On 21 October 1966, part of the 67m high Merthyr Vale Colliery spoil tip, perched high on the side of Taff Valley in South Wales, collapsed. It sent 107,000m3 of material plunging downslope, overwhelming the village of Aberfan below. The slide engulfed 20 houses and the village school, where most of the 144 victims lost their lives.
The disaster revealed a lack of knowledge of the physical properties of colliery spoil and of any rational basis for designing spoil heaps and lagoons. In response the National Coal Board mobilised considerable effort to deal quickly with its 2,000 colliery spoil tips to ensure long term stability.
In 1967, Wimpey Laboratories was called in to assist the NCB and an extensive research programme of field laboratory and design projects started. The stability of a large number of tips required immediate investigation (some 15% of those investigated in South Wales have since revealed some kind of structural failure). An interim code of practice was produced in 1968 and superseded by a comprehensive technical handbook in 1970.