Crack Propagation Mechanism Of Fractured Rock Under Static-dynamic Loading And Seepage Water Pressure

Abstract:To reveal the water inrush mechanics behavior of underground rock mass subjected to dynamic disturbance such as blasting, the crack propagation mechanism of fractured rock under static-dynamic loading and seepage water pressure is analyzed through theoretical research, experimental study, acoustic emission monitoring, numerical simulation and engineering test.The compression-shear rock crack initiation rule and the evolution of crack tip stress intensity are analyzed under static-dynamic loading and seepage water pressure on the basis of theoretical deduction and experimental research. It is drawn that the major influence factors of the crack tip stress intensity factor are seepage pressure, dynamic load, static stress and crack angle. The seepage water pressure's existence aggravates branch cracks' propagation. With the seepage pressure increasing, the branch crack becomes unstable extension from stable propagation. The dynamic load in the direction of maximum main stress increases type I crack tip stress intensity factor and its influence on type II crack intensity factor is relative with crack angle and material property. Crack initiation angle changes with the dynamic load. The initial crack initiation angle of type-? dynamic crack fracture is70.5. The compression-shear crack initial strength relates to seepage pressure, confining pressure, dynamic load. Experimental results verify the initial crack strength increases with the confining pressure's increase, and decreases with the seepage pressure's increase.The seepage pressure loading device was developed. The uniaxial cyclic loading experiments were done by loading prestress on crack rocklike materials i.e. concrete samples to contrast the compression failure characteristics with or without the action of seepage water pressure. The experimental results showed that finite cyclic loading in the elastic stage helps to strengthen rocklike materials. The strength of rock after uniaxial cyclic loading is reduced with the action of seepage water pressure, and the depressed degree is related to the prestress when loading cyclic pressure. When cyclic loading compressed in the elastic stage, the rock strength is higher than the one in the damage stage. The rock strain under the action of seepage water pressure is greater than the one without seepage water pressure. As for rock bursting tendency rock, loading and unloading can reduce its rock-burst proneness because of formatting damage, and lower seepage water pressure can greatly decrease the rock burst orientation.To reveal the mechanical response of deep rock under the coupled action of ground stress and seepage water pressure when facing dynamic disturbance such as blasting, the containing crack rocklike specimen were compressed on the seepage pressure loading device. The pressed crack rocklike specimen's uniaxial cyclic compression failure process was monitored by acoustic emission test and analysis system, when it respectively subjected to seepage water or without seepage water pressure. It was found that acoustic emission has big difference such as acoustic emission interval and energy when with or without seepage water pressure. The acoustic emission's energy from pressed crack rocklike specimen under seepage water pressure is higher than the acoustic emission's energy from pressed crack rocklike specimen without seepage water pressure during loading the cyclic loads when the pressed crack rocklike specimen is its elastic stage. The acoustic emission's energy from pressed crack rocklike specimen under seepage water pressure is much higher than the acoustic emission's energy from pressed crack rocklike specimen without seepage water pressure during loading the cyclic loads when the pressed crack rocklike specimen is its damaged stage. AE events generated in the lower or higher stress levels when crack contained rocklike concrete specimen subjected to cyclic loads in the seepage water pressure, but their mechanism is different. The difference is that low tensile stress appeared in the pressed crack rocklike specimen resulting specimen's damaging and acoustic emission under the low stress and high compressive stress appeared in the pressed crack rocklike specimen resulting specimen's damaging and acoustic emission under the high stress.In order to better observe the crack initiation, propagation and failure process, and explain its failure mechanism, the RFPA2D software is used to simulate the crack rock compression process. Through the computer simulation the failure process of the one crack rock under uniaxial compressive loading, it is found that the compressive strength has the very big relations with the crack angle, with or without the action of seepage water pressure. With the increase of crack angle from0degree to90degree, the compressive strength of specimen decreases firstly and then increases.When the crack angle is45°, the compressive strength is the minimum, the same to the crack initiation strength. With the action of seepage water pressure, the specimens compressive strength is lower than the one without water pressure effect. Through the computer simulation the failure process of two parallel crack specimen under uniaxial compression, it is found that two parallel crack initiation sequence is random in pure stress field. While for two parallel cracks in the seepage water field, the one with the seepage water pressure effect first initiates. By simulating two non-parallel crack specimen failure process under uniaxial compression, it is found that in pure stress field two non-parallel crack initiation sequence is related to the crack angle. The order of infiltration of water into the crack affects the strength of the rock mass.To testify the authenticity of laboratory experiment and numerical simulation results, the broken rock zone in seepage tunnel of Jian-Lin-Shan mining area Daye iron mine is tested under production blasting vibration. During testing process, it is found that blasting vibration causes internal cracks of surrounding rock initiate and develop, the fracture density increases, the acoustic wave velocity of rock mass decreases, the broken rock zone expands. At the same time, through on-site observation, it is found that blasting vibration results in crack development, connected and formation water pathway, leads to surface water into the ground. The FPA2D calculation software is used to simulate the mechanical response of seepage tunnel under blasting vibration. It is Found that in the seepage water pressure, stress and cyclic loading, internal cracks of roadway surrounding rock initiate, propagate and through out gradually, and the fracture range is expanding, the broken rock zone also expands. It is consistent with the engineering test conclusion. Also found that under the effect of seepage water pressure, the tensile stress appear around some cracks, leading to part of the cracks more likely to generate shear failure, consistent with the indoor experiment conclusion.