Seepage Characteristics of Triaxial Compression-Induced Fractured Rocks under Varying Confining Pressures

In water-rich rock masses from underground excavations, the flow pressure and confining pressure significantly affect the seepage properties of the fractured rock induced by complex stress. To understand the seepage mechanism of fractured rocks under stress, seepage tests on fractured granite and slate specimens after triaxial compression were carried out under confining pressures from 1 to 10 MPa by using the self-developed temperature–stress–seepage coupling test system. The evolution from linear to nonlinear seepage process is comprehensively investigated by changing the seepage pressures gradient. The flow rate increases when increasing the seepage pressure gradient and decreases when increasing the confining pressure. To illustrate the influence of fracture morphology on the physical mechanism of seepage, fracture surfaces of rock specimens after seepage tests were scanned by a three-dimensional structural optical scanner. The fracture surface morphology of different rock types influences the seepage mechanism in different ways. Under the same seepage condition, the first-order waviness has a greater influence on reducing the critical Reynolds number. The larger the waviness, the more easily the nonlinear seepage occurs in the rock fractures. The relationship between the seepage pressure gradient and the flow rate has been well described by the Forchheimer equation, and the seepage process can be divided into Darcy flow and nonlinear flow. Under the same seepage pressure gradient, the Reynolds number decreases as the confining pressure increases. The nonlinear seepage feature tends to be more evident when increasing the confining pressure. The nonlinear factor E = 0.1 was determined to be the key point to solve the critical Reynolds number and the threshold for nonlinear flow. The Euler numbers of granite fractures are two orders of magnitude larger than that of slate fractures. This is because the first-order waviness on granite fractures fluctuates more significantly than those of slate fractures, leading to a large local resistance loss.