卸围压深埋大理岩扩容特征与裂纹演化规律

Dilatancy Characteristics and Crack Evolution of Deep-Buried Marble under Unloading Confining Pressure

深埋硬脆岩石开挖卸围压破裂过程与裂纹的扩展密切相关,利用MTS815 Flex Test GT岩石力学试验平台,对锦屏二级2 400 m深埋大理岩展开25、50、80 MPa三种不同初始围压的三轴卸围压试验,依据卸围压过程中扩容变形与能量转换过程,深入探究不同卸围压状态下深埋大理岩变形破坏过程中的裂纹演化规律。研究结果表明:深埋大理岩卸围压变形破坏过程阶段性明显,通过横向应变差法与体积应变极大值点确定起裂应力与扩容损伤应力,基于此对体积变形过程进行了阶段性划分,并发现其中屈服扩容阶段可进一步划分为稳定扩容和加速扩容两阶段;初始围压对大理岩扩容变形影响显著,随着初始围压增大,其扩容特性减弱且承受扩容变形能力增强。裂纹的扩展伴随着能量的耗散,为明确卸围压深埋大理岩扩容破坏机理,综合塑性应变与能量耗散特征,获得其变形破坏过程中的裂纹演化规律,发现:初始围压控制裂纹类型与发育程度,初始围压增大,裂纹总体发育愈充分,剪切裂纹占比增加,其破坏模式由张剪破坏向剪切破坏转变;提出横向裂纹应变比率POC3以表征深埋大理岩卸围压阶段裂纹演化趋势,POC3曲线转折点对应岩石内部主裂纹的延展,多为极限围压卸荷量的60%左右,可据此进行深埋岩石卸围压破坏前兆预判。

The fracture process of unloading confining pressure in deep-buried rock is related to crack propagation. The triaxial unloading tests under different initial confining pressures of 25,50 and 80 MPa were carried out on the MTS815 Flex Test GT Rock Mechanics Test Platform for the 2 400 m deep-buried marble of Jinping II. The deformation and dilatation characteristics and energy conversion law in the process of unloading confining pressure were analyzed. Based on this, the crack evolution law in the deformation and failure process of deep-buried marble under different initial confining pressure conditions was explored. The research results indicate as follows: The deformation and failure process of deep-buried marble under unloading confining pressure has phase characteristics. The crack initiation stress and dilatancy damage stress were determined by the transverse strain difference method and the maximum point of volumetric strain. Based on this, the volume deformation process is divided into stages. And the volumetric deformation process is divided into two stages: stable dilatancy and accelerated dilatancy. The initial confining pressure has an effect on dilatancy of marble, with increase of initial confining pressure, the dilatancy property decreases but the ability to bear dilatancy increases.The propagation of cracks is accompanied by dissipation of energy. In order to clarify the dilatancy failure mechanism of deep buried marble under unloading confining pressure, the crack evolution law in the process of deformation and failure was obtained by integrating the characteristics of plastic strain and energy dissipation. It is found thatinitial confining pressure controls the type and development of cracks. With increase of initial confining pressure, the cracks develop more fully and the proportion of sliding cracks increases, and the failure mode changes from tensile shear failure to shear failure. The ransverse crack strain rate POC3was proposed to characterize the crack evolution trend in the unloading confining pressure stage. It is pointed out that the turning point of POC3curve corresponds to the extension of the main crack in the rock, and most of them are about 60% of the unloading value of the limit confining pressure. According to this, prediction of unloading confining pressure failure of deep buried rock can be conducted.