Research On Pre-Split Crack Formation Mechanism Of Water Coupling Presplit Blasting Under High In-Situ Stress

In the presplit blasting of deep underground engineering,the existence of high in-situ stress will cause difficulties in crack development,poor quality of the formed contour,and serious overbreak and underbreak,and then lead to issues such as safety and low efficiency in blasting and mining operations.Water-filled hole is a common engineering phenomenon in the downward hole blasting of underground stope,which has an important effect on the rock blasting.However,the mechanism of crack propagation in water coupling blasting under high in-situ stress is not clear,and there is also no relevant technical solution to be adopted.Therefore,it is of great theoretical significance and engineering value to study the pre-split crack formation mechanism of water coupling presplit blasting under high in-situ stress to control the contour quality and the stability of the deep stope.In terms of the crack propagation of water coupling presplit blasting under high in-stress,this paper uses the methods of theoretical analysis,numerical simulation,physical model test and field test to carry out the research.The results reveal the borehole wall pressure(BWP)characteristics of water coupling blasting,the attenuation law of blasting stress,the mechanism of crack propagation driven by blasting stress and gas-water media under high in-sit stress,and puts forward the optimization scheme of presplit blasting technology under high in-situ stress.The primary work and achievements are as follows:(1)Based on the traditional theoretical formulas,the peak value of BWP in water coupling blasting is calculated.It is clarified that the difference in the characteristics of BWP is the essential reason affecting the blasting stress attenuation,and the decoupled cylindrical charge blasting stress wave attenuation model is established to analyze the influence of the wave source parameters on the stress wave attenuation in the rock.The models of crack propagation driven by the blasting stress and the quasi-static pressure of gas-water medium for presplit blasting under high in-situ stress are established,and the maximum hole spacing for the pre-crack formation under different static loads are calculated.The results show that when the peak value of BWP is the same,compared with the air medium,the water coupling blasting with a lower loading rate of BWP has the characteristics of slower decay of blasting stress and longer initial crack.The static load can inhibit the crack growth.Compared with the air medium,the water medium is beneficial to increasing the presplit hole spacing,and the greater the static load,the greater the advantage of increasing the hole spacing.(2)Based on the cement mortar block,single-hole decoupled charge blasting model tests were carried out.The PVDF piezoelectric film was used to directly collect the BWP signals in air or water coupling blasting with various decoupling ratios.Combined with the finite element simulation results,the traditional calculation formula of BWP was revised,and the influences of decoupling coefficient and filling medium on BWP were analyzed.The results show that the modified power function calculation model can give more accurate peak value of BWP.As the decoupling ratio increases,the ratio of the peak values of BWP between the water and air coupling blasting gradually increases.When the peak value of BWP is the same,the loading rate of BWP in water coupling blasting is smaller than that in air coupling blasting.(3)The blasting strain wave was measured by pre-embedded strain gauges,and the influences of decoupling ratio and filling medium on the blasting stress attenuation index were analyzed combined with the simulation results.Based on the improved numerical model and the response surface method,the individual and interactive effects of peak value of BWP,loading rate and velocity of detonation(VOD)on the attenuation index were analyzed,and the stress wave attenuation model derived in this paper was used to explain the influence mechanism.The results show that the attenuation index has no obvious relationship with the decoupling ratio in air coupling blasting,but decreases with the increase of the decoupling ratio in water coupling blasting.The attenuation index is negatively correlated with the peak value of BWP and VOD,and positively correlated with the loading rate.As the VOD increases,the effects of peak value and loading rate becomes smaller.The stress attenuation index in water coupling blasting is smaller than that in air coupling blasting,especially at low VODs.(4)Based on the blasting test platform coupled with high static load,presplit blasting model tests were carried out.The effects of water medium and static load on pre-split crack,contour flatness,and block damage were analyzed.Combined with the model of crack propagation driven by the quasi-static pressure,the control principle of crack propagation direction,and the dynamic and static combined stress field,the influence mechanism is revealed.The results show that compared with air coupling presplit blasting,water coupling blasting is easier to form pre-split cracks and has better contour flatness.The static load parallel to the direction of blasthole arrangement can promote the formation of pre-split crack,improve the contour flatness,and reduce the block damage.The opposite is true for the static load perpendicular to the direction of borehole arrangement.The greater the biaxial equivalent confining pressure,the harder it is to form pre-split crack,the poorer the contour flatness,but the smaller the block damage.Compared with the presplit holes arranged perpendicular to the maximum principal stress direction,the presplit holes arranged parallel are easier to form pre-split crack and the block damage is smaller.(5)In view of the presplit blasting of the deep stope,three technical schemes are proposed,namely,reducing the hole spacing,arranging the boreholes in the direction of the maximum principal stress,and using water as the filling medium.Based on the numerical model and theoretical model,the presplit hole spacing under the three technical schemes is optimized.The effectiveness and practicability of the relevant optimization schemes are verified by the field test of the presplit blasting of the kilometer-deep stope in Chambishi Copper Mine.This paper contains 140 figures,35 tables,211 references.