Search

Abstract: A systematic pre-drilling campaign was undertaken during the excavation of the Sedrun South lot of the Gotthard Base Tunnel. This tunnel section is located 897 to 2026m below a high alpine topography and located in fractured crystalline basement rocks of the Gotthard massif. The section covered by the systematic drilling campaign extends over 5km in the Nalps hydropower lake region and aimed to support decisions and counter measures related to tunnel drainage induced surface deformations. The pre-drillings conducted in both tunnel tubes included 30 cored and 94 destructive drillings with a length ranging from 17.5 to 358.5m. This paper presents in detail the results from pre-drilling and tunnel inflow monitoring. A new methodology is presented and discussed that allows for quantitative analysis of fracture transmissivity and equivalent continuum hydraulic conductivity from basic pre-drilling inflow and pressure observations, using a simplified form of the Jacob and Lohman solution for transient inflow rate to a well of constant drawdown in a confined aquifer. Uncertainties with respect to the derived hydraulic conductivity values are estimated through Monte Carlo simulation and compared with results from digitally controlled borehole outflow and pressure build-up tests. The spatial distribution of fracture transmissivity and equivalent continuum hydraulic conductivity is presented and compared with tectonic and lithological properties. It is suggested that both steeply dipping, regional brittle faults and lithologic contacts with significant stiffness contrasts impact the distribution of preferential groundwater pathways at depth. Along fault strike strong small scale variations in brittle fault architecture lead to low hydraulic conductivity correlations between the two tunnel tubes separated by 40m. Normal to fault strike larger groundwater inflows cluster within domains of several hundred meters in length. The hydraulic conductivities from pre-drilling analysis are used for inflow prediction for the excavated tunnel. Compared to the small-scale (50m) variations of hydraulic conductivity as derived from pre-drilling analysis the sampled volumes of long term tunnel inflows are at least one order of magnitude larger (>500m). This leads to very uniform steady state inflow rates of 500–1000m long monitoring sections for cumulative tunnel inflows. These inflows range between 3 and 5l/s·km and compare very well with the predicted tunnel inflows from geometric mean hydraulic conductivity as derived from pre-drilling analysis. [Copyright &y& Elsevier]

Highlights: [•] Experiences from one of the most demanding sections of the Gotthard Base Tunnel. [•] Detailed information on geology, excavation, support and rock response to tunnelling. [•] Identification of schistosity as an important cause of squeezing variability. [•] Empirical correlation between deformations and characteristics of the rock. [Copyright &y& Elsevier]