Study on Pressure Relief Technology of High-Pressure Water Jet of Residual Coal Pillar in Overlying Goaf in Close Seam Mining

To solve the problem of strong ground pressure behaviour under a residual coal pillar in the overlying goaf of a close-distance coal seam, this paper proposes the technology of weakening and relieving the residual coal pillar in the overlying goaf by a high-pressure water jet. Based on the geological occurrence of the No. 3 coal seam and mountain No. 4 coal seam in the Yanzishan coal mine, the high-pressure water jet pressure relief technology of residual coal pillars in the overlying goaf of close-distance coal seams was studied by theoretical analysis and field industrial tests. First, the elastic-plastic zone of the residual coal pillar and the stress distribution law of the floor are obtained by theoretical analysis, and the influence degree of the residual coal pillar on the support of the lower coal seam working face is revealed. Then, a high-pressure water jet combined with mine pressure is proposed to weaken the residual coal pillar. Finally, through the residual coal pillar hydraulic cutting mechanical model and “double-drilling double-slot” model, the high-pressure water jet drilling layout parameters are determined, and an industrial field test is carried out. The single knife cutting coal output and 38216 working face hydraulic support monitoring data show that high-pressure hydraulic slotting can weaken the strength of the coal body to a certain extent, destroy the integrity of the residual coal pillar, cut off the load transmission path of the overlying strata, and reduce the working resistance of the hydraulic support under the residual coal pillar to a certain extent, which is beneficial to the safe mining of the working face.

The field monitoring experiment of the high-level key stratum movement in coal mining based on collaborative DOFS and MPBX

The deformation and movement characteristics of high-level key stratums in overlying strata are important for estimating ground subsidence and understanding failure characteristics of ultrathick strata during mining. In this study, a distributed optical fiber sensor (DOFS) and multipoint borehole extensometers (MPBXs) were collaboratively employed to monitor the deformation of high-level key stratums in situ during the mining process at working face 130,604 of the Maiduoshan Coal Mine. DOFS monitoring results showed that the distance from advance influence of mining on the ground surface is 219.2 m. The deformation of the shallow stratums were greater and was affected earlier than that of the deep stratums. The deformation in the strata did not occur continuously and the boundary curve of the impact from advance mining was not a straight line with the advancement of the working face. By the MPBX technology, we measured the strata movement and obtained four-stage characteristics of high-level key stratum movement. The subsidence of the primary key stratum and the sub key stratum were monitored to reach 1389 and 1437 mm; their final relative displacement differed by 48 mm. No bed separation was observed in between the strata, and the key stratums tended to sink as a whole with the advancement of the working face. This research guides the analysis the movement of thick high-level key stratums.

Analysis of coal face stability of lower coal seam under repeated mining in close coal seams group

Aiming at the problem of coal face failure of lower coal seam under the influence of repeated mining in close coal seams, with the working face 17,101 as a background, the coal samples mechanics test clarified the strength characteristics of the coal face under repeated mining, through similar simulation experiments, the development of stable roof structure and surrounding rock cracks under repeated mining of close coal seams are further explored. And based on this, establish a coal face failure mechanics model to comprehensively analyze the influence of multiple roof structural instabilities on the stability of the coal face. Finally, numerical simulation is used to further supplement and verify the completeness and rationality of similar simulation experiment and theoretical analysis results. The results show that: affected by repeated mining disturbances, the cracks in the coal face are relatively developed, the strength of the coal body is reduced, and the coal face is more prone to failure under the same roof pressure; During the mining of coal seam 17#, the roofs of different layers above the stope form two kinds of "arch" structures and one kind of “voussoir beam” structure, and there are three different degrees of frequent roof pressure phenomenon, which is easy to cause coal face failure; Under repeated mining of close coal seams, the roof pressure acting on the coal face is not large. The main controlling factor of coal face failure is the strength of the coal body, and the form of coal face failure is mostly the shear failure of soft coal. The research results can provide a theoretical basis for coal face failure under similar conditions.

A Novel Method of Monitoring Surface Subsidence Law Based on Probability Integral Model Combined with Active and Passive Remote Sensing Data

For the accurate and high-precision measurement of the deformation field in mining areas using different data sources, the probability integral model was used to process deformation data obtained from an Unmanned Aerial Vehicle (UAV), Differential InSAR (DInSAR), and Small Baseline Subset InSAR (SBAS-InSAR) to obtain the complete deformation field. The SBAS-InSAR, DInSAR, and UAV can be used to obtain small-scale, mesoscale, and large-scale deformations, respectively. The three types of data were all superimposed by the Kriging interpolation, and the deformation field was integrated using the probability integral model to obtain the complete high-precision deformation field with complete time series in the study area. The study area was in the WangJiata mine in Western China, where mining was carried out from 12 July 2018 to 25 October 2018, on the 2S201 working face. The first observation was made in June 2018, and steady-state observations were made in April 2019, totaling four UAV observations. During this period, the Canadian Earth Observation Satellite of Radarsat-2 (R2) was used to take 10 SAR images, the surface subsidence mapping was undertaken using DInSAR and SBAS-InSAR techniques, and the complete deformation field of the working face during the 106-day mining period was obtained by using the UAV technique. The results showed that the subsidence basin gradually expanded along the mining direction as the working face advanced. When the mining advance was greater than 1.2–1.4 times the coal seam burial depth, the supercritical conditions were reached, and the maximum subsidence stabilized at the value of 2.780 m. The subsidence rate was basically maintained at 0.25 m/d. Finally, the accuracy of the method was tested by the Global Navigation Satellite System (GNSS) data, and the medium error of the strike was 0.103 m. A new method is reached by the fusion of active and passive remote sensing data to construct efficient, complete and high precision time-series subsidence basins with high precision.

Reasonable Layout of High Drainage Roadways for Jiaojiazhai Mine

To ensure that the gas concentration at the top corner does not exceed the limit, a reasonable level of the high drainage roadway layout in Jiaojiazhai Mine should be determined. In this work, based on the actual conditions of the working face, an SF6 tracer gas was used to test the connectivity between the high drainage roadway and the working face. A discrete element analysis program was used to simulate the deformation law of the overlying strata in the goaf, and a corresponding caving control program for the surrounding rock was written based on the obtained parameters and “O” ring theory. A fluid simulation software was used to simulate and analyze five goaf models with different high drainage roadway layouts (10, 15, 20, 25, and 30 m). The gas drainage data for two layers (10 m and 20 m) of the high drainage roadway were measured. The results showed that the height of the caving zone in the goaf is approximately 20 m, and when the high drainage roadway is arranged along the roof (when the layout layer height is 10 m), the roadway will be directly connected to the working face, thus pumping fresh air to the working face. The gas extraction effect of the 20 m stratum was better than those of the other strata. The simulation results of the gas extraction were consistent with the measured data. The proposed scheme was practically applied, and its effect was found to be evident, thus solving the problem of high gas concentration at the top corner and increasing the mine output.

Study on the Bearing Characteristics and Application of the Filling Body in Original Roadway Filling and Nonpillar Driving

Original roadway filling and nonpillar driving can effectively solve the difficulty facing mining replacement in the stope of deep mines. As the bearing characteristics of the filling body in the original roadway play a crucial role in the structural stability of the overlying strata, with the recovery and geological conditions of 62210 working face in Xinzhuangzi Coal Mine, Huainan Mining Group, China, as the background, this study analyzed the stability characteristics of the filling body in the original roadway through comprehensive research methods of theoretical analysis, laboratory tests, and onsite monitoring. The results disclose that the filling body in the original roadway should boost early strength, strong bearing capacity, and long-term weakening. When the water-cement ratios are 1 : 1, 1.5 : 1, 2 : 1, 2.5 : 1, and 3 : 1, the strengths of the filling body are 1.12 MPa, 0.93 MPa, 0.57 MPa, 0.33 MPa, and 0.21 MPa at 2 h and 5.63 MPa, 4.66 MPa, 2.87 MPa, 1.65 MPa, and 1.02 MPa at 48 h, respectively. The strengths surge by 5 times within 2 d on the whole and reach the maximum at about 7 d, i.e., 8.12 MPa, 6.91 MPa, 6.60 MPa, 3.95 MPa, and 2.20 MPa, respectively. As time goes, the water content of the filling body gradually decreases and the compressive strength plunges. This demonstrates that the rapid solidification material with a high water content can satisfy the requirements of the bearing characteristics of the original roadway filling body. With reference to numerical simulation and the data monitored onsite, it can be known that the filling body in the original roadway can support the roof effectively and control the surrounding rock deformation of newly excavated roadways in the lower section. The research results provide theoretical guidance for coal mining under similar geological conditions and serve as reference for safe and efficient coal mining.

Law of coal caving behind the flexible shield support in pseudo-inclined working face

Complex boundary conditions are the major influencing factors of coal caving law in the pseudo-inclined working face. The main purpose of this study is to analyze coal caving law of flexible shield support and then to establish the internal relations among coal caving parameters under complex boundary conditions. Firstly, the law of coal caving in different falling modes is simulated physically. Secondly, the coal caving shape, displacement field, and contact force field is simulated. Then, coal caving law and process parameters is analyzed theoretically. Finally, the test was performed in Bai-Ji Mine. The research shows that ellipsoidal ore drawing theory has universal applicability in coal drawing law analysis and parameter optimization. After the Isolated Extraction Zone and Isolated Movement Zone reach the roof, the expansion speed is marked by a short delay, and then, while expanding to the floor, two butted incomplete ellipsoids are formed. There is a time-space difference in coal caving after the support, and some coal will be mined in the next round of coal caving. There are obvious differences in the coal loosening range, displacement field, and contact force field on both sides of the long axis. When the support falls along with the bottom plate, it is more conducive to the release of coal. The test shows that the research is of great significance for optimizing the caving parameters of flexible shield support in the pseudo-inclined working face of the steep seam.

The Research on Stability of Surrounding Rock in Gob-side Entry Driving in Deep and Thick Seam

The gob-side entry driving is driving in low pressure area, which bears less support pressure and is easy to maintain, so it is widely used. Taking the gob-side entry driving in thick coal seam of Dongtan Coal Mine as an example, the reasonable size of pillar and the section of roadway are numerically simulated by combining numerical with measurement, and the roadway support is designed. According to the distribution of lateral stress in working face, eight pillars of different sizes are designed. By simulating and comparing the stress distribution of surrounding rock and the development range and shape of plastic zone in different positions, the pillar size of gob-side entry driving is optimized to be 4.5m. According to the results of optimization of roadway section, the section of straight wall semi-circular arch roadway is adopted. According to the analysis, the roadway is supported by bolt + steel mesh + anchor cable. By observing the stability of roadway, it provides experience for the stability study of roadway the gob-side entry driving with small pillar in thick seam.

Influence of Roadway Cross-Section Shape on Gas Explosion Shock Wave Law in U-Type Ventilation Working Faces

In U-shaped ventilation working face, different tunnel section shapes are one of the important factors affecting the propagation of gas explosion shock wave. In order to study the propagation law of gas explosion shock wave in working face, the numerical simulation study was carried out by using Fluent simulation software combined with the actual situation of gas explosion in #415 working face of Chenjiashan Coal Mine in Shaanxi Province. By constructing a three-dimensional mathematical and physical model, a simulation study of the upper-corner gas explosion was carried out. The results are described as follows. (1) After the gas explosion shock wave propagates 40 m, the overpressure peak equidistant difference tends to be stable and attenuates and propagates in the form of a single shock wave. The study determines that the effective length of the U-shaped ventilation inlet/return tunnel is 40 m. (2) When the tunnel section is trapezoidal, the initial overpressure of the gas explosion shock wave propagating to the inlet/return airway is the highest, followed by rectangular and semicircular arches, but the internal overpressure attenuation trend of different cross-sectional shapes is the same. (3) The gas explosion shock wave propagates radially along the working face section during the working face propagation. The farther away the location is from the upper corner of the tunnel during a gas explosion with different cross-sectional shapes, the closer the cutoff overpressure peak is. The attenuation trend of overpressure with the propagation distance conforms to the power function law. The research results provide an important theoretical direction for the numerical simulation of gas explosions in coal mining faces.