-axis compression test can be divided into linear stage, elastic-plastic stage and ideal plastic stage, and the shear strength of sand-cobble soil increases with the increase of stone content. In the process of tunnel excavation, the stability of tunnel face increases with the increase of stone content in the stratum; while the tunnel buried depth and diameter increase, the stability of tunnel face decreases. These results provide a reference for predicting the instability and failure of sand-cobble tunnels, which is critical for the construction and safety of tunnels in sand-cobble stratum.

Stability of complex rock tunnel face under seepage flow conditions using a novel equivalent analytical model

The longitudinally inclined layered rock masses under seepage flow conditions have an important impact on tunnel face stability. To solve this issue, a novel equivalent analytical model is proposed in this paper. This model settles the puzzle that the traditional model of limit analysis cannot directly determine the hydraulic head by treating the distribution hydraulic head at the anterior aspect of the tunnel face with the equivalent method, that is, without the tedious calculation by the numerical software. The seepage force is directly considered as a body force in this analytical model. Then, combined with the mirror-image method, layered discretization technique, limit analysis method, and Hoek-Brown yield criterion, an analytical expression for stability determination can be derived. The main factors affecting the stability of tunnel face are analyzed, such as the inclination angle of the flow line, the inclination angle of the rock layer, hydraulic conductivity, rock quality, excavation disturbed zone (EDZ), and the groundwater table height. The calculated diagrams have a certain guiding effects for tunnel construction.

Unlined Length Effect on the Tunnel Face Stability and Collapse Mechanisms in c-ϕ Soils: A Numerical Study with Advanced Mesh Adaptive Strategies

This paper presents a stability study on the collapse mechanisms of a plane-strain tunnel face in c-ϕ soils using the upper bound finite element method with rigid translatory moving elements (UBFELA-RTME) and nonlinear programming technique. Practical considerations are given to the unlined length influence behind the tunnel face. An advanced mesh adaptive updating strategy is adopted, aiming to improve the computational efficiency, the accuracy of upper-bound solutions, as well as the produced collapse mechanisms. The unlined length influence on the face stability and collapse mechanism of the tunnel face are determined with various combinations of tunnel depth ratios, soil friction angles, and dilatancy angles. Using the UBFELA-RTME with the Davis’s approach and a mesh adapting strategy, the non-associated plasticity flow rule can be well approximated. The developed technique was validated against different numerical methods, and it is concluded that the tunnel face stability can be improved by increasing soil friction and dilatancy angles, and yet weakens as the unlined length increases where a mesh-liked collapse zone gradually appears on the tunnel vault top. It gradually evolves to a global collapse failure till the ground surface. The findings contribute to a better understanding of the ground surface failure under the unlined support length influence in tunnel construction.

Seismic stability analysis of 3D tunnel faces in unsaturated soils

This paper aims to develop a framework using the kinematic approach to assess the face stability of tunnels in unsaturated soils under seismic conditions, allowing for a rigorous upper-bound solution of the required face pressure. The stress behavior of the soil under steady unsaturated infiltration is analytically described using a suction stress-based framework. Three methods, i.e., the pseudostatic (PS) method, conventional pseudodynamic (CPD) method, and modified pseudodynamic (MPD) method are employed to represent the earthquake acceleration for the seismic analysis of tunnel face stabilities. The temporal and spatial variations of seismic accelerations and suction stresses are incorporated into the work-energy balance equation within the 3D discretization-based failure mechanism. A hybrid optimization routine is employed to search for the maximum required face pressure. The developed framework is validated through a comparison analysis with prior research, and the outcomes obtained from the three methods of representing seismic accelerations are compared. Several parameter analyses are performed to discuss the effects of the unit weight of unsaturated soils, dynamic characteristics, and hydraulic hysteresis. The framework can provide a useful tool for quantitatively analyzing the impact of potential seismic forces, as well as steady unsaturated infiltrations paired with hydraulic hysteresis, on the tunnel face stability.

Transparent soil model test and numerical study on the effect of adjacent spring on the stability of tunnel face in composite strata

To study the instability and failure mechanism of tunnel face in composite stratum and the evolution law of supporting pressure in the areas with spring, this paper used two different types of transparent soil and a self-designed 3D model test system. Six large transparent soil model tests were carried out by considering different confined water heads of spring and tunnel burial depth. The optical laser and high-speed camera were controlled to move on a high-precision linear platform. CT scanning was performed to obtain the failure model under different conditions. The finite element method considering a two-way fluid–structure coupling was used to validate the model test. The research results indicate under spring, the support pressure curves can be divided into three stages: rapid decline, rebound-rise, and constant. There is no rebound-rise stage under the condition of no spring. With increased confined water heads or reduced tunnel burial depth, limit support pressure shows an increasing trend. 2D and 3D damage models for different working conditions were obtained by PIV technology and 3D reconstruction technology. If there is a spring, the maximum displacement moves to the top of the tunnel with the increase of water head and the failure mode is a combination of “silo shape” and “inverted prism”. When there is no spring, the maximum displacement appears at the interface of the soil layer, and the failure mode is a combination of “silo shape” and “wedge shape”. The presence or absence of springs and the change of the confined water head have no significant effect on the height of the loosening area. With the tunnel burial depth ratio of 0.5 to 2.0, the height of the loose area increases from 0.17 to 0.83 D, and the soil arch area develops outward.