Ground fracturing technology and application of hard roof in large space

The high strength and large caving span of hard roofs results in a strong mine pressure which is a major problem in roof control.Especially in the condition of extra thick coal seam mining,due to the large mining thickness,the hard roofs in the overlying large space would break,and resulting in complex and strong mining pressure in working face.The study shows that the breakage of the hard thick strata (HTS) with a distance more than 100 m above the coal seam is the main factor causing the strong mining pressure in working face,however,there are no existing pre fracturing techniques to be used to control the HTS.For this purpose,the authors put forward a technical proposal on ground fracturing HTS to control the mine pressure for the first time,and the crack propagation law in triaxial in situ specimens were conducted,the propagation pattern of hydraulic fractures and the stress and strain evolution of specimens during the whole fracturing process were revealed.The key range for ground fracturing is given,and by integrating the mining position,fracturing horizon,overburden stress and fracture development,a theoretical model and selection criteria for determining fracturing position were established.The model of fracturing area-flow-time was given,and the relationship between fracturing area and fracturing time and flow rate was obtained.The ground underground combined fractures monitoring technology was developed,and combined with the previous researches,the system of hard roof control by ground fracturing technology was developed and the vertical and horizontal well fracturing engineering practice were carried out.The results showed that the fracture propagation range during vertical well fracturing was large which could reached 218 m and 250 m length,30 m to 120 m width.During horizontal well fracturing,the fracture propagation was 196 to 216 m length,and 43 to 50 m height which covered the area of working face.The underground mine pressure monitoring showed that the support resistance was reduced by 21%,coal wall spalling reduced by 23% when the working face mined into the fracturing crack propagation zone,there was no bending or damage of props,and the surrounding rock in the working face was stable,which proved the great effect of ground fracturing.The study showed that the ground fracturing avoided the strong mine pressure in working face and opens up a new way of hard roofs control in coal mine.The ground fracturing is of great significance to solve the mine pressure disaster caused by the breakage of high-level HTS.