Prediction of water inflow in Tsingtao subsea tunnel based on the superposition principle

•Permeability coefficients are obtained based on water inflow in probe drilling holes.•Formulas of the water inflow are derived based on the superposition principle.•Factors affecting the water inflow in subsea tunnels are analyzed.•Prediction formulas of water inflow are verified by field test and numerical simulation. Prediction of water inflow is a key scientific problem in construction of subsea tunnels. Tsingtao subsea tunnel is taken as the research object, and a calculation method for predicting and analyzing the water inflow based on superposition principle is proposed by applying theoretical analysis, numerical simulation and field monitoring. First of all, the computation model of seepage field and water inflow in the subsea tunnel is established. The permeability coefficients of strongly weathered layer and moderately weathered layer are determined according to the permeability coefficient of slightly weathered layer. Secondly, based on the well flow model of infinite confined aquifer, the double water gushing points calculation model is presented, and then the model is simplified into single water gushing point calculation model. The equivalent calculation formulas of water inflow under the condition of multi-horizontal strata are derived. The water inflow which is affected by the tunnel depth, radius and seawater depth is analyzed. Thirdly, the proposed method is compared with the engineering practice and numerical simulation, which verifies the accuracy and feasibility of the method. The results show that: (1) The water inflow of subsea tunnels increases with the permeability coefficients of strongly and moderately weathered layers. (2) The water inflow of subsea tunnels first decreases and then increases with the tunnel depth increasing. (3) The water inflow of subsea tunnels increases linearly with the tunnel radius and seawater depth.