An analysis method for evaluating the safety of pressure water conveyance tunnel in argillaceous sandstone under water-weakening conditions

In a pressure water conveyance tunnel with surrounding rock of argillaceous sandstone, the water-weakening properties of argillaceous sandstone pose a major challenge to engineering safety. In addition to the conventional features of fluid-solid coupling, the argillaceous sandstone has two additional features during inside-to-outside seepage in pressure water conveyance tunnels: (1) water infiltration significantly weakens the mechanical properties of the rock and (2) variation in the rock stress tempts an observable effect on permeability due to variation in rock microstructures. In this investigation, an analysis method was proposed for the safety of pressure water conveyance tunnels in argillaceous sandstone under water-weakening conditions. Three parts were included to propose this method such as, experimental, numerical calculation, and safety evaluation. Firstly, we studied the water-weakening mechanical properties and permeability characteristics of argillaceous sandstone based on laboratory tests. Subsequently, based on the test results, we carried out simulation calculations considering the water-weakening mechanical properties and permeability characteristics of argillaceous sandstone. Next, we evaluated tunnel safety by the relevant evaluation criteria based on the calculation results. The rationality of this method was verified by successfully simulating a permeability test on large-size cylindrical specimens of argillaceous sandstone. Finally, we adopted the method to evaluate the engineering safety of pressure water conveyance tunnels with surrounding rocks composed of Cretaceous argillaceous sandstone in the Lanzhou water-source construction project. The obtained results play an important role in guiding the design and construction of the engineering. Compared with the traditional empirical strength reduction method, the proposed method provided a better option for the accurate evaluation of the safety of pressure water conveyance tunnels in argillaceous sandstone during operation.