The Influence of Heading Rate on Roof Stability in Coal Entry Excavation

Coal entry heading is one of the most hazardous activities in coal mine operations because a certain area of an unsupported roof inevitably forms and poses a significant threat to the safety of miners. In order to accelerate the coal entry heading, a simplified method including theoretical analysis and laboratory and in situ tests was developed to predict the influence of heading rate on the stability of the unsupported immediate roof. The results demonstrate that the deflection of the unsupported immediate roof at the heading face is on a scale of millimetre; hence, monitoring the deformation by conventional observation methods is difficult. The proposed model shows that, within the unsupported immediate roof, the peak values of normal stressesσ_x(perpendicular to the direction of excavation) andσ_y(parallel to the direction of excavation) and shear stressesτ_xz(perpendicular to the direction of excavation) andτ_yz(parallel to the direction of excavation) have different changing trends. The peak values ofσ_xandσ_yboth rise with the increasing advancing distance; however,σ_yreaches the tensile strength within a shorter range thanσ_x. Moreover, the peak values ofτ_xzandτ_yzinitially increase with the increasing advancing distance and then stabilize or decline. The major threat to roof stability at the heading face is tensile failure parallel to the heading direction. According to the industry practices, it is proved that our method can make a good prediction of the mechanical state of the unsupported immediate roof, further deriving the heading rate with a considerable safety margin.

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