ABSTRACT
Brazilian split experiments were carried out on coarse sandstone, obtained from a coal seam roof passed by the Shihuoshan tunnel in Xinjiang, and treated at different temperatures (room temperature 25 °C and high temperature 100 °C ~ 900 °C). The physical and mechanical characteristics of the samples were studied. The results showed that: after heat treatment, the color of the coarse sandstone samples gradually changed from dark gray to brownish red-pink; the higher the treatment temperature was, the darker the sample color. Microcracks and mineral composition changes occured in the coarse sandstone samples after heat treatment, which decreased the longitudinal wave velocity of the samples. The longitudinal wave velocity of the coarse sandstone samples decreased as a quadratic function of the treatment temperature. With the increase in longitudinal wave velocity, the tensile strength of the samples first increased and then decreased, changing as a quadratic function relationship. After heat treatment, the tensile stress-strain curve of the coarse sandstone could be divided into compaction, elasticity, plasticity, and failure stages. The energy was continuously accumulated in the coarse sandstone before its failure, and it was released suddenly after the failure. With increasing treatment temperature, the cumulative energy in the prepeak stages first increased and then decreased, reaching a maximum value at 500 °C. The prepeak energy and tensile strength of the coarse sandstone samples satisfied a linear function fitting relationship, indicating that a higher tensile strength in the coarse sandstone, led to more accumulated energy in the samples.
ABSTRACT
With the extension of mines to deep mining, horizontal stress has become an important factor in the stability of the roadway surrounding rock, and the influence of horizontal stress on the structure of a layered roof is more complex. The simulation test of a layered roof of a roadway under different lateral pressure coefficients was carried out by using a self-designed test device and discrete element simulation software. The crack coalescence behavior of a layered roof in a roadway was analyzed, and the critical stress of rock beam failure in each layer was determined. The results show that the larger the stress levels, the larger the number and height of cracks in the layered roof were, and the relationship between them can be characterized by a quadratic polynomial. With the increase of stress levels, the layered roof was damaged gradually from the bottom to the top along the bedding plane. The maximum height of the roof caving was 9, 23, 31, and 39 mm, respectively. The width of the upper roof caving was 72, 35, 31, and 27 mm in turn. The left angle of the caving range had little change (about 60°). With the increase of stress level, the maximum caving height of the layered roof tended to increase, while the width of the top surface tended to decrease. The relationship between the stress level and the height and width can be represented by a quadratic polynomial and exponential function. According to the stability theory of the compression bar, the critical stress of failure and the decrease of the span of the rock beam in each layer were determined respectively, and the reasons for the instability of the layered roof caving are explained.
