Considering that (i) the hydraulic conductivity of compacted clays is smaller on the wet side of optimum; (ii) the plastic limit is the water content below which the soil develops fissures under small stresses; (iii) the plastic limit and the optimum standard Proctor water content are similar for many natural soils; and (iv) the strength of compacted clays, thus the limit of trafficability, is a function of (w – wopt)/Ip, relevant conditions for the design of clay liners and the evaluation of their hydraulic conductivity are proposed. Key words : compacted clay, liner, hydraulic conductivity, strength, design.