The longevity of grouting has a significant effect on the safe and sustainable operation of many engineering projects. A 500-day experiment was carried out to study the long-term mechanical behavior of nano silica sol grouting. The nano silica sol was activated with different proportions of a NaCl catalyst and cured under fluctuating temperature and humidity conditions. The mechanical parameters of the grout samples were tested using an electrohydraulic uniaxial compression tester and an improved Vicat instrument. Scanning electron microscope, X-ray diffraction, and ultrasonic velocity tests were carried out to analyze the strength change micro-mechanism. Tests showed that as the catalyst dosage in the grout mix is decreased, the curves on the graphs showing changes in the weight and geometric parameters of the samples over time could be divided into three stages, a shrinkage stage, a stable stage, and a second shrinkage stage. The catalyst improved the stability of the samples and reduced moisture loss. Temperature rise was also a driving force for moisture loss. Uniaxial compressive stress-strain curves for all of the samples were elastoplastic. The curves for uniaxial compression strength and secant modulus plotted against time could be divided into three stages. Sample brittleness increased with time and the brittleness index increased with higher catalyst dosages in the latter part of the curing time. Plastic strength-time curves exhibit allometric scaling. Curing conditions mainly affect the compactness, and then affect the strength.

The pressure relief of coal pillars in close-distance multi-coal seam goaf is a complex engineering problem with the characteristics of “dynamic mine pressure”. Hence, this paper studies such problems. First, the influence factors of the coal pillar in the goaf on the mine pressure of the mining face of the lower coal seam under this condition were theoretically analyzed, and it was concluded that vertical stress is the most important element, followed by horizontal stress. Next, a physical similarity simulation experiment was designed to study the stress distribution law of the coal pillar floor in the goaf before and after pressure release and the damage depth. Finally, a technology and monitoring method for coal pillar blasting pressure alleviation in goaf were introduced and implemented in engineering practice. After the pressure is alleviated, the surrounding rock stress of the lower coal seam mining face is redistributed, and the vertical stress is decreased by 20%. The adjacent rock’s deformation is improved. This technology’s cost and safety advantages are extraordinary and helpful for mining coal seams over close distances.

This laboratory study investigated the damage evolution of sandstone specimens under two types of cyclic loading by monitoring and analyzing changes in the elastic moduli and the ultrasonic velocities during loading. During low‐level cyclic loading, the stiffness degradation method was unable to describe the damage accumulations but the ultrasonic velocity measurements clearly reflected the damage development. A crack density parameter is introduced in order to interpret the changes in the tangential modulus and the ultrasonic velocities. The results show the following. (1) Low‐level cyclic loading enhanced the anisotropy of the cracks. This results from the compression of intergranular clay minerals and fatigue failure. (2) Irreversible damage accumulations during cyclic loading with an increasing upper stress limit are the consequence of brittle failure in the sandstone’s microstructure.

Currently, bolt-support technology is commonly applied throughout the world during mining activities, civil engineering, and hydraulic projects. The theory of a pretensioned and fully bonded bolting system has long been proposed for use as tunnel supports; however, this technology is difficult to implement in coal mining, particularly for roadways that require immediate support. In this study, a modified cement with water-to-cement ratio of 0.25 was used to work in tandem with a traditional resin cartridge to create a pretensioned, fully bonded bolting system. The fast-setting epoxy resin is positioned at the bottom of the borehole for pretension, and the cement is positioned along the rest of the borehole for full-length bonding along the bolt. Based on a series of pullout tests on the proposed bolting system with different pretension forces, this study shows that a pretensioned, fully bonded bolting system is more durable and stable than the end-encapsulated resin bolting system that is currently utilized in most of China’s coal mines. Additionally, an acoustic emission detection test was simultaneously conducted to further describe the inner fracture mechanisms of the bolting system under different pretension forces ranging from 50 to 140 kN, with an increment of 30 kN. The results of this test indicate that a higher pretension force can decrease the damage events in the bolting system. Additionally, the failure form of a pretensioned, fully bonded bolting system is more likely to be dominated by the fracture of the bolt rod in most circumstances. Finally, this combined bolting technology is successfully implemented in a coal mine, and the monitored results confirm that the proposed bolting measure is effective and reliable.