Failure Mechanism Of Tunnels Research Articles

Longitudinal and cross-sectional partitioned failure mechanism of tunnels subjected to stick-slip action of strike-slip faults

Longitudinal and cross-sectional partitioned failure mechanism of tunnels subjected to stick-slip action of strike-slip faults

Limit analysis on roof failure mechanism of tunnels subjected to sequential excavations

Compared to traditional full-face excavation methods, sequential excavation methods (SEM), such as central diaphragm (CD) and sidewall drift (SD) methods, can significantly reduce rock disturbance and minimize the extent of roof failure in tunnel crowns. Therefore, they are particularly suitable for tunnel projects in weak rock formations and large cross-sectional underground chambers. This paper presents a kinematic approach to predict the roof failure mechanism of tunnels excavated using the CD method. The collapse blocks at the top are influenced by excavation steps and initial in-situ stresses, and the direction of their failure velocity field is typically non-vertical, posing challenges for limit analysis applications. To address this, a tilted velocity field for the collapse failure of the tunnel crown in the CD method is constructed based on the initial normal stress distribution along the tunnel’s top contour, obtaining an upper bound solution for tunnel collapse failure. The proposed method is compared with the full-face excavation method, demonstrating the superiority of the CD method over the full-section method. The rationality of the proposed tilted velocity field mechanism is verified through numerical validation using finite difference methods. Building upon the upper bound solution, safety assessment indicators for tunnels excavated using the CD method, namely allowable rock stress release and safety factor, are proposed. Furthermore, we extend this method to analyze the roof failure mechanism in tunnels constructed using the SD method and confirm its effectiveness through comparison with FDM analysis. Theoretical analysis and numerical validation both indicate that sequential excavation methods can significantly reduce the potential collapse range at the top and improve the safety of tunnel construction.

Failure mechanism and deformation prediction of soft rock tunnels based on a combined finite–discrete element numerical method

The large deformation mechanisms and predictions of soft rock tunnels have always been important but difficult to solve problems in the field of geotechnical engineering. A combined finite–discrete element numerical simulation method (FDEM) was used to study large deformation mechanism, classification and prediction. The deformations or failure mechanisms of tunnel surrounding rock were revealed, and the failure modes and displacement value prediction of surrounding rock with different strength-stress ratios were also investigated. The following conclusions were obtained: (1) Critical hysteresis damping can be adopted to simulate the progressive large deformation process of soft rock tunnels, which can obtain the final deformation of unsupported tunnels and avoid a dynamic response. (2) Under concentrated tangential stress, the surrounding rock undergoes X-shaped conjugate shear fracture, and this type of fracture network continues to propagate toward the depth of the surrounding rock until the model reaches a stable state; the reduction in tunnel cross-section is mainly caused by the macroscopic movement and volume expansion of rock fragments, and the latter is due to the generation of a large number of macroscopic voids. (3) As the strength-stress ratio decreases, the deformation or failure modes of the surrounding rock can be divided into four categories: elastic–plastic deformation, closed fracturing, shear dilation and broken expansion. (4) Finally, a prediction equation for isotropic and homogeneous soft rock unsupported tunnel deformation with general size driven by hydrostatic in situ stress is obtained, which indicates that with an increasing strength-stress ratio, the surrounding rock displacement decreases as an exponential function, with a correlation coefficient of R2 = 0.997. In addition, the robustness and reliability of the prediction equation is verified.

Dynamic Response and Failure Mechanism of Deep-Buried Tunnel with Small Net Distance under Blasting Load

Under blasting load, a series of safety problems, such as lining cracking and surrounding rock instability, are prone to occur in deep-buried tunnels with a small net distance. It is significant to understand the dynamic response and failure mechanism of tunnels under blasting. The blasting attenuation formula is optimized through theoretical analysis and field experiments. The measuring point vibration is monitored in real time and the tunnel blasting model is established by ANSYS/LS-DYNA software. The model was set as having no reflective boundary and an uncoupled charge structure was used. The attenuation law of blasting seismic waves is studied from the adjacent tunnel lining and the direction of the tunnel cross-section and length. The inner and outer sides of the tunnel lining are investigated, respectively. The displacement and acceleration of lining measuring point are also analyzed. The dynamic response of the tunnel lining under blasting excavation is analyzed from multiple angles. The results show that the arch foot on the inner side of the lining (the side in contact with the tunnel headroom) is the first to generate vibration. On the outside of the lining (the side in contact with the rock),the peak vibration velocity is reached after blasting load unloading. There is little difference in the vibration velocity at different positions of the transverse section, but great difference in the vibration velocity of the longitudinal section. The influence of the horizontal displacement was greater than that of the vertical displacement. The vibration acceleration of the measuring point at the arch foot of the section is the largest and the detonation is also the largest.

Study on large deformation and failure mechanism of deep buried stratified slate tunnel and control strategy of high constant resistance anchor cable

Aiming at the significant deformation problem of deep buried stratified soft rock tunnel, taking the No.3 inclined shaft of Muzhailing tunnel as background, the deformation and failure mechanism of the tunnel and the control strategy of high constant resistance anchor cable are studied. The deformation and failure mechanism of stratified tunnel surrounding rock is studied by physical model test, including the tunnel's temperature field, strain field, and displacement field from excavation to failure. Lateral stress significantly influences the deformation of the surrounding rock of the horizontal stratified tunnel. The surrounding rock of the tunnel forms a dumbbell-shaped tensile stress zone under the action of stress. The high tensile stress of the roof and floor leads to dislocation and layer separation, which causes floor heave failure and temperature change. The indoor tensile test verified the super strength characteristics and large deformation of the high constant resistance anchor cable. Based on the excavation compensation method and the coupling control mechanism of high constant resistance long and short anchor cables, a new high prestress constant resistance coupling control strategy is proposed. The numerical simulation and field test studies according to the new design are presented, which show that the high constant resistance anchor cable coupling support significantly reduces the tensile stress zone, effectively controls the deformation of tunnel surrounding rock, and avoids the damage of supporting structure and the cracking and deformation of the lining. The research results can provide a reference for constructing and supporting deep-buried, stratified soft rock tunnels.

Limit equilibrium models for active failures of large-diameter shield tunnel faces in soft clay reinforced with soil–cement walls

To prevent face collapses during shield tunneling, it is necessary to reinforce soft ground; however, thus far, the stability of tunnel faces in reinforced ground has not been investigated thoroughly. In this paper, finite-element analyses were first conducted to explore the failure mechanism of tunnel faces in reinforced soft soil, with a focus on the influence of the distance between the tunnel face and the cement wall, L. The limit support pressure acting on the tunnel face was determined, and the failure zone of the soil in front of the tunnel face was elucidated for cases involving different values of L. Thereafter, three types of failure modes—the full-penetration, partial-penetration, and non-penetration modes—were identified based on the development of the shear strain in the soil. The corresponding failure models were established based on the limit equilibrium method, and analytical expressions for calculating the limit support pressure were derived. The prediction results obtained using the proposed formulas were in good agreement with the results of numerical simulations; this indicates that the proposed collapse models can accurately reflect the impact of the face–wall distance, L, on the stability of shallow tunnel face in soft clay reinforced with soil–cement wall.

Influence of fault forms on the evolution of concrete damage patterns in tunnels

Quickly and accurately estimating the seismic weak surface of a fault tunnel is one of the most severe challenges in tunnel seismic design. Therefore, the strong nonlinear response of the Jinping II Hydropower Station under the dislocations of positive, reverse, and slip faults was investigated through the finite element method using a static elastoplastic model. The results reveal the damage and failure mechanism of tunnels under different faults. By using the IDA damage rating index, the damage initiation, evolution, and development process of tunnels under different types of faults are analyzed. The results showed that the affected area of fault dislocation is concentrated and intense, which is mainly distributed along the two sides of the fault surface. The damage of the positive and reverse faults to the tunnel extends from the arch waist to the vault and the invert of the arch, while the influence of the slip fault on the tunnel is the greatest at the vault and invert of the arch and then extends to the arch waist. In terms of the impact range, the reverse fault is the biggest, followed by the slip fault, while the positive fault is the lowest. This study contributes to the design and construction of tunnels through the faults.

Deformation and failure of overburden soil subjected to normal fault dislocation and its impact on tunnel

In this study, a large-scaled model test was conducted to explore the deformation and failure mechanism of tunnel with fault rupture. The strain evolution, ground fissure and displacement, earth pressure and tunnel deformation are collected in the test. Then, tunnel mechanical behavior under the fault dislocation is revealed based on the test result. The results show that a prismoid-like 3D shear zone appeared within fault fracture zone. The tunnel experiences an elongated ‘S’-shaped deformation along longitudinal axis of tunnel and the displacement changes of sectional linings within the shear zone are more significant. In transversely, the linings exhibit an oval horse-shoe deformation pattern. Three types of failure mode defined as Type Ⅰ, Type Ⅱ and Type III, corresponding to slight damage, moderate damage and severe damage, are summarized transversely in this paper. Under normal fault dislocation, the tunnel within the shear zone experienced a combination failure of bending moment and shear force. But compression failure of linings far from the fault would occur with longitudinal cracks on tunnel subjected to normal fault. Taking fault width (Wf) and tunnel span (St) into consideration, a reasonable tunnel fortification length was proposed with maximum of 2.25Wf and 3.5St under a specific fault dip of 60°. In the model test, when the fault dislocation reaches 10 mm, a slight crack begins to appear on arch foot of lining and then dislocation of joint occurs. Thus, corresponding to a prototype tunnel, a safe vertical displacement to ensure no damage to tunnel should be 34.6 cm when a segmentally designed tunnel is subjected to normal fault dislocation.

Dynamic failure mechanism of tunnel under rapid unloading in jointed rockmass: A case study

Field observations demonstrate that rapid unloading can induce intensive dynamic failure of a tunnel in a jointed rockmass. In this study, we demonstrate the characteristics of the dynamic failure and establish a series of validated discrete element models to investigate the mechanism of the dynamic failure. Our results indicate that the excessive release of strain energy and the complex nonlinear behavior of joints together resulted in the dynamic failure. Primarily, joints can enlarge the unloading disturbance zone (UDZ) and lead to the excessive release of strain energy, especially for a model with smaller joint spacing and higher unloading rate. Secondly, joints can significantly decrease the dependence of the dynamic effect on the unloading rate and expand the unloading time threshold that can induce the dynamic effect, more importantly, the nonlinear variation of joint stiffness not only induces vibration localization but also increases vibration amplitude and reduces vibration frequency.

Experimental Study on the Failure Mechanism of Tunnel Surrounding Rock under Different Groundwater Seepage Paths

The influence of groundwater on tunnel engineering is very complicated. Due to the complexity of water flow water pressure transfer and uncertain defects in the stratum, all of which are key factors with regard to the design of tunnel engineering. Therefore, the variation of surrounding rock during excavation and the deformation and failure of soft surrounding rock under different seepage paths of underground water after excavation systematically. Experimental results showed that the stress change of surrounding rock caused by tunnel excavation can be divided into 3 stages: stress redistribution, stress adjustment, and stress rebalancing. In the process of water pressure loading, water flow rate is closely related to the experimental phenomenon. The between stable loading water pressure pore water pressure of the tunnel surrounding rock and the distance from the measuring point to the edge of the tunnel obey the exponential function of the decreasing growth gradient. With the increase of loading pressure, the pore water pressure and stress at the top of the tunnel increase, and the coupling of stress field and seepage field on both sides of surrounding rock more and more intense. The failure process of the tunnel can be divided into 6 stages according to the damage degree. The final failure pattern of the surrounding rock of the tunnel is mainly determined by the disturbed area of excavation. The arched failure area and the collapse-through failure area are composed of three regions. The surrounding rock is characterized by a dynamic pressure arch in the process of seepage failure, but it is more prone to collapse failure at low water pressure. The results of this study are the progressive failure mechanism of tunnel under different groundwater seepage paths and would be of great significance to the prevention of long-range disasters.

Instability mechanism of shallow tunnel in soft rock subjected to surcharge loads

The tunnel instability accidents caused by surcharge loading could even lead to ground settlement or collapse, damage to the existing municipal pipelines or inclination of the surgical buildings. Series of large-scale physical model test was conducted in this paper to reveal the failure mechanism of tunnel in soft rock subjected to surcharge loading. The non-contact full-field displacement measurement (i.e. Digital Image Correlation, DIC) was employed to obtain a detailed deformation of surrounding rock after excavation. A coupled numerical method combing the continuum (Finite Difference Method, FDM) and dis-continuum (Discrete Element Method, DEM) analysis was used to deal with the failure mechanism of tunnel excavation in rock mass. Good agreements between physical model test and numerical analysis approve the accuracy of the proposed FDM-DEM numerical model. It is found that the collapse mainly occurred in tunnel roof rather than in tunnel sidewall. The collapse zone shows a step-type increase with the increase of surcharge loads. The stress loosening zone (SLZ) in tunnel roof is larger than that in tunnel sidewall. The thickness of SLZ in tunnel roof can be used to design the length of the formed pressure anchors. The proposed large-scale model test with DIC method is applicable to the tunnel engineering study. Moreover, the results in this paper give some insights to secure excavation works and optimize supporting structures.

Effects of seepage and weak interlayer on the failure modes of surrounding rock: model tests and numerical analysis.

The presence of weak interlayers and groundwater are common adverse geological conditions in tunnels. To investigate the modes of failure of rock masses surrounding tunnels owing to weak interlayers and groundwater, model tests and numerical simulations were conducted in this study based on two cases, and a model that considers only the weak interlayer was conducted for comparison. Based on the tests, differences between two models in terms of rock pressure, displacement, cracks and strain were analysed. The results reveal that the presence of groundwater has a significant effect on the space–time distribution of stress, displacement and cracks in the surrounding rock. Furthermore, based on the numerical model, the seepage field was analysed in terms of pore water pressure, permeability and the seepage process to understand the joint action of groundwater and weak interlayer on the failure mechanism of tunnels. The results show that the groundwater and interlayer complement each other to induce the failure mode of the surrounding rock. The water accelerates slip in the interlayer and the development of cracks. Conversely, low strength, muddy weak interlayers serve as the channels of water flow, resulting in deformations and cracks at different locations and different failure modes.

A 3D Discrete-Continuum Coupling Approach for Investigating the Deformation and Failure Mechanism of Tunnels across an Active Fault: A Case Study of Xianglushan Tunnel

For water transmission tunnels constructed in high-risk seismic regions of western China, active faults pose threats of serious ruptures to the tunnels. To overcome this issue, a 3D discrete-continuum coupling approach is introduced into the study. By this approach, spherical discrete-element-method (DEM) particles are used to represent the surrounding rock mass, and the tunnel is considered to be the continuous finite-difference-method (FDM) zone. In this way, a 3D coupling model was established to study the longitudinal displacement profile and stress response of the tunnel lining under various fault dislocations. The failure pattern of the surrounding rock mass was investigated from a micro perspective. Meanwhile, the design strategy of flexible joint was investigated with the present numerical model. The results from a parametric study show that the smaller segment length, wider width and weaker strength of the flexible joints are beneficial to the anti-dislocation performance of the tunnel. Moreover, an orthogonal array test technique was utilized to investigate the influence level of the main design parameters of the flexible joint on the lining internal stress. With the obtained knowledge, the optimal combination for flexible joint design was presented. Findings may provide references for the anti-dislocation issue of tunnels across active faults.

Viscous damping effects on the seismic elastic response of tunnels in three sites

Time-domain commercial codes are widely used to evaluate the seismic behavior of tunnels. Those tools offer a good insight into the performance and the failure mechanism of tunnels under earthquake loading. Viscous damping is generally employed in the dynamic analysis to consider damping at very small strains in some cases, and the Rayleigh damping is commonly used one. Many procedures to obtain the damping parameters have been proposed but they are seldom discussed. This paper illustrates the influence of the Rayleigh damping formulation on the tunnel visco-elastic behavior under earthquake. Four Rayleigh damping determination procedures and three soil shear velocity profiles are accounted for. The results show significant differences in the free-field and in the tunnel response caused by different procedures. The difference is somewhat decreased when the soil site fundamental frequency is increased. The conventional method which consists of using solely the first soil natural mode to determine the viscous damping parameters may lead to an unsafe seismic design of the tunnel. In general, using five times site fundamental frequency to obtain the damping formulation can provide relatively conservative results.

Analysis of Seismic Factors to Tunnels in the Earthquake

Underground structures have mechanical characteristics and seismic responses very different from common structures on the ground in the earthquake due to the constraints of the surrounding soil. Tunnel destruction has occurred many times in domestic and international earthquake. In order to study seismic factors and failure mechanism of tunnels in the earthquake, numerical simulation of the seismic responses of the circular shield tunnel is carried on. Harmonic P-waves and S-waves are entered to make the tunnel vibrate and deform. Viscous-spring artificial boundary condition is applied in the numerical simulation to consider the semi-infinity of soil. The elastic modulus of soil, the frequency of harmonic waves and the depth of the tunnel are regarded as the main factors. The seismic responses such as stress and relative displacement are analyzed in different parameters to get the failure mechanism of circular tunnel.

Numerical Simulation of Effect of Fracture Distribution on Failure Mechanism of Tunnel

In order to study the effect of rock fracture distributions on stability of tunnel, the effect of joints with different strike dip, the size and density was considered by using probability and statistics method. Based on the Monte Carlo random simulation, the statistical network model of 2D jointed rockmass was established to describe and characterize jointed rock mass. On the basis of this, the numerical code RFPA2D is employed to analyze effect of fracture dip angle on the tunnel stability. Numerical results show that the fracture distribution has a little effect on tunnels failure mechanism and failure type of tunnel because the failure of tunnel in the jointed rockmass is similar to the one without joints and fissures. However with the change of the fracture dip angle, the loading capacity of the tunnel in the jointed rockmass varies accordingly.

Numerical Simulation of Effect of Fracture Distribution on Failure Mechanism of Tunnel

In order to study the effect of rock fracture distributions on stability of tunnel, the effect of joints with different strike dip, the size and density was considered by using probability and statistics method. Based on the Monte Carlo random simulation, the statistical network model of 2D jointed rockmass was established to describe and characterize jointed rock mass. On the basis of this, the numerical code RFPA2D is employed to analyze effect of fracture dip angle on the tunnel stability. Numerical results show that the fracture distribution has a little effect on tunnels failure mechanism and failure type of tunnel because the failure of tunnel in the jointed rockmass is similar to the one without joints and fissures. However with the change of the fracture dip angle, the loading capacity of the tunnel in the jointed rockmass varies accordingly.

Physical Modelling of Joint Effects on Deformation and Failure of Tunnels

The deformation and failure mechanisms of tunnels in jointed rock mass with variable orientation and overburden pressures were studied by physical modelling. The deformation and expansion characteristics and regularities of joints and its influence on the stability of tunnels were analyzed. The results shown that the effects of unloading and reactivation of joints, and subsequent shear slip and deformation induced by excavation cause the structural instability of tunnels. The deformation, failure, and instability of surrounding rocks are essentially the continued deformation and repeated failure of joints in varied stress field caused by excavation and overburden pressure. When the dip angle of the joint sets is 75°, the joints have extremely striking influence on the stability of tunnels. In addition, with the increasing of the overburden pressures, the deformation extent and failure rate are sped up prominently due to the rapid increasing of the shear stresses on joint planes.

Numerical analysis of failure mechanism of tunnel under different confining pressure

More and more deep-buried underground excavations have been carried out in some large-scale hydroelectric stations at southwest of China. When underground development continues at great depths, the danger of surrounding rock instability will inevitably increase in underground powerhouses and tunnels. To in-depth analysis the failure mechanism and quantify deformation of the structures is one of the hottest problems to be probed in rock mechanics and rock engineering field. The objective of this study is to investigate the effect of different confining pressures on the failure phenomena of tunnels. A realistic failure progressive analysis code named RFPA was adopted to display the full failure processes of tunnel under different confining pressures. RFPA code has the capability of simulating the whole fracturing process of initiation, propagation and coalescence of fractures around excavations. The numerical model presented here can be used not only to display fracturing patterns of tunnels, but also to predict fracturing patterns under different confining pressures. The simulated results demonstrate that confining pressures play a great role in the failure mode of tunnels. With the increase of lateral pressure, the initiation, migration and expansion of cracks can be delineated. Based on these fracturing patters, failure mechanisms are identified and reasonable support can be thus performed later on.

Numerical study on failure mechanism of tunnel in jointed rock mass

During underground excavation many surrounding rock failures have close relationship with joints. The stability study on tunnel in jointed rock mass is of importance to rock engineering, especially tunneling and underground space development. In this paper, a numerical code called RFPA was used to study the influence of different dip angle of layered joints and the lateral pressure coefficient on the stability of tunnel in jointed rock mass. Numerical analysis indicated that both the dip angle of joints and the lateral pressure coefficient have significant impacts on the failure mode and displacement characters of tunnel. The progressive failure processes of tunnel in jointed rock mass were presented and the mechanisms were discussed. The applicable condition of geographical method by Goodman is also discussed. These results offer a guideline in support design.