Study on Reasonable Size of Coal and Rock Pillar in Dynamic Pressure Roadway Segment of Fully Mechanized Face in Deep Shaft

The deformation mechanism of the protective coal and rock pillar area outside a stope is an important parameter for setting a reasonable size. In this paper, based on the geological condition of working face 1231(1) in a mine in Huainan, a method that combines the use of a borehole and Brillouin optical time domain reflectometry (BOTDR) was proposed to analyze the stress variation laws of coal and rock pillar areas, and the parameters of the monitoring borehole and installation technique of the sensing optical cables were designed. Based on the monitoring data, the strain distribution characteristics of the sensing optical cables and their relationship with the rock strata were analyzed, the development law of coal and rock strata deformation during the mining process was revealed, and the transverse influence range of the coal and rock pillar affected by mining was reasonably divided. According to the results, the sensing optical cables show an overall trend of tensile strain, with a maximum value of 1800 με, and the main areas of rock strata deformation occur near the interface of rock strata. The range of rock strata disturbance along the borehole direction was approximately 38 m, and the maximum deformation of rock strata after the disturbance, namely, the displacement, was 24.87 mm. A numerical model was constructed to acquire the strain variation characteristic within 100 m in the outer floor of the working face. The transverse range of the floor disturbance was analyzed to be 30–36 m. The field test had good correspondence with the numerical simulation results, which indicates that the optical fiber testing technology can effectively describe the stress variation in the coal and rock strata. The test results can provide technical support for the rational setting of coal and rock pillars and disaster prevention and control. The research direction of deep rock mass testing is discussed, and optical fiber testing in boreholes is considered an effective method for studying deep dynamic disaster control.

Water Inrush from Pregrouting Fractures Induced by Mining Activities and Its Engineering Control Method Optimization

Water inrush accidents during coal mining still occur after reinforcement of fissured water bearing strata underlying coal seams through grouting, which seriously troubles field technicians. In order to further prevent the water inrush, especially the water inrush after grouting reinforcement, the grouting technique needs to be improved. Experimental design, theoretical analysis, and numerical simulation have been carried out in sequence. On the microscale level, the relationship between strains and water flow rate was analyzed through laboratory experiments; on the macroscale level, the propagation mechanism of grouting cracks resulting from the rock strata deformation during coal production was analyzed with rock beam theory, and then a new model of casing pipes preventing water inrush by controlling deformation of the floor rock strata is proposed. The laboratory test results show that the response of the radial strain and the water flow rate to mining operation is consistent and positively correlated and verify that it is feasible to reduce water inrush disaster by controlling deformation of the rock strata. The deformation governing equation of the casing pipes is deduced theoretically. At last, the numerical calculation was done to prove the effectiveness of controlling the floor rock strata deformation by the casing pipe group, which indicated that the displacement of the monitoring points after grouting is smaller than that before grouting practice. It is practicable to realize grouting technique optimization by the innovative adoption of the grouting casing pipes in the future.

Strata Movement and Fracture Propagation Characteristics due to Sequential Extraction of Multiseam Longwall Panels

Multiseam longwall mining-induced strata deformation and fracture propagation patterns are different from those of single-seam mining. This difference is due to interaction of the caved zones as a result of longwall mining activity at different coal seams, which severely impacts formation of subsidence and permeability of the strata after multiseam mining. Understanding this phenomenon is of great importance in order to predict the multiseam subsidence reliably, evaluate the risk of water inrush and take suitable preventive measures, and determine suitable locations for placing gas drainage boreholes. In this study, scaled physical modelling techniques are utilised to investigate strata deformation, fracture propagation characteristics, and vertical subsidence above multiseam longwall panels. The results show that magnitude of the incremental multiseam subsidence increases significantly after multiseam extraction in comparison with single-seam mining. This increase occurs to different extent depending on the multiseam mining configuration. In addition, interstrata fractures above the abutment areas of the overlapping panels propagate further towards the ground surface in multiseam extractions compared with single-seam extractions. These fractures increase the risk of water inrush in presence of underground/surface water and create highly permeable areas suitable for placing gas drainage boreholes.