Tunnel Face Failure Research Articles

Model test and numerical analysis of shield tunnel face instability in sandy soil with strain-softening properties

During excavation of a shield tunnel in a sandy stratum, variations in the state of the sand affect the stratum stability of the shield tunnel face. When dense sand is loaded, its strength decreases gradually owing to dilatancy and it exhibits strain-softening behavior. Model tests and finite element numerical simulations were performed to clarify the failure characteristics of the tunnel face induced by strain-softening. In the model tests, a dense sand stratum was constructed; the movement of the supporting plate of the excavation face was controlled to simulate the progressive failure of the shield tunnel face. Based on a material state-dependent plasticity model, a numerical analysis model of the tunnel face was established to capture the strain-softening behavior of dense sand by including a nonlocal regularization technique. Comparing the model test and numerical simulation results, the evolution of the material state of the sand with progressive failure of the tunnel face was analyzed, and the failure characteristics caused by the strain-softening of sand were revealed.

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.

Limit equilibrium models for passive failure of a large-diameter shield tunnel face in reinforced soft clay

Limit equilibrium models for passive failure of a large-diameter shield tunnel face in reinforced soft clay

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.

Numerical modelling of shield tunnel face failure through a critical state sand plasticity model with nonlocal regularization

During shield tunneling in sand strata, variations in the state of sand can lead to localized instability and strength loss, which brings about numerical difficulties. This paper numerically investigates how nucleation and propagation of plastic deformation zones, including those leading to localized banding, possibly promote failure of shield tunnel faces excavated in sand. To this end, a material state-dependent model is used to capture the mechanical responses of sand. Nonlocal enhancement is introduced, through the volumetric strain increment that drives the change of void ratio and related material hardening/softening behavior, to regularize ill-posed boundary value problems caused by the activation of strain localization. Simulations of tunnel face failure indicate that lower initial void ratios result in more concentrated plastic deformation in the vicinity region near the face and a strain-softening trend in global deformation responses. Conversely, initial looser states drive the deformation zone to propagate spatially to the ground surface, and the resultant global deformation responses exhibit strain-hardening. The second-order work is used to explain the link between material instability around the tunnel faces and initial density. It was found that material instability usually occupies a fraction of plastic deformation regions, and shrinks as sand initial density increases.

Analysis of the Tunnel Face Failure during the Excavation and Evaluation of Excavation Method and Sequential on Ciuyah Tunnel, Lebak Regency, Banten Province

The difference in geological and geotechnical conditions during design and implementation are frequent due to the lack of data collection and diverse rock variations. An important step in designing the excavation method, stages, and support system are determining the rock mass’s actual condition. Inaccuracy in evaluating rock mass quality can cause a collapse during tunnel excavation. This study used an example of a collapse case on the inlet side of the Ciuyah tunnel. The classification system used in the design was the JSCE 1975, but collapses occurred at several locations during excavations. An additional assessment of the tunnel stability was carried out by doing surface surveys and evaluations in STA 0+123 using the RMR89, GSI, and JSCE 2016 classification systems. The rock mass in this location consists of tuffaceous claystone and tuffaceous sandstone with very poor quality. The empirical evaluation showed that Daraei’s multigraph could be used to determine the collapse pattern for very poor rock. For the excavation method, stages, and support systems, it is preferable to apply the RMR classification system with consideration for the maximum stand-up time. This research aims to provide information that may be used for other projects with similar geological circumstances.

Physical modeling of the influence of tunnel active face instability on existing pipelines

In congested urban areas, earth pressure balance (EPB) shielding is commonly adopted to alleviate tunneling effects on surrounding structures. Tunnel active face instability can cause excessive ground movements, which threaten the serviceability of nearby underground pipelines. However, previous studies mainly focused on tunnel face support pressure and tunnel face failure induced ground movements, while pipeline deformations due to tunnel active face instability are seldomly explored. In this study, a series of physical model tests are designed and conducted to investigate the effects of tunnel cover to diameter ratio (C/D) and normalized horizontal distance (H/D) between tunnel face and existing pipeline on three-dimensional pipeline deformation mechanisms. Tunnel active face instability induces excessive settlements and tensile strains in the existing pipeline when it is located at 0.1 D ∼ 0.3 D in front of tunnel face. By increasing H/D ratio from 0.1 to 0.5, the maximum settlement and tensile strain of the existing pipeline decrease by up to 64.8% and 59.9%, respectively. But the maximum settlement and tensile strain of the pipeline only decrease by up to 14.9% and 7.3%, respectively, as C/D ratio increases from 1.0 to 1.5. Obviously, the effects of C/D ratio on pipeline response are less prominent than that of H/D ratio. The measured results are used to verify calculation charts for directly predicting maximum tensile strains of existing pipelines due to tunnel face active instability induced differential ground movements.

A Multi-Layer Blowout Model for the Tunneling Face Stability Analysis

The stability of the tunnel face during tunneling is one of the major criteria for the design and construction of the tunnel. Collapse and blowout are two modes of tunnel face failure during the excavation. The cover-to-diameter ratio is one of the main parameters controlling these failure modes. Several analytical solutions have been proposed to estimate the range of support pressure applied on the tunnel face to avoid both the collapse and the blowout. However, most of those models deal with homogeneous soils. This paper aims at proposing an analytical model to predict the blowout of the tunneling face of a tunnel in multi-layered soils. The derivation is based on a limit equilibrium analysis, which considers the water tCiable. The proposed model is validated against the real blowout data reported from the tunneling in the Second Heinenoord Tunnel project in the Netherlands. Then, the maximum support pressure exerted on the tunneling face is predicted as a function of the cover-to-diameter ratio, the tunnel diameter, and the water table level for five representative soils. Finally, the model is applied to an underground segment of the Hanoi Metro Line 3 project (in Vietnam) to show the role of the multi-layer aspect.

Collapse mechanism of shield tunnel face and evaluation method for the minimum required support pressure considering soil strain softening effect

During shield tunneling, accurately revealing the collapse mechanism and evaluating the minimum required support pressure are key issues to avoid tunnel face failure. The strain softening effect of sandy soil is one of the vital factors for tunnel face stability analysis. However, it has not been explicitly considered in previous research. In this study, a practical strategy was proposed to consider the strain softening effect in finite element simulations. A novel method was also proposed for the evaluation of the minimum required support pressure. The results show that the stratum failure zone can be divided into three types during tunnel face collapse, which correlates with the cover to diameter ratio of the tunnel and can be uniformly delineated by the wedge-prim model. The process of tunnel face collapse was accompanied by the development of local stratum strain softening. The soil’s friction angle at the collapse zone boundary was found in a range between the peak value and the residual value. A coefficient representing the strain softening degree was defined to improve the traditional wedge-prim model. Moreover, by comparing with existing tests and theoretical models, the effectiveness of this method was well validated. The proposed method could accurately assess the minimum required support pressure for construction safety concerns.

Identification and Treatment of Collapse and Mud Inrush Caused by Deep Karst Trough in Tunnels

To predict and prevent the collapse and mud inrush caused by the deep karst trough in tunnels, an engineering case is studied. Firstly, a comprehensive identification method for the hazard-causing structure of the deep karst trough in tunnels is proposed and successfully applied. Secondly, the morphological characteristics and developmental mechanism of the deep karst trough are studied. The hazard-causing structure is mainly developed at the entrance section of the karst tunnel, which is funnel-shaped, mostly filled with clay and a small amount of gravel. The development scale is affected by the geological structure, tectonic movement, formation lithology and inclination, sunshine and precipitation, and surface soil. Thirdly, four hazard-causing modes of the collapse and mud inrush are proposed, i.e., rock mass failure of tunnel face, direct exposure of tunnel face, direct exposure of tunnel roof, and rock mass failure of tunnel roof. Finally, a treatment technology for collapse and mud inrush is put forward. The treatment technology is to “remove the mud, fill the karst cave, conduct advance grouting, strengthen the support, waterproof the collapse area, backfill the collapse pit.” The collapse and mud inrush of Qiyueshan Tunnel was successfully treated by using the above technology.

Failure responses of rock tunnel faces during excavation through the fault-fracture zone

It is essential to cast light on the construction risks in tunnel excavations through the fault-fracture zone (FFZ). This study adopts the material point method (MPM) to simulate the failure responses of a rock tunnel face during excavation through the FFZ. A numerical study was conducted to compare a physical model test and validate the feasibility of using the MPM in simulating tunnel face failure. One hundred ninety numerical simulation cases were constructed to represent a rock tunnel excavation project with different site configurations. The simulation results suggest that the cohesion and the friction angle significantly influence failure responses. The tunnel cover depth can magnify the failure responses, and the FFZ thickness significantly affects the mobilized rock mass volume when the FFZ consists of a weak rock mass. The numerical simulation results suggest three deformation patterns: face bulge, partial failure, and slide collapse. The failure responses can be characterized by stress arch, slip surface, angle of reposing, and influence range. The insights suggested by the face failure responses during excavation through the FFZ can aid field engineers in determining the scope of possible damage, and in establishing emergency measures to minimize losses if such failure occurs.

Experimental study of frictional resistance influences on tunnel face stability

In recent years, there have been several ground surface collapses due to tunnel excavation in Japan. Although such accident is caused by ground loosening and tunnel face collapses due to excavation, there is no accepted theory for estimating the face stability because this phenomenon is quite complicated and highly influenced by the supports and lining types. Specifically, the loosened area is relatively large for the urban NATM method, which applied in a shallow sedimentary rock layer. Therefore, a method to quantitatively evaluate the tunnel face stability by this kind of mechanical property is essential. In this study, 2D tunnel pull-out test was conducted to observe the influence of frictional resistance on tunnel face failure and loosening area by changing the shear resistance angle using three particle sizes of dry silica sand and different variation of earth covering. The result demonstrated that failure region doesn’t reach the ground surface when the shear resistance angle is 46.4° and covering thickness to tunnel height ratio H/D is over 5 or the angle is 55.8° and H/D is over 3. This study considered that there is an arch effect depending on frictional resistance and earth coverings occurred. Furthermore, even with the same earth covering, the smaller the shear resistance is, the earlier the ground surface collapses. Therefore, it could be said that the higher the frictional resistance is, the later the collapse proceeds.

Face failure analysis of a shield tunnel with slurry penetration into the ground

This paper focuses on the face stability analysis of a tunnel driven by a slurry shield machine in the water-bearing ground. A multifield model coupling seepage flow and face stability of the tunnel is developed. The slurry-soil interaction in the ground is considered as a multiphase flow process in porous media. The kinematic approach of limit analysis is used to assess the tunnel face failure. The pressure transfer mechanism of the bentonite suspension is time-dependent and shows good agreement with the field measurement. This model allows the consideration of the slurry support efficiency and its influence on the failure shape through the discretization technique. The results obtained indicate that the support effectiveness decreases with the slurry penetration into the ground, and the traditional membrane model leads to an overestimation of the face stability. Several charts are provided for parametric analysis to discuss the influence of the hydrogeological and slurry properties.

Failure analysis and control measures for tunnel faces in water-rich sandy dolomite formations

Revealing the mechanism of sand gushing triggered by failure of tunnel face in water-rich sandy dolomite strata contributes to the safe construction of tunnels. In this paper, we find that sandy dolomite and the water environment are the key factors causing sand gushing through geophysical prospecting at the tunnel face where 12,000 m3 sand surge occurred. Through the flow-solid coupling model, the extrusion deformation at the tunnel face is the main deformation form in the tunnel of water-rich sandy dolomite strata. The influence of different factors on the deformation value of the tunnel face is from strong to weak: sandification degree, head height, and tunnel depth. Combined with the study of pressure arch characteristics of the model, the limit equilibrium theory considering seepage effects and pressure arch characteristics is proposed. We get that sand gushing will occur in the tunnel in the intensively sandy dolomite strata where the head exceeds 80 m. For the intensively sandy dolomite strata with head below 80 m, the stability of the tunnel face is controlled by the degree of seepage damage. According to the reason of sand gushing, the comprehensive control measures consisting of risk identification by over-detection, double-layer close-packed pipe shed, grouting sealing technology and dewatering technology, and Milling and blasting combined construction method are proposed, and the effective control of tunnel working face is realized in practical application.

Seepage effect on failure mechanisms of the underwater tunnel face via CFD–DEM coupling

Seepage plays a significant role affecting the tunnel face failure process. However, its effect on the stability of the underwater tunnel face remains unclear. To study the seepage effect on the failure mechanism of the underwater tunnel face, this study models the failure process by the Computational Fluid Dynamics-Discrete Element Method (CFD–DEM) approach, in which the seepage force is simulated by fluid–particle interaction between CFD and DEM. The contact parameters of the sand particles in water were obtained by calibrating the results of underwater sand column collapse tests. In this study, three hydraulic conditions including dry, undrained, and seepage are considered for the tunnel face. The simulation results indicate that seepage affects the failure mechanism and the support pressure significantly. Then, a comparison between CFD–DEM results and solutions in previous literature was conducted, which suggests that the tunnel face permeability is the key factor that causes the difference in the support pressure under different hydraulic conditions. Additionally, failure mechanisms with different water depths were analyzed by displacement field, water pressure field, flow field, and support pressure. The results show that under the seepage conditions the failure zone is enlarged as the water depth increases. Besides, the microscopic contact information under the seepage condition in different regions was presented. This work provides evidence for evaluating the support pressure of a stable tunnel face when considering the seepage.

Modeling Hydraulic Fracturing and Blow-Out Failure of Tunnel Face During Shield Tunneling in Soft Soils

A common conflict of objectives for shield tunneling is to provide sufficient support pressure to maintain the stability of the tunnel face while avoiding blow-out of the tunnel face. Slurry-induced hydraulic fracturing, which is an important cause of blow-out failure in slurry-type shield tunneling, is investigated in this study. A fracture initiation mechanism for soft soils is developed and the fracturing pressure is described using a combined shear and tensile failure criteria within the cavity expansion theory framework. A slurry-induced fracture propagation model, coupled with the fracture initiation mechanism, is then proposed for predicting the fracture length and growth rate in front of tunnel face. The model is successfully applied to test data from the literature, and used in a parametric analysis. The results show that a fracture may easily extend to the ground surface when tunneling in ground with low strength parameters or shallow depth, and hydraulic fracture can be mitigated by using a high-density slurry. Finally, some design charts are provided to guide engineers in selecting a reasonable range of slurry pressure during shield tunneling, thus mitigating the risks of blow-out as well as maintaining face stability.

Face failure in cobble-rich soil: Numerical and experimental approaches on 1 g EPB reduced scale model

Recent years have witnessed several accidents associated with tunnel face failure in cobble-rich soil in the city of Chengdu, China. Due to its lack of cohesion, cobble-rich soil can be easily disturbed by shield tunneling. Based on the general conditions of the Chengdu Metro Line 1 project, the mechanisms of face failure of tunnels in cobble-rich soil driven with earth pressure balance (EPB) machines are studied. Specifically, we present results of tests carried out using a laboratory reduced-scale model of EPB tunneling operations in cobble-rich soil. The failure kinematics and limit face pressures are presented and analyzed. Then a three-dimensional (3D) discrete-element method (DEM) model, which is able to simulate the main EPB excavation processes is employed to gain further insight into the mechanisms of face failure in cobble-rich soil. Comparisons of these results with the observations based on previous studies are discussed. The results reveal a fundamentally different tunnel-face failure mechanism in cobble-rich soil in contrast with that in clayey or sandy soils. It shows that the ground movement during face failure is sudden in cobble-rich soil, which is different from the progressive mechanism in frictional–cohesive materials. The observed sinkhole at surface takes the shape of an oval, and the failure zone behind tunnel face extends almost as far as that ahead of the face, which is different from the observations in previous studies. The failure zone is found to be wider than that of sandy soils in both the transverse and longitudinal directions.

Reliability Analysis of Seismic Stability of Shield Tunnel Face under Multiple Correlated Failure Modes

In this paper, the seismic stability of shield tunnel face under multiple correlated failure modes is studied by combining the upper bound theorem of limit analysis with the response surface method. The pseudo-static method is employed to research the influence of seismic forces on the assessment of active and passive failure of shield tunnel face. The critical collapse pressure and blowout pressure of tunnel face are solved by the upper bound method. Furthermore, the reliability model of shield tunnel face is established in terms of these two limit states, and the range of supporting pressure to ensure the stability of tunnel face is obtained by the response surface method. Comparison of the solutions of this paper with existing results verifies the present method. The research shows that, the safe range of supporting pressure of tunnel face based on the reliability model of multiple correlated failure modes is more reasonable and effective. The soil shear strength and seismic force exert an obvious effect on the stability of tunnel face. In addition, the dispersion of random variables reduces the stability of shield tunnel face. This work can provide some guidance for the anti-seismic design of tunnel face in earthquake-prone regions.

An efficient probabilistic design approach for tunnel face stability by inverse reliability analysis

In order to maintain the safety of underground constructions that significantly involve geo-material uncertainties, this paper delivers a new computation framework for conducting reliability-based design (RBD) of shallow tunnel face stability, utilizing a simplified inverse first-order reliability method (FORM). The limit state functions defining tunnel face stability are established for both collapse and blow-out modes of the tunnel face failure, respectively, and the deterministic results of the tunnel face support pressure are obtained through three-dimensional finite element limit analysis (FELA). Because the inverse reliability method can directly capture the design support pressure according to prescribed target reliability index, the computational cost for probabilistic design of tunnel face stability is greatly reduced. By comparison with Monte Carlo simulation results, the accuracy and feasibility of the proposed method are verified. Further, this study presents a series of reliability-based design charts for vividly understanding the limit support pressure on tunnel face in both cohesionless (sandy) soil and cohesive soil stratums, and their optimal support pressure ranges are highlighted. The results show that in the case of sandy soil stratum, the blowout failure of tunnel face is extremely unlikely, whereas the collapse is the only possible failure mode. The parametric study of various geotechnical uncertainties also reveals that ignoring the potential correlation between soil shear strength parameters will lead to over-designed support pressure, and the coefficient of variation of internal friction angle has a greater influence on the tunnel face failure probability than that of the cohesion.

Study on Tunnel Face Failure Mechanism in Two-Layer Soils

Study on Tunnel Face Failure Mechanism in Two-Layer Soils