Limit Support Pressure Research Articles

Model test on the effects of shield machine cutterhead vibration on tunnel face stability in sandy ground

Face stability is one of the essential problems in shield tunneling. When tunneling in cobble stratum or mixed face ground conditions, significant cutting-induced cutterhead vibration would occur and affect the face stability. To reveal the mechanism and effect of vibration on the tunnel face stability, a transparent tunnel model with a movable vibration exciter was designed and a series of model tests were performed under different vibration magnitudes Aa and frequencies f. Meanwhile, particle image velocimetry was used to reveal the displacement field and the failure pattern of the tunnel face. The test results indicate that the cutting-induced vibration produces a significant reduction effect on the tunnel face stability, as expressed by the increase of the face support pressure and the failure zone when the vibration magnitude and frequency increase. Compared with the static unloading conditions, the width of the failure wedge Lwt increased by about 5.75% and 35.66% for the loose and dense sand, respectively, under dynamic unloading conditions (Aa = 0.2 g, f = 10 Hz). The limit support pressure increased up to about 0.20γD at a vibration of 0.3 g and 50 Hz, much larger than those of static conditions, which were about 0.08γD–0.09γD. An observable self-stabilizing arch can be formed in dense sand under static unloading conditions, while under dynamic unloading conditions, the long-time stable soil arch would not occur. The contributions of this paper could provide an insightful understanding of the effects of cutterhead vibration on tunnel face stability.

Influence of confined water on the limit support pressure of tunnel face in weakly water-rich strata

Influence of confined water on the limit support pressure of tunnel face in weakly water-rich strata

Face stability assessment of a longitudinally inclined tunnel considering pore water pressure

AbstractThe face stability analysis of a longitudinally inclined shield tunnel using an analytical approach in water‐rich areas is still a research gap. To solve this face stability problem, a numerical simulation based on the FLAC3D is first conducted to calculate the seepage field behind the inclined tunnel face. An improved rotational failure mechanism is developed to make it possible to investigate the face stability of inclined tunnels using analytical approaches. In the framework of the kinematic approach of limit analysis, the limit support pressures and corresponding failure surfaces of the inclined tunnel face are determined to analyze the face stability issue. The interpolation tool (griddata) in MATLAB is adopted to involve the obtained numerical values of pore water pressures into the analysis of the stability issue. The analytical solutions obtained from the proposed method are validated by comparisons with existing results from published literatures and numerical results. For a quick estimation of the inclined tunnel face stability in water‐rich areas, a series of design charts are then presented for various soil strength parameters, water tables, and inclined angles. Finally, an application of the proposed method to a practical tunneling case is provided, which further illustrates the effectiveness of the proposed method.

A Method for calculating jacking force of parallel pipes considering Soil Arching Effect and Pipe-Slurry-Soil Interaction

For multiple parallel pipe jacking projects, the calculation of the jacking force is crucial for design and construction. This paper presented a method for the calculation method of the jacking force required by each pipe, which includes the jacking resistance of the excavation face and the side friction around pipe. Firstly, the improved composite arch model in front of the excavation face was established under the limit support state, and jacking resistance of the excavation face was obtained by calculating the limit support pressure according to the model. Then, based on four typical pipe-slurry-soil contact modes, an improved calculation method for side friction was proposed and then corrected for multiple pipes under mutual disturbance. The reliability of the calculation method was subsequently proven by comparison with the model test results and engineering case data available in the literature. The mutual disturbance level among pipes decreases with increasing Poisson’s ratio of surrounding soil. Compared with a single pipe under the same conditions, pipe under the influence of mutual disturbance usually requires a larger force for jacking, so that the required jacking force of the following pipes in multiple parallel pipe jacking projects is usually larger than that of the initial pipe.

Numerical Simulation of Tunnel Face Support Pressure in Layered Soft Ground

In shield construction, the limit support pressure of the tunnel face has an important influence on ground settlement and construction safety. In this study, MIDAS GTS NX software was used to conduct a series of three-dimensional finite element simulations to investigate variations in excavation face support pressure under different layered ground conditions. By changing the shear strength parameters of the top or bottom layers, the influence of composite layers with different formation boundaries on the support pressure of the excavation face was studied. It was observed that when the formation boundary is placed above the axis, the support pressure of the excavation face is more sensitive to a change in the parameters in the lower part of the formation than in the upper part. Conversely, when the formation boundary lies below the axis, this sensitivity becomes reversed. Additionally, we derived a robust and accurate equation to estimate the limiting face support pressure in layered soft ground based on numerical modeling.

Limit equilibrium model for active failure of large-size rectangular tunnel face considering effect of height–width ratio

The stability of a rectangular tunnel face (RTF) has received less attention than that of circular tunnels; in particular, the stability of RTFs with different height–width (H/W) ratios has not yet been investigated comprehensively. In this study, extensive numerical simulations are conducted to investigate the limit support pressure (Pa) of the active failure of an RTF with different H/W ratios and to determine the appropriate failure mechanism without assuming the failure shape in advance. The result shows that, as the H/W ratio increases, Pa decreases gradually for specified heights and overburden depths of the RTF. The failure shape of the soil in front of the RTF is determined by analyzing the contours at plastic points of the soil. Subsequently, three types of failure modes, namely, partial semicircular, semicircular, and supersized semicircular modes, are identified based on the progression of plastic points in the soil. Corresponding failure models are established using the limit equilibrium method. Compared with relevant theoretical models, the proposed model predicted results that are more consistent with the numerical simulation results. Furthermore, a parametric study is performed to investigate the stability of the RTF. Finally, the predicted results are compared and analyzed with the pressure monitored inside the chamber, thus providing a theoretical reference for the jacking construction of the Shasan project.

Deformation mechanism and limit support pressure of cutting steel plate during connection between pipes in large spacing using pipe curtain structure method

The pipe curtain structure method (PSM) is a novel construction method to control ground deformation strictly. Compared with the traditional pipe-roofing and pipe jacking method, the connection between pipes in large spacings using PSM is widely acknowledged as a unique construction procedure. Further study on this connection procedure is needed to resolve similar cases in that the pipes are inevitably constructed on both sides of existing piles. Cutting the steel plate during the connection procedure is the first step, which is crucial to control the safety and stability of the surrounding environment and existing structures. The deformation mechanism and limit support pressure of the cutting steel plate during the connection between pipes in large spacings are studied in this paper, relying on the undercrossing Yifeng gate tower project of Jianning West Road River Crossing Channel in Nanjing, China. A modified 3D wedge-prism failure model is proposed using the 3D discrete element method. Combined with Terzaghi loose earth pressure theory and the limit equilibrium theory, the analytical solutions for the limit support pressure of the excavation face of the cutting steel plate are derived. The modified 3D wedge-prism failure model and corresponding analytical solutions are categorised into two cases: (a) unilateral cutting scheme, and (b) bilateral cutting scheme. The analytical solutions for the two cases are verified from the numerical simulation and in-situ data and compared with the previous solutions. The comparative analysis between the unilateral and bilateral cutting schemes indicates that the bilateral cutting scheme can be adopted as a priority. The bilateral cutting scheme saves more time and induces less ground deformation than the unilateral one due to the resistance generated from the superimposed wedge. In addition, the parametric sensitivity analysis is carried out using an orthogonal experimental design. The main influencing factors arranged from high to low are the pipe spacing, the cutting size, and the pipe burial depth. The ground deformation increases with the increased cutting size and pipe spacing. The pipe burial depth slightly affects the ground deformation if the other two factors are minor. Cutting steel plates in small sizes, excavating soil under low disturbance, and supporting pipes for high frequency can effectively reduce the ground surface subsidence.

Numerical and analytical studies on the coupling effects of unloading and cutterhead vibration on tunnel face in dry sandy ground

When tunnelling in difficult ground conditions, shield machine would inevitably produce significant ground loss and vibration, which may disturb the ground ahead of the tunnel face. In this paper, discrete element models calibrated by model tests were established to investigate the response of tunnel face under the coupling effects of unloading and cutterhead vibrations. The results show that the friction angle reduction under cyclic loading and vibration attenuation in the sandy ground are significant and can be estimated by the fitted exponential functions. Under cutterhead vibration, the tunnel face stability is undermined and the limit support pressure (LSP) increases to 1.4 times as that in the static case with the growth of frequency and amplitude. Meanwhile, the loosening zone becomes wider and the arching effect is weakened with the reduction of peak horizontal stress and the increase of vertical stress above the tunnel. Based on the numerical results, a pseudo-static method was introduced into the limit equilibrium analysis of the wedge-prism model for calculating the LSP under vibration. With an error rate less than 5.2%, the proposed analytical method is well validated. Further analytical calculation reveals that the LSP would increase with the growth of vibration amplitude, vibration frequency and covered depth but decrease with the increase of friction angle. This study can not only lay a solid foundation for the further investigation of ground loss, ground water and soft-hard heterogeneous ground under cutterhead vibration, but also provide meaningful references for the control of environmental disturbance in practice.

Numerical analyses for three-dimensional face stability of circular tunnels in purely cohesive soils with linearly increasing strength

The tunnel face stability in purely cohesive soils with linearly increasing strength was investigated using three-dimensional finite element limit analysis (FELA). Both the collapse (active failure) and blow-out (passive failure) of the tunnel face were considered in the analysis. The rigorous upper bound (UB) and lower bound (LB) solutions of the load factor were calculated with a wide range of ground conditions to cover a broad scope of practical application. The results showed that the whole surface of the face is at failure in the collapse case; while in the blow-out case, there exists a gradual evolution process from partial failure to global failure within the tunnel face with increasing buried depth. Later, based on 960 finite element limit analysis results, a series of practical equations were proposed for tunnel face stability analysis in purely cohesive soils. These equations can be employed to quickly calculate the UB and LB solutions of the limit support pressure and the stability number of a tunnel face in both the collapse and blow-out cases. Finally, the calculation results from these equations were compared with those from previous studies in detail. The comparisons showed that the proposed equations make an improvement over existing methods and can be used as an efficient tool in practical engineering.

An efficient approach for safety factor-based and reliability-based design of tunnel face support pressure using BP Neural Network

ABSTRACT This paper develops an efficient approach for safety factor-based design and reliability-based design of tunnel face support pressure by constructing a surrogate model to predict limit support pressures. The Back Propagation Neural Network is utilised to fit a surrogate model based on a set of training samples with limit support pressures computed by a proposed numerical method. Several scenarios are used to demonstrate the applications after the prediction performance of the model has been evaluated. A safety factor-based design is implemented by predicting the limit support pressure of a scenario with strength parameters divided by the target safety factor from those of the original scenario, whereas a reliability-based design is performed by predicting the limit support pressures of a certain number of Monte Carlo simulation samples and then choosing a quantile value from the sorted limit support pressures. The design results are checked by direct numerical simulations, demonstrating that the model is valid for conventional scenarios within the specified design domain. The approach is convenient and efficient for applicable scenarios because the most complex and time-consuming process of constructing a surrogate model is no longer needed.

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.

Stability analysis of shield tunnel face in sandy pebble strata considering the route slope angle

Unique physical characteristics of sandy pebble strata amplify disadvantages of the route slope angle for tunnel-face stability. This study uses Lanzhou Metro Line 1 as support project to investigate tunnel-face stability in sandy pebble strata. Using PFC 3D to simulate the tunnel-face failure mechanism of the horizontal tunnel and the longitudinal inclined tunnel. The results show that the slip line ahead of the tunnel face is like a logarithmic spiral, likewise, an ellipsoid failure region forms above the tunnel vault. Based on the above, the theoretical analysis model of tunnel-face stability considering the route slope angle is established. The logarithmic spiral-elliptical model comprises an upper elliptical loose region I and a logarithmic spiral slip region II. Moreover, the limit support pressure is derived by combining the upper bound theorem and associated flow rule. Finally, existing theoretical analysis models are compared with the proposed model, showing that the proposed model fits well with the tunnel-face failure mechanism in sandy pebble strata.

Face failure of EPB shield tunnels in dry dense sand: a model test and DEM study

This study aims at addressing the face failure of earth pressure balance (EPB) shield tunnels in dry dense sand by model tests and discrete element method (DEM) models. The model tests incorporated a miniature EPB shield which could fully reproduce the real tunnel construction of excavation and support. DEM models simulating the model tests were developed to capture the underlying face failure mechanism. Results show that both the limit support pressure obtained at chamber board and tunnel face increase with increasing C/ D ( C is tunnel cover depth, and D is tunnel diameter). The ratio of the former to the latter approximates 0.60 due to the soil retaining of cutter head panel, and it is independent of C/ D. The local face failure initializes around tunnel face and develops directly to the global failure outcropping the ground surface in one phase with C/ D ≤ 1.0, while the local failure develops to the global failure in three phases with C/ D = 2.0 due to the soil arching evolution. The soil arching gets weaker when it propagates upward, and the horizontal stress concentration in the longitudinal direction is stronger than the transverse direction due to the difference of arch foot.

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.

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.

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.

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.

Intelligent decision method for stability assessment of shield tunnel based on multi-objective data mining.

Due to improper operation of the shield construction process and unknown geological surveys, shield construction faces many risks in passing through complex strata, among which the excavation face instability is the most serious, potentially leading to disastrous accidents. To address these issues, this research focuses on the limit support pressure and the excavation face stability in the soil when crossing the Yangtze River. First, an analytical formula for the limit support pressure of the excavation face is established through the wedge model. The support safety coefficient is used to assess the excavation face stability quantitatively. Then the rough set algorithm is used to analyse the sensitivity of each index to establish the reduced evaluation index system for the excavation face stability. The back propagation (BP) neural network is used to train the learning data, and a neural network evaluation model with a prediction error of 5.7675 × 10-4 is established. The prediction performance of BP is verified by comparison with the TOPSIS prediction model and the cloud model. The evaluation method proposed in this paper provides an essential reference for evaluating the underwater shield tunnel excavation face stability. This article is part of the theme issue 'Artificial intelligence in failure analysis of transportation infrastructure and materials'.

A Study on the Stability of Reinforced Tunnel Face Using Horizontal Pre-Grouting

As tunnel excavation under poor geological conditions is liable to cause ground collapses, reinforcement measures are necessary to reduce construction risks. The stability of a tunnel face that was reinforced using horizontal pre-grouting was investigated through numerical simulation and theoretical analysis in this study. An analytical model was developed using the limit equilibrium method and taking into account the impact of horizontal grouting reinforcement (HGR) on the tunnel face. Subsequently, a comprehensive numerical simulation was conducted to confirm the validity of the model. By performing a parametric analysis, it was found that the limit support pressure exhibited a linear relationship with the cohesion and friction angle. In addition, the limit support pressure is more sensitive to changes in the cohesion and friction angle than changes in the stiffness ratio and thickness of HGR. The HGR more effectively decreases the limit support pressure in conditions of low cohesion (or friction angle) compared with conditions of high cohesion (or friction angle).

Numerical investigation and prediction of the excavation face stability for river-crossing shield tunneling: An intelligent prediction model for limit support pressure

The instability of the shield tunnel's excavation face, particularly underwater tunnels, is one of the most hazardous factors in subsea tunnel excavation projects. Therefore, an accurate prediction of the limit support pressure to maintain the excavation face's stability is essential to minimize the possible risk of damage. This study proposed an intelligent prediction model to address these issues. First, the analytical formula of the limit support pressure of the excavation face was established through mechanical analysis to give the mechanics criterion for active instability of the excavation face. Subsequently, the numerical simulations were conducted to reveal the evolution mechanism of the excavation face instability. The result revealed that the buried depth of the tunnel, water depth, driving speed, internal friction angle, water content, cohesion, and support safety factor were the main factors affecting the limit support pressure. Furthermore, the support vector machine (SVM) model was established for predicting the limit support pressure. The prediction results agreed well with the actual data, thereby indicating the feasibility and convenient implementation of the SVM predictor. The proposed model was validated as an effective method for predicting the limit support pressure for further engineering applications.