Abstract: To investigate the permeability characteristics of filling media and its mechanical properties and failure mechanisms under high water pressure conditions, a large-scale testing system was developed for the measurement of coupled seepage and triaxial stress. We proposed a pressure chamber especially for coupling three-dimensional (3D) seepage-stress, and designed a perfect sealing and loading system. This system has multiple functions including automatically applying triaxial stress, surrounding rock pressure and osmotic pressure, gushing water and sand acquisition and monitoring parallel multivariate information. It can be used to perform the tests of permeability and correlative mechanics measurement for large-scale and different patterns karst conduit and karst fissure under the initial confining pressure. The maximum applied lateral pressure and permeability pressure are up to 2 MPa and 5 MPa, respectively. The maximum axial loading capacity is 300 kN. The precise acquisition and analysis of seepage pressure, strain and displacement in the water environment can be achieved by the fibre technology, flexible waterproof technology of resistor elements and automatic data acquisition system. The permeability tests were conducted on filling medium under the condition of occurrence, and seepage characteristics of different kinds of filling medium were systematically analysed. The seepage failure model of filling medium was further revealed. Therefore, the developed system provides a new testing method for hydropower and tunnelling engineering.

Key words: filled-type karst conduit, filling medium, seepage characteristics, experimental system