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Abandoned waste coal pile in Pennsylvania

A slag heap (also called Boney piles, culm, waste coal, gob piles, or slate dumps) is a pile built of accumulated tailings, which are by-products of mining. These waste materials are mostly composed of shale, as well as smaller quantities of carboniferous sandstone and various other residues.

Slag heaps may be conical in shape, hence appearing as conspicuous features of the landscape, or they may be much flatter and eroded, especially if vegetation is established thereon. The highest in Europe is in Loos-en-Gohelle in the former mining area of Pas-de-Calais. It comprises a range of five cones, of which two reach 180m (590 ft), surpassing the highest peak in Flanders, Mont Cassel.

The oldest coal-based slag heaps may still contain enough coal to begin spontaneous slow combustion. This results in a form of vitrification of the shale, which then acquires sufficient mechanical strength to be of use in road construction. Some can therefore have a new life in being thus exploited; for example, the flattened pile of residue from the site of the 9/11 terrorist attacks. Conversely, others are painstakingly preserved on account of their ecological wealth. With the passage of time, they become colonised with a variety of flora and fauna, sometimes foreign to the region. This diversity follows the mining exploitation. For example, because the miners threw their apple or pear cores into the wagons, the slag heaps became colonised with now more or less forgotten varieties of fruit trees. One can even observe the proliferation of buckler-leaved sorrel (French sorrel - Rumex scutatus), the seeds of which have been carried within the cracks in the pine timber used in the mines. Furthermore, on account of its dark colour, the South face of the slag heap is significantly warmer than its surroundings, which contributes to the diverse ecology of the area. In this way, the slag heap of Pinchonvalles, at Avion, hosts 200 different varieties of higher plants. Some thirty species of birds nest there.

Some cultivate vines, as in the case of slag heap No7 of the coal-mining region of Mariemont-Bascoup near Chapelle-lez-Herlaimont (province of Hainaut) which produces some 3,000 litres of wine each year.

Slag heaps on the site Écopôle 11/19 in Loos-en-Gohelle (right). The town of Liévin is on the left (Picture taken in 2005).

Finally, some slag heaps provide for various sporting activities. The slopes of the slag heaps of 11/19 at Loos-en-Gohelle, or again, at Nœux-les-Mines, are used for winter sports, for example ski and luge, since the provision of a piste on the flank of the heap.

In Belgium, a long distance footpath along the slag heaps (French: terrils) (GR-412, Sentier des terrils) has been opened in 2005. It leads from Bernissart in western Hainaut to Blegny in the province of Liège.

In the United States, mining companies have not been allowed to leave behind abandoned piles since the Surface Mining Control and Reclamation Act was passed in 1977.

The question of re-utilisation of slag heaps is sometimes raised.

• At Nœux-les-Mines, an artificial ski slope has been provided on an old slag heap, the building of which has provided it with a novel use.
• Because they are inappropriate for building purposes, old slag heaps provide valuable green spaces.
• Some slag heaps contain residual coal and evolving techniques allow renewed exploitation of this asset.

Heaps sometimes grew to millions of tons, and, having been abandoned, remain as huge piles today. They trap solar heat, making it difficult for vegetation to take root; this encourages erosion and creates dangerous, unstable slopes. The piles also create acid rock drainage, which pollutes streams and rivers.

As they contain a high proportion of coal, some slag heaps can commence subterranean combustion, which can be followed by surface fires.

Such fires can follow slow combustion of residual coal. Their extinction can require complete encasement, which can prove impossible for technical and financial reasons. Sprinkling is generally ineffective and injecting water under pressure counter-productive, because it carries oxygen, bringing the risk of explosion.

The weak environmental and public health impact of these fires leads generally to waiting for their natural extinction, which can take a number of decades.

With slag heaps there is a danger of landslip. An example was the Aberfan disaster, of Friday, 21 October 1966, in which a total 144 people were killed, 116 of whom were school children, mostly between the ages of 7 and 10. Five teachers also died in the accident.

Wikimedia Commons has media related to:

Slag heaps

• Spoil heap 11/19 in Loos-en-Gohelle (in French)
• http://chaine.des.terrils.free.fr (in French)
• http://www.terrils.be/ (in English,French,German,Italian)
• Turning coal waste into light, 8 October 2004
• Photo of a typical coal breaker with culm bank
• Breaker Boys and Bony Pickers, 28 November 2006