Study On The Effect Of Destressing Boreholes On Mechanical Properties And Failure Characteristics Of The Coal And Rock

Rock burst is a severe problem in China’s coal mines,and destressing boreholes is a common method to avoid and control rock burst.Borehole diameter and stress during destressing are essential factors.These factors have a significant impact on the mechanical properties and failure characteristics of coal and rocks.At present,the quantitative relationship study between borehole diameter,stress during destressing and mechanical properties,failure characteristics of the coal and rock is still inadequate.It is difficult to determine borehole diameter and optimum stress during destressing.Based on this,this paper uses experimental research,theoretical analysis and numerical simulation methods to study the mechanical response and failure characteristics of coal and rock after destressing boreholes.The main research results are as follows.(1)The similar material of bursting liability coal rock was obtained.Similar materials specimen are brittle,with a uniaxial compressive strength of 21.22 MPa,dynamic failure time of 56.7ms,elastic energy index of 6.11,and burst energy index of 12.65.The specimen has a strong burst tendency.(2)The ratio of hole diameter to sample width is proposed as index,the shear mechanical properties and failure characteristics of simulative simple after destressing boreholes were studied.And the calculation of plastic zone of surrounding rock was derived,the variation law of plastic zone radius is analyzed.With the increase of boreholes diameter/shear angle,the shear peak strain is gradually reduced,and the shear peak strength decreases linearly,the number of cracks is gradually reduced and the shape is gradually regular,acoustic emission activity becomes inactive.The cohesion force and internal friction angle are quadratic functions with D/W,cohesion decreases gradually,and the angle of internal friction increases first and then decreases.Combined with the elastoplastic theory,the calculation formula of the plastic zone of surrounding rock is obtained,and the evolution law of the plastic zone of surrounding rock is analyzed.(3)The variation law of compressive mechanical properties and failure characteristics of simulative simple was analyzed,the strength influence model of coal rock was established,and the reasonable pressure relief aperture was determined.With the increase of pore diameter,the compressive strength and elastic modulus decrease gradually,while the elastic modulus drop decrease gradually,the crack propagation evolution is more abundant.The influence model of compressive strength of coal rock is established,and the correctness and adaptability of the model are verified.Based on the laws of elastic modulus and modulus reduction,the impact liability of the drilled sample was analyzed.When D/W≥0.16,the drilled sample had no impact liability,the acoustic emission characteristics have a significant mutation.According to the law of D/W and impact liability,plastic zone,compressive strength and acoustic emission,the reasonable D/W is 0.16.(4)The effect of drilling stress pressure relief on mechanical properties and failure characteristics of drilling borehole simulative simple under load was investigated and the optimum drilling stress was determined.With the increase of drilling stress,the elasticity and softening modulus change little,the compressive strength increases first and then decreases.The maximum compressive strength is obtained when the stress during destressing is 0.45σ_c.When the drilling stress is greater than 0.45σ_c,the cumulative ae energy decreases significantly.Considering the mechanical properties,failure characteristics and acoustic emission characteristics of the sample,the optimum drilling stress is 0.45σ_c.