Failure of the partially finished earthfill Carsington Dam at the beginning of June 1984 was sudden and catastrophic. In the space of 36 hours, a 400m section of the 1,250m long earthfill dam in Derbyshire slipped, leaving a 15m deep, 30m wide chasm along the crest. At its peak, the slip was moving at 120mm per hour.
At first, the reason for the slip was unclear, but detailed analysis revealed that the unusual boot shape of the clay core (with its 'toe' on the upstream side) set up a non uniform distribution of strain through the dam. Under the large strains induced, the foundation clay - for which there had been very limited strength testing - suffered local brittle failure, resulting in progressive failure of the dam's upstream side.