Stress Distribution and Failure Characteristics of Stope Overburden of an Inclined Coal Seam

The stress distribution, failure depth, and shape and range of overlying strata of the stope are important bases for the prevention of roof water hazards and determination of reasonable locations of roof roadways. Based on the hydrogeological data of the E9103 workface, FLAC numerical simulation software was used to establish a numerical calculation model of the overlying strata of the E9103 inclined coal seam, and the stress distribution and failure characteristics of the overlying strata were analyzed. The development height of the caving and water-flowing fractured zones in the overlying strata of the workface was determined. Results showed that the stress reduction area appeared above the goaf in the form of an “arched” distribution, and tensile stress occurred in the local area of the overburden. The overburden relief arch of the workface was symmetrically distributed along the advanced direction and asymmetrically distributed along the inclined direction, with the arch crown deflecting above the workface. The horizontal and vertical displacements of the overlying strata of the stope increased with the advancing distance of the workface. The horizontal displacement in the x -direction presented two obvious regions, and the critical points of the two regions moved forward with the advancement of the workface and showed a certain degree of symmetry. The horizontal displacement in the y -direction presented an “inverted bowl” distribution and increased with the advancement of the workface. The main failure forms of the overlying strata of the workface were a tensile and shear failure, and shear failure was dominant in the upper direction. The height of the overburden caving zone in the workface had little relationship with the advancing distance of the workface and increased slowly as the advancing distance of the workface increased. The development height of the caving zone is 7.2–18.13 m. The development height of the water conduction fissure zone increased rapidly with the increase in the advancing distance of the workface. When the advancing distance was equal to the length of the workface, the development height of the water conduction fissure zone was flat and basically maintained at a stable value. The development height of the water conduction fissure zone is 30.8–62.2 m. These research findings have important engineering importance for ensuring safe and efficient mining of E9103 workface.

Numerical Study on the Movement Laws of Overlying Strata in Shallow-Buried Stope Based on the Goaf Compaction Effect

This study presents an integrated approach including the theoretical analysis and numerical modelling to investigate the failure characteristics of the overlying strata in the shallow-buried stope. The mechanical characteristics of the caving zone are first revealed and then calibrated by using the double-yield model. The theoretical results show that the mechanical properties of the collapsed rock mass are closely related to its crushing expansion coefficient and uniaxial compressive strength. The vertical stress of the collapsed rock mass increases slowly with the strain and then increases exponentially after a certain critical strain. The simulation indicates that the fracture zone volume is 1.7-1.8 times that of the caving zone in the 31108 working face, and the failure volume of the overlying strata is 9-10 times that of the stope. The simulated height of the caving zone and fracture zone is 9 m and 20 m, respectively. The comparison between the numerical and field measurement results demonstrates that the new evaluation method using shear-tensile strain behaviors can accurately predicate the height of the two zones. The proposed numerical method could be a viable alternative approach to two zones height calculation.

Study on the Evolution Characteristics of Two-Zone Failure Mode of the Overburden Strata under Shallow Buried Thick Seam Mining

There are risks of water burst and sand inrush in the working face of the Northwest Mining Area in China. Based on the 22407 working face of Halagou coal mine, the evolution characteristics and mechanism of a two-zone failure mode of the overburden strata in shallow buried thick seam mining were thoroughly analysed using physical modelling, theoretical analysis, on-site observation, and other research methods. A method to calculate the overburden fissure width was also proposed. The analysis results indicated that the evolution of a two-zone failure mode of the overburden strata mainly includes four stages: gestation, formation, transformation, and stabilization. In the transformation stage, a fracture zone is transformed into a caving zone. The caving zone and fracture zone are separately transferred to the working face direction based on the structure type of key strata of voussoir beam and cantilever beam after the heights of the two zones stabilize, and the “two-belt” cracks are mainly composed of inclined and horizontal fissures. Based on this study, the mechanism of the two-zone failure mode of overburden strata development was analysed according to the mining height and overburden strata key layer structure. This paper serves as a guide for safe and green mining on shallow buried thick seams.

Deformation and Subsidence prediction on Surface of Yuzhou mined-out areas along Middle Route Project of South-to-North Water Diversion, China

The Middle Route of the South-to-North Water Diversion Project crosses the coal mined-out area in Yuzhou city. There are 4 mined-out areas along Yuzhou section of the water diversion project. The collapse, fracture and deformation of overlying rock mass occurred due to strata loss and stress field change, which then resulted in the subsidence, collapse fissures on the ground. The deformation of the upper rock mass is characterized by “three-zone”: caving zone, fault zone, and bend zone. Thus, it is very important to predict the surface subsidence of the mined-out areas. Highly precise deformation observation network were set along the channel. The monitoring data show that the surface tilt, curvature and horizontal deformation of the mined-out areas were less than critical values. However, the forecast and monitoring values are relatively larger in Guocun and Xinfeng mined-out areas due to large mining range and the possible location of two mined-out areas in the affected region. Therefore, the monitoring should focus on the two mined-out areas.

Patterns of gob-water inrush in Chinese coalmines

Gob-water is a serious hazard in Chinese coalmines. Such inrushes have caused more than 80% of the coalmine water accidents recorded in recent years. This study is based on reviews of past gob-water hazard cases in China. Six inrush patterns are suggested, caused by gobs exposed by tunneling, channeled by faults, or fissure or caving zone channeling, damage to or collapse of water-resistant coal pillars and multi-factor interactions. The results are of relevance in all coal-producing countries wishing to contain and treat gob-water hazards.