Prediction And Control Of Blasting Damage During The Excavation Of Left Bank Rock Slope In Baihetan Hydropower Project

During the blasting excavation of high rock slope,the rock is fragmented by blasting load and the reserved rockmass will be also unavoidably damaged,resulting in the formation of the so-called Excavation Damage Zone(EDZ).With the elevation of the follow-up excavations decreasing,the slope becomes steeper and steeper,thus the blasting effect on the stability of high rock slope should be strictly controlled.In order to reduce the size of EDZ,the blast parameters and charge structures of blast-holes should be optimized.According to the blasting excavation of the left bank abutment slope of Baihetan Hydropower Project,three blasting tests have been done to determine the blasting parameters and charge structures of blast-holes preliminarily,and the prediction and control of EDZ are studied based on blasting vibration by using the comprehensive research method of combining theoretical analysis,field tests and numerical simulation.In order to provide reasonable blasting parameters to the excavation of the left bank abutment slope of Baihetan hydropower project,three pre-splitting blasting tests are carried out according to the terrain conditions and site conditions,and the parameters of latter test are optimized based on the previous.Through optimizing of blasting parameters and charge structures of blast-holes,the formed pre-splitting crack isolated the propagation of stress wave induced by explosion of main blast holes effectively,thus the damage control effect are good enough that the optimized data can provide a reference for the subsequent slope excavation.Therefore,blasting excavation at the altitude of 834.0?770.0 m on the left bank abutment slope of Baihetan hydropower project was carried out.Through vibration monitoring and acoustic detection,the vibration attenuation rules and damage depth of each bench are obtained,then the effect of pre-splitting blasting is evaluated.The results indicate that pre-splitting blasting has good vibration isolation effect and both the blasting damage and the maximum blasting vibration can be thought induced by pre-splitting blasting.Although the correlation between the extent of blasting damage and mediate-to-far-field PPV is not theoretically rigorous,there is an indirect relationship between them.According to the damage and vibration data of the left bank abutment slope of Baihetan hydropower project,two damage prediction methods that based on monitored vibration,empirical formula prediction method and BP neural network prediction method,are studied through analyzing the relationship between peak particle velocity(PPV)and damage depth.The damage depth of subsequent bench is calculated by using both methods,and the prediction accuracy is high and can meet engineering requirement.It is simple and convenient to predict the damage depth by using these presented methods.This real-time assessing approach can significantly reduce the massive sonic wave testing workload during excavation of large-scale rock slope and greatly improve the work efficiency in similar project.The blasting excavation of the left bank abutment slope of Baihetan hydropower project is simulated by using software LS-DYNA,and the monitored vibration and damage depth are in good agreement with numerical simulation results,which indicates that the numerical simulation is reasonable and reliable.Through analyzing the simulation results,the relation between damage-threshold PPV and damage depth is obtained,and the damage-threshold PPV of the 7th bench is calculated.Based on the vibration attenuation formula proposed by Lu and Hustrulid,the vibration attenuation rule near the blasting area is obtained by regression analysis.Then combining the obtained damage-threshold PPV of the 7th bench with vibration attenuation rule,the damage depth of the 7th bench is calculated and agrees well with the monitored result.Under the condition of constant charge structure,a method to control the blasting damage of rock excavation is proposed based on LS-DYNA simulation,which can provide a reference for the blasting parameters determination of subsequent slope excavation and guide the blasting damage control during rockmass excavation.