初应力条件下超临界CO_2气爆致裂规律研究

Mechanism of Fracture Caused by Supercritical CO_2 Explosion under the Impact of Initial Stress

为研究地下爆破工程中初始地应力条件下超临界CO2气爆的致裂规律,利用自主研发的三轴加载式超临界CO2气爆实验系统,对不同应力组合工况下混凝土试件进行了超临界CO2气爆致裂实验,实验结果表明超临界CO2气爆爆破峰值压力低且高压持续时间长,致裂过程不同于传统炸药爆破,分为动态和准静态过程:应力波将气爆孔附近介质压碎形成粉碎区,在环向拉应力作用下粉碎区周围介质产生径向裂隙的动态过程,高压CO2气体进入裂隙形成气楔,促使裂隙继续扩展的准静态过程,得到了气爆后试件沿最大初始压应力方向开裂的规律.通过理论计算的方法分析了初应力作用下气爆过程中介质应力状态的变化规律,揭示了初应力影响裂纹起裂和扩展的机理:位于气爆孔最大初始压应力方向的介质产生初始环向压应力最小,在超临界CO2径向冲击产生的环向拉应力作用下首先发生开裂;位于气爆孔最小初始压应力方向介质中初始环向压应力最大,开裂所需的径向冲击压力增大,开裂滞后;垂直裂隙方向的应力抑制裂隙的张开而阻碍CO2气体的进入,同时增大了裂隙扩展所需的气楔压力,气楔作用效果大幅减弱,对裂隙扩展的阻碍作用显著.裂隙的扩展速度与扩展距离呈"S型"曲线关系,初始压应力越大,裂隙扩展相同距离降低的速度值越大,且压碎区和裂隙扩展范围逐渐减小.

A customized triaxial loading explosion system is developed to investigate concrete fractures under coupled action.The supercritical CO2 explosion experiments are carried out to explore the process of crack propagation with initial stress.The experiments indicate low peak pressures and long durations of high pressure during the explosion,which are different from the traditional explosion.Stress of the medium and mechanism of crack propagation are studied in the process with initial stress.The process is divided into two stages,i.e.the dynamic stage and the quasi-static stage.During the first stage,the medium near a blasting hole is crushed,a crush zone comes into being,and a radial crack is formed outside the crush zone due to the circumferential tensile stress.Then the high-pressure CO2 diffuses into the fracture and forms a gas wedge,so cracks continue to extend.It is found that cracks form in the direction of the maximum initial stress.The initial circumferential compressive stress of the medium in that direction is the minimum,and therefore,the specimen is cracked because of the tensile stress caused by supercritical CO2.The initial circumferential compressive stress of the medium in the vertical direction is the highest,so a sufficiently high stress is required to cause cracks,leading to a delay in crack initiation.The stress in the vertical fracture direction prevents gas from diffusing,and a higher pressure is needed to form the wedge.Influence of the wedge is therefore weakened,and the crack propagation is considerably hindered.The velocity of crack propagation follows an S-shaped curve with the distance from the hole.At the same distance in propagation,the greater is the initial stress,the greater is the velocity decline,and the more reduced are the crush zone and crack area.Above all,it is suggested that the distance between holes is broadened and the hole size is reduced to improve the efficiency of explosion with initial stress.