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Rock bursts are aggressive dynamic failure processes that involve the rapid release of strain energy stored in a rock under unloading conditions. The dynamic fracture and strain energy transformation characteristics of rock unloading failure must be investigated to predict and control rock bursts. This study adopts the discretized virtual internal bond (DVIB) method to investigate this problem. The element partition method is performed in the DVIB method to investigate the effect of cracks on rock unloading failure. Three indicators, i.e., fracture area, the fractal dimension of the failure pattern, and energy release ratio (kinetic energy over strain energy), are adopted to evaluate the failure intensity. Simulation results show that a critical strain energy density (SED) for rock unloading failure exists. The unloading failure occurs only when the SED exceeds the critical value. Subsequently, under the same SED conditions, the effect of the lateral pressure coefficient (LPC), heterogeneity, and size on rock unloading failure is investigated. As the LPC increases, the failure intensity increases slowly and then decreases significantly. The critical LPC is approximately 0.5, indicating that the unloading failure is the most severe. The simulation results suggest that the crack can reduce the energy release capacity of the rock and then restrain rock unloading failure. When the precrack angle is 0° or 90°, the precrack barely affects the rock unloading failure, and the failure intensity is similar to that of the intact rock. When the precrack length or the density of random precracks is larger, the rock unloading failure is weaker. [ABSTRACT FROM AUTHOR]

• A plastic Stillinger-Weber potential for soft rocks is proposed. • The plastic fracture behavior of soft rocks is effectively simulated. • The dependence of simulation results on mesh size is effectively mitigated. • Influence factors on the failure behaviors of soft rock tunnels are investigated. Soft rocks present some undesirable behaviors, such as low strength, high plasticity and softening. These traits pose challenges in the design and construction of soft rock tunnels. To address these issues and model plastic fracture behaviors of soft rocks, we proposed a plastic Stillinger-Weber (PSW) potential and integrated it into the discretized virtual internal bond (DVIB) model. The PSW-DVIB model combines the strengths of the plastic DVIB and the SW-DVIB models. It not only effectively reproduces the plastic fracture behavior but also accurately reflects the Poisson's ratio. Moreover, due to the inclusion of fracture energy in the PSW potential, the simulation results are basically mesh size independent. The PSW-DVIB model is then utilized to investigate the failure behaviors of soft rock tunnels. The effects of the coefficient of lateral pressure, in-situ stress, Poisson's ratio and tunnel shape on tunnel failure are investigated. The simulated results indicate that the coefficient of lateral pressure has significant influence on the failure zone distribution of the surrounding rock. With the coefficient of lateral pressure increasing, the failure degree of surrounding rock decreases firstly and then increases. The optimum coefficients of lateral pressure for the stability of circular, arched and square tunnels are about 1.00, 0.75 and 0.75, respectively. When the in-situ stress or Poisson's ratio is larger, the influence of the coefficient of lateral pressure on tunnel failure is more obvious. The influence of the coefficient of lateral pressure on square tunnels are more significant than the arched tunnels, and then the circular tunnels. These findings provide valuable insights for the design and construction of soft rock tunnels. [ABSTRACT FROM AUTHOR]

• The dynamic fracturing behaviors of fissured rock under unloading condition are studied. • The tensile-shear transition of fracture pattern is found when increasing the initial stress level. • The unloading failure of rock with parallel and randomized multiple flaws are analyzed; • The findings can improve the understanding on rock burst. Unloading can cause many geotechnical disasters, e.g., the rock burst. The underlying mechanism of rock failure is the propagation and coalescence of cracks. To get insight into rock unloading failure, the discretized virtual internal bond (DVIB) model in conjunction with element partition method (EPM) is used to simulate the dynamic fracturing process under unloading conditions. At first, the samples with the single flaw are simulated under unloading conditions. It is found that the fracturing pattern exhibits the tensile-shear transition with the initial stress level increasing. When the flaw inclination is closer to 45 degree or the flaw is longer, shear cracks are more likely to occur. Then, the unloading failure process of rock with multiple flaws are simulated. It is found that the inclination of the parallel flaws has remarkable influence on the unloading failure of rock. When the inclination angle is about 45 degree, the rock is easier to fail. For the rock with multiple randomized flaws, the amount of flaws has slight influence on acoustic emission counts. However, the configuration of randomized flaws can significantly affect the fracture process and failure patterns. These findings can improve understanding of rock burst and provide valuable references for the prediction of unloading failure of rock. [ABSTRACT FROM AUTHOR]

The mechanical responses of tunnels with stiff and yielding linings were predicted using an analytical method. A theoretical model of a stiff lining-supported tunnel was established, which considered the effects of tunnel face advancement and lining installation delay. The deformation process in the yielding lining can be divided into three stages based on the elastic–plastic–stiff deformation characteristics of the yielding elements. The circumferential shortening of the yielding lining during Stage 1 was caused by the elastic deformations in the segmental linings and yielding elements. The theoretical stiffness of the yielding lining at this stage was determined, and analytical solutions were derived from the theoretical stiffness in the mechanical model of the stiff lining. The yielding lining deformation during Stage 2 was induced by plastic compression of the yielding elements, and the lining resistance pressure remained unchanged. The yielding lining was reduced to a stiff lining during Stage 3, and analytical solutions for the tunnel displacement and lining resistance pressure during Stages 2 and 3 were provided. The analytical solutions for a stiff lining-supported tunnel were compared with those provided in previous studies, and an analytical model of a tunnel that used a yielding lining was applied to an actual tunnel. The effectiveness and reliability of the solutions were verified by comparing them with the application results. Theoretical analysis showed that tunnel deformation could be controlled using a stiff lining, because the increasing rate of pressure is considerably less than that of the lining bearing capacity. However, a 1.4 m lining thickness was required to prevent lining damage. The yielding lining resistance pressure decreased from 3.46 to 0.68 MPa when the lining was 0.2 m thick, and the yielding displacement increased from 50 to 150 mm. The maximum stress in the lining decreased by 56.95 MPa when the yielding lining was displaced by 50 mm, and the thickness increased from 0.2 to 0.5 m. The application of a stiff lining could be considered for rocks that exhibit weak or average deformability; however, attention should be paid to the yielding lining in rocks that exhibit strong rheology. [ABSTRACT FROM AUTHOR]