Geotechnical Summary
Table of geotechnical properties
Geology and hydrology
Geological maps and cross-sections
Precursor events
Figures and plates from the Tribunal Report
Other figures




In the mining industry, there has been a history of serious slope failures, but it is hard to think of one so tragic as the debris flow that struck the village school at Aberfan in Wales on October 21, 1966.  A portion of the colliery waste pile above Aberfan liquefied and more than 150,000 cubic meters flowed down hill at high speed. The flow engulfed a section of the village and its school with debris, killing 144 people, of whom 116 were children.

The series of spoil piles, referred to as “tips”, were the biggest feature on the landscape, reaching more than 200 feet above the surrounding hillside. This waste, mostly sand-sized shale fragments but with appreciable clay-size material, was deposited as loose, unengineered fill at the angle of repose of the material (geotechnical summary).

This poor construction was fatally aggravated by poor siting. The tips were underlain by the contact between the Brithdir Sandstone, a highly  transmissive aquifer with appreciable fracture permeability and a low-permeability Quaternary deposit of periglacial origin referred to as “head”. A line of springs near the base of the Brithdir outcrop is the result. Some of these springs had been identified on earlier ordnance survey maps, and the tipping area crossed this spring line, a fact that was not appreciated by the mining engineers.

During the inquiry, representatives of the Coal Board maintained that the development of a spring beneath Tip #7 was a completely unpredictable event, whereas from the geologic map the potential for spring activity should have been obvious. Flow of water into the base of the spoil pile from these springs was judged to have been the critical element in producing the failure. Apparently the presentation of data as maps was not a format that could easily be incorporated by engineers into their planning.

Aberfan presents many of the classic aspects of mine failures. There was poor site selection, followed by poor engineering and a series of precursor events whose significance was not appreciated at the time. These human failures were a result of a focus of the Coal Board and its regulators on the extraction of coal to the exclusion of all other considerations. What happened to the waste material once it was extracted from the ground was simply not part of their conception of the industry.