水力耦合作用下裂纹扩展演化机理的试验和颗粒流分析

Experiment and Particle Flow Analysis of Crack Propagation Evolution Mechanism Under Hydraulic Coupling

在地应力不断变化的过程中,裂隙水压力的作用机理会变得十分复杂,为阐述裂隙水压力对裂纹扩展规律的影响,基于离散元理论和室内试验,研究了含单裂隙的水泥砂浆试件在单轴压缩和内水压共同作用下的裂纹演化机理。结合岩石颗粒低渗透性的特点,修正了流体与颗粒之间相互作用的计算法则,并改进了流体域参数的计算方法,提出了一种更加适用于脆性岩石的流固耦合模型。研究结果表明:当裂隙倾角为450,内水压为1 MPa时,翼裂纹在初始萌生阶段时会沿着最大应力降方向扩展,其扩展方向基本与裂隙平面垂直,并在扩展的过程中使裂纹尖端附近的拉应力消散。试样轴向应力达到峰值后,次生裂纹大量萌生,微裂纹数目随轴向应变增加呈指数关系增长,同时预制裂隙尖端的压应力场得到释放。与传统水压致裂机理不同,水压力并不会沿着萌生的新裂纹一直扩散,在恒定内水压作用下,由于裂隙尖端一直存在部分压应力场,水压力只会沿着翼裂纹扩散,并没有扩散到已经贯通的次生裂纹中。在轴向应力不断变化的情况下,颗粒之间的孔径处于动态变化之中,反过来对水压力的变化规律产生影响,从而形成了3种不同类型的水压变化规律,类型Ⅰ:水压力随轴向应变增加至峰值后迅速下降,但下降幅度不大,之后水压会随轴向应变增加而上升;类型Ⅱ:水压力随轴向应变增加一直增加;类型Ⅲ:水压力随轴向应变增加至峰值后迅速跌落至0 MPa,水压力最终消散。

The mechanism of fissure water pressure is highly complicated in the process of changing in situ stresses.To elucidate the influence of water pressure on crack propagation law,based on discrete element theory and laboratory test,the evolution mechanism of a single flaw in cement mortar specimens under uniaxial compression and internal water pressure is studied.According to the characteristics of low permeability of rock particles,a more suitable model of fluid-solid coupling for rock is proposed by correcting the calculation rules of interaction between fluid and particles and improving the calculation of the parameters of the convective area.The results show that when the crack angle is 450 and the internal water pressure reaches 1 MPa,the wing crack will propagate along the maximum stress drop in the initial stage,and the tensile stress near the flaw will be dissipated because of the crack propagation.After the specimens reach the peak stress,a large number of secondary cracks are initiated and propagate,and the number of micro-cracks increases exponentially with the increasing axial strain,which cause the compressive stress field at the tip of the pre-existing flaw to release.Unlike the traditional hydraulic fracturing mechanism,the water pressure does not penetrate along newly generated cracks.Under the constant internal water pressure,the water pressure will only spread along the wing cracks rather than the already existing secondary cracks owing to the existence of partial compressive stress field at the crack tip.The pore size between the particles is dynamically changing due to the constant variation of axial stress,which in turn affects the variation law of water pressure,thus forming three different types of water pressure changes as follows:TypeⅠ:the water pressure decreases rapidly when the axial strain reaches the peak value,while the decrease is not significant,and then the water pressure increases with increasing axial strain;TypeⅡ:the water pressure increases with the increase of axial strain;TypeⅢ:the water pressure drops rapidly to 0 MPa when axial strain reaches the peak value,and finally the water pressure dissipates.