In order to protect surface buildings and maximize the recovery of coal resources, there are a large number of strip coal pillars and unfilled goafs left in strip mining areas in China. Affected by the comprehensive factors such as long-term overlying rock load, water accumulation in goafs, and adjacent mining, the instability probability of strip coal pillar increases greatly, which becomes a potential hidden danger of surface disaster. However, the law of overlying rock and surface movement caused by strip coal pillar instability is not clear up to now. Therefore, a series of experimental studies on the law of overlying rock movement and deformation induced by strip coal pillar instability are carried out in this paper. The results show that it is feasible to use paraffin-containing materials to make a strip coal pillar model and simulate strip coal pillar instability by heating paraffin. After mining, the subsidence curve of the L5 monitoring line and its upper strata is “disk.” After the instability of the strip coal pillar, the subsidence curve changes from “disk” to “basin.” The instability of strip coal pillar can lead to significant and more harmful overburden movement between overburden and surface than that of strip working face. However, because the unstable coal pillar still has a certain bearing capacity, the maximum subsidence of the surface is still far less than that under the condition of longwall mining.

Conventional borehole pressure relief can meet the requirements for preventing rock bursts, but it can also easily destroy the roadway, resulting in system support failure. Taking coal-like samples with boreholes as the research object, the pressure relief effects of the same-diameter boreholes and variable-diameter boreholes are compared and analyzed by a uniaxial compression test. The joint pressure relief mechanism of the variable-diameter drilling hole is discussed. The experimental results show that the stress–strain curve of variable-diameter drilling sample occurred the phenomenon of pre-peak stress drop and post-peak stress plateau, which indicates that the variable-diameter drilling hole can enhance the plastic characteristics of coal-like samples. The borehole size dramatically influences the variation of various pressure relief indexes. The evolution law of AE counting the pre-peak and post-peak of borehole samples verified the abovementioned law. Variable-diameter drilling can enhance the plasticity of samples, weaken the brittleness and reduce the impact tendency. Under the maximum size of the variable-diameter drilling hole and the same-diameter drilling hole is same, the pressure relief effect is more significant. The results obtained in this paper can provide a new theoretical basis and technical guidance for borehole pressure relief and roadway support.

Rock fractures are considered as favorable objects for enhanced geothermal development. The fracture morphologies play an important role in enhanced geothermal development. Therefore, the study of fracture morphologies has a certain guiding significance for the geothermal reservoir. Water cooling and water cooling cycles can change the morphology of fracture surfaces formed by the shear failure of intact granites. To date, however, there is little work on the effect of water cooling and water cooling cycles on the morphology of fracture surfaces formed by direct shearing of intact granites. In this study, the direct shear tests of intact granites treated by water cooling cycles at different temperatures were conducted, and the variations in the laws of shear strength of intact granites and morphologies of fracture surfaces with temperature or cycle times were analyzed. Test results showed that the shear strength and shear stiffness of intact granites decreased nonlinearly with the increase of temperature or cycle times, but the height and apparent dip angle of asperities on the fracture surface increased with the increase of temperature or cycle times, and the overall uniformity of the fracture surface was improved. The height distribution frequency of asperities on fracture surfaces can be divided into four types: right-biased peak type, left-biased peak type, left-biased middle peak, and left-biased flat peak. The asperities on the fracture surface formed by the shearing of intact granites have asymmetric characteristics. The maximum apparent dip angle and average apparent dip angle in the reverse shear direction are larger than those in the shear direction, and the initial contact area ratio between the shear direction and reverse shear direction is in the range of fluctuation between 1.4 and 2.