Mechanical interaction between swelling compacted clay and fractured rock, and the leaching of clay colloids

We consider the interaction between a saturated clay buffer layer and a fractured crystalline rock engineered disturbed zone. Once saturated, the clay extrudes into the available rock fractures, behaving as a compressible non-Newtonian fluid. We discuss the modelling implications of published experiments carried out in Sweden and more recently in Japan. Extrusion is halted either when the advancing clay front reaches a narrow enough aperture along the fracture (relative to the yield stress of the gel-like front), or when enough of the clay mass has extruded so as to reduce the density back in the buffer, and hence the swelling pressure (which is exponentially dependent upon density). In the latter case a relatively small reduction in density may be sufficient. As the clay extrudes, the gel-like front may be a source of clay colloids, being sufficiently hydrated so as to allow clay platelets to escape the matrix and diffuse away. We show that such mass loss is limited as a mechanism for leaching away the emplaced barrier, yet may still be significant in mobilising otherwise highly sorbing radionuclides within the buffer.