Desiccation of clays can lead to the development of cracks and this is an important issue for mineral barriers used in lining and capping of landfills. The mechanisms of post-compaction moisture loss in a clay layer are reviewed and the results from a field test cell investigating clay desiccation are presented. A large scale instrumented test cell was constructed comprising a clay layer overlain by a geomembrane. Parts of the test cell were covered with a stone protection layer and parts of the test cell were sloping. Temperatures throughout the clay layer, relative humidity just above the clay, ambient air temperatures and crack patterns and dimensions wereobserved during a 36 day summer period. The amount of cracking was quantified using various crack indices. Cracks in the covered and flat sections were minimal whereas in the sloping uncovered portion cracking was initiated rapidly and then spread until the network of cracks at the surface stabilized and further dryingresulted in crack depth propagation. The geomembrane, exposed to the sun, was found to have a significant heating effect on the soil below. Significant moisture was lost over the entire thickness of the exposed barrier resulting in a modelled flow through the barrier of 19 times the construction value. Immediate covering of the geomembrane was found to greatly reduce the temperature variations, moisture loss and cracking. The results are discussed in the context of current crack theories and implications for landfill design and construction.