Abstract: Regarding the leakage of fractured limestone in the underground cavern of a hydropower station, the rock samples and reservoir water were collected on site, and two schemes of water-rock interaction tests which lasted 360 days were carried out. The scheme #1 regularly changed the soaking solution during the soaking process to simulate the long-term immersion and flow renewal phenomenon of groundwater, while the scheme #2 mainly simulates the long-term immersion process. Based on the micro-morphological characteristics of the fracture surface and the changes of ion concentration in soaking solution, the characteristics and mechanism of the seepage characteristics of fractured limestone under water-rock interactions were analyzed. The research results show that in the course of soaking, the permeability coefficient of the single-fractured limestone increases first sharply then gently. After 360 days of soaking, the permeability coefficients of the single-fractured limestone with 2.5 MPa, 3.0 MPa, 3.5 MPa, and 4.0 MPa confining stresses in the two schemes increased by 729.90%~384.17%、549.04%~297.45%, respectively. The smaller the confining stress is, the more obvious the permeability coefficient increases. Moreover, the growth trends of permeability coefficient stabilized after 120 days for scheme #1 and 90 days for scheme #2. In comparison, the permeability coefficient increase rate and amplitude of single-fractured limestone in scheme #1 are significantly higher, which is 1.34~2.07 times that of scheme #2. In scheme #1, the immersion reservoir water was periodically changed to maintain the disproportion of the immersion solution, which resulted in a large concentration gradient between the rock sample and the solution. This concentration gradient increased the reaction chemical potential, and accelerated the rate of mineral dissolution and erosion on fracture surface. In scheme #2, the long-term immersion facilitated the solution ion concentration to approach the equilibrium state gradually, leading to the dissolution and erosion rate of the minerals on fracture surface being significantly lower than that in the scheme #1. Long-term water-rock interaction weakens the roughness and fluctuation of fracture surface significantly accompanied with the progressively decrease of local slope and undulation, and the increase of overall erosion depth. Based on this, a seepage channel evolution model for single-fractured limestone under long-term water-rock interaction is established. The seepage channels of fractured rock samples gradually evolved to the direction of low anastomosis and large opening, which leads to significantly increases of equivalent hydraulic opening and permeability coefficient of fractured rock samples. The ideas and results of this study provide an excellent reference for the simulation of water-rock interaction in fractured rock masses.

Key words: water-rock interaction, periodically change water, long-term immersion, seepage characteristic, microscopic morpho- logical, seepage channel