Lihui Sun

Weakly cemented rocks have a loose structure, poor mechanical properties, and soften and disintegrate upon contact with water. Mining operations cause damage and ruptures to rocks under cyclic loading and unloading, leading to serious disasters. This study investigated the effects of cyclic loading and unloading on the mechanical properties of weakly cemented sandstone (WCS) with various water contents (0–7.72%). A numerical model based on the particle flow theory simulated the behavior of WCS particles. The stress–strain relationships, damage and rupture patterns, energy evolution, and damage properties of WCS were examined using loading–unloading simulations. Water negatively affected the strength and elastic modulus of WCS. High water contents (>2.31%) increased the rupture probability and affected the rupture modes. Ruptures mainly occurred via the main fissure and caused cleavage damage; however, instances of tensile damage and shear slippage increased with an increasing water content. The elastic, dissipation, and total energies gradually increased with increasing cyclic loading and unloading. The damage factors of WCS with different water contents gradually increased with the growth rate. The mechanical properties of the sandstone were deteriorated by water, which increased the peak value of the damage factor from 0.77 for 0% moisture to 0.81 for 7.72% moisture.

Weakly cemented rocks are characterized by low strength, loose structure, and easy disintegration. High-intensity mining activities can damage and rupture such rock bodies and induce damage, such as flaking and roofing on roadways. To reveal the mining intensity influence on the weakly cemented rocks’ deformation and damage, a numerical particle flow model of weakly cemented sandstone was established based on particle flow theory. Uniaxial compression simulation tests were conducted at four loading rates of 0.01, 0.1, 0.5, and 1 mm/min to study the weakly cemented sandstone’s stress–strain relationship, damage rupture, acoustic emission, and energy evolution. The results show that, with an increased loading rate, the uniaxial compressive strength of weakly cemented sandstone increases exponentially, and the rupture mode transforms from brittle damage to ductile damage; the greater the loading rate, the greater the degree of damage and crushing range of the rock. Further, with an increased loading rate, the peak hysteresis of rock acoustic emission events decreases, and the number of events increases; the energy accumulated in the rock increases, thus intensifying the degree of rock damage. Therefore, the possibility of engineering disasters should be considered when conducting high-speed underground mining activities.

In weakly cemented rocks, the mesoscopic parameters have a great influence on the macroscopic mechanical properties. One example of a typical weakly cemented rock is the Cretaceous coarse sandstone in the Hongqinghe Coal Mine. In this study, rock samples were subjected to physical and mechanical experiments, from which a sample model was constructed based on particle flow theory. Uniaxial compression numerical simulation experiments and analyses were conducted, and sensitivity analyses of various microscopic parameters in relation to the macroscopic mechanical properties of the rock were performed via a control variable method. A response mechanism between the macroscopic and mesoscopic parameters was then inferred. On the microscopic scale, the rock is porous with a loose structure and extremely low average uniaxial compressive strength, indicating looseness and weakness. The mesoscopic parameters were then divided into three grades based on their degrees of influence from high to low on the peak strength, peak strain, and elastic modulus. Laboratory experiments revealed that the fracture form of weakly cemented coarse sandstone is typically due to single-section shear failure, whereas through simulation, cracks are caused mainly by tension failure. These two failure modes were inferred to be consistent with each other.

Bond strength is one of the most important parameters and can affect the macroscopic mechanical properties and the damage state of rock to some degree. Coarse-grained sandstone was studied using the controlled variable method. The influence of parallel bond strength on the peak strength and failure mode of coarse-grained sandstone was simulated, and the evolution law of peak strength and the failure mode of bond strength were comprehensively analyzed. The results show that the peak strength of the rock was positively correlated with the bond strength; the difference in quantity between the tensile and shear cracks was negatively correlated with tensile bond strength and positively correlated with shear bond strength. With a tensile-shear bond strength ratio of less than 0.5, the peak strength of the rock was usually stable at the certain extreme value under a constant tensile bond strength. The tensile cracks were negatively correlated with the tensile-shear bond strength ratio, and the shear cracks were positively correlated with the tensile-shear bond strength ratio. The main failure mode of the coarse-grained sandstone in the weakly cemented stratum of the Hongqinghe coal mine is shear failure. The research results can be used to guide the ground control of other mine stopes or roadways with weak cementation lithology.