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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 R 2 = 0.997. In addition, the robustness and reliability of the prediction equation is verified. [ABSTRACT FROM AUTHOR]

The combined finite-discrete element method (FDEM) is an advanced finite element and discrete element coupling method, which is commendably suitable for simulating the slope's entire evolution process from cracking, expansion, penetration, sliding, collision to deposition under seismic load. The original FDEM has apparent shortcomings in simulating the dynamic instability process of earthquake-induced landslides, and some targeted improvements are made to the original program. Numerical tests are carried out to prove the accuracy and robustness of the improved FDEM, and the influence of the structural plane on the dynamic evolution mechanism and process of the slope is studied. The results show that the improved FDEM can accurately reproduce the entire dynamic evolution processes and failure forms of different jointed rock slopes under seismic load and can quantitatively evaluate the dynamic stability of jointed rock slopes (dynamic safety factor and critical failure surface). In addition, the results also emphasize that the slope failure degree significantly affects the dynamic response of the slope and suggest that the combination of the cut-through of the failure surface and the increase of kinetic energy should be used as the criteria for slope dynamic instability simulation by FDEM. This study provides scientific and technological support for revealing the failure mechanism of jointed rock slopes and disaster reduction and prevention. [ABSTRACT FROM AUTHOR]

Rock materials are typically heterogeneous. Traditional digital image processing (DIP) technology and the grain-based model (GBM) method are difficult to apply to engineering-scale problem studies, such as tunnel excavation. Based on the combined finite-discrete element method (FDEM), a random parameter assignment method is proposed to simulate the mechanical properties and failure behavior of heterogeneous rock. The elastic moduli of triangular elements, as well as the strength parameters of quadrilateral joint elements, assigned by this method obey a Weibull distribution. The simulation results of uniaxial compression, triaxial compression and Brazilian disc show that with the increase in the heterogeneity m value, the uniaxial compressive strength, elastic modulus, triaxial compressive strength, equivalent cohesion, equivalent internal friction angle and tensile strength all increase exponentially and approach those of the homogeneous rock sample. In addition, the heterogeneous rock samples mainly shear fail along their corresponding theoretical failure angle under uniaxial and triaxial compression. The simulation results of tunnel excavation indicate that for heterogeneous rock with different m values, the overall fracture morphology of the surrounding rock remains unchanged, and X-shaped conjugate shear failures mainly occur followed by a small amount of tensile failures, which are similar to the homogeneous surrounding rock. However, with the increase in the m value, the failure degree and the maximum fracture propagation range of the surrounding rock decrease exponentially. Moreover, the simulation results of uniaxial compression and tunnel excavation with different element sizes suggest that the random parameter assignment method proposed in this paper is reliable. [ABSTRACT FROM AUTHOR]

The combined finite-discrete element method (FDEM) is an advanced finite and discrete element coupling method, which can commendably simulate the large deformation and fracture process of materials. The explicit solution method in FDEM is naturally suitable for solving dynamic problems, and the key to dynamic analysis is to establish artificial boundary conditions that can be prepared to simulate infinite domain motion. At present, there is only the viscous boundary condition in the original FDEM, but this is only applicable for solving the internal source wave problems (e.g., blasting, mechanical vibration), and there are no artificial boundary conditions for solving the external source wave problems (e.g., earthquake). Therefore, three artificial boundary conditions have been developed in the present study to enhance the ability of FDEM to conduct dynamic response analysis. Firstly, the basic principle of FDEM and the existing viscous boundary condition are briefly introduced. Then, three newly added boundary conditions are introduced in sequence: (1) viscous-spring boundary condition, which can absorb stress wave energy at the boundary and restore the residual displacement to meet the actual engineering; (2) free-field boundary condition, which is coupled with free-field motion to absorb scattered waves at the lateral boundaries of the model, and the algorithms of mesh matching retrieval and coupling calculation are also introduced; (3) static-dynamic unified boundary condition, which can accurately transform the fixed boundary under the quasi-static analysis into the non-reflective boundary condition under the dynamic analysis. Finally, several classic models are built to verify the feasibility of the newly added boundary conditions. The comparison with the shaking table test also shows that the improved FDEM can be used for seismic response analysis under actual earthquakes. [ABSTRACT FROM AUTHOR]

The combined finite-discrete element method (FDEM) is an advanced finite and discrete element coupling method. This study further applies it to the problem of stress wave propagation in the jointed rock mass. Firstly, the fundamental theory of FDEM is briefly described and the methods commonly used in FDEM to characterize structural planes are comprehensively analyzed. Then, based on the viscous boundary condition (VBC), the viscous-spring boundary condition (VSBC) is added to absorb the reflected wave at the artificial boundary and restore the residual displacement to meet the actual engineering. In addition, the application range of the VBC and VSBC is verified, which indicates that the VBC and VSBC in the improved FDEM can be well applied not only to the continuum range but also to the case of the new fracture formed. Finally, several classic models are solved to verify the stress wave propagation across different types of joints by FDEM. The results reveal that results from FDEM agree well with analytical solutions based on the displacement discontinuity model and numerical solutions from universal distinct element code (UDEC), which means that the improved FDEM can capably and accurately simulate wave propagation in the jointed rock mass. [ABSTRACT FROM AUTHOR]

The failure mechanism and failure mode of anisotropic rock masses are very different from those of isotropic rock masses. For unsupported tunnels, the failure modes of anisotropic rock masses can be summarized as composite failure, V-shaped notch failure and spalling of bedding planes. Different failure modes are controlled by the mechanical parameters of the rock masses, the environmental occurrences and the excavation conditions. The combined finite-discrete element method (FDEM) was employed to investigate the failure mechanism and failure process of layered rock masses and the influence of different factors on the failure mode. The study results indicate that (1) composite failure is the basic failure mode of the layered rock masses and that it is caused by the combined action of shear-slip fractures F1 parallel to the bedding plane, tensile fractures F2 perpendicular to the bedding plane, and conjugate shear fractures F3; the rotation movement of slab-like rock fragments induced by fractures F1 and F3 into the tunnel causes significant deformation of the surrounding rock masses; (2) on the basis of composite failure, when fractures F1 disappear and fractures F2 obliquely intersect with the bedding plane, V-shaped notch failure occurs; furthermore, spalling of the bedding plane occurs when the propagation of fractures F3 is blocked at the bedding plane; and (3) with increasing rock mass strength, lateral pressure coefficient of in situ stress and layer thickness, or decreasing tunnel span, the surrounding rock masses change from composite failure to V-shaped notch failure and then to spalling of the bedding plane. However, the effect of the elastic modulus on the failure mode of anisotropic rock masses is weak, and the influence of the dip angle of the rock layer on the failure mode is complicated. [ABSTRACT FROM AUTHOR]

Core material softening method has been widely used in two-dimensional tunnel excavation simulation by the combined finite-discrete element method (FDEM). In numerical studies, softening stress paths (generally described by critical kinetic energy (CKE), softening time step (STS) and softening curve (SC)) usually have a significant influence on simulation results and computational efficiency. There is not a direct way to determine these three parameters. In the present study, a series of numerical studies by the FDEM are performed to discuss the effects of these three parameters on the simulation results and calculation efficiency in a circular tunnel excavation project with a 4 m diameter. The results show that the CKE is closely related to the model size and rock density. The CKE can be obtained from the model kinetic energy curve, and a selection method is proposed. The STS value of 50 000 steps is optimal. To obtain a uniform and stable propagation process of the surrounding rock crack, the SC should be in an exponential function. The engineering case shows that the simulation results are consistent well with the field observations as the softening stress paths are calibrated by the proposed method. [ABSTRACT FROM AUTHOR]

Deformation and fracturing of the rock mass are heavily affected by complex thermal-hydraulic-mechanical (THM) processes in the subsurface environment. This study develops a THM coupling formulation based on the 2D combined finite-discrete element method (FDEM) to investigate the rock deformation and fracturing behavior driven by coupled THM phenomena in the rock mass. The coupled scheme consists of three subsolvers, i.e., a hydraulic solver, thermal solver, and mechanical solver. In the developed hydraulic solver, a flow network searching algorithm is proposed based on the pore network models and the special mesh discretization in the FDEM. The laminar viscous flow in the flow network is solved using the cubic law approximation, and the fluid pressure field is determined by a linear compressibility model. The developed thermal solver accounts for the complex thermal transport processes decomposed into conduction in the solid and the fluid, advection by fluid flow, and fluid-solid thermal convective transfer. The coupled THM formulation is implemented by iterating the three subsolvers using an explicit, partitioned scheme. This approach is then progressively verified by a series of benchmark problems. Finally, the deformation and branching of a single fracture embedded in the rock mass due to cold fluid injection are simulated, in which the gradual aseismic fracture slip/opening, as well as the branch fracture initiation and propagation, and their potential impact on seismic events are discussed. The results show the potential of the developed numerical tool in modeling the coupled THM process-driven deformation and fracturing of the rock mass. Improvements in the implementation are needed, such as improvements in modeling the thermo-poroelastic effects and temperature- and pressure-dependent physical properties. [ABSTRACT FROM AUTHOR]