The long-term strength and creep behavior of fully saturated shaly Opalinus Clay

Long-term deformation in tunneling is typically associated with consolidation and creep, two time-dependent processes that may occur simultaneously and are superimposed. However, from tunnel convergence measurements these two processes cannot be distinguished. Thus, an accurate laboratory characterization of creep mechanisms under long-term in-situ conditions (i.e., fully saturated and drained) is required to improve numerical predictions for deep geological nuclear waste repositories. The laboratory study investigates the pure rheological creep behavior of shaly Opalinus Clay. After full re-saturation and consolidation of the specimens, a fully drained multi-stage creep test was performed. Time-dependent axial and radial deformations were monitored during creep stages of constant effective stress. The results show that the creep strain rates increase exponentially with increasing differential stress accompanied by a change in the dominant creep mechanism. Creep strain rates at low differential stresses up to 10 MPa are in the magnitudes of 10-11 s-1 and 10-10 s-1, whereas creep rates in the magnitudes of 10-10 s-1 and 10-9 s-1 are observed at elevated differential stresses of more than 10 MPa, before initiation of tertiary creep, i.e., creep failure. Two thresholds for possible creep failure are presented, defining a stress-related long-term strength and a strain-related onset of tertiary creep.