Study On Crack Evolution Mechanism Of Surrounding Rock By Shaped Charge Blasting In Tunnel Engineering

At present,drilling and blasting method is still the main construction method of tunnel engineering in China.Inadequate control of the conventional drilling and blasting method would easily result in engineering and social issues such as vibration hazard,environmental pollution,instability of surrounding rock,serious over and under excavation,and occupational diseases,which severely impede the tunnel engineering project implementation.The advantages of tunnel shaped charge blasting include reducing the disturbance of surrounding rock,preventing over and under excavation,accelerating construction period,improving the working environment,and presenting a wide range of potential application.Studying the rock-breaking mechanism of shaped charge blasting and seeking a solution to the issue that theoretical research lags behind the engineering practice are crucial from a practical standpoint.This paper systematically investigates the dynamic evolution law of stress wave propagation and crack propagation generated by shaped charge blasting in single-hole,double-hole,multi-hole,and peripheral holes of tunnel ultilizing the research methods of theoretical analysis,model test,numerical simulation,and field application,with a focus on the evolution mechanism and engineering application of surrounding rock crack induced by shaped charge blasting in tunnel engineering.The primary research contents and conclusions are as follows:(1)Research on the shaped charge action principle and the division of surrounding rock fracture caused by shaped charge blasting.This paper analyzes the mechanism of shaped charge with elliptical bipolar linear shaped charge structure collapsing to form shaped charge jet,the distribution characteristics of detonation energy,the propagation law of explosion stress wave,the penetration effect of shaped charge jet on rock mass,and the distribution range of surrounding rock crack under shaped charge blasting load using the theories of explosion mechanics,detonation theory,and impact dynamics.The results demonstrate that under the influence of shaped charge blasting,the rock mass is subjected to the combined action of shaped charge jet penetration,shock wave crushing,stress wave stretching,and quasi-static pressure of explosive gas.The crushing zone,fracture zone,and vibration zone are subsequently formed in the rock mass from close to far along the blasting aperture.Four stages make up the shaped charge blasting process: shaped charge jet penetration,explosion shock wave,stress wave,and detonation gas.A crushing zone forms around the blast hole as a result of the shock wave’s effect,the stress wave drives secondary crack to form,and the quasi-static pressure of detonation gas promotes the crack to continue to widen.(2)Research on optimization of shaped charge structure parameters based on machine learning.The formation of shaped charge jet is analyzed using SPH numerical method,an orthogonal test is designed,and the correlation degree between the structural parameters of shaped charge and the jet head speed and jet length is determined through the gray correlation theory.The support vector machine network regression model is employed,and the parameter optimization is developed using a genetic algorithm.The findings indicate that the structure parameters of the elliptical linear shaped charge have a mostly similar impact on the jet head speed,while they have a varied impact on the jet length.To maximize the performance of the shaped charge jet,it’s crucial to choose the appropriate structure parameters.The parameters of the elliptical linear shaped charge are optimized using a support vector machine network regression model and a genetic algorithm,which can accurately establish the relationship between the structural parameters of the elliptical shaped charge and the jet performance.For different structural parameters of the shaped charge,the performance of the shaped charge could be precisely predicted,which can better assist the structural design of the shaped charge.(3)Study on crack development characteristic of tunnel shaped charge single-hole blasting.Two-dimensional single-hole shaped charge blasting model and three-dimensional shaped charge hydraulic blasting are established,meanwhile,the propagation characteristics of stress wave,crack propagation law,hole wall pressure,and surrounding rock stress distribution of shaped charge single-hole blasting are analyzed.The results show that under the blasting of elliptical linear shaped charge,the penetration damage capacity of the shaped charge direction to the rock is significantly more than that of the non-shaped charge direction.The maximum equivalent stress value of the shaped charge direction near the blast hole is about 2.3times that of the maximum equivalent stress value of the non-shaped charge direction.The shaped charge blasting with different blast hole diameters can create two primary cracks on the left and right in the direction of rock-shaped charge,as well as random secondary cracks in other directions.There is a correlation between the length of the primary crack and the diameter of the blast hole.The main crack is the longest when the blast hole is 85 mm in diameter through curve fitting,whilsthe optimal uncoupling coefficient is 3.45.The shaped charge hydraulic blasting improves the utilization rate of blasting energy,enhances the destruction effect in the vicinity of the blasting source,and reduces the stress wave disturbance in the distant area.It provides a better directional blasting impact than the shaped charge air medium blasting.The impact of shaped charge hydraulic blasting is significantly influenced by different charge structures.The water interval charge structure at both ends of the shaped charge device has a better blasting effect,and the optimal axial uncoupling coefficient is1.36.(4)Study on crack development characteristics of tunnel shaped charge double hole blasting.The numerical analysis model of shaped charge double hole blasting is established,as well as the propagation characteristics of stress wave,the stress state of rock mass,the crack penetration rule of surrounding rock with different hole spacing,and the effect of stress wave superposition on crack propagation are investigated when two holes are simultaneously detonated and millisecond detonated.The results suggest that the length of the crack generated by blasting is limited.Only when the spacing between blastholes is appropriate,a through crack would be formed between adjacent blastholes.A small spacing between blastholes is conducive to the directional propagation of the crack generated by blasting.With the increase of the spacing between blastholes,the peak stress at the center point of the blastholes is gradually reduced and the superposition of stress wave is also weakened.Directional through crack can be formed when the spacing between blastholes is within 100 cm;Under the shaped charge double hole millisecond blasting,the length of the crack generated by the first blasting hole increases with the increase of delay time,the proportion of the crack propagation distance before the stress wave action to the total distance increases in turn,and the crack penetration gradually approaches the blasting hole later.(5)Research on the dynamic evolution law of multi-hole shaped charge blasting around tunnel.The numerical analysis model of shaped charge multi-hole blasting is established,moreover,the propagation law of stress wave,the stress state of rock mass,the propagation and penetration law of surrounding rock crack,and the application of shaped charge blasting in tunnel peripheral holes are explored when the multi holes are simultaneously detonated.The comparative analysis with traditional blasting is carried out,and the blasting scheme of shaped charge peripheral holes is optimized.The findings indicate that the multi-hole shaped charge blasting is the result of the superposition of explosive stress waves and the superposition of quasi-static stress field of explosive gas.The results show that the crack coalescence between holes in the multi-hole shaped charge blasting is the result of the superposition of the explosion stress wave and the quasi-static stress field of explosive gas after the initial crack is formed.The smaller load after the initial crack is formed could realize the initiation and propagation of the initial crack.In practical application,it can reduce the charge in the hole and expand the hole spacing.Based on the engineering practice and numerical simulation,the crack penetration process and the surrounding rock damage attenuation law of the traditional blasting and shaped charge blasting of peripheral holes are analyzed and compared.It is proved that the shaped charge blasting can not only effectively form the blasting contour,but also has fewer secondary crack near the hole wall,which greatly reduces the damage and disturbance to the retained rock mass.Through the optimization of the shaped charge blasting scheme of peripheral holes,thus greatly reducing the number of holes and bringing remarkable economic and social benefits.