Experimental Study On Dynamic Responses Of Rock-like Medium With Multi Prefabricated Cracks Under Explosive Load

In the tunnel blasting construction,there are a lot of joints,cracks and other defects in the surrounding rock.When the structural plane of the rock mass and the design contour are oblique,the phenomenon of over excavation and under excavation often occurs,which seriously affects the blasting effect and stability of the tunnel.In this paper,a new type of digital laser dynamic caustics test system is used to simulate the conditions of two holes and single-hole blasting around the tunnel.Four kinds of test models are designed respectively to analyze the effects of different inclination angles and spacing of the prefabricated cracks on the explosion stress wave and the explosion gas.The final blasting effect of the test sample and the dynamic parameters changed law of the crack tip are studied,such as the crack initiation angle,the propagation speed,stress intensity factor and energy release rate of crack tip.Summarize the dynamic response of crack under the action of explosion stress wave and explosion gas.Reduce over excavation and under excavation in blasting engineering under the guidance of theory.The main conclusions of this paper are as follows:(1)In the early stage of crack propagation,the crack tip is mainly subjected to explosive stress wave.Under the action of explosive stress wave,the medium is often brittle.The dynamic parameters tend to increase first and then decrease.And the dynamic parameters of crack tip are higher than those of the dynamic parameters of crack tip under the action of explosive gas.In the later stage of crack propagation,the crack tip is mainly affected by the quasi static pressure of the explosive gas,resulting in the second crack propagation.Under the action of explosive gas,the medium is quasi-brittle,and the dynamic parameters of the crack tip change in an oscillatory way.The action of explosive gas can be divided into steady state and unsteady state.(2)According to the second propagation of crack and the action time of explosive load,the dominant action of explosive stress wave and explosive gas is simply distinguished.In a period of time before the second crack propagation,the area of the crack’s caustics only changes,but the position changes little,that is,the crack does not expand,and the second crack propagation time is 100μs later.Therefore,according to the second crack propagation and time distinguish the explosive stress wave and the role of the explosive gas.The explosive stress wave plays a major role before the second crack propagation,and the explosive gas plays a leading role after the second crack propagation.(3)Under the condition of single-hole blasting,among the factors affecting the final propagation length of the derived crack,the influence of the inclination angle of the prefabricated crack is greater than that of the crack spacing.Under the influence of the guidance of adjacent prefabricated crack or empty hole,the derived crack will start and expand along the direction of the minimum resistance line or the direction of the empty hole,and the derived cracks will intersect and run through in a "zigzag" shape;(4)Compared with the single-hole blasting,the dynamic parameters such as crack propagation speed,stress intensity factor and energy release rate are higher under the condition of double holes blasting.The results show that the stress wave and explosive gas produced by the other side of the blast hole will at least double to enhance the stress field at the crack tip,and the energy gathered at the crack tip is also higher;(5)In the actual blasting process,when there are multiple cracks between the blast holes,the distance between the peripheral holes can be appropriately shortened.Or the angle between the blast holes line and the joint crack can be increased within a certain range,so that a better fracture range will be formed between the main crack and the joint crack,which is closer to the design contour line;when there are empty holes between two adjacent blast holes,empty holes more conducive to the penetration of surrounding holes,improves the smooth blasting effect,and then effectively controls the phenomenon of over excavation and under excavation.