Research On Pressure-relief Technology Of Drilling Slits And Roof Cutting In Hard Roof Working Face

In the field of coal mining at hard top working face,the impact ground pressure disaster has seriously restricted the safety production of coal mining enterprises.Therefore,it is of great engineering value to shift the stress distribution,improve the top plate riser,and reduce the probability of impact ground pressure events by cutting the top to relieve pressure.Under the background of macro regulation,the traditional roof cutting and pressure relief techniques represented by blasting roof cutting and hydraulic fracturing roof cutting have been widely used,but the traditional roof cutting techniques do not have controllability and is difficult to fulfill the request of coal mine safety production guidelines.Based on the above research background,this paper constructs a mechanical model of coal seam roof damage,establishes a mechanical model of drilling seam,proposes a technical method of measuring ground stress by spatial three-hole hydraulic fracturing method,and finally forms a set of design process of drilling seam cutting and pressure relief at hard top working face,based on the damage form of coal seam roof,the characterization of fracture parameters before and after top cutting,and the measurement of ground stress environment.Specific research findings include the following four areas.(1)Mechanical model of coal seam roof damage.A mechanical model of coal seam damage based on the stacked beam stress potential function is proposed,which divides the damage of the top slab into delamination caused by horizontal joints and fracture caused by vertical joints;and it is found that when the direct top and the indirect top jointly resist compression,the ground stress will be transferred to the rock stratum with greater stiffness.(2)Borehole seam formation mechanical model.Based on the sequence of crack emergence,micro-extension and macro-extension at the hole edge,the fracture parameters are characterized under the corresponding conditions.Specifically,the method of determining the upper and lower thresholds of the hole spacing of borehole seam formation;the method of calculating the stress intensity factor of crack tip under the influence of round hole;the method of calculating the stress intensity factor of common line multi-crack.(3)Spatial three-hole hydraulic fracturing method to measure ground stress.Firstly,according to the assumption of fracture in the direction of maximum principal stress,a basic fracture model is constructed,and according to the stress transformation matrix,the overall ground stress measurement can be completed by arranging the associated three holes;the field measurements show that this method is simple and effective,and is suitable for the ground stress measurement of underground projects with good rock properties.(4)The method of determining the hole spacing of drilled holes.It is considered that tensile stress is the fundamental force for crack emergence,and when the holes are close to each other,the stress concentration at the hole edge is slowed down,which is not conducive to the emergence of cracks.The lower threshold of hole spacing is determined according to the double-hole stress potential function,and the upper threshold of hole spacing is determined by the section bending modulus.The theoretical analysis,laboratory tests,numerical simulation and field measurements show that the cracks can be extended along the direction of the circle center line in the range of hole spacing proposed by this method;it proves the correctness of the design process of cutting and unloading the top of the hard top working face by drilling into slits based on this method.It is expected to provide a design basis for the same type of hardtop working face cutting and decompression scheme.Figure [45] Table [12] Reference [118]...