Permeability 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. 

In Figure 13, one can observe suction as far as I meter from the wall whereas local measures show desaturated cracks 50 cm to 1 meter behind the wall. Along the tunnel wall, the simulation, which does not model the concrete wall, overestimates the damaged zone 2 meters instead of 50 cm observed. According to equation 20, a permeability evolution can be estimated. After excavation, the ration k/ko (current and initial permeabilities) reaches 10 and after 10 months of consolidation, its value is equal to lO". This second value is in good agreement with local measures. 

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