Groundwater inflow into tunnels: semi-empirical methods for estimating steady state inflow associated with excavation induced drawdown

Estimation of groundwater inflow is essential for tunnel design and construction; however, analytical solutions used in current engineering practice have limited applicability, especially in cases where drawdown of the water table occurs due to excavation associated drainage. Existing methods for estimating steady state groundwater inflow under a drawn-down water table require improvement, as they typically assume fixed water table boundaries. This article unites and compares classical inflow formulations and reviews their applicability and limitations. Based on numerical modelling, the study proposes two novel methods for estimating inflow into tunnels based on drawdown: one for inflow into shallow tunnels where full drawdown of the water table occurs (trench-like inflow), and one for inflow into moderately deep tunnels, where partial drawdown of the water table occurs. The inflow-drawdown relationship devised accounts for the influence of tunnel size and depth on drawdown shape, and is independent of assumptions of boundary conditions, which limit applicability of other equations. Additionally, practical guidance for numerical modelling of tunnels below the water table is provided, including sensitivity analysis of boundary conditions.