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| Theory and technology of controlling hard roof with hydraulic fracturing in underground mining |
| HUANG Bingxiang1，ZHAO Xinglong1，CHEN Shuliang2，LIU Jiangwei1 |
(1. State Key Laboratory of Coal Resources and Safe Mining，China University of Mining and Technology，Xuzhou，Jiangsu 221116，China；2. School of Safety Engineering，China University of Mining and Technology，Xuzhou，Jiangsu 221116，China)
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Abstract The presence of hard roof in underground coal mine is usually accompanied by the characteristics of high hardness，good integrity and huge thickness，which results in numerous problems in ground control and safety. Hydraulic fracturing transforms the roof structure and controls the roof caving above the working face. A framework of theory and technology for controlling hard roof by hydraulic fracturing were put forward. The typical oblate spheroidal morphology and spatial reorientation propagation morphology of hydraulic fractures were revealed. Additionally，the influence of the differences of principal stresses，water injection rates，bedding plane and pre-existing fissures on the propagation of hydraulic fractures were presented by using the experimental system of true triaxial hydraulic fracturing. The principle of the breaking location of overhanging hard roof for directional hydraulic fracturing was identified according to the stress and strain conditions of the surrounding rock and the effect of roof control. The time-space relationship and the determination method of directional hydraulic fracturing for rock mass affected by the mining were presented. The systematic methods of controlled fracturing such as the directional water jet slotting fracturing was suggested to control the crack morphology. On this basis，a complete set of equipment for high pressure(60 MPa) hydraulic fracturing have been developed to implement in underground coal mine. The hard roof caved timely and fully by controlling the extension of the main hydraulic fractures and wing cracks and the wetting effect due to water absorption. It also achieves the weakening of surrounding rock，the stress transformation，and the coal crushing by inducing ground pressure. A complete set of techniques was developed to solve the problems caused by hard roof，including the control of hanging roof in the face end，hanging roof in the opening cut，hanging roof in the central section of working face，hard top coal weakening，gas ultralimit in the initial mining stage of longwall mining with top coal caving，rock burst and large deformation in gob-side entry. The complete set of technology and equipment has been adopted in Datong coal mining area and Shendong coal mining area. Compared to the traditional methods of weakening the hard roof by blasting，the hydraulic fracturing is more advantageous. It is easy to manage，has less disturbance to the surrounding rock，requires less engineering work. It also works at large scale，long-distance control，and costs 10% less than the cost of explosive blasting.
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2017, Vol. 36 
