The severe floor heave in gob-side entry retaining is the major restriction factor of the wide application of pillarless mining thin coal seams. Reinforcement and stress-relief floor heave control methods are the most promising. However, in practice, floor restoration is widely used. Therefore, floor heave control technology in gob-side entry retaining needs to be improved. This study proposes anti-shear pile technology to control floor heave in gob-side entry retaining. The research was mainly carried out by numerical simulation. It was found that the transformation of high vertical stresses in the entry floor underneath the filling wall and coal seam body into horizontal stresses starts the floor heave process. The vertical dilatancy of rocks under the roadway span and their subsequent unloading lead to the delamination of the floor strata and uplift of the entry contour. In this paper, the best pile installation scheme was found. It is a 2pile 5+2 scheme with the installation of two piles, each 2 m long. After that, it was shown that filling piles are more than 3.3 times cheaper than comparable analogs, and pile installation is less labor-intensive. The implementation of the proposed floor heave control method leads to a reduction in heaving by 2.47 times.

The floor heave is one of the key factors that can restrict high-efficiency and safety mining, especially in the deep roadways with soft rock. Considering the influences of rock fracturing over time on rock mass properties, a case study of the floor heave evolution and rock bolts reinforcement technology was performed in this paper. A numerical simulation was used to study the stress-strain state and displacement of surrounding rocks. It was found that significant floor heave caused by nonlinear deformation of laminated immediate floor under an increase in rock fracturing. The post-peak strain regions appear in the bottom corners of the roadway, after which strata in the immediate floor are destroyed one by one. The joint spacing of 0.45 m on the immediate floor is critical. At this step, post-peak strain regions merge in the central part of the roadway floor, which is the cause of uncontrolled floor heave. Rock bolts reinforcement was proposed to control the floor heave. Three floor support schemes with two types of support elements, different bolt orientations, and lengths of reinforcement were studied. The numerical simulation demonstrated that after reinforcement, post-peak plastic strain in the floor strata was reduced effectively. The optimal floor support scheme and depth of reinforcement were determined by the allowable floor heave. Ideally, the floor heaves could be reduced by rock bolts with a steel belt installed according to the support scheme III and reinforcement length of 2.0 m for outer bolts and 3.0 m for central bolts.

The process of fully mechanized coal seam mining under aquifers and surface water bodies has been a challenge in recent years for different countries. During the evolution of subsidence and the overburdening of rock mass movement above the longwall goaf, there is always a potential risk of connecting the water-conducting fracture zone with aquifers. The water inflows in the coal mine’s roadways have a negative impact on the productivity of the working faces and pose significant hazards to miners in the event of water inrush. Therefore, the assessment of the height of the water-flowing fractured zone has an important scientific and practical significance. The background of this study is the Xinhu Coal Mine in Anhui Province, China. In the number 81 mining area of the Xinhu Coal Mine during the mining of the number 815 longwall, a water inflow occurred due to fractures in the sandstone in the overburden rock. The experience of the successful implementation of the water inrush control method by horizontal regional grouting through multiple directional wells is described in this paper. This study proposes an algorithm for the assessment of the risk of water inrush from aquifers, based on an ANSYS 17.2 simulation in the complex anticline coal seam bedding. It was found that the safety factors based on the stress and strain parameters can be used as criteria for the risk of rock failure in the aquifer zone. For the number 817 longwall panel of the Xinhu Coal Mine, the probability of rock mass failure indicates a low risk of the occurrence of a water-flowing fractured zone.

Rock fracturing is one of the main processes in the modern mining, mineral recovery, and energy industry, which is associated with the numerous environmentally harmful influences. Environmentally friendly alternative to conventional fracturing methods is non-explosive technology, which based on using of soundless chemical demolition agents (SCDA). In this study, method of directional fracturing, which based on creating tensile stress concentrators on the hole surface in the required location, was proposed. Considering the influences of additional installation in the borehole of stress concentration cartridge on stress field nearby the hole, a case study of the effectiveness of proposed rock fracturing method was performed in this paper. A numerical simulation was used to study the stress distribution around the borehole with SCDA in different rock types. It was found that cartridge installation leads to the occurrence of the tensile stress concentrations only near to the gap of the cartridge. The value of tensile stress concentration in the proposed rock fracturing method significantly depends on a cartridge gap distance. An increase in the gap distance leads to a decrease in the stress concentration factor. For granite, sandstone and mudstone, relationships of stress concentration factor to cartridge gap distance that was divided on hole diameter were obtained. The equation for calculating of the hole spacing for the directional fracturing method was proposed. The results of the study are confirmed by experiments in-situ on the directional fracturing of rocks in a quarry and in a coal mine.

Identification of material parameters for numerical simulation of the behavior of rocks under true triaxial conditions