Fracture characteristics of basic roof and mechanism of strata behavior in a pillarless working face

As a research hotspot, pillarless coal mining technology has a high resource recovery rate and low roadway surrounding rock stress. To grasp the three-dimensional fracture characteristics of the basic roof is the basic to reveal the strata behavior mechanism in the pillarless working face. Thus, aiming at pillarless coal mining, the analytical solution of three-dimensional fracture mechanics models of a basic roof was analyzed by elastic thin plate theory; the principal stress distribution of a basic roof being cut by continuous artificial fractures and discontinuous artificial fractures was analyzed; the fracture characteristics of the basic roof was revealed and the strata behavior mechanism was obtained. The following conclusions can be drawn: (i) an ‘O + X’ fracture occurred in the basic roof of a traditional working face, while an opposite-trapezoid-shaped or ≡-shaped fracture was generated in the pillarless working face. (ii) When the basic roof broke, the trapezoidal or rectangular hinged plate traversed the entire pillarless working face, causing the end supports to be pressured, while the trapezoidal hinged plate did not traverse the entire traditional working face and the end supports were not pressured. (iii) The break of a basic roof induced by artificial fractures in pillarless mining reduced or even eliminated the triangular hinged plate area along the goaf edges, making a roadway in the stress relief zone. (iv) Compared with the fractures in traditional roadways and in a discontinuous roof-cutting roadway, continuous fractures could minimize surrounding rock stress and make it easy to maintain a roof-cutting roadway.

Roof Fracture Characteristics and Strata Behavior Law of Super Large Mining Working Faces

Exploiting the working face in coal mines using a super long mining length and large mining height has become important for intensive production with high yield and high efficiency. The paper develops a roof structure model to analyze the influence of 195 m, 242.4 m, and 376 m working face lengths at large mining height in Wangzhuang Coal Mine in China as the case study. The roof fracture characteristics, migration law, and strata behavior law under different working face lengths are compared and studied by numerical simulation, and the reliability of support selection in the working face at large mining height is analyzed by field measurement statistics. The results show that the roof fracture mode of a super large working face is a successive layered fracture. The length of the working face has little effect on the roof fracture step length, and the fracture step length is positively correlated with the thickness of the rock stratum. The roof subsidence law for a super large working face is different from the intermittent subsidence of the unimodal Gaussian distribution curve of ordinary working faces, which shows the intermittent subsidence of multiple ordinary working faces. The roof periodic weighting of a super large working face, which fluctuates violently within 100 m at both ends, is more drastic than that of an ordinary working face as a whole. Field statistical analysis shows it is more appropriate to choose high-strength support for a super large working face.

Mechanism of Support Optimization and Confined Blasting of Thick and Hard Rock with a Wedge-Structure Immediate Roof: A Case Study

Based on the occurrence conditions of a thick and hard main roof and wedge-structure immediate roof in the Zhuxianzhuang Coal Mine, the fracture characteristics and instability migration law of a thick and hard roof (THR) were examined via physical simulations. Mining zones were divided with respect to the strata behaviors and roof control difficulty levels, and the principles and methods of zonal control under THR were put forward. This study proposed a coordinated control strategy of using confined blasting in water-filled deep holes, and reasonable support optimization, which could effectively reduce the roof fracture size, increases the supporting intensity and eliminate roof-control disasters. The length of confined blasting blocks and supporting intensity were calculated using a mechanical model for roof control in the strong strata behavior zone and less-strong strata behavior zone. These key parameters were determined as 20–25 m and 1.15–1.28 MPa, respectively, and the mining strategy was successfully applied in working face 880, performing high security and reasonable economical efficiency.

Effect of Face Length in Initial Mining Stage of Fully Mechanized Top-coal Caving Face With Large Mining Height

Based on the field measurement of the end resistance of the support during the initial weighting of the basic roof and the macroscopic mine pressure behavior during the weighting period of 101,22211,103 and 301 fully mechanized caving face in Changchun Xing Coal Mine, the mine pressure law of the working face is summarized and compared, and the relationship between the working face length and the working resistance of the support ( the weighting strength ) and the macroscopic mine pressure behavior is obtained. In the range of face length 126-230 m, with the increase of face length, the end-of-cycle resistance of the support gradually increases and the dynamic load coefficient of the support gradually increases when the coefficient of the support gradually increases, and the strata behavior of the working face changes from strong to very strong. When the face length is short ( 126-140.5m ), the hanging top area is too large to cause hurricanes when the working face is pressed, which threatens and damages the personal safety and equipment of the working face staff. Based on the above research, the problem of optimizing the surface length is proposed, and the surface length is determined to be within the range of 140-230 m according to the measured results.

Failure Mechanism and Control of Lower Retracement Channel in Close-Distance Double-Thick Coal Seams

Under the condition of close-distance double-thick coal seams’ mining, the serious strata behavior occurred in the lower retracement channel, which is affected by the double disturbance with upper coal-pillar static load and mining dynamic pressure. Field investigation, laboratory test, and numerical simulation were used to study the failure characteristics and stress distribution of N0381 retracement channel. The results show that the plastic failure and stress of the surrounding rock are obviously asymmetric; specifically, the closer the roof is to N0381 gob, the stronger the strata behavior is. In addition, when the upper coal pillar is in critical stable or stable state ( w / h > 2 ), high stress concentration in the upper coal pillar is transmitted downward through floor rock so that two dangerous areas with severe strata behavior are formed in N0381 retracement channel. In view of this, a partition control strategy with “high-pressure water jet + asymmetric high-strength cable-beam net + three-hole anchor cable group + roof grouting” as the core is proposed to ensure the stability of the lower retracement channel. This study provides a reference for coal mines with similar production geological conditions.

Study on strata behavior law of deep inclined coal seam

In order to understand and grasp the law of roof pressure on the working face of deep inclined coal seams, the law of support resistance distribution, the law of leading support stress distribution and the law of surrounding rock deformation of the two roadways, the 94101 working face of Zhangshuanglou Coal Mine was taken as the engineering background. Through a combination of field measurement, numerical simulation, theoretical analysis, etc, this paper analyzes the laws of roof migration and rock pressure manifestation in deep inclined coal seams.

Strata behavior and control strategy of backfilling collaborate with caving fully-mechanized mining

This article analyzes the technical difficulties in full-section backfill mining and briefly introduces the technical principle and advantages of backfilling combined with caving fully mechanized mining (BCCFM). To reveal the strata behavior law of the BCCFM workface, this work establishes a three-dimensional numerical model and designs a simulation method by dynamically updating the modulus parameter of the filling body. By the analysis of numerical simulation, the following conclusions about strata behavior of the BCCFM workface were drawn. (1) The strata behavior of the BCCFM workface shows significant nonsymmetrical characteristics, and the pressure in the caving section is higher than that in the backfilling section. φ has the greatest influence on the backfilling section and the least influence on the caving section. C has a significant influence on the range of abutment pressure in the backfilling section. (2) There exits the transition area with strong mine pressure of the BCCFM workface. φ and C have significant effect on the degree of pressure concentration but little effect on the influence range of strong mine pressure in the transition area. (3) Under different conditions, the influence range of strong mine pressure is all less than 6 m. This article puts forward a control strategy of mine pressure in the transition area, which is appropriately improving the strength of the transition hydraulic support within the influence range (6 m) in the transition area according to the pressure concentration coefficient. The field measurement value of Ji15-31010 workface was consistent with numerical simulation, which verifies the reliability of control strategy of the BCCFM workface.

Safe and Efficient Coal Mining Below the Goaf: A Case Study

Mining at the fully mechanized working face below the goaf of the short-distance coal seam is influenced by the upper goaf. To address this problem, methods such as theoretical analyses, numerical simulation, and on-site measurement are used to study the strata behavior characteristics of the Ningxia Lingxin Coal Mine 051508 working face in this study. The roof weighting intervals of the working faces below the goaf and the non-goaf are obtained via theoretical calculations. The stoping processes of the working faces below the goaf and the non-goaf are simulated with FLAC3D to obtain the distribution law of the bearing pressure and plastic zones before the working face. Based on the statistical analysis of the measured working resistance of the supports and its distribution, the roof weighting interval of the working face mining below the goaf is obtained. The results show that the roof weighting interval and the advanced abutment pressure during mining at the working face below the goaf are smaller than those below the non-goaf, providing a reasonable theoretical basis for mining below the goaf, and having important significance for safe and efficient mining.