Study On Blasting Cracking Mechanism And Blasting Forming Control Method Of Jointed Rock Mass

Tunnel blasting excavation in rock mass is needed for water resources development,infrastructure construction and mineral resources exploitation.Joints are widely distributed in the natural rock mass.The existence of joints makes the propagation of explosive stress wave during tunnel blasting excavation extremely complicated.This affects the blasting cracking mode of rock mass and the blasting excavation forming effect of contour surface.Therefore,it is of great significance to study the blasting excavation effect of jointed rock tunnel contour surface and ensure the safety of tunnel construction.In this paper,the propagation characteristics of explosion stress wave and the cracking mode of rock mass under single-hole and double-hole initiation conditions of jointed rock mass are studied by means of theoretical analysis and numerical simulation.The effects of joint geometric parameters and strength parameters on the propagation of explosion stress wave and the cracking mode of rock mass are analyzed.The influence of hole spacing,air interval charge structure and radial decoupling charge coefficient on the blasting forming effect of jointed rock mass contour blasting is studied,and the control method of blasting excavation forming of jointed rock mass tunnel contour surface is proposed.The main research results are as follows :(1)Jointed rock mass plays a role in blocking and reflecting the propagation of explosion stress wave,The larger the joint width and the weaker the strength parameter,the stronger the barrier and reflection effect on the explosion stress.The reflected stress wave is superimposed with the incident stress wave.The larger the joint width and the weaker the strength parameter,the greater the tensile stress in front of the joint in the horizontal direction along the center of the blast hole.With the joint inclination angle from 30° to 90°,the tensile stress increases.Under the action of explosive stress wave,the tensile stress on the back wave side of the joint end will increase.The wider the joint width,the weaker the strength parameters,and the smaller the tensile stress on the back wave side of the joint end;the greater the joint dip angle,the greater the tensile stress on the back wave side of the joint end.(2)In the case of double-hole blasting,the superposition of explosion stress wave will lead to the increase of tensile stress in the direction perpendicular to the center of the hole.The existence of joints has an effect on the superposition of explosion stress waves: the larger the joint width and the weaker the strength parameters,the greater the tensile stress in the direction perpendicular to the center of the borehole;when there is an angle between the center line of the borehole and the joint,the tensile stress at the midpoint of the center line of the borehole increases with the increase of the joint angle,and the tensile stress is the largest at 90°.(3)The barrier effect of joints on explosion stress blocks the propagation of blastinduced cracks,and there is no crack in the rock mass behind the joints.The propagation of blast-induced cracks at the end of the joint plays a guiding role,and the joint will cause the blast-induced cracks in the rock mass to propagate along the dip direction of the joint.The reflection of the joint to the explosion stress wave increases the degree of cracking and fragmentation of the rock mass between the blast hole and the joint,and the tensile cracks become more.The larger the joint width,the weaker the strength parameter,and the higher the degree of rock fragmentation between the blasting hole and the joint.In the case of double-hole blasting,when the joint inclination angle is 45° and 60° from the center of the double-hole,the cracks between the holes cannot penetrate,and the rock between the holes is completely penetrated at90°.When the angle between the hole connection and the joint is 45°,the rock mass along the direction of the joint inclination angle has the largest cracking range and the most sufficient crushing.(4)For the blasting of the contour surface of the jointed rock tunnel,with the increase of the spacing of the surrounding holes,the amount of over-excavation decreases,resulting in the failure of the rock between the holes;in the case of different air interval charges,with the increase of the number of air interval segments,the amount of overbreak decreases,and the rock between the surrounding holes can be penetrated without underbreak;in the case of different radial uncoupling charge coefficients,with the increase of radial uncoupling coefficient,the overbreak amount gradually decreases;a better tunnel profile can be obtained by appropriately reducing the spacing of surrounding holes,increasing the number of air intervals and increasing the radial uncoupling coefficient.