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.

Quantitative Characterization of Overburden Rock Development Pattern in the Goaf at Different Key Stratum Locations Based on DEM

The overburden rock mining fissures are the main cause of coal spontaneous combustion, gas pooling, and mine water inrush caused by goaf air leakage. Rapid and accurate determination of the development and evolution law of mining fissures have great significance for the application of coal spontaneous combustion prevention and control, gas disaster prevention and control, and water damage prevention and control measures. In this paper, a preliminary judgment of the development height of the water-conducting fracture zone is made based on the theoretical analysis, and the physical model size of the numerical simulation is determined according to its judgment result. It is judged that the development height of its water-conducting fracture zone is between 49 and 64.2 m, which is in line with the actual results. Based on this, a three-dimensional solid model was established in PFC (Particle Flow Code) software to analyze the fissure development pattern of the overburden rock and the development height of the water-conducting fracture zone when the main key stratum of the rock seam is in different positions by simulating the excavation process of the coal seam. The results show that when the main key stratum is located in the “original crack belt boundary,” the development of water-conducting fracture zone is significantly inhibited; when the main key stratum is located in the “original caving zone,” the water-conducting fracture zone is fully developed, and the crack belt finally develops to the top of the model. In order to verify the accuracy of the numerical simulation, similar material simulation experiments were performed under the same scheme. The results are consistent with the numerical simulation conclusions, effectively verifying the accuracy of the numerical simulation. Finally, the extraction of porosity of the goaf was carried out based on numerical simulation, and the permeability zoning of the goaf was performed; the results show that the development of the water-conducting fracture zone has a significant influence on the permeability of the mining area, and the more fully developed the fissure is, the greater is its permeability. In this paper, the fissure development law in the goaf under different key stratums is explored by various research stratums, and the results show a good consistency, which provides a scientific basis for the prevention and control of disasters such as water inrush and coal and gas outburst in mines, and provides theoretical guidance for safe mining.

Accurate Perception of Rock Strata Movement for Environmental Protection in Coal Mining: Taking Thick and Hard Roof Cooperative Control as an Example

Accurate perception of the key stratum instability can improve the safety of coal mining and also provide a basis for alleviating overlying rock strata destruction and environmental disturbance. To efficiently evaluate the instability of the key stratum and its threat to safe mining and environmental protection, the fracture characteristics and weakening mechanisms were studied through physical simulation, theoretical analysis, and field measurement. A scheme and the parameters of confined blasting in water-filled deep hole presplit technology (CBWDHPT) for thick and hard roof (THR) weakening were proposed. Research studies showed that, after the THR fractured into large blocks, the subsequent sliding instability induced serious support-crushing accidents; however, increasing the support strength could only provide limited control. Confined water and infiltrated modified rock mass functioned as the transfer load medium of the explosives, and the CBWDHPT fully utilized high explosion energy to break rocks. Consequently, the collapse and filling of the immediate roof and low-positioned THR, as well as the timely cutting off the middle-positioned THR, could be realized, which alleviated the migration space of THR blocks, overlying strata destruction, and earth-surface step subsidence. Finally, the environmentally friendly strategy (including the CBWDHPT and hydraulic support optimization) for overlying rock strata protection was proposed. In the industrial test, the THR was broken into blocks of different sizes after utilizing the CBWDHPT, and the support working resistance was significantly decreased. It was concluded that the environmentally friendly strategy could effectively reduce the risk of overlying rock strata destruction.

Analytical Solution for the Deformation and Support Parameters of Coal Roadway in Layered Roof Strata

In order to meet the security and high-efficiency production needs, high-strength bolt (cable) reinforcement technology is usually used to maintain the stability of roadways. However, due to the great variability of lithology and mechanical properties, the failure form and stability of the layered roof in coal roadways are significant differences. The traditional supporting design method of the layered roof support in coal roadways is the engineering analogy method, which depends on experiences rather than theoretical analysis. Based on the theory of the elastic foundation beam and key stratum, this paper establishes a simplified analytical model of layered roof strata in coal roadways. Based on the Mohr-Coulomb theory, this paper gives the failure criteria of the layered roof strata, and the failure range of the layered roof strata is obtained. The length and pretightening force of bolt (cables) of the layered roof strata can be calculated based on the suspension theory and composite beam theory, which providing a quantitative theoretical basis for the determination of supporting parameters. Finally, as a case, the layered roof strata failure range and supporting parameters of the S1301 auxiliary transportation roadway in Gucheng coal mine are calculated.

Study on the Working Resistance of a Support under Shallowly Buried Gobs According to the Roof Structure during Periodic Weighting

The phenomenon of dynamic pressure in the panel under shallowly buried gobs is obvious, resulting in limited and challenging support type selection. In this paper, theoretical analysis, numerical simulation and field measurement were combined to study the reasonable working resistance of the support in panels under shallowly buried gobs. First, the definition of the equivalent main key stratum (EMKS) was proposed. Then, a method of identifying the structure of the EMKS and broken key stratum blocks was given. The roof structure of the panel under a shallowly buried gob (PSBG) during strong periodic weighting could be divided into 12 types. Mechanical models of the roof structure were established, and the method to calculate the working resistance of the support was given. The Bulianta coal mine and Fengjiata coal mine in the Yushenfu Mining Area were taken as research objects. Based on the measured working resistance curve of the support, the structural morphology of key stratum blocks during strong periodic weighting was distinguished. On this basis, the working resistance of the support was calculated. Finally, FLAC2D numerical software was used to test the working resistance of the support. Based on the subsidence of the roof, horizontal displacement of the coal wall and the development range of the plastic zone in the surrounding rock, the working resistance of the support and adaptability of the surrounding rock control were verified and evaluated. The results show that it is reasonable to calculate the working resistance of the support based on the roof structure during strong periodic weighting. The research results can provide a reference for the scientific and rational selection of the support in a PSBG.

Experimental Study on Physical Similar Model of Fault Activation Law Based on Distributed Optical Fiber Monitoring

The impact ground pressure in coal mining is closely related to the fault structure, and the fault activation pattern is different when the working face advances along the upper and lower plates of the fault, respectively. In this paper, the F16 positive fault in the southern part of Yima coalfield is used as a prototype to carry out the physical similar model test simulating the process of the working face advancing from the upper and lower plates of the fault, and PPP-BOTDA optical fiber sensing technique is used to study the overburden deformation law and fault activation law when the working face is located in the upper and lower plates of the fault, respectively. The study shows that the key stratum breakage is closely related to the fault movement, and the shear stress concentration range occurs within the key stratum. The additional shear stress concentration at the fault surface caused by the working face advancing in the lower plate is much larger than that at the upper plate, which is the reason for the serious fault destabilization phenomenon at the lower plate. The upper rock layer on the fault face is affected by the mining action of the working face before the lower one, and the working face is affected by the fault in a larger range when advances in the lower plate than that in the upper plate, and the risk of fault activation instability occurs earlier when the working face advances in the lower plate than that in the upper plate. The distributed optical fiber sensing technology is used to verify the basic conclusions that the impact of the working face advancing from the lower plate is much greater than that from the upper plate, which is more likely to cause fault activation. The preferential placement of the working face in the upper plate in the fault area will be beneficial to mine pressure control. The results of the study provide an experimental basis for the application of distributed optical fiber sensing technology to the study of fault activation law.

Subsidence prediction of overburden strata and ground surface in shallow coal seam mining

Shallow coal seam with thick soil layer is widely reserved in the Jurassic Coalfield, Western China, mining-induced subsidence represents complex characteristics. Combining with physical simulation, theoretical analysis and in-situ observation, the overburden strata structure in dip direction were revealed, and the subsidence prediction models were established, based on this, the subsidence equations of overburden strata and ground surface were proposed. The results show that after shallow coal seam mining, based on the subsidence and movement characteristics, the overburden strata structure can be divided into three zones, which are "boundary pillar F-shape zone" (BPZ), "trapezoid goaf zone" (TGZ) and "coal pillar inverted trapezoidal zone" (CPZ). The subsidence of overburden strata depends on the key stratum, while the subsidence of soil layer depends on the bedrock subsidence basin, which is between the bedrock and thick soil layer. The bedrock subsidence is mainly related to mining height and bulking coefficient in TGZ, while it is mainly affected by mining height and distribution load on the key stratum in BPZ and CPZ. According to physical simulation and theoretical model, the maximum surface subsidence of No.1-2 seam mining in Ningtiaota coal mine are 1.1 m and 1.07 m respectively, which is basically consistence with the result of in-situ observation (1.2 m).

Fracture Characteristics and Zoning Model of Overburden during Longwall Mining

The fracture characteristics and zoning model of overburden during longwall mining are the basis of coal mine disaster prevention. However, the existing theoretical model is inconsistent with the field measurement. In order to further research into the strata’s fracture characteristics and optimize the overburden’s zoning model, we used the elasticity and Winkler foundation theory to establish first fracture and periodic fracture mechanics models of clamped boundary supported by an elastic foundation with a key stratum as the research object. We analyzed the stress distribution characteristics and fracture evolution pattern of the mining-induced key stratum. We analyzed the zoning characteristics of mining-induced overburden and established the zoning model according to different fracture mechanisms. The results show that the key stratum formed a double “O-X” shaped interconnected fracture zone after the first fracture. The key stratum formed a double “C-K” shaped interconnected fracture zone after the periodic fracture. We divided the mining-induced overburden into three zones along the horizontal direction: the original rock zone, the inverted triangular compression-shear fracture zone, and the trapezoidal tensile fracture zone. The study revealed the mechanism of inverted step fracture in the separation zone, explained the fracture mechanism of the coal pillar support zone, and has significant theoretical value for the prevention and control of water disasters, gas outbursts, and strata movement.

Evolution Law of Gas Discharge of Carbon Monoxide in Mining Extra-Thick Coal Seam of Datong Mining Area

In order to reveal the evolution law of gas discharge of carbon monoxide in mining an extra-thick coal seam of the Datong mining area by the numerical simulation and field monitoring test, the 8202 working face and 8309 working face in the Tongxin coal mine are chosen as the test sites. The results show that the seepage flow of carbon monoxide gas reaches 1.854 × 10 − 8 m 3 / s in the #1 fracture after the #3 key stratum in the far field breaks in the 8202 working face, the seepage flow of carbon monoxide gas reaches 1.307 × 10 − 7 m 3 / s in the #2 fracture, the seepage flow of carbon monoxide gas reaches 4.276 × 10 − 7 m 3 / s in the #3 fracture, the seepage flow of carbon monoxide gas reaches 4.192 × 10 − 7 m 3 / s in the #4 fracture, and the seepage flow of carbon monoxide gas reaches 1.623 × 10 − 7 m 3 / s in the #5 fracture. The initial caving of the #3 key stratum in the far field occurs and collapses to the gob, when the working face in the #3-5 coal seam advances to 180 m, and the voussoir beam forms in the #3 key stratum. Besides, a shower shape was formed by the seepage flow of carbon monoxide gas, and the maximum flow in the working face reaches 4.562 × 10 − 4 m 3 / s . When the 8309 working face advances from 521.2 m to 556.4 m, the air pressure at the working face gradually rises and reaches the maximum magnitude and then begins to decrease; when the working face advances to 556.4 m, the air pressure at the working face reaches the maximum magnitude of 91.35 kPa. The gas discharge disaster of carbon monoxide in mining the extra-thick coal seam of the Datong mining area is effectively controlled by the dynamic balance multipoint control technology. The research results can be treated as an important theoretical basis for the prevention and treatment for carbon monoxide discharge disaster in mining the extra-thick coal seam of the Datong mining area.

Roof structure of shallow coal seam group mining in Western China

In order to realize roof control of shallow coal seam group mining in Western China, combining with engineering statistics, physical simulation and theoretical analysis, the roof weighting characteristics during lower coal seam mining were revealed, and the classification of shallow coal seam group was proposed. Based on this, mechanical models of roof structure were set up, and the calculation method of support resistance was determined. The results show that the roof weighting is closely related to the interburden thickness and the mining height of lower coal seam, considering the ratio of interburden thickness to the mining height, as well as the key stratum structure, the classification of shallow coal seam group was put forward. The first type is shallow coal seam group with no key stratum (SCSG-No), its roof pressure is mainly affected by caving roof of upper coal seam, and the interburden roof forms slanting pillar-beam structure. The second type is shallow coal seam group with single key stratum (SCSG-S), interburden roof represents step voussoir beam structure. The third type is shallow coal seam group with double key strata (SCSG-D), interburden roof can form double key strata structure, the lower key stratum forms slanting step voussoir beam structure, while the upper key stratum forms voussoir beam structure, besides, longwall face represents large—small periodic weighting. Through establishing the roof structure models, the calculation formulas of support resistance were determined, it can provide basis for roof control and promote safe mining in Western China.