Hydraulics tunnel excavation

Pusch R, 1989, Alteration of the hydraulic conductivity of rock by tunnel excavation, Int J Rock Mech Min Sci Geomech Abstr 26(l) 79-83... 

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 simulation started after the installation of heaters, bentonite and plug. The influences against water pressure and temperature in the host rock due to excavation of the tunnel did not considered. In this analysis, initial hydraulic head is specified 100m in the host rock. The initial temperature is 10°C in the all region, and the initial water content of buffer is 14%. 

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.)... 

Figure 4 shows the comparison carried out between the measured and simulated pressures. The general shape of the curves obtained with hydraulic simulation is quite similar to measurement results, except there is a constant difference of about 1 bar between the P4 measurements and the simulation. This point will be discussed in Section 4.5 below. Our simulation approach (a hydraulic steady flow analysis with four steady-state steps) was not able to reproduce pore pressure increase (led to a higher horizontal stress than vertical stress) as excavation neared the monitored borehole intervals and as post-tunnel face dissipation was completed. 

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