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AbstractFloor heave is a disaster that is frequently encountered in tunnel engineering. Existing explanations of floor heave mechanisms have many limitations, and the most important of which is that the stress concentration, release and transfer phenomena and the influences of the in-situ stress lateral coefficient and tensile strength are ignored. Therefore, the combined finite-discrete element method (FDEM) is used to study the failure process of floor rock masses and propose a new mechanical mechanism of floor heave that can consider the influences of the in-situ stress lateral coefficient and tensile strength. The type I fracture energy corresponding to different tensile strengths is calibrated using a direct tensile simulation test. Then, the floor heave mechanism is investigated under different lateral coefficients and tensile strengths, and five different floor heave modes are proposed under different in-situ stress lateral coefficients and tensile strengths. The floor heave modes controlled by the various in-situ stress lateral coefficients and tensile strengths are different, but they can all be explained by the maximum concentrated tangential stress.

AbstractTo investigate the effects of the oval hole on the strength and the failure mechanisms of rock materials, a series of uniaxial and biaxial compression numerical simulations were carried out using the combined finite-discrete element method (FDEM). Based on the numerical simulation results, the effects of dip angle (β), major–minor ratio (ω) and position of the oval hole on rock strength characteristics crack initiation and development process and final failure modes were analysed in detail. The numerical simulation results indicate that: (1) Under uniaxial compression, the ultimate failure modes of rock containing oval holes can be mainly classified into single inclined plane shear failure, X-shaped tensile-shear composite failure and Y-shaped tensile-shear composite failure. Under biaxial compression, the ultimate failure mode is single-inclined plane shear failure dominated by shear cracks. Whether it is uniaxial compression or biaxial compression, the crack initiation position of the main fracture surface is near the end of the major axis of the oval hole. (2) Under uniaxial compression, the axial stress–strain curve of rock containing the oval hole is smooth before and after the peak, but under biaxial compression, there is obvious fluctuation in the post-peak stage due to crack initiation and propagation. In both uniaxial and biaxial compression, the pre-peak elastic stage of the axial stress–strain curve of the rock containing the oval hole is significantly shortened and the peak stress is significantly reduced compared with the intact rock, which indicates that the bearing capacity of the rock is seriously reduced by the existence of oval hole. (3) When the position and major–minor axis ratio of the oval hole remain unchanged, the uniaxial compressive strength and biaxial compressive strength increase with the increase of the dip angle of the oval hole. When the position and dip angle of the oval hole remain unchanged, the uniaxial compressive strength and biaxial compressive strength decrease with the increase of the major–minor axis ratio of the oval hole. (4) At different dip angles, the brittleness index Biof rock containing oval holes decreases with the increase of confining pressure, and the brittleness degree shows a gradual weakening trend. At the same time, when the dip angle increases, the decreasing amplitude of brittleness index Bidecreases with the increase of confining pressure.

• A new hysteretic damping model is proposed. • The critical hysteretic damping coefficient acquisition method is proposed. • The effects of different hysteretic damping ratios on the simulation results of quasi-static tunnel excavation and quasi-dynamic rock slope slip are investigated. In the field of mechanical numerical simulation, damping is always a vital parameter. However, in the combined finite-discrete element method (FDEM), the current application of damping is insufficient. Therefore, in this paper, a new hysteretic damping model and its corresponding critical damping coefficient acquisition method are proposed. Furthermore, simulations of quasi-static tunnel excavation and quasi-dynamic rock slope slip are employed to investigate the influence of different hysteretic damping ratios on the numerical results. The study results show that the new hysteretic damping model, which is independent on the calculation time step, considers the effects of the element size, Young's modulus and material density and can obtain the critical damping coefficient by comprehensively using cantilever beam vibration numerical modelling and theoretical equations. In addition, the sensitivity of quasi-static tunnel excavation to the hysteretic damping ratio is very weak; however, for quasi-dynamic rock slope slip simulation, the damping ratio η has a substantial influence on the simulation results. The new hysteretic damping model proposed in this paper is important for improving the accuracy of FDEM numerical simulation results, especially for quasi-dynamics simulation. [ABSTRACT FROM AUTHOR]

The shear mechanical behavior is regarded as an essential factor affecting the stability of the surrounding rocks in underground engineering. The shear strength and failure mechanisms of layered rock are significantly affected by the foliation angles. Direct shear tests were conducted on cubic slate samples with foliation angles of 0°, 30°, 45°, 60°, and 90°. The effect of foliation angles on failure patterns, acoustic emission (AE) characteristics, and shear strength parameters was analyzed. Based on AE characteristics, the slate failure process could be divided into four stages: quiet period, step-like increasing period, dramatic increasing period, and remission period. A new empirical expression of cohesion for layered rock was proposed, which was compared with linear and sinusoidal cohesion expressions based on the results made by this paper and previous experiments. The comparative analysis demonstrated that the new expression has better prediction ability than other expressions. The proposed empirical equation was used for direct shear simulations with the combined finite-discrete element method (FDEM), and it was found to align well with the experimental results. Considering both computational efficiency and accuracy, it was recommended to use a shear rate of 0.01 m/s for FDEM to carry out direct shear simulations. To balance the relationship between the number of elements and the simulation results in the direct shear simulations, the recommended element size is 1 mm.

•A novel ensemble classifier is proposed based on four individual classifiers and two optimization algorithms.•Ten imputation methods are proposed to fill the incomplete database of tunnel squeezing.•The present model predicts with more accuracy than the announced models and could provide warning information for timely support measures.•A new SHAP-LightGBM method revealing the features' importance is presented.