Micro-mechanism of water stability and characteristics of strength softening of rock in deep mines

In order to study the properties of water stability and the rules of strength softening of rock, systematic hydrolysis tests are carried out on rock samples from deep mines of Juye Coal Field at depth below one kilometer. The results show six types of hydrolysis deformation and failure. Based on the limit expansion quantity feature, four types of water stability properties are summarized, to demonstrate the rationality of classification by voidage. On this basis, the micro-mechanism of difference in water stability is analyzed, and the main reason lies in the difference in micro-structure and the other reasons include the relative quantitative contents of clay mineral of kaolinite, illite, illite/montmorillonite mixing layer. The strength softening feature of rock samples changes obviously after saturation and is affected by the type of their water stability properties. Moreover, it has good consistency. Layered failure mainly appears in the saturated tensile strength tests. Change in internal friction angle is the key to change in shear strength softening feature. The tensile strength tests on saturated rock samples show that the tensile strength of rock samples decreases with the increase of saturated time with a trend of an exponential decline and has a stable value at the end. The repeated immersion compression tests show that the final failure stress drops on condition of repeated immersion compression of rock samples, and strength fatigue appears. Its value is less than the failure stress of saturated samples under one-time pressure condition, and is much less than the failure stress of natural state samples. This is because the expansion effect of the internal structure of rock samples after immersion makes their internal structure soften, which reduces the connection strength of rock structure.