Study On Crack Propagation Characteristics Of Rock Mass Under Blast Loading

The essence of rock blasting is the process that the impact load transiently acts on the blasthole wall and induces rock mass rupture damage.To solve the engineering problems,such as incomplete crushing,large fragmentation,over-excavation of rock mass,and excessive environmental vibration,it is especially important to study the dynamic crack propagation characteristics of rock mass under blast loading.However,due to the "high temperature,high pressure and high speed" characteristics of blasting,it is difficult to dynamically monitor the internal damage of the rock mass in the process,and the complexity of the rock mass material makes the theoretical analysis even less effective.Taking into account the above factors,this paper mainly uses the numerical simulation method to explore the expansion characteristics of the explosion crack under different working conditions.The main research contents and conclusions are as follows:(1)Using the method of acoustic approximation and elastic mechanics,by introducing the dynamic fracture failure criterion of rock,the radius expression of the complete rock mass crushing ring and the crack ring under single hole blasting is derived.Ignoring the effect of explosive gas,the radius of the crushing ring is generally 3 to 7 times the radius of the blasthole,and the crack ring is 20 to 50 times the radius of the blasthole.(2)The numerical simulation of blasting in intact rock mass and fractured rock mass is carried out,which proves that the primary crack has a great influence on the crack propagation,and the effects of crack's distance,angle and length on the crack propagation are mainly investigated.The research shows that the existing crack suppresses the main crack propagation of the vertical fracture surface while promotes the crack extension of the parallel fracture surface.When the fissure is located directly above the borehole and close to the borehole,the spallation phenomenon occurs in the rock mass between the fissure and the blasthole,which is beneficial to increase the rock fracture rate.Wing cracks which generate at both ends of the fracture,increase the fracture range of the rock mass.(3)Porous blasting into a seam is the result of a dynamic expansion of cracks.It main experiences six periods which are 'the formation of smashing circle of each blasthole','the independent expansion of cracks around the smashing circle','the cracks extend in the opposite direction',' the cracks between the blastholes penetrating with each other','the branch cracks beginning to appear',and 'cracks arresting';When the free surface is close to the borehole,the rock mass between the borehole and the free surface produces spallation cracks,which increases the area of blasting rock-breaking.Under different initiation sequence,there are some differences in the propagation of cracks between the boreholes.The longer the initiation delay,the farther the longitudinal crack propagation distance of the borehole is.When it comes to smooth blasting,the effect of simultaneous blasting is better than delayed.(4)The empty hole has a guiding effect on the crack,which effectively promotes the blasting process.It can ensure that the joints in rock mass between the holes is smoothly formed when the hole spacing is increased so as to reduce the unit consumption of the explosive.Under the coupled charge condition,the larger the hole diameter,the stronger the guiding effect,and the optimal diameter of the blast hole is three times of charge poles.The circular hole with horizontal grooving has the strongest guiding effect,which is beneficial to the formation of the smooth blasting or the presplit blasting.