Landfills of hazardous waste like radioactive rest products with low activity, or ashes from incinerated organic waste, need to have top clay liners for minimizing penetration and percolation of precipitated rain and meltwater. Temperature and dry weather vary interchangeably with wet periods making the clay desiccate and fissure, and subsequently wetted etc. Top liners are commonly made of smectite clay, which is the best isolating soil material, undergoing swelling and shrinkage to an extent that depends on the clay content and density. The most important question is whether such liners, in unfrozen condition and covered by erosionresisting coarse soil, maintain their coherence and tightness after centuries of hydration/dehydration cycles. The present study, made on physically confined soft Iraqi clay with about 30% smectite indicates that initially homogeneous dense clay shrinks and desiccates and becomes fissured at 30oC and room RH, but partly recovers by becoming water saturated by infiltrated water. A limited number of drying and wetting sequences seem to give approximately the same change, suggesting that, under common weather conditions and lack of external disturbance, such liners retain a considerable part of their initial water tightness. Thick liners with moderately to high density and exposed to loading by overlying coarse fill are expected to serve particularly well.