水-力耦合作用下单裂隙灰岩三轴压缩与声发射试验及压剪断裂模型

Triaxial compression and acoustic emission tests on single cracked limestone and compression-shear fracture model under hydraulic-mechanical coupling action

深部煤炭资源开采面临着高渗透压和高应力耦合作用的地质环境,水-力耦合作用下裂隙岩体力学行为及破裂特征研究对于揭示深部岩体失稳破裂规律有重要意义。通过在灰岩标准圆柱体试件中部预制裂隙形成单裂隙灰岩试件,采用MTS815岩石力学试验系统和PCI-2声发射监测仪,对单裂隙灰岩试件进行不同围压和裂隙水压下的常规三轴压缩试验和破裂过程的声发射监测,其中预制裂隙倾角设置分别为15°、45°和75°三种不同产状,围压分别设为10,15,20和25 MPa共4个系列,裂隙水压分别设置为0,2,4,6,8 MPa的5种工况。试验结果表明:与完整灰岩试件相比较,单裂隙灰岩试件的应力-应变曲线呈现出独特的双峰值性态:前高后低双峰型和前低后高双峰型,经历第1峰值点后,其承载结构的刚度明显下降;单裂隙灰岩的起裂、扩容和峰值强度随水压的增加存在明显的降低趋势,而随围压的增加显著提高。在相同围压和水压作用下,裂隙倾角α=75°的试件起裂、扩容和峰值强度最高,α=15°的试件次之,α=45°的试件最小;裂隙倾角和水压显著影响了单裂隙灰岩的岩石破裂过程中声发射信号变化规律。随着裂隙倾角的增大,裂隙起裂和稳定扩展阶段振铃计数变得越来越稀疏,相对于高水压下的声发射特性,在低水压作用下声发射信号更为强烈。基于试验结果建立了水-力耦合用下单裂隙灰岩的压剪断裂判据,明确了压剪参数与裂隙倾角之间的关系,并发现破裂模式与|K;/K;|存在相关性,即随着裂隙倾角增大,|K;/K;|也相应增加,压剪复合应力状态下岩石裂隙越趋向于剪切开裂。

The extraction of deep coal resources faces a geological environment with the coupling of high water pressure and high stress. The study of mechanical behavior and fracture characteristics of the fractured rock mass under hydraulic-mechanical coupling is of great significance to reveal the instability and fracture law of deep rock mass. In this paper, a single cracked limestone specimen is made by prefabricating a fissure in the middle of a limestone standard cylindrical specimen. The MTS815 rock mechanics test system and PCI-2 acoustic emission monitor are used to perform conventional triaxial compressive tests on single cracked limestone specimens under different confining and water pressures, and the acoustic emission monitoring during the fracture process, respectively. The inclination angles of the prefabricated crack is set to 15°, 45° and 75°, respectively, and the confining pressure is set to 10, 15, 20 and 25 MPa, respectively, and the water pressure is set to 0, 2, 4, 6, and 8 MPa, respectively. The results show that the stress-strain curve of a single cracked limestone under hydraulic-mechanical coupling presents a unique bimodal behavior of high front and low back or low front and high back, moreover, the stiffness of bearing structure decreases obviously after the first peak point. The crack initiation stress, dilatancy stress and peak stress of single cracked limestone all decrease obviously with the increase of water pressure, but increase significantly with an increase in confining pressure. Under the same water pressure and confining pressure, the above three stress levels with the crack inclination angle α=75° are the highest, followed by with α=15° and those with α=45° are the lowest. The crack inclination angle and water pressure significantly affect the change law of acoustic emission signal during the rock fracture process of single cracked limestone. With the increase of the crack inclination angle, the ringing count in the crack initiation and stable growth phase becomes more and more sparse. Compared with the acoustic emission characteristics under high water pressure, the acoustic emission signal is stronger under the action of low water pressure. Based on the experimental results, the compression-shear fracture criterion of single cracked limestone under hydraulic-mechanical coupling action is established. The relationship between compression-shear parameter and crack inclination angle is clarified. It is found that there is a correlation between the fracture mode and |K;/K;|, that is, |K;/K;| increases with the increase of crack inclination angle, and the rock crack subjected to combined shear and compression mode tends to shear fracture.