The effect of over excavation on the time-dependent convergence of a drift in Callovo-Oxfordian claystone

Convergence measurements recorded for a drift excavated in two stages in Callovo-Oxfordian claystone are analyzed. In the first stage the drift is excavated with a diameter of 3.8 m. Then, three months later, the drift is over excavated to a final diameter of 5.2 m. In-situ measurements show an anisotropic convergence during the first stage with maximum and minimum values in horizontal and vertical directions respectively. In the second stage, the convergence is quasi isotropic. The closure of walls is fitted using a semi-empirical convergence law. The results show that in the first stage, the values for the parameters describing the time-dependent behavior are very close to those obtained for drifts excavated in the same direction (5 m diameter). It is observed that these values are lower in the second stage, suggesting that the over excavation could limit the delayed response of the ground. In contrary, this excavation method yields to a greater extent of the fractured zone in descending vertical direction.