Effect of strain rate and stress corrosion on the long-term strength of crystalline rocks. Final report

A key factor in the storage of radioactive waste is the long-term stability of the repository. In order to model the rheological behavior of a site, it is necessary to know the time-dependent deformational characteristics of the rock as well as its fracture strength as a function of time or loading rate. In brittle crystalline rocks, the principal mode or deformation is the opening of and propagation of cracks parallel to the loading axis. In the presence of water or water vapor, crack tips are hydrated; and thus cracks propagate time dependently at stresses well below their theoretical bond strength. The rate of crack propagation is proportional to the water pressure, temperature and applied stress. This stress corrosion cracking gives rise to such time-dependent properties as creep, static fatigue failure and the reduction in strength with decreasing strain rate. A series of experiments were done to determine the effect of pore water pressure, temperature and strain rate on the strength of brittle crystalline rocks. Specifically, granite was tested at an effective confining pressure of 300 bars at strain rates of 2 x 10-7 and 7 x 10-9 sec-1 at temperatures of 200, 1000, and 3000C. The pore pressure was varied between 1 and 1200 bars. For a fixed temperature and strain rate, the fracture strength decreased with increasing pore pressure. Typically increasing the pore pressure from 1 to 1200 bars caused a 7% or so reduction in strength. Increasing temperature and decreasing strain rate also resulted in reducing the failure stress