FEBEX tunnel excavation

The development and dissipation of excess pore water pressures in the vicinity of the advancing tunnel (at the time of the FEBEX tunnel excavation) was a clear example of hydromechanical interaction. It was concluded that the development of pore pressures was controlled by the initial stress field state, by the rate of excavation and by the permeability and drainage properties of the granite. However, the available information on the intensity and direction of principal stresses in the area was found inconsistent with the actual measurements. The problem posed by this discrepancy was essentially unsettled since a precise determination of the initial stress state in the vicinity of the FEBEX tunnel was not available. 

Based on the available geological, hydraulic and mechanical characterizations of the Site as well as on results of hydraulic tests performed on boreholes, a hydro-mechanical model for the zone around the FEBEX tunnel was to be prepared. Using this model, changes in water pressure induced by the boring of the FEBEX tunnel in the near vicinity, as well as the total water flow rate to the excavated tunnel was required. 

The long term reaction of the pressure measured in the borehole section P4 is to show a steady decrease since the excavation of the tunnel implies a neighboring boundary at a given relative humidity (the RH prevailing in the FEBEX tunnel prior to the buffer and heater installation). Most of the models show this trend although the rates of water pressure decay may change. 

For the purposes of the organization of the exercise into specific tasks the Benchmark was divided into three main parts A Rock behaviour during the excavation of the FEBEX tunnel, B Buffer behaviour and C Rock behaviour during the heating and (partial) hydration of the buffer. This distribution has been maintained in the paper. Specific conclusions for each of the mentioned parts have been given before. Only a few concluding remarks will be added here ... 

HYDROMECHANICAL RESPONSE OF JOINTED HOST GRANITIC ROCK DURING EXCAVATION OF THE FEBEX TUNNEL... 

The FEBEX in situ test started in 1995 with rock-mass characterization and tunnel excavation. After the installation of heaters, the bentonite barrier, and monitoring equipment, the heating... 

On the whole, flow is moving from within the massif towards the laboratory tunnel (from west to east). Before excavation of the FEBEX tunnel, the equipotentials were roughly parallel to the wall of this main tunnel. A zero pressure has been assumed in the laboratory tunnel. To the north and south, the model is constrained by two major shear zones S1-1-S2, as specified in Section 2.2. Figure 3 displays the hydraulic boundary conditions adopted, in a plane view. The head and gradient adopted were deduced both from measurements conducted within the boreholes intersecting our... 

As part of the "DECOVALEX" project, we were asked to predict the total water flow rate to the excavated tunnel over the last 17.40 meters of FEBEX tunnel section (between 54.00 and 71.40 m). A purely-hydraulic computation, using the previous calibration, yields an estimated steady-state flow of between 6 and 7.6 ml/min. This size scale is to be compared with total water inflow estimated from a semi-quantitative hydrogeological map of the FEBEX tunnel 7.8 ml/min (with an estimation of about 27% of inflow water coming through the matrix). In comparison with the continuous prediction (Alonso et al, 2001), this estimation is rather good and the discontinuous approach allows localizing water output with greater precision. (This kind of comparison was not included within the "DECOVALEX" task.)... 

For the prediction of water head changes induced by FEBEX tunnel boring, it was impossible to simulate a transient evolution due to changes in geometry with HM3D. We chose to model tunnel boring in 4 excavation phases and then simulate the steady state corresponding to the end of each phase. 

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