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Coal is a common porous sedimentary rock whose microstructure and mechanical behavior are often affected by high temperature. In order to deeply investigate the effect of high temperature on the structure and mechanical properties of coal, six groups of coal samples with different heating temperatures were prepared. Various testing methods, including ultrasonic test, scanning electron microscopy, nuclear magnetic resonance and uniaxial compression test, were used in this study. The results showed that the physical and chemical change processes of coal samples under high temperature can be divided into two stages: the stage of drying and degassing and the stage of pyrolysis of the coal matrix. With the increase of heating temperature, the total porosity of coal samples increases, while the integrity and the deformation resistance of coal samples decrease. In addition, the elastic modulus and uniaxial compressive strength of the coal sample decrease with the increase of heating temperature. The Poisson’s ratio decreases firstly and then increases with the increase of heating temperature.

Four groups of numerical models of Brazilian tests on rock-shotcrete interfaces were successfully conducted by PFC2D. The tensile strength and Young’s modulus of shotcrete were considered. Six different undulations of rock-shotcrete interface were set up. The influences of multiple parameters on the bearing characteristics of the rock-shotcrete interface were studied. The results showed that a better support performance can be obtained by increasing the Young’s modulus of shotcrete rather than the tensile strength of shotcrete. For different tensile strength and Young’s modulus, the increase of sawtooth height has different effects on the support performance. The failure mechanism of the rock-shotcrete interfaces was analysed in detail. The stress shielding effect and stress concentration effect caused by the shape characteristics of rock-shotcrete interface were observed. The influence of these parameters on the overall support performance should be fully considered in a reasonable support design.

Acoustic emission (AE) localization algorithms based on homogeneous media or single-velocity are less accurate when applied to the triaxial localization experiments. To the end, a robust triaxial localization method of AE source using refraction path is proposed. Firstly, the control equation of the refraction path is established according to the sensor coordinates and arrival times. Secondly, considering the influence of time-difference-of-arrival (TDOA) errors, the residual of the governing equation is calculated to estimate the equation weight. Thirdly, the refraction points in different directions are solved using Snell’s law and orthogonal constraints. Finally, the source coordinates are iteratively solved by weighted correction terms. The feasibility and accuracy of the proposed method are verified by pencil-lead breaking experiments. The simulation results show that the new method is almost unaffected by the refraction ratio, and always holds more stable and accurate positioning performance than the traditional method under different ratios and scales of TDOA outliers.

The tunnels might be suffered from damage and destruction under the dynamic loads generated by trains moving on the upper railways, which can in turn threaten the safety of the railways. Understanding the dynamic response of tunnel under the train moving load is of significance for maintaining the operation safety of both the tunnel and upper railway. However, there is currently a lack of relevant research on this topic, especially regarding model experiments. To address this gap, this paper conducts physical model experiments on the train-rail-bridge-shaking table test system to investigate the dynamic response of tunnels subjected to train moving load. The experiment results revealed that except for the average peak particle velocity (PPA) at tunnel vault, the average PPAs at other locations and the vibration dominant frequency (f0) at all locations increase as the train moving speed (vtr) increases. With the exception of some special cases, the average PPAs on tunnel spandrels are generally less than that on the tunnel vault and greater than that on the tunnel invert. When the vtrexceeds 7.03 m/s, except for the average PPA at tunnel invert, the average PPAs on the tunnel wall are generally larger than those inside the surrounding rock mass. As vtrincreases, the average f0on tunnel wall changes more obviously compared to average f0inside the surrounding rock mass. When the vtrexceeds 10.54 m/s, the average f0on tunnel wall are noticeably larger than those inside the surrounding rock mass. The high coefficients of determination obtained from the nonlinear regression analysis indicates a power relationship between the average PPA and vtr, while the average f0shows an exponential association with vtr. The research findings hold great importance in ensuring the safe operation of tunnels and railways.

Abstract: Numerical simulations were performed using a 2D particle flow code (PFC2D) to study the dynamic behavior of two deep buried tunnels subjected to blasting disturbance. Depending on the distances between the two tunnels and blasthole, the two tunnels were divided into adjacent tunnels and nonadjacent tunnels, respectively. The propagation characteristics of the blasting stress wave were first examined. The evolution of strain energy around the two deep buried tunnels was then evaluated. The results show that at different locations around the tunnels, the variation characteristics of strain energy are different. Moreover, in the region where the strain energy accumulates, the safety factor decreases, while in the region where the strain energy releases, the safety factor increases. The effects of tunnel spacing and tunnel shape on the nonadjacent tunnel were further investigated. For the nonadjacent tunnel, the safety factor at the left and right sidewalls is positively related to the tunnel spacing, while the safety factor at the roof tends to increase first and then decrease. When the tunnel shape subsequently changes from circular to U-shaped and rectangular, the safety at the right sidewall and roof show the trend of decreasing and increasing respectively, and the safety factor at the left sidewall decreases first and then increases. Article highlights:

The stress evolution characteristics are closely related to the location of surrounding rock.

The tunnel setting is closely related to the stability of non-adjacent tunnel.

The variation of strain energy is negatively correlated with the variation of the safety factor of tunnel.