Ture Triaxial Experimental Study Of Failure Characteristic Of Granite Under Dynamic Disturbance Of Low Strain Rate

With the development of economy and industry,hydraulic engineering,transportation engineering,mining,radioactive waste disposal engineering,deeply buried protective engineering and other underground constructions migrate to deep grounds,and the engineering disasters are inevitable and frequent.The excavation of underground openings exposes free surfaces and leads to stress redistribution,i.e.,radial stress release(zero at the excavation boundary)and tangential stress concentration,in the vicinity of the excavation boundary.After excavation,the rock mass near the excavation boundary is under true triaxial compression characterized by large stress difference,or even close to capacity.As the excavation advances,these rock masses are often subjected to various kinds of low strain rate dynamic disturbance resulting from drilling,mechanical operation,remote blasting and so on,and consequently would be suddenly destroyed,even rockburst.Therefore,it is of great academic value and practical significance to explore the mechanical response and failure characteristics of the deep rock mass when subjected to low strain rate dynamic disturbances,to study the mechanism of rockburst and to predict and control rockburst.This paper uses the high voltage servo dynamic true triaxial rigid test system to carry out the true triaxial test of granite under low strain rate(2×10-3～5×10-3s-1)dynamic disturbances,including the slope dynamic disturbance,low-frequency cyclic dynamic disturbance and high-frequency weak dynamic disturbance.And the influence of different initial static stress on rock failure characteristics is analyzed.The damage mechanics behavior and damage mechanism of rock under dynamic disturbance of low strain rate are revealed and compared with the experimental results of rock stress-strain curves,failure forms,damage evolution laws and acoustic emission characteristics under the kinds of dynamic disturbance.The experiment results indicate that:(1)Low strain rate dynamic disturbance load and medium-high strain rate impact load under rock failure showed significant difference:①The failure forms of the specimens under the action of low strain rate dynamic disturbance load are destroyed by multiple partitions,after the failure,the specimen still has a certain bearing capacity.Under the action of medium-high strain rate impact load overall instability.②The mechanism of rock failure under the action of medium-high strain rate impact load is mainly caused by the input energy,which is similar to that of ramp dynamic disturbance,the cyclic dynamic disturbance and weak dynamic disturbance are mainly caused by fatigue damage.(2)The low strain rate dynamic disturbance is easy to trigger the true triaxial failure of the high stress rock mass near the excavation boundary,and the failure characteristics are also different from the static loading:①There is no obvious yield point before the rock failure peak under the ramp dynamic disturbance and stress-strain is mainly linear elastic response.②The failure forms of the rock specimen under the action of low strain rate dynamic disturbance and static loading have similar characteristics of multiple partitioning,that is,in the region of the two sides of σ3 direction,there are split cracks parallel to the loading direction,and shear cracks appear in the middle area of the sample.But failure extent of the specimen under the action of low strain rate dynamic disturbance is more serious;③Under the action of low strain rate dynamic disturbance,the acoustic emission count and the acoustic emission absolute energy accumulation during the rock failure process are larger than those of static loading.(3)The results of rock true triaxial failure under the action of ramp dynamic disturbance show that the rock peak strength and peak strain first increase and then decrease with the increase of σ1 direction static stress,and the damage of rock during dynamic disturbance is stepped,and there is obvious moment sudden increase feature.The results of rock true triaxial failure under the action of cyclic dynamic disturbance and weak dynamic disturbance show that there exists a strain threshold for rock failure,and the damage of rock during dynamic disturbance is inverted S-type,showing obvious fatigue damage characteristics.The σ1 direction static stress of the rock is higher,the smaller the strain increment is required for σ1 direction strain when the specimen is destroyed under dynamic disturbance and the shorter the time required for dynamic disturbance to failure under the action of dynamic disturbance.(4)The comparative analysis of acoustic emission results shows that under the action of static loading and low strain rate dynamic disturbance,the acoustic emission count before rock failure will appear "relatively calm period" and it’s not the maximum when the rock failure.The acoustic emission absolute energy accumulation increases in the form of parabola during the disturbance process,and the acoustic emission absolute energy accumulation is maximum when the rock is destroyed under the action of ramp dynamic disturbance.