Effect of fluid pressure in ground disposal site

Relations between compressive stress and fluid pressure are discussed on the basis of a three-dimensional elasticity theory using a model for a disk-like crack. A two-dimensional void is more readily collapsed than spherical one. Furthermore, collapse of a void occurs more easily as the fluid pressure in the void increases. Regarding the fluid pressure, the effective stress rule can generally be applied. Fracture of rock bed depends on the effective stress, i.e., the difference between stress within the rock bed and void fluid pressure. An increase in fluid pressure in a void, or a decrease in effective stress, is likely to decrease the rupture strength and increase the brittleness of the rock, leading to the production of cracking fracture. If this occurs, the permeability will sharply increase causing an extreme decrease in isolation performance of the rock bed. The shape of a repository is also an important factor to its performance. Ideally, it should be spherical to avoid concentration of stress. A pressure in a repository can act to stabilize the cavity if there is no stress concentration and if the stress within the rock bed is isotropic. Absortive and desorptive properties as well as rheology of filling materials such as clay and zeolite should be studied more detailedly. (Nogami, K.)