Study On Fracture Characteristics And Control Technology Of Thick Hard Sandstone Roof

Thick and hard roof is an important factor that causes strong mining pressure in working face,which seriously threatens safe and efficient mining of coal mine.It is of great practical significance to study the breaking characteristics and mechanism of thick and hard roof and take effective measures to control it.In this paper,aiming at the problem of strong initial pressure in coal seam mining under thick and hard roof,taking the geological conditions of Ⅱ1011 working face as engineering background,the breaking characteristics of thick and hard roof and the movement law of overburden rock in the stope during coal seam mining were studied by means of theoretical analysis,numerical simulation,similarity test and field practice.The fracture mechanics characteristics,overburden fracture characteristics and stress and displacement evolution law of thick hard sandstone roof under the influence of coal mining were obtained,and the hydraulic fracturing technology was proposed to control the fracture and collapse of thick hard sandstone roof,which weakened the pressure strength of the roof and achieves good results.The main conclusions of this paper are as follows:(1)Based on the geological conditions of the working face,the physical and mechanical parameters of the surrounding rock of the working face were measured,the location of the key strata of the overburden was determined,and the structural characteristics of the overburden strata of the stope were revealed.It was concluded that there are five key strata in the height range of 90m above the roof of the working face.The basic fine-grained sandstone with a thickness of 9.4m is the sub-key strata Ⅰ.(2)The mechanical model of thick and hard roof was established,and the critical expression of fracture step was obtained for the initial and periodic fracture of thick and hard roof,which revealed the mechanical characteristics of thick and hard roof.The maximum normal stress σxmax and σymax before fracture of the basic roof increased gradually with the advance of working face,and σxmax>σymax.When the first fracture occurred,the normal stress was symmetrically distributed along the cross section of the midpoint on both sides of the adjacent area.The roof started to fracture from the middle position of the working face and the opening hole and the central goaf.During periodic fracture,the normal stress was symmetrically distributed along the section of the midpoint of the working face,and the roof started to fracture from the middle position of the working face.(3)Based on the fracture mechanics characteristics of thick and hard roof,the failure law of overlying strata in stope and the characteristics of ore pressure were studied in depth,and the evolution law of mining stress and displacement of overlying strata in working face mining process under the condition of thick and hard roof was revealed.(4)The hydraulic fracturing mechanical model of thick hard roof was established,and the theoretical calculation formula of the minimum fracturing pressure and the hydraulic fracturing height of thick hard roof was obtained,which provided the theoretical basis for the hydraulic fracturing of thick hard roof.After the thick and hard roof was cracked by water pressure,the pressure strength was weakened,the breaking of thick and hard roof was effectively controlled,the initial pressure step distance was reduced,and the safe and efficient mining of working face was ensured.Figure[65]Table[9]Reference[92]...