On the non-Darcian seepage flow field around a deeply buried tunnel after excavation

When a tunnel is excavated in a water-bearing rock mass, groundwater may flow toward the tunnel and cause some adverse effects. Although the non-Darcian hydraulic characteristics of fractured rock masses have been extensively studied by laboratory and field tests, the non-Darcian seepage flow was seldom taken into account in engineering practice so far. This paper investigates the non-Darcian seepage flow around a deeply buried tunnel after excavation. In-situ water injection tests were conducted to investigate the hydraulic characteristics of the rock mass, and a formula was derived to quantify the non-linear hydraulic conductivity from the test data. A numerical method based on non-linear finite element method was then proposed to simulate the non-Darcian seepage flow toward the tunnel. By comparing the numerical results of non-Darcian seepage flow with the results based on a traditional Darcian assumption, it is found that the difference of hydraulic head distribution is slight, but the difference of flow velocity and total discharge is obvious. The Darcian assumption provides rational discharge estimation only when the actual flow velocity around the tunnel is similar to the flow velocity in in-situ tests; however, the non-Darcian assumption is able to obtain a more rational result especially when the number of test pressure steps used in in-situ tests is limited.