Tunnel Failure Research Articles

A new classification of failure mechanisms at tunnels in stratified rock masses through physical and numerical modeling

Stability of tunnels in stratified rock masses under low stress conditions is governed by the rock mass structure (blocks) and excavation geometry and dimension. A physical model was built and the effects of bedding dip, discontinuity spacing, and tunnel dimensions on tunnel failure mechanisms were examined. Based on the image analysis, three zones were recognized around the excavation consisting: stationary, collapsed and buckling zones. The buckling zone was more affected by layer dip angle. The collapsed zone occurred in three modes of block falls and sliding, and toppling. Also, numerical simulation using distinct element method was performed and verified. After model calibration, numerical simulations were extended to a wide range of block size and discontinuity gradients. Finally, considering physical test results, image analysis, and numerical simulation results, failure mechanisms in stratified rocks were classified using “bedding dip” and so called “dimensional ratio”. Results lead us to a deep insight of ground behavior around the shallow underground excavations in stratified blocky rock masses under low stress condition. Additionally the deformation mechanisms of blocks such as buckling, shearing, sliding are discussed in detail.

Experimental simulation analysis of the process and failure characteristics of spalling in D-shaped tunnels under true-triaxial loading conditions

Spalling is a typical failure mode encountered on the sidewalls after the tunnel face excavation of deep D-shaped tunnels or caverns. It significantly affects the safe and efficient construction and long-term stability of tunnels. To study the process and failure characteristics of sidewall spalling in D-shaped tunnels, four different three-dimensional initial stress states were established to perform a series of true-triaxial tests on cube specimens with a D-shaped hole. During the tests, a wireless micro camera was used to monitor and record the failure process of the sidewall in real time. Based on the test results, the process and failure characteristics of sidewall spalling were summarised, and the influence of cross-section shape on tunnel failure characteristics and tunnel stability were discussed. The results indicate that both sidewalls perpendicular to the maximum principal stress are cut into approximately parallel rock slabs, demonstrating typical spalling features. Failure zones on both sidewalls are located primarily between the corner and spandrel, and their outlines are “V” shaped (i.e., V-shaped notch). Spalling slabs are thin in both wings, and thick in the middle; they are cut into multilayer thin slices that are not completely detached from one another. With the increase in failure depth, the shape of the cracks first evolved from a straight shape into an arced shape, and subsequently evolved into a “V” shape. When the maximum principal stress is perpendicular to the tunnel axis, tunnel excavation along the minimum principal stress is conducive to the prevention of spalling. Compared with the circular tunnels, the D-shaped tunnels can effectively reduce the severity of rock failure and transform the rockburst into a static spalling. In a relatively low stress environment, the D-shaped cross section can improve the stability of the tunnel, while it is not conducive to the stability of the tunnel in a high stress environment. The result of this study benefits the design and prevention of spalling and rockburst of hard rock tunnels.

Shear velocity-based uncertainty quantification for rock joint shear strength

The shear strength of rock joints is an important property required in order to analyze the stability of rock slopes and tunnels. However, estimation of the shear strength of rock joints for in situ conditions is a complex task due to various influencing factors present in the field. Among these factors, the shear velocity or the shear displacement rate along the rock joints are important parameters which are relatively less studied since their effect is considered to be of second order compared to other factors. However, some recent studies in the literature suggest that shear velocity has a significant influence on the shear strength of rock joints, and hence the shear strength of joints estimated at low shear velocities in laboratories cannot be used under in situ conditions where the possibility of higher shear velocities exist due to the presence of different factors, such as blasting, excavation, and thermal and seismic loads. In this paper, we have addressed these issues in three steps. In the first step, an experimental study on jointed rock specimens is presented to investigate the influence of the displacement rate on the shear strength of rock joints. In the second step, a probabilistic method is developed based on the experimental results and the compiled data from the literature to estimate the in situ shear strength of joints under higher displacement rate conditions, i.e., blasting, excavation, and seismic loads from laboratory-estimated shear strength at the International Society for Rock Mechanics suggested low displacement rates. In the third step, a case study of a Himalayan rock tunnel was used to demonstrate the described approach. It was observed that the shear strength of discontinuities reduced with ncreasing shear velocity and that the rate dependency was higher for low-density rocks and under high confining stress. Further, a considerable effect was observed on the probability of failure of the rock tunnel when the effect of shear velocity was considered in the stability analysis.

Instability analysis of a deep tunnel under triaxial loads using a three-dimensional numerical method with strength reduction method

Influence of triaxial stress states on failure behaviors and safety factor of deep tunnels, involving the intermediate principal stress and the ratio of the vertical stress to the horizontal stress, is investigated using a three-dimensional numerical model and the strength reduction method (SRM). The numerical model is able to consider the heterogeneity of rock masses described by the Weibull distribution based on the statistical damage theory. The strength reduction method is applied to implement the progressive failure modelling of the surrounding rocks, and the tunnel stability under different stress conditions is obtained. The results show that the failure behaviors of the surrounding rocks are complex under the effect of triaxial stress states. Both the lateral pressure coefficient and axial stress have significant influence on the tunnel failure patterns. When the direction of the intermediate principal stress is parallel to the tunnel axis, the safety factor of the tunnels increases with the increasing of the intermediate principal stress. When the direction of the intermediate principal stress is perpendicular to the tunnel axis, the safety factor increases in the beginning and then decreases as the intermediate principal stress increases gradually until reaching the maximal principal stress. When the direction of the maximum principal stress is parallel to the tunnel axis, the zonal disintegration is observed because of the large axial stress. Overall, the numerical model is successfully adopted to model the fracture patterns and evaluate the tunnel stability. The simulated results have contributed to reveal the nonlinear failure behaviors of deep tunnels and understand their safety condition under complex triaxial stress states.

Analysis of a tunnel failure caused by leakage of the shield tail seal system

This paper presents an analysis of a tunnel failure accident during shield tunnel construction on Foshan Metro Line 2 in China. The failure is caused by the leakage of the multilayer seal system, which consists of several brush seals at the tail of the shield. Four different failure modes for the multilayer seal system are discussed. A simple structural analysis of the brush seals is then conducted, and failure mode 4 (failure due to brush seal deformation) is identified as a major reason for the Foshan tunnel accident. A finite element method (FEM) analysis is employed to validate the conclusions drawn from the simple structural analysis of the brush seals.

Instability Process of Crack Propagation and Tunnel Failure Affected by Cross‐Sectional Geometry of an Underground Tunnel

The process of crack propagation and tunnel failure is affected by the cross‐sectional geometry of an underground tunnel. In order to quantify the effect of section shape on the process of crack propagation in deep tunnels under high ground stress conditions, a total of four physical models with two cross‐sectional shapes and twelve stress levels were designed and several large‐scale physical model tests were conducted. The results indicated that, when the vertical stress is 4.94 MPa, the length and depth of the cracks generated in the rock surrounding the horseshoe tunnel are about eight times that around a circular tunnel. The position where the circumferential displacement of the horseshoe tunnel begins to be stable is about two, to two and a half, times that around a circular tunnel. After the deep chamber was excavated, continuous spalling was found to occur at the foot of the horseshoe tunnel and microcracks in the surrounding rock were initially generated from the foot of the side wall and then developed upwards to form a conjugate sliding shape to the foot of the arch roof, where the cracks finally coalesced. Discontinuous spalling occurred at the midheight of the side wall of the circular tunnel after excavation, and microcracks in the surrounding rock were initially generated from the midheight of the side wall and then extended concentrically to greater depth in the rock mass surrounding the tunnel. Tensile failure mainly occurred on the surface of the side wall: shear failure mainly appeared in the surrounding rock.

The influence of impacting orientations on the failure modes of cracked tunnel

Underground tunnels are often subjected to dynamic loadings, and the dynamic loadings may come from various directions. To study the influence of loading orientations on the failure modes of cracked tunnels, laboratory impact experiments were conducted by a drop weight testing apparatus with synchronized measuring system. Green sandstone was selected as the experimental material to fabricate the tunnel samples, a pre-crack emanating from tunnel roof was designed to simulate the natural crack around a tunnel. In the process of dynamic loading tests, strain gauges were employed to determine crack time-to-fracture and crack velocity. Numerical simulation was carried out by the finite element software ABAQUS and the finite difference software AUTODYN. Crack initiation fracture toughness (IFT) was evaluated by experimental-numerical technique and the failure behavior of tunnel model under different orientation impacts was presented in this paper. The studying results show that as impacts come from tunnel roof, tunnel failure mainly occurs near the crack tip and the tunnel bottom (or the tunnel foot); as impact come from tunnel sidewall, the tunnel failure only occurs at the two sides of arch shoulder. The dynamic initiation fracture toughness of the pre-crack in the tunnel models is greatly related to the impact loading orientations.

Analysis on the Precursor Information of Water Inrush in Karst Tunnels: A True Triaxial Model Test Study

Water inrush in karst tunnels usually results in casualties, equipment damage, project delays and other serious consequences. An important measure to ensure the safety of tunnel construction is to provide accurate prediction of water inrush during tunnel excavation. To study the stability of the surrounding rock under effect of a water-bearing cave in front of the tunnel face, a true triaxial geomechanical model test was carried out based on the engineering background of the Xiema Tunnel in Chongqing Province. The response regularities of multivariate physical information of the surrounding rock during rock failure and water inrush were obtained by monitoring the displacement, stress and seepage pressure of different monitoring sections. The results show that some internal connections exist among the kinds of physical information. When the tunnel face was adjacent to the disaster source, the change of displacement and stress was in a decreasing trend, and the seepage pressure increased first and then decreased during the tunnel excavation. In addition, by fusion analysis, the sensitivity priority of precursor information for a rock failure and water inrush disaster in karst tunnels was summarized as stress, displacement and seepage pressure. Finally, the theoretical basis of the prediction of water inrush in karst tunnels had been established, which proved the feasibility of predicting disasters according to the precursor information.

Experimental simulation investigation on rockburst induced by spalling failure in deep circular tunnels

Spalling and rockburst are two common failure modes in deep hard-rock tunnels and they exhibit a strong correlation. In this paper, the process of rockburst induced by spalling is investigated. The uniaxial compressive strength and rockburst tendency of red sandstone are measured. Four different initial stress conditions are set up, and cubic red sandstone samples with a prefabricated hole are tested by a true-triaxial test system. During the experimental process, the failure of the hole sidewall is monitored and recorded in real time by a micro-camera. The process of rockburst induced by spalling damage in deep hard-rock tunnels is reproduced, and the mechanism by which spalling damage on induces rockburst is revealed. In addition, the evolution process and failure characteristics of rockburst induced by spalling damage are analysed. The experimental results indicated that the red sandstone has a moderate rockburst tendency, and its rockburst process can be divided into four periods: the calm period, the small grain ejection & spalling damage period, the slab buckling & fragment ejection period and the violent ejection period. The mechanism of spalling damage on inducing rockburst is mainly embodied in two aspects: promoting large buckling deformations (providing energy for rockburst) and to weakening the strength of the rock mass (creating conditions for the suddenly release of energy). The effect of lateral stress on spalling damage and rockburst is more obvious than that of axial stress, and the severity of rockburst can be significantly reduced by increasing the lateral stress. The diameter of the hole has a strength size effect on the sidewall damage, producing a certain inhibitory effect on the spalling damage and rockburst. Smaller lateral stress corresponds with greater depths of the V-shaped notch and much smaller width-to-length ratios in the rock fragments.

A probabilistic method to analyze collapse failure of shallow rectangular tunnels

It is difficult to forecast the collapse failure of rectangular tunnel roof because mechanical properties of surrounding rocks or soils are strongly affected by uncertainties. A uniform design (UD)-based response surface method (RSM) is proposed to handle uncertainties and to forecast the stability of rectangular tunnel roof with the help of Tunnel Seismic Prediction 203 plus (TSP203PLUS). Firstly, the influence factors of shallow rectangular tunnels are studied by the excavation half-width (Lu), which is derived using Mohr envelope. Since TSP203PLUS is an essential approach to predict the geohazard in tunnels, and a series of parameters such as the density (ρ), P-wave seismic velocity (V) and dynamic Young's Modulus (E) of geomaterials can be read from TSPwin (the software of TSP203PLUS). Then, the relationship among Lu and ρ, V and E are established, so that ρ, V and E read from TSPwin are treated as input variables in the UD-based RSM. Finally, a tunnel located in China is illustrated to demonstrate the efficiency of this approach and the result obtained from the proposed method is validated by Monte Carlo simulations (MCS). The proposed method provides a quick and effective method to study the collapse failure of rectangular tunnel roofs.

An Improved Crack Initiation Stress Criterion for Brittle Rocks under Confining Stress

The stress level associated with crack initiation is an important damage threshold in rock failure process, which can be applied in the prediction of brittle failure in tunnels. In the present study, the uniaxial and triaxial compression tests are conducted on two sets of hard sandstones, while the dilation strains and AE counts are employed to characterize the crack accumulation. The crack initiation stress is then determined through the inflection point on strain and AE curves. Furthermore, two crack initiation modes: the splitting mode for low confinements and the sliding mode for high confinements are presented by analysing the fracturing mechanism. Based on the derivation of maximum tensile stress around the rock defects, an improved crack initiation criterion is proposed to predict the σci of confined rocks. In the new criterion, the frictional components on crack surface are considered by introducing the parameter mci, which can be applied for various types of rocks as well. At last, the triaxial compression test data of different rock types are reviewed to verify the reliability and applicability of the new criterion. The results indicate that the new criterion can predict the crack initiation stress of confined rock with high accuracy.

Numerical modeling of rockburst near fault zones in deep tunnels

Many factors that influence rockburst damage have been identified but the mechanism that drives rockburst is not fully understood. Weak planes such as faults and shears have been observed near excavation boundaries in many rockbursts and these structures must have played a role in these rockburst events. In this paper, the role of weak planes around tunnels in rockburst occurrence and damage was studied. A heterogeneous Abaqus2D explicit code was used to simulate dynamic rock failure in deep tunnels. Firstly, rock failure near the excavation boundary of a tunnel without any adjacent weak plane was modeled. Then a fault with different lengths, inclinations, and distances to the tunnel was added to the model and its effect on rock failure was simulated. The velocity and the released kinetic energy of failed rocks, the failure zone around the tunnel, and the deformed mesh were studied to identify stable and unstable rock failures. The simulation results showed that the presence of a fault near a tunnel could induce rockburst if the fault is positioned and oriented in such a way that it promotes high stress and low mine system stiffness. Finally, a rockburst occurred in the Jinping II drainage tunnel with an observed nearby fault was simulated. The modeling results captured the field observation of rockburst damage and confirmed that the presence of weak planes in the vicinity of deep tunnels is a necessary condition for the occurrence of rockburst. The presented methodology in this paper can be useful for rockburst anticipation and control during mining and tunneling in highly stressed grounds.

An approximate nonlinear modified Mohr-Coulomb shear strength criterion with critical state for intact rocks

In this paper, the Mohr-Coulomb shear strength criterion is modified by mobilising the cohesion and internal friction angle with normal stress, in order to capture the nonlinearity and critical state concept for intact rocks reported in the literature. The mathematical expression for the strength is the same as the classical form, but the terms of cohesion and internal friction angle depend on the normal stress now, leading to a nonlinear relationship between the strength and normal stress. It covers both the tension and compression regions with different expressions for cohesion and internal friction angle. The strengths from the two regions join continuously at the transition of zero normal stress. The part in the compression region approximately satisfies the conditions of critical state, where the maximum shear strength is reached. Due to the nonlinearity, the classical simple relationship between the parameters of cohesion, internal friction angle and uniaxial compressive strength from the linear Mohr-Coulomb criterion does not hold anymore. The equation for determining one of the three parameters in terms of the other two is supplied. This equation is nonlinear and thus a nonlinear equation solver is needed. For simplicity, the classical linear relationship is used as a local approximation. The approximate modified Mohr-Coulomb criterion has been implemented in a fracture mechanics based numerical code FRACOD, and an example case of deep tunnel failure is presented to demonstrate the difference between the original and modified Mohr-Coulomb criteria. It is shown that the nonlinear modified Mohr-Coulomb criterion predicts somewhat deeper and more intensive fracturing regions in the surrounding rock mass than the original linear Mohr-Coulomb criterion. A more comprehensive piecewise nonlinear shear strength criterion is also included in Appendix B for those readers who are interested. It covers the tensile, compressive, brittle-ductile behaviour transition and the critical state, and gives smooth transitions.

Squeezing failure of tunnels: A case study

The Laodongshan Tunnel, as a part of Guangtong-Kunming railway in China is one of the most important connections in the whole railway line. The large deformation and failure of primary support occurred in the early tunneling stage, and deformation characteristics and failure causes of primary support were obtained by means of field investigation and analysis. There have been many factors that result in the occurrence of squeezing failure of tunnel during construction, such as high in situ stress, soft-weak rock, groundwater seepage, weak support, delay closure of primary support, etc. The lessons to be learned is that for tunnels construction in weak rock mass under high in situ stress, the controlling principle of tunnel deformation ought to be based on combination of stress releasing and support resistance. The support type and method during tunnel excavation needed to be adjusted dynamically based on the pre-excavation geological forecast (e.g. tunnel seismic prediction (TSP), ground penetration radar (GPR), and horizontal pre-boreholes, etc.). Besides, many countermeasures were proposed during construction, including strengthened pre-support, improvement of support stiffness, grouting reinforcement, installing additional supports, closing the tunnel ring early, and timely installation of secondary lining. Finally, from feedbacks of monitoring results, squeezing failure of Laodongshan tunnel was effectively controlled. Thus these countermeasures can guarantee safety construction in weak rock mass under high in situ stress was achieved.

Application of transparent soil model test and DEM simulation in study of tunnel failure mechanism

With fast growing demand for modern transportation, more shallow tunnels are being constructed and planned. Understanding the deformation and failure mechanism of a shallow underground tunnel has been a topic of research. The influences of the surrounding material (rock and soil) strengths and buried depths on the deformation and failure mechanism are investigated through the transparent soil model test technique and PFC3D numerical simulation in this study. It can be observed from model tests that the failure mode of test 1 (relative density of 30%, buried depth of 60 mm) is similar with that of test 2 (relative density of 70%, buried depth of 60 mm), both showing the funnel shape. The difference lies in that the tunnel stability in test 1 is lower. On the other hand, tests 2 and 3 (relative density of 70%, buried depth of 120 mm) are of different failure shapes as the latter displays a chimney shape. PFC3D simulation is carried out to numerically examine the failure modes under different testing conditions. The stress field and the displacement field derived from the numerical results can help to interpret the tunnel failure mechanism. In addition, it is obvious from this study that the Peck formula (1969) is less applicable for the surrounding materials with low strength. Therefore, it should be amended according to the surrounding materials, especially for the granular soils.

Methods Applied in the Prediction of Brittle Failure in Tunnels and Underground Caverns

Tunnels and underground caverns located at greater depth (high rock cover or overburden) are subjected to high in-situ stress environment. Those rock mass that are relatively unjointed and massive are exposed to the brittle failure, which is famously known as rock spalling/ rock bursting phenomenon. Establishing state of the stress and evaluating stress-induced instability in tunnels passing through such rock mass at relatively greater depth is therefore a challenge. The aim of this manuscript is to describes existing brittle failure (rock burst) prediction methods that are being practiced worldwide and propose necessary editions so that quality of assessment is enhanced. The methods described are very practical and the author is confident that professional engineers will use them to evaluate and predict potential rock burst/ rock spalling scenario in the tunnels during planning, design and construction phases. Each method of prediction is explained, applicability extent is highlighted and comparisons between the methods are made. HYDRO Nepal JournalJournal of Water Energy and EnvironmentIssue No: 22Page: 5-9Uploaded date: January 14, 2018

Catastrophe analysis of active-passive mechanisms for shallow tunnels with settlement

In the note a comprehensive and optimal passive-active mode for describing the limit failure of circular shallow tunnel with settlement is put forward to predict the catastrophic stability during the geotechnical construction. Since the surrounding soil mass around tunnel roof is not homogeneous, with tools of variation calculus, several different curve functions which depict several failure shapes in different soil layers are obtained using virtual work formulae. By making reference to the simple-form of Power-law failure criteria based on numerous experiments, a numerical procedure with consideration of combination of upper bound theorem and stochastic medium theory is applied to the optimal analysis of shallow-buried tunnel failure. With help of functional catastrophe theory, this work presented a more accurate and optimal failure profile compared with previous work. Lastly the note discusses different effects of parameters in new yield rule and soil mechanical coefficients on failure mechanisms. The scope of failure block becomes smaller with increase of the parameter A and the range of failure soil mass tends to decrease with decrease of unit weight of the soil and tunnel radius, which verifies the geomechanics and practical case in engineering.

Analytical Solution of Tunnel Surrounding Rock for Stress and Displacement Based on Lade–Duncan Criterion

The deformation and failure of tunnel surrounding rock is the result of tunnel excavation disturbance and rock stress release. When the local stress of surrounding rock exceeds the elastic limit of rock mass, the plastic analysis of surrounding rock must be carried out to judge the stability of tunnel. In this study, the Lade–Duncan yield criterion is used to calculate the analytic solutions for the surrounding rock in a tunnel, and the radius and displacement of the plastic zone are deduced using an equilibrium equation. The plastic zone radius and displacement based on Lade–Duncan criterion and Mohr–Coulomb criterion were compared by using single‐factor analysis method under the different internal friction angles, in situ stresses, and support resistances. The results show that the solutions of the radius and displacement of plastic zone calculated by the Lade–Duncan criterion are close to those of Mohr–Coulomb criterion under the high internal friction angle and support resistance or low in situ rock stress; however, the radius and displacement of the plastic zone calculated by the Lade–Duncan criterion are larger under normal circumstances, and the Lade–Duncan criterion is more applicable to the stability analysis of the surrounding rock in a tunnel.

The effect of radial cracks on tunnel stability

The surrounding rock mass contains cracks and joints which are distributed randomly around tunnels, and in the process of tunnel blasting excavation, radial cracks could also be induced in the surrounding rock mass. In order to clearly understand the impact of radial cracks on tunnel stability, tunnel model tests and finite element numerical analysis were implemented in this paper. Two kinds of materials: cement mortar and sandstone, were used to make tunnel models, which were loaded vertically and confined horizontally. The tunnel failure pattern was simulated by using RFPA2D code, and the Tresca stresses and the stress intensity factors were calculated by using ABAQUS code, which were applied to the analysis of tunnel model test results. The numerical results generally agree with the model test results, and the mode II stress intensity factors calculated by ABAQUS code can well explain the model test results. It can be seen that for tunnels with a radial crack emanating from three points on tunnel edge, i.e., the middle point between tunnel spandrel and its top with a dip angle 45o, the tunnel foot with a dip angle 127o, and the tunnel spandrel with 135o with tunnel wall, the tunnel model strength is about a half of the regular tunnel model strength, and the corresponding tunnel stability decreases largely.

Upper bound limit analysis of uplift failure in pressurized sealed rock tunnels

SummaryThe rock around tunnels used for gas storage is subject to high pressures, reaching 30 MPa in the case of compressed air energy storage. Uplift failure of the overlaying rock mass up to the surface represents the main hazard scenario in such cases. The present paper investigates this problem by using the upper bound theorem of limit analysis assuming a continuum rock mass model obeying the Mohr Coulomb failure criterion with tension cut‐off. Tools of the calculus of variations are used to assess the geometry of the failure surface. The effects of geometrical and geotechnical parameters on uplift pressure are analyzed systematically. Charts are then provided, which enable a quick estimation of the upper bound of uplift pressure across a wide range of geotechnical and geometrical conditions.