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Aberfan remembered: An enduring legacy

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Fifty years after the Aberfan landslip which claimed 144 lives, lessons are still being learned.

The Aberfan disaster, 50 years ago, remains deeply etched on the collective memory of the UK populace; indeed, it is one of only a few British peacetime disasters that are familiar even to those born years later.

Fifty years on, there remains tremendous power in the images of the aftermath of the accident, which caught the moments in which hardened miners dug through mounds of mine waste that they had generated with their own hands, in a desperate race to save the children of their community.

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With shovels or bare hands, rescue workers tear into the mud and rubble burying the ruins of one of a row of seven houses which were engulfed by a moving mountain of coal slurry at Aberfan

Their faces show the grim reality that miners, all too familiar with accidents in which victims are buried, knew well – that the likelihood of survival when engulfed by rock and soil is terrifyingly low, rendered worse by the saturated nature of the filthy coal mine waste that engulfed the school, and by the high vulnerability of children to the effects of burial.

The events of that day are well-known, but bear repeating. The flowslide occurred from Tip 7 of the Merthyr Vale colliery on the valley slopes above the village of Aberfan on 21 October 1966. The progressive failure was first noted when the crew responsible for waste tipping arrived on site at around 7.30am on the morning of the disaster.

They reported that the tip had deformed by about 3m overnight, and tried to send a message down to the mine office that a landslide was developing. However, this had to be delivered in person as they did not have a telephone, introducing a huge delay.

An hour later, shortly before the collapse, the same crew noted that the slope had deformed by a further 3m or so, and that the movement was now sufficiently rapid to be observed with the naked eye. When the final collapse occurred at around 9.10am, the tipping crew reported that the transition from a visibly creeping mass to a flowslide appeared to be spontaneous and dramatic; they then reported the mass moving as a series of waves down the slope.

The volume of the landslide was estimated to have been about 105,000m3, which is not especially large for a flowslide of this type. The landslide travelled around 500m before entering the village. The official report suggested that the velocity of the landslide front was between 17km/h and 34km/h.

The landslide struck and relentlessly engulfed 16 houses and the Pantglas Junior School, killing

144 people, of whom 116 were children, mostly aged between seven and 10 years old. Five teachers were also killed and a further 29 children and six adults were injured. The last survivor was rescued less than two hours after the accident.

Aberfan was a key moment in our understanding of landslides, and of making mining safer. Sitting alongside the 1963 Vajont landslide disaster in Italy, the aftermath of which was also graphically depicted in monochrome photographs, this landslide was hugely significant in terms of the development of science and understanding.

While elements of the aftermath of the landslide were handled in a manner that we might now question – the use of £150,000 of charitable funds to remove spoil tips from above the village even though the money had been donated by the public for the victims, for example – the scientific investigation of the disaster was undertaken rigorously and involved acknowledged authority figures in geotechnical engineering.

Professor Alan Bishop, along with colleagues at Imperial College London, led the scientific investigation. But it also had input from the Meteorological Office, the Institute of Geological Sciences (now the British Geological Survey), the National Coal Board and various other organisations.

The investigation demonstrated that the failure in Aberfan’s Tip 7 was just the latest in a long succession of similar events. A significant flowslide had occurred nearby at Abercynon in 1939, an event that was actually larger than Aberfan, and which came close to causing loss of life. Aberfan’s Tip 4 had failed in 1944; and Tip 7 itself had failed in 1963.

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The moving mountain of coal sludge after the disaster at Aberfan when the coal tip avalanched through the Pantglas Junior School, killing 116 children and 28 adults

Indeed the report of the public inquiry noted: “Tip slides are not new phenomena. Although not frequent, they have happened throughout the world and particularly in South Wales for many years, and they have given rise to quite an extensive body of literature available long before the disaster.” But it added that “there was no general apprehension in the National Coal Board regarding tip stability”.

The public inquiry report, although carefully written, was highly critical of the way in which tip stability had been managed. Thus, Aberfan sent a message through the geotechnical and mining industries about the importance of collecting data on, and learning from, previous events. This message remains current.

But it is also true to note that the mechanics of flowslides of this type were poorly understood prior to Aberfan. Flowslides in finer-grained materials were reasonably well described, particularly in relation to coastal failures in the Netherlands and quick clay slides in Norway. Bishop wrote a summary of his analyses of

Aberfan, and other flowslides, in a 1973 paper in which he highlighted the tendency of soil mechanics at the time to focus on processes up to the point of failure, and to ignore what happens thereafter.

He demonstrated the importance of the brittleness index – the ratio between peak and residual strength, in determining post-failure behaviour. In particular he noted that where the Brittleness Index is large, high strain rates could develop in the post-failure domain. He used multiple examples to highlight and interrogate this behaviour. This observation has proven to be astute.

The Aberfan flowslide also depended on the availability of large volumes of water. In this case the tip had been constructed on top of a spring that created saturated conditions after periods of heavy rainfall. Thus, Aberfan highlighted the importance of understanding site hydrogeology prior to and during tipping.

Nonetheless, the investigations immediately after Aberfan did not fully solve the problems of flowslides, and in particular static liquefaction behaviour. Bishop himself noted that aspects of this behaviour remained poorly constrained, and even speculated that air entrainment might play a role, which we now know is not the case in most flowslides of this type. These problems had to wait for several more years to be resolved, but built upon the Aberfan work.

But the investigations of Aberfan were more than sufficient for the public inquiry to make strong recommendations in terms of mine waste management. In particular, they highlighted the need for proper understanding of the hydrology and hydrogeology of tip sites, and of the materials upon which the tip is constructed, as well as proper management of the tips themselves.

The government and the National Coal Board subsequently took up these recommendations, and to a large degree they have also been implemented internationally. Many dangerous spoil heaps were removed or re-engineered, even though in the public inquiry the National Coal Board argued that this might be prohibitively expensive.

Far greater attention was paid to the properties of the location of spoil heaps, in particular in relation to the hydrogeology; to the mechanical properties of the waste itself, in particular in relation to the amount of fine-grained material present; and to the density of the tipped waste.

No large-scale repeat of the Aberfan disaster has occurred in the UK, and given the limited amount of mining that now occurs, and the strict regulatory environment, a similar event does not seem likely.

Of course the accident at Aberfan also had a profound impact in other areas of understanding of such disasters. Subsequent studies looked at the impact of trauma on communities by: evaluating changes in the birth rate in the village after the disaster; looking at the impact of post-traumatic stress disorder on survivors; and considering the interactions between governments and large-scale disasters. Many of these studies have proven to be as influential in their own fields as was the investigation of the geotechnical aspects of the failure.

However, even after 50 years the lessons of Aberfan have not been learnt universally. In many less developed countries the rate of mining and quarrying related flowslides remains surprisingly high, and these take a terrible toll. The above table above lists major fatality-inducing mining, quarrying and waste management flowslides in the last five years; most of these involve sliding in mine waste, and thus are analogous at some level to Aberfan.

Some are remarkably similar. The impact has been almost 1,000 deaths, mostly in China and Myanmar.

A cumulative plot of human losses over this period shows acceleration in the last three years. This is driven primarily by the sequence of major flowslides in the Hpakant area of Myanmar in the last two years, which have involved the collapse of waste heaps created from jade mining. The high death tolls reflect the dangers of unregulated extraction of jade from the waste piles by artisan, usually poverty-stricken and often blameless miners.

The socio-economic background to these mining operations is complex, but the intensity of jade mining in this area has increased dramatically in recent years as operators try to maximise extraction rates prior to anticipated increased levels of regulation as Myanmar transitions towards a democratic governance system. The consequences for the poor, artisan miners in the Hpakant area are severe.

And of course the failure of tailings dams, and the transition of the waste into highly mobile flowslides, as illustrated by the Bento Rodriguez failure in Brazil in 2015, remains alarmingly common. It is hard to think of any other area of geotechnical engineering that has such a high failure rate with such devastating impacts, and at least superficially it remains deeply surprising that this level of impact is tolerated.

The legacy of Aberfan lives on, and will continue to do so for the foreseeable future. Aberfan highlighted the risks posed by types of materials, changed the ways that mining operations are undertaken in most countries, drove an understanding of the processes of static liquefaction and inspired considerable future research. The challenge now is to ensure that these lessons are applied everywhere.

  •  This article was written by University of East Anglia pro-vice chancellor Dave Petley

 

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