Shallow Shield Tunnel Research Articles

Three-Dimensional Active and Passive Seismic Stability Analysis of Shallow Shield Tunnel Faces

Three-Dimensional Active and Passive Seismic Stability Analysis of Shallow Shield Tunnel Faces

Experimental study of face passive failure features of a shallow shield tunnel in coastal backfill sand

Experimental study of face passive failure features of a shallow shield tunnel in coastal backfill sand

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.

Investigation on overburden thickness considering face and anti-floating stability of shallow shield tunnel

Ensuring the safety of water-rich shield tunnels requires a reasonable design of overburden thickness. However, few studies have comprehensively investigated the design of overburden thickness for shield tunnels in terms of both face and anti-floating stability. In this study, the traditional wedge-prism model was optimized, and formulas for active and passive limit support pressure of tunnel faces were derived for various excavation inclination angle and seepage conditions. The analytical solution for two limit support pressures, with special attention on the proposed passive failure model were verified using finite element numerical simulations. Design procedures for a reasonable range of overburden thickness were provided based on the analytical solution of the uplift resistance and limit support pressure. The passive limit support pressure was found to increase with the cohesion and internal friction angle of soil stratum and decrease with the excavation inclination angle, while the active limit support pressure exhibited the opposite trend. The two limit support pressures increased with rising groundwater levels.

Upper bound solution of passive instability on the face of longitudinal inclined shallow buried shield tunnel based on rotation–translation mechanism

To better evaluate the passive failure mode of shallow shield tunnel faces in sandy soil strata, tunnel faces passive failure mode was investigated considering the longitudinal inclination in this study. A three-dimensional rotation–translation mechanism that simultaneously considers the longitudinal inclination of the tunnel and partial failure of the tunnel face based on the upper bound limit analysis method was presented. The mechanism comprised two rigid translation blocks and one rigid rotation block. Subsequently, the reliability of the proposed mechanism was verified using the horizontal rigid translation mechanism and numerical simulation considering the longitudinal inclination. Finally, the effects of tunnel longitudinal inclination (δ), cover depth ratio (C/D), soil internal friction angle (ϕ), and the possible presence of uniform surface surcharge (σS) on the limit support pressure of the tunnel face and the partial failure range was assessed. The results indicated that the partial failure range of the tunnel face gradually increased with δ When C/D<1. Conversely, global failure of the tunnel face occurred at different longitudinal inclinations when C/D≥1. The rotation angle (θ) of the rotational failure zone was sensitive to the longitudinal inclination (δ) and decreased with δ at the same coverage depth ratio.

A method of calculating water and soil loading on top of shallow shield tunnels near water areas

Long-term water seepage in shield tunnels has a serious impact on water and soil loading on the outer surfaces of a shield segment near water areas. A theoretical analysis was used to obtain a formula to express the average vertical seepage gradient at the top of the tunnel. A formula for calculating the coefficient of lateral earth pressure for the principal stress arch effect was utilized. A model that takes into consideration the effects of long-term water seepage on the shield tunnel’s water and soil load was designed. Based on this calculation model, the variation law of the water and soil loading on top of shield tunnel near water area with the internal friction angle in the soil body, the density of the soil, the tunnel depth-to-diameter ratio, the water head of the external section, and the amount of water seepage per unit length is studied. Based on the geological conditions and field survey results of water and soil loading of two typical segments of the Maliuzhou Waterway section of the Hengqin Tunnel, a comparative analysis of the theoretical results and field survey measurements was performed for different calculated conditions. The research shows that the proposed model is able to perform a reasonably effective evaluation of the water and soil pressure at the top of the shield tunnel for the marine and land segments of the shield tunnel; and, when compared with Dimitrios Kolymbas’ effective stress method and Terzaghi’s Principle, the method shown in this paper has fewer errors. The results of the associated research are sufficient to reasonably design and propose a theoretical basis for underwater shield tunnels.

Numerical Investigation of Slurry Fracturing during Shield Tunneling under a Reservoir

The Jinan Jiluo Road Crossing the Yellow River Tunnel North Extension Project will intersect the Queshan reservoir, which currently supplies 60% of the domestic water in Jinan City. During the excavation process of the large-diameter slurry type shield used in this project, it may lead to slurry fracturing of the stratum in front of the excavation face and slurry blow-out from the surface if the slurry support pressure is too high. The leakage of shield slurry will pollute the reservoir water, and the safety of domestic water in Jinan will be threatened. Shield slurry blow-out may also lead to water inrush accidents. It is difficult to prevent slurry blow-out during shallow shield tunnel construction due to an insufficient understanding of the shield slurry fracturing mechanism. The initiation and extension of shield slurry fracturing are very complex and difficult to observe in the stratum. Currently, there is no effective method to study the slurry fracturing mechanism of shield tunneling. This paper presents a numerical simulation method of shield tunneling slurry fracturing based on the extended finite element method (XFEM). The risk of slurry blow-out in shield tunnel crossing reservoir engineering is analyzed. The advantages of the XFEM for simulating crack propagation are fully exploited. Considering the coexistence of tensile and shear failures in soft soils, embedding the combined tensile and shear failure criterion is realized in the XFEM by the secondary development of the ABAQUS extended finite element. Compared with the slurry fracturing test of blind-hole clay samples, the rationality of the simulation method for slurry fracturing in cohesive soil is verified. Through the establishment of the slurry fracturing extension model, the slurry fracturing process of shield tunneling in cohesive soil layer is simulated. The variation law of slurry pressure in the process of fracture extension is studied, and the influence of shield slurry support pressure, overburden thickness, formation shear strength, and slurry viscosity on fracture extension pressure and extension path is analyzed. Based on this numerical simulation method, the risk of slurry blow-out is analyzed in the shield tunneling intersecting the Queshan Reservoir of the Jinan Jiluo Road Crossing the Yellow River Tunnel North Extension Project.

Influence of non-associated flow rule on passive face instability for shallow shield tunnels

This paper aims at analyzing the influence of non-associated flow rule on passive face instability for shallow shield tunnels. The influence exerted by dilation angle on the limit support pressures, the failure zones and partial failure ratios is investigated by numerical simulations. Based on the failure zone obtained from numerical simulation, a log-spiral mechanism is proposed. The upper-bound theorem is employed to acquire the limit support pressures, failure zones and partial failure ratios. The analytical results indicate that dilation angle has a significant influence on the limit support pressure and failure mechanisms. Finally, by comparison with the numerical models and existing analytical models, the results indicate that the proposed log-spiral model has a certain accuracy in analyzing the face instability of blow-out.

Rock–liner interaction mechanism involving a nonuniform deformation pattern in shallow shield tunnels

A numerical plane-strain model is established on the basis of the nonuniform deformation pattern proposed by Loganathan and Poulos. A 2D analysis method of the rock– liner interaction is proposed for the shallow shield tunnel. The accuracy of the proposed method is verified by comparing the surface settlement curve obtained through the analytical method and field measurement. Variations in rock deformation and rock stress are investigated using the proposed method in terms of the influence of gap parameter, deformation modulus, unit weight of the rock, and diameter and depth of the shield tunnel. Results indicate that the liner stress is significant for a shield tunnel with a large diameter, and the buoyancy effect is evident, indicating that the liner is apt to the failures for the shield tunnel with a large diameter. The unit weight significantly affects the liner and rock stress, the effect of the depth of the shield is less, and the effect of the deformation modulus is the smallest.

A Simplified Limit Equilibrium Model for Calculating Limit Support Pressure on Shallow Shield Tunnel face

Shield tunnel excavation face may collapse if the applied support pressure is not enough. Based on an elaborated failure mode of shallow shield tunnel face, the least limit support pressure to maintain stability is analysed by using the limit equilibrium method. The calculated limit support pressure decreases nonlinearly with the increase of friction angle, decreases linearly with the increase of cohesion, and decreases with the ratio of buried depth to tunnel diameter. By comparing the calculated results with existing tests, the theoretical model was shown to be reasonably predict the limit support pressure of tunnel excavation face in sand under drained condition and in clay under undrained condition.

Deformation Response Induced by Surcharge Loading above Shallow Shield Tunnels in Soft Soil

In the case of sudden surcharge loading, shallow shield tunnels in areas with soft soil experience substantial deformation responses. It is very important to understand the different loading modes and control measures above the shallow shield tunnels in soft soil for improving the safety of tunnel structure and reducing the influence of deformation. In this study, a three-dimensional numerical model of shallow shield tunnels in soft soil is established with FLAC. Ground and tunnel deformations are analyzed under different loading modes, and the effects of different deformation control measures are also studied. The numerical simulations in this paper show that the surface and tunnel deformation responses vary when induced by different loading modes above shallow shield tunnels in soft soil. After surface hardening with a 20-cm-thick layer of C20 concrete, the surface settlement is effectively controlled, and the uneven longitudinal settlement of the tunnel vault is improved. However, controlling the height of the surcharge is the most direct deformation control method. When the height of the surcharge is reduced from 6 m to 4 m and 2 m, the maximum ground settlement is reduced by 37.8% and 69.4%, respectively, and the maximum longitudinal settlement of the tunnel vault is reduced by 35.3% and 65.2%, respectively. During the operation of shallow shield tunnel in soft soil area, sudden surcharge loading should not be allowed. In the inevitable case, the surcharge loading on one side of the tunnel should be prevented and the surcharge loading height should be strictly limited.

Analysis of the Limit Support Pressure of a Shallow Shield Tunnel in Sandy Soil Considering the Influence of Seepage

The objective was to optimize the existing solution for the limit support pressure of a tunnel face. Firstly, based on the numerical simulation results, the existing three-dimensional analytical solution for pore water pressure distribution is expanded to a three-dimensional solution considering the pore water pressure distribution in the upper formation behind the tunnel face. Then, according to the results of physical model tests, a failure model considering the failure range in the upper formation behind the tunnel face is established, and the newly established three-dimensional solution for pore water pressure is introduced into the model, and then the limit effective support pressure of the tunnel face considering seepage is obtained by the method of soil–water joint calculation. Finally, the calculation results in this paper are compared with the experimental results, numerical simulation results and existing theoretical solutions. The major findings are as follows. The distribution of pore water pressure in the front and back strata above the tunnel face is basically symmetrical. The limit effective support pressure of the tunnel face will increase linearly with an increase in the hydraulic head difference between the tunnel face and the ground surface. The calculated results of the new limit equilibrium theory are obviously larger than those of the existing theory and numerical simulation and closer to the results of the physical model tests. Therefore, the new limit equilibrium model can better predict the limit effective support pressure of the tunnel face considering seepage and provide a reference for actual projects.

An analytical method of continuous upper bound limit analysis for the stability of shallow shield tunnels in cohesive-frictional soils

For stability analysis of shallow shield tunnels in cohesive-frictional (clayey–sandy) soils, an analytical method of continuous upper bound limit analysis is developed to determine the most critical slip line position and the minimum required tunnel support pressure. The ground displacement field due to composite volume loss is calculated using a complex variable solution, and the optimization problem is solved by employing the particle swarm optimization technique. Energy dissipation at the potential slip boundary, which is determined through a system of tracking schemes, and in the corresponding potential plastic zone is taken into account. Example calculations are carried out to verify the proposed method via finite element simulations and comparisons with solutions from existing methods, and to study the effects of soil strength properties and tunnel size and depth. The results show that both the internal friction and the cohesion of the surrounding soil may help reduce the range of the potential collapse zone and the magnitude of the necessary support pressure. This study represents the first attempt to deal with cohesive-frictional soils via an analytical and continuous approach, and the developed method can be extended for various tunnel and other geotechnical stability problems involving cohesive-frictional soils.

Research on the reinforcement measures of close-parallel tunnels

Based on shallow parallel underground tunnel and double-line shield tunnel of Jinan Rail Transit Line R3, this paper proposes two reinforcement measures including bored piles and curtain grouting in the middle of the tunnel, making a numerical analysis and comparison of the reinforcement effect. The results show that the influence of later tunnel construction on the deformation of earlier tunnel can be effectively reduced after taking two reinforcement measures respectively and the reinforcement effect of the bored piles is better.

An upper-bound analytical model of blow-out for a shallow tunnel in sand considering the partial failure within the face

This paper proposes a three-dimensional analytical model of blow-out for a shallow tunnel in sand. Upper bound theorem is adopted to deduce the analytical solutions of limit support pressure, and a dimensionless parameter called partial failure ratio is defined to describe the range of partial failure within the tunnel face. A series of numerical simulations are carried out to analyse the limit support pressure and failure zone on the limit conditions. Finally, the analytical solutions including the partial failure ratio and limit support pressure obtained by the proposed model are compared with those obtained by the numerical simulation and the existing global failure model approaches. Results show that the proposed model in this paper can not only analyze the partial failure with better accuracy, but can also consider the global failure. The proposed model has more extensive applicability in analyzing the face stability of blow-out for a shallow shield tunnel.

A continuous upper bound limit analysis for the transverse stability of shallow shield tunnelling in undrained clays

For a cross sectional ‘convergence-oval deformation-sagging/uplifting’ composite volume loss model of shallow shield tunnels in undrained clays, the ground displacement field is calculated using complex variables, and the most critical tunnel stability number as well as its supremum are determined by using continuous upper bound limit analysis. Illustrative calculations reveal: (1) the method can be verified via finite element modelling and comparisons with other limit analysis solutions and centrifuge model test results; (2) with the increase of the tunnel sagging coefficient or oval-deformation coefficient, the ground surface settlement increases mainly in magnitude but minor in trough width; (3) the influence of the tunnel uplifting coefficient on ground displacement is similar in pattern but opposite in direction to that of the tunnel sagging coefficient; (4) the effect of the tunnel oval-deformation coefficient on the supremum of the tunnel stability number is more significant than that the tunnel sagging/uplifting coefficient; (5) the uniform convergence volume loss model can reflect the most critical pattern of tunnel instability and the most critical tunnel stability number with sufficient accuracy. The proposed method and the relevant observations can serve as references for limit support pressure calculations concerning shallow shield tunnel excavations in undrained clays.

Back Analysis of Water and Earth Loads on Shield Tunnel and Structure Ultimate Limit State Assessment: A Case Study

A method based on the least-squares method was proposed for the back analysis of water and earth loads using in situ monitoring data of a limited amount of structural strain values for a shallow shield tunnel buried in the soil stratum. Theoretical formulas were used to calculate the internal forces, conduct an assessment of the ultimate limit state, and determine the safety factors of compression and shear. Based upon all of the above, an assessment system was established, and its procedures were illustrated. The highlights of this assessment system are that only a few strain monitoring points are required for the back analysis of the external loads, the internal forces in every position of the segment are calculated by the uniform rigidity ring model, and the safety assessment is conducted by the proposed ultimate limit state formulas. According to the in situ monitoring project in Nanjing subway line 1, the monitoring data of internal forces were close to the back analysis data, which proves the reliability of this method.

Limit Analysis for the Face Stability of a Shallow-Shield Tunnel Based on a Variational Approach to the Blow-Out Failure Mode

Limit Analysis for the Face Stability of a Shallow-Shield Tunnel Based on a Variational Approach to the Blow-Out Failure Mode

Numerical simulation of slurry fracturing during shield tunnelling

The slurry type shield method is an important tunnelling method, especially for underwater tunnels in soft soil. If the slurry pressure is too high, it may lead to slurry fracturing of the stratum in front of the excavation face and cause water inrush accidents. Controlling the face stability of shallow shield tunnels is difficult owing to the inadequate understanding of the slurry induced fracturing mechanism. Most importantly, understanding the details of the slurry fracturing propagation process is not enough, as the process is especially complicated. However, the propagation process cannot be directly observed in the field or in small-scale laboratory tests because it occurs in bodies of soil. Thus far, an effective approach to investigate the slurry fracturing mechanism in the surrounding stratum of a slurry shield tunnel has not been reported. In this paper, a numerical simulation method for evaluating tunnelling-induced slurry fracturing is presented. The developed numerical model considered the couplings of stress distribution, fluid flows, and fracturing. The fracture initiation, branching, and propagation in the stratum could be reproduced numerically. A series of simulations for different ratios of cover depth to tunnel diameter were carried out to investigate the propagation process of slurry fracturing. The effects of slurry pressure at the tunnelling face and the cover depth above the tunnel were investigated. The results of the fracturing process induced by slurry, such as the fracture propagation velocity, slurry pressure distribution along the fracture, fracture initiation pressure, fracture extending pressure, and stratum deformations, were analyzed.

Uplift Model Analysis for Shallow Overlaying Slurry Shield Tunnel Based on Time-Varying Behavior of Cement Grouting Material

As a necessary and key procedure of shield technology, synchronous grouting affects ground layer deformation, and controls the tunnel uplift and ground subsidence, usually exerting negative influence on underground engineering construction and surrounding buildings. This paper establishes the 2D uplift model of the soil and lining by ABAQUS, based on a static buoyancy varying with solidification of grout and dynamic buoyancy produced by grouting pressure. The effect of buoyancy on lining is simulated in the lining and soil through a series of specific magnitude of interference fit. Effectiveness of the model is validated by comparison of data measurement of South Hongmei Rd. tunnel with predicted results for vertical displacement of ground surface. East line of South Hongmei Rd. tunnel with super large diameter is chosen for the simulation and central burial depths vary as a key parameter. The results show t4hat the differences between predictions and data measurement are limited, hence providing a basis of numerical analysis for the design and optimization of shallow slurry shield tunnel with super large diameter.