Step effects of hydraulic pressure of metro tunnels in loess under sudden high-pressure seepage

The leakage of a high-pressure water source has sudden, rapid and energy-release effects. Its destructive behavior of surrounding rock of loess and deterioration of tunnel structures is more severe than static infiltration. To reveal the transmission characteristics and inducement of the step effects of surrounding rock of loess under high-pressure seepage as well as the influences on the tunnel structures, the temporal and spatial evolution of seepage behavior of surrounding rock of loess and the mechanical response of tunnels are studied through the similar model tests. The research shows that the seepage process has successively experienced local collapsible erosion of loess → generation of the hidden cavity → formation of the preferential channels → step fluctuation of water pressure of surrounding rock, and structural internal force→seepage stability. The release of the local collapsible potential and the development of the preferential seepage channels are the main reasons for the step fluctuation of the water pressure of surrounding rock and structural stress. Under different water source locations, the changes of internal force of linings below the arch springing line are significantly greater than those of the upper structures, and the arch bottom location is most affected by the force. The maximum step increase of its axial force is 324 kN under the upper water source, and that of its bending moment is 84 kN·m under the lower water source. The surrounding rock area at the lower side of the dominant preferential channel is most affected. The influence degree of the internal force of linings under different water source positions is shown as upper water source > lower water source > side water source.