Abstract: Argillaceous sandstone shows the typical characteristics of water-weakening due to the high content of clays. Tunnelling in argillaceous sandstone stratum with rich (trapped) water is a challenging issue for underground engineering. This study examined the stability and supporting time for a water conveyance tunnel in the argillaceous sandstone stratum with weakened and un-weakened conditions. Firstly, the water-weakened mechanical test results of argillaceous sandstone were presented. Then, to evaluate the mechanical parameters of the argillaceous sandstone in both water-weakening and un-weakening conditions, the rock mass evaluation method on basis of the Hoek-Brown strength criterion was adopted. Next, Lanzhou water source tunnel project was taken as the research object, and the stability of surrounding rock mass for the tunnel in argillaceous sandstone in both conditions was analyzed by a numerical modelling method. The researches indicate that the water-weakening of argillaceous sandstone significantly influenced tunnel safety. Finally, the convergence confinement approach was utilized to analyze the necessary supporting time to ensure tunnel safety during construction. The results indicate that if the argillaceous sandstone does not weaken, early support measures for the tunnel surrounding rock mass can be considered at the distance of 4 to 5 meters away from the excavation face. However, in the case of argillaceous sandstone water-weakening, early support measures should be employed immediately when the tunnel face is excavated. The research findings can provide the foundation for the safe construction of tunnels in a weak surrounding rock mass with water-weakened conditions.

Key words: argillaceous sandstone, water-weakening, tunnel stability, numerical analysis, the convergence confinement method, support timing