Microstructural stability controls the hydraulic conductivity of smectitic buffer clay

The hydraulic conductivity of smectitic buffer clays is determined by the volume fraction and continuity of permeable parts of the microstructure, i.e., soft and medium-dense clay gels. The bulk conductivity calculated by use of microstructural parameters agrees well with experimental data except for soft Ca bentonite, which is significantly less permeable than predicted. The microstructural stability of its softest parts is poor, which causes erosion, transport and accumulation of particles yielding clogging of voids and reduction in conductivity. The presence of soft parts explains why water under relatively high pressure can penetrate to a few centimeters depth in partly water-saturated clay and why gas makes its way through channel-like paths in saturated clay.