Photo from The Geological Society of London: The extent of the final landslip.
Text below from EA/DEFRA paper: Lessons from Historical Dam Incidents:
The dam had a rolled clay core with an upstream extension (the boot) and shoulders of compacted mudstone with horizontal drainage layers of crushed limestone about four metres apart (Davey and Eccles, 1983). The upstream slope was 1:3 and the downstream slope 1:2.5. Fill placing began in May 1982 and took three summers, with winter shutdowns. A small berm was placed at the upstream toe to compensate for a faster rate of construction in August 1983. Earth filling restarted in April 1984 and was one metre below the final crest level on 4 June 1984 when the upstream slope slipped.
Text below from EA/DEFRA paper: Lessons from Historical Dam Incidents:
The dam had a rolled clay core with an upstream extension (the boot) and shoulders of compacted mudstone with horizontal drainage layers of crushed limestone about four metres apart (Davey and Eccles, 1983). The upstream slope was 1:3 and the downstream slope 1:2.5. Fill placing began in May 1982 and took three summers, with winter shutdowns. A small berm was placed at the upstream toe to compensate for a faster rate of construction in August 1983. Earth filling restarted in April 1984 and was one metre below the final crest level on 4 June 1984 when the upstream slope slipped.