| The problems of high-strength boulders and bedrock strata encountered by urban subway tunnels during shield tunneling are still very common in the southeast coastal areas of China.Surface drilling and pre blasting is one of the effective methods to deal with such problems.However,with the increasing buried depth of urban subway tunnels,the vibration speed control of important structures is becoming more and more strict,and the special slag discharge requirements of shield machines,it is more and more difficult to deal with high-strength boulders,Only the methods of increasing explosive unit consumption,improving hole network parameters or applying delayed blasting can not meet the needs of engineering.Therefore,it is very necessary to study from the perspective of improving explosive energy utilization.This paper creatively proposes to apply the circumferential slotted tube to the problem of blasting pretreatment of boulders,and use the energy accumulation effect of the circumferential slotted tube to control the energy flow density in a specific direction,in order to provide a new idea for solving the above problems.In this paper,theoretical analysis,numerical simulation and model test are used to study the mechanism of circular shaped charge blasting and the application of pretreatment of boulders.The main research contents and results are as follows:(1)Based on the basic equations of detonation wave and shock wave,the energy flow density of shock wave under the constraint of slotted shell and the transmission and reflection at different media are analyzed.It is pointed out that the circumferential slotted tube can theoretically increase the pressure of shock wave acting on the blast hole wall.Combined with the quasi-static effect of explosive gas,the energy accumulation effect of circumferential slotted shell is revealed.(2)By establishing a two-dimensional numerical model of the cross section and vertical section at the circumferential slit,the influence of the circumferential slit tube on the shape of the shock wave and its energy accumulation mechanism are studied.It is found that the shock wave will produce phase difference in the propagation process,resulting in shear stress difference,which reveals the fundamental principle of directional cracking,verifies the energy accumulation effect of the circumferential slit tube and the feasibility of applying it to Boulder Blasting,The influence of radial decoupling coefficient on energy accumulation effect is preliminarily explored.(3)Based on the principles of material similarity,geometric parameter similarity and blasting power similarity,the Boulder model test of circular shaped charge blasting is carried out,and the blasting block screening data of each strength test block under all levels of overburden and different explosive unit consumption are obtained.Through the analysis of screening curve,the influence law of overburden thickness and explosive unit consumption on fragmentation under the action of circular shaped charge,as well as the average fragmentation and the best matching charge under various working conditions are obtained.(4)Based on the least square method,the surface fitting of the test data is carried out by using Origin software,and the polynomial function relationship between the block size of each strength boulder and the unit consumption of overburden and explosive is obtained,and then the best unit consumption standard of explosive for each strength boulder under different thickness overburden is obtained;By comparing the fragmentation distribution and explosive unit consumption of conventional blasting and circumferential slotted tube shaped charge blasting,the control effect of circumferential shaped charge blasting on blasting fragmentation and explosive unit consumption is further verified;It is proposed that the wave impedance ratio of circumferential slit charge to rock mass is the key factor affecting the effect of energy accumulation.The matching coefficient of wave impedance affects the failure time of slotted tube,and then determines the energy utilization rate of explosive explosion. |
