From design and performance assessment of clay-based barriers to leakage of contaminated water from repositories for hazardous waste the following properties are important:

• Fluid penetration and hydraulic conductivity • Gas penetration and conductivity • Diffusion of ions and fluids

In designing clay seals, the problem of predicting flow through them is of fundamental importance. Unfortunately, the understanding of the mechanisms related to penetration of water and gas is incomplete, and significant mistakes in determining the penetrability in laboratory and field tests have caused false conclusions through the years. A major reason for this is that geotechnicians pay too little attention to the role of the hydraulic gradient: use of very high gradients for saving time in laboratory testing causes microstructural changes that can lead to substantial underestimation of the hydraulic conductivity. On the other hand, overestimation can also occur and lead to unnecessarily expensive technical solutions because the gradients are in practice often lower than those one can use in testing. The dilemma is hence that the very low gradients that prevail in practice cannot be applied in laboratory testing for time and cost reasons and that the very high gradients used in laboratory testing give irrelevant and nonconservative results. These issues are dealt with in this chapter, in addition to a number of practical ones, such as the role of temperature.