Development of 3D top coal caving angle model for fully mechanized extra-thick coal seam mining

During high-intensity, fully mechanized mining of extra-thick coal seam, the top coal would cave to a certain 3D form. Based on the data collected during drilling, a 3D model of top coal caving surface space was established to determine the relationship between the location of the stope roof and the caving surface,enabling the mathematical computation of the top caving angle(φ). The drilling method was employed to measure the top caving angle on two extra-thick fully mechanized coal caving faces under the conditions of three geological structures, namely, no geological structure, igneous rock structure, and fault structure.The results show that the value of top caving angle could be accurately estimated on-site with the 9-parameter 3D top coal caving surface model built with the drilling method. This method is a novel onsite measurement that can be easily applied. Our findings reveal that the characteristics of the coalrock in the two mining faces are different; yet their caving angles follow the rule φigneous rock structure<φno geological structure< φfault structure. Finally, through the data fitting with two indexes(the top coal uniaxial compressive strength and the top caving angle), it is found that the relationship between the two indexes satisfies an exponential decay function.