The hard roof in underground coal mines is generally considered as a main cause that induces disasters in working faces and entries, such as rock burst and windblast. Hydraulic fracturing technology is an effective approach to weaken hard roof to reduce ground pressure and control rock burst. The state and achievements associated with hydraulic fracturing technology applied in Chinese coal mines are summarized in this paper. Theories related to hydraulic fracturing are of the essence in fracture design to determine the fracture initiation and propagation pressure and predict the geometry and the characteristics of hydraulically induced fractures. Initially, theory of elasticity was employed to determine the breakdown pressure for a vertical borehole where the rock medium is assumed to be elastic. However, porosity and pore fluid have an influence on the borehole breakdown pressure, and then the poroelasticity theory was applied. Arbitrary deviated boreholes in practice are rather common and the maximum tensile stress criterion was employed to determine borehole breakdown pressure. The effect of natural crack in the rock medium on the initiation pressure has been taken into consideration through fracture mechanics. Meanwhile, considerable achievements have been made on hydraulic fracturing tools and equipment, and a set of mature hydraulic fracturing technologies for hard roof control are available. Some comprehensive investigations on the hydraulic fracturing for hard roof weakening are presented, and successful field applications in the Jincheng and Luan coal mining districts are introduced. Finally, the unsolved problems related to hydraulic fracturing are also put forward.
The roof composed of hard, thick and intact strata is usually considered as the hard roof, which is quite common in Chinese coal mines. Hard roof generally cannot cave in time with advance of working face, which will lead to extensive overhanging areas in the gob, and strong loading on supports in working faces and entries. Furthermore, hard and thick rock is frequently conducive to accumulation of vast elastic energy resulting in dynamic disaster, such as coal and rock burst in deep coal mines (Shi, Ning, & Wei, 2003). Thus, forced caving technologies for hard roof are always needed. Conventionally, blasting in hard roof is commonly employed. However, blasting implemented in hard roof in coal mines can result in numerous problems, such as a lot of boreholes drilled and consumption of explosive, generation of mass carbonic oxide polluting mine air, triggering gas and coal dust explosion. Hydraulic fracturing is recognized as an efficient forced caving technology for hard roof, and the problems caused by blasting can be avoided.