Tunnel inflow, prognosis, and results - Evaluation of inflow to rock tunnels in the Gothenburg

Abstract

Grouting design has been shifted from being based on a rather empirical basis, to having more focus on calculations of fracture apertures and having the design being carried out in accordance with the observational method. In the literature, limited data is available regarding follow-up on prognoses of grouting and tunnel inflow from projects based on either of the two grouting design methods. This thesis therefore aimed at illuminating how the theoretical method used to estimate inflow coincides with the empirical results from four tunneling projects in the Gothenburg region. The aim of the thesis also included to analyse how the parameters used in the inflow equation affected the estimation of inflow, and in particular the choice of skin factor. Collected data from hydraulic tests performed during the construction of the four tunneling projects, e.g. through probe drillings and preinvestigation boreholes, were used to evaluate the hydraulic conductivity of the rock mass. Since different grouting designs were used, the evaluation of the hydraulic conductivity had to be treated differently depending on the information available. Predictions of the inflow were performed using selected best fit values of the input parameters for the inflow equation based on collected data, which were then compared to the actual measured inflows. Through a sensitivity analysis, it was found that the main parameters affecting the inflow was the hydraulic conductivity of both the grouted and ungrouted rock mass and the hydraulic head. These three input parameters (K, Kgr, and H), also had the largest uncertainties in the estimation of the parameter values. It was also found that the ratio of K and Kgr influenced the results to a quite large extent. It was concluded that when a small sealing effect is expected (for example when the rock mass already have a low hydraulic conductivity), more care should be taken when choosing the skin factor. Generally, the inflow equation overestimated the predicted inflow (approximately with 30%). It was also concluded that the inflow equation worked well in conditions of "normally" fractured rock mass. When the hydraulic conductivity of the rock mass was very low, the equation did not perform as good.