Abstract: To investigate the functional relationship between different seepage states of tunnels and the water pressure behind the lining, as well as the bearing safety of cracked and seeping lining structures, a set of constant-pressure and constant-flow water supply equipment suitable for large-scale model tests is developed using stabilized air pressure method. This equipment enables the stable generation of local water pressure under a large flow rate and the control of water head height at the decimeter level. Furthermore, a large-scale model test with a scale of 1∶50 is conducted indoors to examine and map out the magnitude of water pressure behind the lining based on directly measurable existing apparent phenomenon indicators such as seepage volume and crack opening degree, thereby assessing the bearing safety of cracked and seeping lining structures. The results indicate that: (1) Affected by the roughness and tortuosity of cracks, in actual engineering, the seepage volume of cracked and seeping linings varies non-linearly with the water pressure behind the lining. The functional relationship between the seepage volume and the water head height is y=19.69e^(x/2.45) -18.06. (2) Under the combined action of water pressure and soil pressure, the axial force at the seepage part of the cracked lining significantly decreases; the direction of the bending moment changes, demonstrating a "hinge effect", and the safety factor drops sharply in an exponential form with an increase in seepage volume.

Key words: highway tunnels, water pressure action, water pressure equipment, load bearing mechanics, similarity test