Physical Model Experimental Study on the Coalface Overburden Movement Law on the End-slope of an Open-pit Mine

Due to technology and safety limitations, the amount of coal resources overlying slopes in open pit coal mines is immense. In recent years, this problem has gradually attracted the attention of researchers. How to realize the efficient recovery of the side overburden resources with the premise of ensuring the stability and safety of the slope has become an important topic for the development of opencast mining technology in China. To study the yield failure characteristics of coal pillars and the rock mass migration law of the end slope mining field under the mining condition of the end slope shearer, 2D/3D, integrated, simulation experimental equipment is developed based on similarity theory and efficient region theory. This equipment overcomes the technical problem that the internal failure of the rock mass is invisible and that deformation data are not easily obtained during the simulation of end slope coal mining on an existing experimental platform. Based on the engineering geological conditions of the Ordos mining area in China, a typical engineering geological model of the slope near the horizontal condition is constructed to simulate the process “formation of mining cave group -failure of support coal pillars - instability of slope rock mass”. Based on laser positioning technology and multiangle, oblique photography technology, a panoramic phase 3D laser scanner, high-resolution digital camera and deep space micromonitoring system are comprehensively employed to carry out the whole process tracking monitoring and analysis of the deformation and failure of the supporting coal pillars and slope rock mass. The experiment is verified by numerical simulation. The results show that under the experimental conditions, with an increase in mining cave depth, the vertical stress of the supporting coal pillar increases linearly. At a certain distance before reaching the end of the mining cave, the peak value is reached. At this time, the depth continues to increase, and the stress value decreases sharply. The vertical stress gradually decreases to the original rock stress after a certain distance beyond the end of the mining cave. A certain length of supporting coal pillar from the end of the mining cave will never collapse, which is approximately 2.5~3 times the width of the mining cave. The triggering condition of slope deformation and failure is under the combined action of dynamic and static loads. The actual stress of the supporting coal pillar in the deep part of the geometric centre along the slope of the mining cave group is greater than the ultimate stress, and then large discontinuous deformation of multiple adjacent coal pillars around the central coal pillar is caused by compressive shear failure. The boundary of the final collapse plane range of the roadway group is approximately a closed curve formed by two paraboloids, which are axisymmetric with the No. Ⅳ coal pillar and open opposite. The parabola opening in the shallow part of the slope area is small, and the parabola opening in the deep part of the slope area is large. There is a significant space-time correspondence between the failure of supporting coal pillars and the deformation of the slope surface. According to the failure process of the rock mass structure and the movement and deformation characteristics of the slope surface, the slope after failure can be divided into three areas, and the upper part of the slope is the key area of deformation and instability of the overlying rock mass in the end-slope mining field. The research results provide a theoretical basis for scientific monitoring and stability control of slope deformation coal mining conditions in open-pit mines.

Mechanism of Coal Burst and Prevention Practice in Deep Asymmetric Isolated Coal Pillar: A Case Study from YaoQiao Coal Mine

Coal pillar bursts continue to be a severe dynamic hazard. Understanding its mechanism is of paramount importance and crucial in preventing and controlling its occurrence. The extreme roadway deformations from the asymmetric isolated coal pillars in the central mining district of YaoQiao Coal Mine have responded with frequent intense tremors, with risky isolated coal pillar bursts. The theoretical analysis, numerical simulation, and field measurements were done to research the impact of spatial overburden structure and stress distribution characteristics on the isolated coal pillar area, aiming to reveal the mechanism of coal pillar burst leading to the practice of prevention and control in the asymmetric isolated coal pillar area. The study shows that the overburden structure of the asymmetric is an asymmetric “T” structure in the strike-profile, and the stress in the coal pillar is mostly asymmetric “saddle-shaped” distribution, with the peak stress in the east side of the coal pillar, and the coal pillar is a “high stress serrated isolated coal pillar.” Numerical simulation results showed that the support pressure in the isolated coal pillar area on the strike profile was asymmetrically “saddle-shaped” distribution. The peak vertical stress in the coal pillar area continued to rise and gradually shifted to the mining district's deep part. As a result, the response of the roadway sides to the dynamic load disturbance was more pronounced. They developed a coal burst prevention and control program of deep-hole blasting in the roof of asymmetrical isolated coal pillar roof and unloading pressure from coal seam borehole. Monitored data confirmed that the stress concentration was influential in the roadway’s surrounding rock in the asymmetric isolated coal pillar area, circumventing coal pillar burst accidents. The research outcomes reference the prevention and control of coal bursts at isolated working faces of coal pillars under similar conditions.

Failure Mechanism of Gob-Side Roadway under Overlying Coal Pillar Multiseam Mining

Roadway deformation and rock burst are the two key challenges faced by the safe operation of coal mines. Aiming at the issue of large deformation of the gob-side roadway under coal pillars in multiseam mining, this study has considered the case of the 8308 panel of Xinzhouyao coal mine in China. Based upon a combination of theoretical analysis, numerical simulations, and engineering practices, the mechanical model of “stress and deformation quantitative calculation of gob-side roadway under overlying coal pillars” was established in this study. The analytical solutions of the vertical stress distribution and the plastic zone of the gob-side roadway under overlying coal pillars were obtained. Finally, the accuracy of the mechanical model was verified using numerical simulations. The results showed that the coal pillar, upright above the gob-side roadway, and the cantilever roof around the gob-side roadway were the main factors leading to stress concentration and deformation around the gob-side roadway. For the particular cases considered in this study, the peak stress of the gob-side roadway could reach 1.8 times of the self-weight stress of overlying strata. The rates of the contribution of the gob-side roadway’s overlying pillar and the cantilever roof around the gob-side roadway to peak stress were 78.3% and 16%, respectively. The obtained results have an essential reference significance for stress calculations and rock burst prevention design of gob-side roadway under overlying coal pillars in multiseam mining.

Experimental research on the haulage drifts stability in steeply dipping seams

Purpose. Substantiation of the conditions for haulage drifts stability using different protection methods in steeply dipping seams based on a set of experimental studies. Methods. To achieve the purpose set, mine instrumental observations have been performed to study the rock pressure manifestations in zonal advance workings adjacent to the stope face on the haulage horizon. The conditions for their maintenance, within the mining site, are assessed by the side rocks convergence value on the drift contour and the change in the cross-sectional area, taking into account the deformation properties of the protective structures. Findings. It is recorded that in the zone of the stope works influence, in the most difficult conditions, haulage drifts are maintained, when coal pillars or clumps of prop stays are used for their protection. It has been determined that a decrease in the section of such mine workings up to 50% is the result of the protective structures destruction. When protecting the hau-lage drifts with the rolling-on chocks, a decrease in the mine working section up to 30% occurs in the process of the protective structures compression. It has been revealed that deformation of coal pillars or clumps of prop stays up to 10-20% leads to a loss of their stability, and an increase to 60% leads to a complete loss of their load-bearing capacity, intensification of rock displacements on the mine working contour and deterioration of its stability. It has been determined that in the process of deformation of the rolling-on chocks from sleepers by 20-60%, they are compressed without loss of load-bearing capacity, which ensures a smooth deflection of the overhanging stratum and restriction of rock displacements on the haulage drift contour. Originality. To study the deformation characteristics of protective structures above the drift, the function of the increment is used of side rock displacements on the haulage drift contour along the mining site length dependent on the relative deformations of protective structures, which makes it possible to assess the real dynamics of the process. Practical implications. When mining steep coal seams, using the specificity of geomechanical processes, which are manifested in an anisotropic coal-rock mass during unloading, satisfactory mine workings stability can be ensured by changing the deformation properties of protective structures above the drift.

Investigation on Hydraulic Fracturing and Cutting Roof Pressure Relief Technology for Underground Mines: A Case Study

Using hydraulic fracturing for cutting roof pressure is a critical technology to protect coal pillars. In this paper, based on the engineering background of 18506 working face in the Xiqu Coal Mine, using the methods of theoretical analysis, numerical simulation, and field measurement, a reasonable coal pillar width and practical parameters of hydraulic fracturing are given. The results show that roof cutting can significantly increase the stress in goaf and relieve the advanced pressure of the working face. Taking 18506 working face as the research object, the industrial test is carried out, and the surrounding rock control scheme of hydraulic fracturing and roof cutting is put forward, the mine pressure monitoring results show that the auxiliary roadway of 18506 working face reaches a stable state within 20 days, the deformation and damage degree of roadway surrounding rock are small, and the integrity of surrounding rock is improved.

Investigation of the Optimal Layout of the Roadway in Closely Spaced Ultra-Thick Coal Seams Mining with Remaining Coal Pillars

The reasonable layout of the roadway in closely spaced, ultra-thick coal seam mining is of great significance to mining safety. Based on the research background of repeated roof leaks in the process of repairing the return air roadway in working face No. 30503 in the Tashan Coal Mine, theoretical analysis, in situ engineering testing, and numerical simulation were jointly adopted to evaluate the stability of the return air roadway under two schemes of repairing the original return air roadway and excavating a new return air roadway. The results show that the vertical mining-induced fissure above the roadway will cause severe damage to the roadway due to the influence of working-face mining when restoration of the roadway excavation is adopted. When choosing to excavate a new return air roadway, the new return air roadway just staggers the vertical cracks located in the top slab of the original return air roadway, putting the roadway in a state of stress reduction, making the roadway itself more stable and conducive to support. Therefore, the new air return tunnel was selected to establish the working face. To ensure safety of the working face during the mining of the original return air roadway, the original return air roadway was filled with high water content materials. Site investigation data show that this material plays a cushioning role in the filling section of the original return air roadway during the mining of the 30503 working face, and the deformation of the new return air roadway during the filling section crossing the original return roadway is stable and well controlled.

Simulation Analysis and Engineering Application of Retaining Width of Waterproof Coal Pillar in Island Working Face

The old mining area in Pingdingshan coalfield has the following problems: long mining service life, many remaining coal pillars, and great difficulty in mining; to extend the service life of the mine, realize cost saving and efficiency increasing, it is urgent to recover the remaining coal pillars, but the mining of isolated island face faces the problem of reasonable retention of waterproof coal pillars, if the protection is not good, it is easy to cause mine water damage and increase the mining cost. Therefore, in view of the practical engineering problems faced by the field, aiming at eliminating or reducing the goaf water disaster, this paper adopts numerical simulation research methods to optimize the original design scheme and carry out comparative analysis, dynamically reappear the surrounding rock stress field, displacement field and plastic failure law under multi face mining and roadway mining, and carry out engineering practice application. The results show that there is a certain thickness of elastic core area before and after mining with 25m coal pillar width. The deformation of surrounding rock is small, which is conducive to roadway maintenance, without obvious stress concentration. It can meet the actual needs of the project. The mining face has achieved safe mining, without water inrush accident in the goaf, and the coal resources have been recovered to the maximum extent. The research results are left over to similar mining areas in China The safe recovery of coal pillar can be used for reference.

Simulation of the Uniaxial Mechanical Properties and Crack Evolution of Coal Pillar-artificial Dam in Abandoned Mines

The key to the construction of underground reservoirs in abandoned mines is the construction of coal pillar-artificial dams, and the choice of bonding parameters between the coal pillars and artificial dams is the deciding factor that determines the engineering stability. Based on the analysis of the force state of coal pillar-artificial dams, the influence of the interface angle was analyzed. Seven sets of coal pillar-artificial dam specimens were prepared and a PFC3D numerical model was constructed to carry out the uniaxial compression test without lateral pressure. Based on the strength, deformation, and energy evolution characteristics of the coal pillar-artificial dam, the influence of the angle of the coal pillar-artificial dam interface on the performance of the specimen was analyzed. The PFC3D model was used to investigate crack evolution, particle displacement, and spatial distribution. The research results showed that the force state of the coal pillar-artificial dam can be divided into three types: split bearing, shared bearing, and coordinated bearing, corresponding to three different constitutive models. The composite simulation curve showed obvious post-peak viscosity. The compressive strength, peak strain, and average dissipated energy curves of the coal pillar-artificial dam showed a unimodal trend that first increased and then decreased. The total energy and elastic energy of the coal pillar-artificial dam showed an increasing trend during loading. The dissipation energy curve increased obviously in the early stage, then flattened, and finally, decayed. The simulated initiation stress and damage stress of the coal pillar-artificial dam specimens were intermediate to that of the coal pillars and the artificial dams, which first increased and then decreased with the increase in inclination, reaching the peak at 70°. The failures of the single and combined models were both dominated by monoclinic splitting. As the inclination increased, the position of the main cracks gradually shifted downwards and then upwards.