Directional fracturing is fundamental to weakening the hard roof in the mine. However, due to the significant stress disturbance in the mine, principal stresses present complicated and unmeasurable. Consequently, the designed hydraulic fracture (HF) extension path is always oblique to principal stresses. Then, the HF will present deflecting propagation, which will restrict the weakness of the hard roof. In this work, we proposed an approach to drive the HF to propagate directionally in the hard roof, utilizing a set of hydraulic fractures and their stress disturbance. In this approach, directional fracturing in the hard roof is conducted via the sequential fracturing of three linear distribution slots. The disturbed stresses produced by the first fracturing (in the middle) are utilized to restrict the HF deflecting extension of the subsequent fracturing. Then, the combined hydraulic fractures constitute a roughly directional fracturing trajectory in rock, i.e., the directional fracturing. To validate the directional fracturing approach, the cohesive crack (representing rock fracture process zone (FPZ)) model coupled with the extended finite element method (XFEM) was employed to simulate the 2D hydraulic fracturing process. The benchmark of the above fracturing simulation method was firstly conducted, which presents the high consistency between simulation results and the fracturing experiments. Then, the published geological data of the hard roof in Datong coal mine (in Shanxi, China) was employed in the fracturing simulation model, with various principal stress differences (2~6 MPa) and designed fracturing directions (30°~60°). The simulation results show that the disturbing stress of the first fracturing significantly inhibits the deflecting propagation of the subsequent fractures. More specifically, along the direction parallel to the initial minimum principal stress, the extension distance of the subsequent hydraulic fractures is 2~3 times higher than that of the deflecting HF in the first fracturing. The fracturing trajectory of the proposed direction fracturing method deviates from the designed fracturing path by only 2°~14°, reduced by 76%~93% compared with the traditional fracturing method utilizing a single hydraulic fracture. This newly proposed method can enhance the HF directional propagation ability more effectively and conveniently in the complex and unmeasurable stress field. Besides, this directional fracturing method can also provide references for the directional fracturing in the oil-gas and geothermal reservoir.

The effect of pore pressure on rock damage is a core problem in underground engineering. In this paper, the pseudotriaxial hydromechanical coupling experiment system is utilized to carry out the physical model experiment of the pore pressure on the rock damage, and the effect of evenly distributed pore pressure on rock strength and pore structure is analyzed. Compared with natural rock sample before the pore pressure effect, the uniaxial compressive strength is reduced by about 35.24%. Pore pressure decreases the strength of the rock and reduces its ability to resist deformation. The stress growth rate at the prepeak stage of the stress-strain curve for rock sample after pore pressure effect decreases. The peak strength decreases, and the strain corresponding to the peak stress increases. The plastic deformation characteristics of the sample are obvious. After the sample has macroscopically broken, the decline rate of postpeak stress slows down, and the residual strength decreases. The rock gradually changes from brittle failure mode to ductile failure mode. Through scanning electron microscopy (SEM) test and comparison analysis, it is found that after the pore pressure effect, the surface of the rock sample becomes rougher. There are multiple pore and fissure concentrated areas in the rock sample. Compared with the sample before the pore pressure effect, the number of pores with an equivalent diameter in the range of 2 μm-3 μm increases significantly. The proportion has increased from 7.7% to 19.1%, and the number has increased from 1468 to 4454. A large number of small-sized pores (less than 3 μm) are generated in the rock sample after the pore pressure effect. Therefore, the overall pore content of the rock sample increases.

The typical thick-hard limestone roof with high failure pressure and easy fracture closure under in situ stress is extremely prone to induce disasters, which seriously threatens the safe production of coal mines. The modification of carbonate minerals by acidic effects can effectively realize the weakening control of hard limestone strata. In this study, a multi-factor orthogonal experiment was designed for limestone acidification reactions, and the evaluation model for the reformation and strength weakening of hard limestone based on the acidic effect was established accordingly. The results showed that there is an order in the influence of various factors on the reaction parameters and strength indices of acidified limestone, and the improvement of rock properties by the level difference of acid concentration is significantly better than that of acid type and acidification time. Through numerical analysis, the evaluation model of limestone reformation and strength weakening considering the acid reaction parameters is given. The reliability of the model passed the credibility test and experimental verification, which can effectively reflect the strength response characteristics of acidified limestone. The simulated annealing (SA) algorithm is introduced to derive the optimal acidification system suitable for limestone weakening. Combined with the model, the control mechanism of the acidic effect on hard limestone strata was analyzed. The acidic effect can not only induce the rapid generation and expansion of micro-cracks at mineral-containing crystal defects, but also make the cracks remain relatively open under in situ stress due to the differential interaction on mineral components, which is conducive to the bearing capacity reduction and structural damage of limestone rock mass. The research results provide theoretical guidance for the acidification control of hard limestone strata in underground mines.