Study On The Fracture Expansion Of Radial Precast Cracks Under Explosive Loads In The Underground Caverns And Its Application

Notched blasting can make the rock crack along a certain direction,which can lead to a good result and obvious economic benefits.Therefore,it is of great significance to study the blasting mechanism and related parameters of the notched blasting.Because of the deep underground cavern,some structures(such as rock anchor beam)of the blasting excavation requires construction in fine scale.The requirements of protection on remain rock is relative high.Therefore it is necessary to control the blasting construction,which will crack along the predetermined direction,so as to control the surrounding rock damage in a certain range.This paper based on the blasting excavation engineering of rock anchor beam of underground main powerhouse of Baihetan Hydropower Station.To analyze the mechanism of notched blasting and the influence of blasting parameter change on notched blasting,a two dimensional plane strain model with multi-slot blasting holes is build.The explicit dynamic finite element analysis program ANSYS/LS-DYNA is used to calculate the stress distribution and changes of the rock around notched blasting hole in the dynamic blasting load.Notched blasting dynamic cracking mechanism is analyzed by the fracture mechanics and damage mechanics.Mmeanwhile,influence of blasting parameters on notched blasting is discussed.In this paper,the effect of the notched blasting and the damage mechanism are studied by investigating the actual effect of rock bolt beam grooving blasting and the ultrasonic testing of the surrounding rock.The main conclusions are as follows:(1)Through analyzing the characteristics of the process of stress change of the surrounding rock mass in single notched blasting hole and two notched blasting holes by the numerical simulation,the process of crack formation and crack coalescence are studied.The results show that the stress distribution of the rock mass on the borehole wall is uneven and the effect of stress concentration in the notch is in the process of notched blasting.According to the structure characteristic of notched blasting hole,rock nearby the two hole line most easily crack,so as to achieve directional fracture blasting result.In two notched blasting holes condition,the stress wave superposition in the two borehole center line center position,the stress is much greater than other non overlap,which has created favorable conditions for the crack grow along the line of two holes.(2)Though the selected the material model and the calculation method of fluid solid coupling,three kinds of coupling coefficient and hole spacing based on double notched blasting model have been build to simulate notched blasting effect with different blasting parameters.The simulation results show that,when the coupling coefficient is K=3.6 is the best,and the damage of the rock mass is the least.The bigger the coupling coefficient is,the better the protective effect of the rock mass is,the better the effect of the crack is.But the coupling coefficient is too large,not only uneconomical,but also difficult to grasp the cartridge in the hole.Due to the advantages of the structure,the hole spacing can be increased properly and the construction cost is reduced.However,the large hole spacing will result in the failure of the blasting effect.Therefore,in the process of actual notched blasting construction,the reasonable blasting parameters should be selected according to the test results and field construction conditions.(3)Field test was carried out in Baihetan Hydropower Station Underground Powerhouse Rock Anchor Beam notched blasting excavation engineering.Through the analysis of notched blasting result,notched blasting can meet the requirements of directional fracture,rock anchor beam fine blasting,rock surface integrity,less damage,moderate block,and little impact on the environment.(4)The rock ultrasonic testing of notched blasting test results show that the round hole blasting cumulative damage effect on remaining rock mass is very strong,which threats the safety and stability of the surrounding rock,and the notched blasting cumulative damage effect on remaining rock mass is smaller,which has certain advantages on the safety and stability of the surrounding rock.Based on the theory that the direct result of cumulative damage times by multiple rock blasting is degradation of rock mechanics parameters.The calculation formula of rock mass strength parameters associated with acoustic velocity is proposed in this paper,which is verified by ultrasonic testing data obtained in the experiment.Its good consistency with practical engineering proved the rationality and applicability of the formula.