Mining in focus: Mining and the timber trail

It is not an exaggeration to say that the timber industry in South Africa grew on the back of mining, writes Dr Nicolaas Steenkamp.

The expansion of underground mines depends on quality support to keep the mine safe and operational. For many years, timber was the choice of underground support for most South African deep-level mines.

M TAU LEKOA STICKS 1 NW ZA 2011 09 03 LL 025
Timber ready to be installed as roof support in the Tau Lekoa Mine close to Orkney in the North West Province.
Image credit: Leon Louw

Historical mines and current mines, especially deeper underground mines, depend heavily on timber support to keep working areas safe. In more recent times, timber stacks and combination timber stacks have been replaced by mechanical props and supports, but it still remains one of the more cost- and time-effective ways to support underground mine workings. The main mine timber plantations are located along the escarpment in Mpumalanga, and its origins are closely related to South Africa’s gold mining history.

Growing timber support

The growth of the South African timber industry is intimately linked to the rise of the gold mining industry in the Sabie and Pilgrims Rest areas and later the Witwatersrand. In the late 1800s, the Transvaal Gold Mining Estates operated mines under the control of The Corner House. Timber supports were initially obtained from the indigenous forests, but this source of timber was quickly nearly exhausted. The farms Elandsdrift, Hendriksdal, and Klipplaat were the location of the first timber plantation in the then Eastern Transvaal of the Zuid-Afrikaansche Republiek.

The climatic conditions and nature of the soil in this area are well suited to growing wattle and eucalypts. The method of ground preparation was by ploughing to a depth of 10–15cm and harrowing it to a loose tilth. Seeds of black wattle was hand sown in rows one metre apart and 3.6m in length. At the age of 10 years, the trees produce marketable tannin bark and poles suitable for mining purposes. A hectare yielded eight to 12 tons (t) of tannin bark and 40t of wood, depending on the depth and nature of the soil. Following the success of the wattle-growing programme, a large commercial timber scheme was launched in 1919 with the planting of pines. Pine trees then became the major cultivar planted and harvested.

M TAU LEKOA FALLING ROCK 1 NW ZA 2011 09 03 LL 029
Timber support combined with other methods in a gold mine on the West Rand of Johannesburg.
Imagr credit: Leon Louw

The high tannin bark was sold off to third parties and the timber transported almost exclusively to the mines on the Witwatersrand. Since that time, the plantations in the area have grown exponentially and have changed ownership several times. The famed geologist Dr Hans Merensky was also involved in the mine timber industry. He operated a timber processing company, owning several sawmills in four different parts of the country and growing eucalyptuses for sawlogs, poles, and mining timber on plantations near Tzaneen.

Best timber for support

The two classes of trees most suitable for mine timbers are the coniferous and deciduous trees. Both types of trees grow from the centre outwards, adding a new layer of wood each year. The most suitable timbers are long-grained, giving it elasticity as well as strength. A dense, resinous, slow-growing wood type is always preferable for mine support purposes. The result is that a prop subjected to pressure on its ends will bow outwards considerably without breaking, while still resisting the applied pressure. If the pressure were to be removed from the prop, it would regain its original shape. A prop should be strong enough to bend without breaking. These wood types shatter slowly when their loading capacity is exceeded and thus warn the miners through a definite creaking.

Collapsed untreated timber stacks due to prolonged wetting
Collapsed untreated timber stacks due to prolonged wetting. Timber employed in moist or wet mines should be treated with a preservative beforehand.
Image credit: Dr Nicolaas Steenkamp

Posts, also called props, may be either round or splits. Round props are the natural logs, cut and used in a green or seasoned state. Splitting rather than sawing is preferred if an individual stick is big enough so that several props can be made from it. Splitting does not destroy the sapwood or cause undue damage to the grain or fibre of the prop. An axe-cut prop with its uneven bearing surface will tend to bend and could pop out of place should a force be applied to it.

Timber packs, or a combination of timber and brick packs, are the simplest roof support method in incompetent rock, for example in faulted zones, and contribute greatly to increasing mine safety. It is, however, disadvantageous for artificial mine ventilation, in that it increases air resistance. The stability of packs also decreases with increasing height; timber packs with height-to-width ratios exceeding 2:1 are generally considered unstable. Untreated timber packs are also the main risk for underground fires. Untreated timber is impregnated with water — its moisture content is raised to about 70% from a typical value of 20%. All the water present must be vaporised into steam before the timber itself can be heated to a temperature of about 280°C, at which combustion can commence and the reaction becomes exothermic.

Installed timber packs
Timber packs, or a combination of timber and brick packs, are the simplest roof support method in incompetent rock, for example in faulted zones, and contribute greatly to increased mine safety.
Image credit: Dr Nicolaas Steenkamp

Timber employed in moist or wet mines, especially for shaft construction in exhaust ventilation shafts or drifts in water-bearing strata, should be treated by a preservative beforehand. Timbers are impregnated by dipping them into special solutions, such as Roman salt or copper sulphate, either with or without pressure.

Timber installation types

Each day in South African gold- and platinum mines, about 10 000 packs are built for roof support. In an underground mining operation, framing or timbering supports usually consist of two props and one roof bar made of wood or iron. There are three main methods of installing timber supports: the German framing method, the Polish timbering method, and the ‘Silesian’ method. The German framing method is characterised by the interlocking or scarfing of bars and props. The Polish timbering method is distinguished by a loose laying of the roof bar on top of the two grooved props, while the ‘Silesian’ method includes a ‘soldier sprag’ brace wedged between the props underneath the roof bar to reinforce the props against lateral compressive forces.

Dwindling supply

The South African government introduced restrictions on the issuing of forestry and planting permits about 20 years ago in light of dwindling water resources. Presently approximately 70% of the sawn timber in South Africa is used for construction purposes. The rest is used in the furniture sector, the joinery sector, and the packaging sector, with a small percentage also going into industrial utilisation, such as mining supports.

Pine plantations
A large commercial timber scheme was launched in 1919 with the planting of pines. Pine trees then became the major cultivar planted and harvested.

Due to a shortage of suitable land, it is estimated that South Africa will have to import nearly half its pine structural wood requirement within the next two decades. Supply solutions are being investigated, which include a review of the planting permit or licensing system, efficiency improvement by the sawmilling industry, the importation of sawn lumber, the establishment of faster-growing species, and the development of more forestry areas.


 

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