Pore water pressure response to small and large openings in argillaceous rocks

Document available in extended abstract form only. In the last decade an important amount of piezometers have been installed in the Bure Underground Rock Laboratory (URL) in the vicinity of ongoing works involving gallery excavations and drilling of boreholes and alveoles both in the major and minor stress directions. Relatively far field piezometers (placed one to four diameters from the excavation wall) showed a qualitatively consistent response at different scales. Here, we investigate whether the pore water pressure response around openings of different scales may be up-scaled. An attempt is made to find a common set of parameters that explains quantitatively the rock response at the different scales. The mechanisms underlying the pore water pressure response around an underground opening are twofold. The first class of mechanisms is usually associated with nearly undrained behaviour and the related pore water pressure changes are induced by the stress redistribution triggered by the creation of the tunnel opening causing a reorientation of the principal stresses and influenced by the initial stress anisotropy. These pore water pressure changes are closely linked to the mechanical constitutive law of the rock and to the damage zone around the opening. The second class of mechanisms is related to the drainage of excess pore water pressure relative to a state governed by the atmospheric water pressure condition prescribed at gallery wall and the water flow law, usually Darcy's. Strong anisotropy effects on the hydraulic response of Callovo-Oxfordian Clay can be observed with reference to Figure 1 that shows the pore pressure response to the drilling of a 150 mm-diameter borehole performed to install a heater for the TER thermal experiment. The borehole is aligned with the major horizontal principal stress. Therefore, in principle, the stress state should be approximately isotropic in a cross section of the borehole. As a matter of fact, however, a degree of initial stress state anisotropy develops because of stress redistribution associated with the excavation of a number of drifts in the vicinity. The results of two pore pressure sensors in a cross-section at 7 m from the borehole mouth are presented: one is located at 0.53 m from the borehole axis in the horizontal direction and the second one is located at 0.67 m from the borehole axis in the vertical direction. In the horizontal direction a very large increase of pore pressure is observed and a somewhat smaller reduction of pore pressure is obtained in the vertical direction, consistent with the fact that it is slightly further away from the borehole. In both cases the water pressure response correlates closely with the drilling advance and a significant increase/decrease of pore water pressure is observed when the drilling tool passes the sensor section. In this case, the main cause of the hydro-mechanical response is the anisotropy in stiffness (the material is more stiff in the direction of bedding, i.e. horizontal), possibly enhanced by the effects of some in situ stress anisotropy. Afterwards, the excess pore water pressure dissipates. The dissipation seem to occur more rapidly in the horizontal direction accordingly to the smaller distance to the draining heater borehole and also, most likely, to the higher permeability in that direction. (authors)