2.6 Underground mining
Coal extraction is of course less straightforward using underground mining techniques. The associated costs are higher, and these begin with the sinking of two shafts, an 'upcast' and a 'downcast' shaft for ventilation (Figure 22). Sinking these to a depth of a kilometre may take a few years and during this time, no coal is extracted. However, not all underground mines involve deep, vertical shafts. Coal seams less than 350 m deep may be reached by inclined tunnels, or drifts. In hillsides, horizontal adits may be opened up directly into the coal seam, producing an earlier return on investment.
The coal seams exploited in most European mines are typically about 1.5 m thick, but can vary from 0.5 m to 3 m. Modern underground mines use longwall extraction methods (Figure 8c), relying on highly mechanized extraction techniques.Acutting machine works its way along the start of the planned extraction to develop a roughly 250 m long coal face (Figure 23). Temporary hydraulic jacks support the roof at the face. Cut coal falls onto a conveyor belt laid out parallel to the coal face, and is carried away.
In the most recent installations the conveyor is articulated, and advances automatically, together with the roof supports, as the cutter moves along. By the end of each traverse, the face is ready for the next run in the opposite direction. As the face advances, supports are removed and the roof is allowed to fall into the cavity (goaf) left when the coal is removed (Figure 24).
Tunnels or gates at either end of the face link it to the colliery's permanent 'road' system. These roads are used for access of miners and machinery, to ventilate the face and remove the coal. In both new and established mines, roads have to be driven to new areas of coal working. Cutting gates and roads is not a profitable operation in itself, but it does provide valuable geotechnical information to assist the mining operation. Like faces, roads and gates require support, using concrete linings, steel arches or bolts secured in the rock with resin. Wire mesh, secured by the latter, prevents minor rock falls.
The road network links the faces (most modern mines have several working at any one time) to the two shafts (Figure 22 and 24) that allow coal and stale air to be removed, and fresh air, workers and supplies to enter the mine.
Figure 25 illustrates the two common types of underground mine layout used in the UK. At an advance face, the coalface is advanced into a block of coal. The access gates at either end of the face are also advanced to keep pace with the face. At a retreat face, the two access gates are first driven to the far boundary of the block of coal before the coalface is opened at their extremities. The face is then worked back towards the main roadways and the roof collapses in the same direction.
Until the late 20th century, most underground mines in the UK were separate operations, dating from the period before nationalization in 1945, although some adjacent mines were linked. The last major underground coal development in the UK focused on newly proven major reserves in North Yorkshire, well away from the existing coalfield of South Yorkshire. It involved mine planning on a huge scale, involving several linked mines (Box 2).
Box 2 The rise and fall of an underground coalfield: the Selby complex in North Yorkshire
The UK's most recently developed underground coalfield, below Selby in North Yorkshire, was initiated between 1972 and the first production of coal in 1983. The Selby complex provides not only a good illustration of the development of a modern coalfield, but also how economic forces can dictate the future of a prospect.
Coal seams in the Yorkshire coalfield dip eastwards under a cover of younger Permian rocks. As demand for coal increased and the exposed coalfield became exhausted, the working area extended progressively eastwards to the deeper levels of the concealed coalfield.
In the 1960s, exploration drilling showed that the principal seam (the Barnsley seam) is between 1.9 m and 3.25 m thick under the Vale of York, north of the town of Selby (Figure 26). This represented an exciting prospect of workable coal covering an area of 260 km2. A major drilling programme commenced in 1972, and from 68 deep boreholes, drilled approximately 1.5-2.5 km apart, reserves of 600 × 106 t were calculated in the Barnsley seam. The coal was found to be very clean, with 2-8% ash content (ideal for the power station customers who specified no more than 18% ash).
Stripping ratios of up to 550:1 at North Selby were far too high for surface mining, so the coal could only ever be extracted by underground methods. The mining complex consisted of two parallel 12.2 km long tunnels that provided the main coal outlet to the railway at Gascoigne Wood. Other tunnels connected working areas in different parts of the coalfield to this main outlet. Shafts served each working area, by providing ventilation and allowing access for workers and machinery (Figures 26 and 27).
The drilling programme had proved nine other seams of workable quality lying at depths between 300 m and 1100 m. However, the land in the Vale of York is only some 7 m above mean sea-level, so surface subsidence with a consequential risk of flooding was an important consideration. Because of this, planning permission limited extraction to just the Barnsley seam, thereby limiting the surface subsidence to less than a metre, with no extraction around the towns of Selby and Cawood (Figure 26) and some industrial areas. Leaving nine seams behind, as well as a substantial part of the Barnsley seam itself reduced the total amount of recoverable coal from 2000 × 106 t to just 224 × 106 t.
What proportion of the coal in the Selby coalfield was recoverable? How long would it take the miners in the Black Thunder strip mine to extract a similar volume of coal?
The proportion of recoverable coal at Selby was
produced compared with the cost of removing the waste material. In addition, surface mining machinery is unable to operate below a certain depth.
The Black Thunder mine produced 51.5 × 106 t of coal in 2003. Therefore, to produce 224 × 106t would take
The £400 million colliery complex started production under very strict environmental controls in 1983 and output peaked in the early 1990s at 10 × 106 t yr-1. At the height of production, coal from ten retreat faces was transported along the network of conveyor belts to the coal-handling point at Gascoigne Wood. There the coal was washed and blended (by mixing coals of different ash contents from different mines) before being transported by rail to the nearby power stations at Ferrybridge, Eggborough and Drax A and B. Although Selby coal then represented about 15% of all coal used for electricity generation in the UK, it also supplied the domestic 'house coal' market too.
Output at the Selby complex fell to 6 × 106 t yr−1 by 1999. This decline in production resulted from an over-reliance on the power stations as customers, with contracts agreed at a time of low global coal prices. In the end, Selby was unable to maximize its potential, despite long-term contracts with the power stations, the expansion of their market beyond power station fuel, and despite repeated initiatives to maximize production and cut costs. The last mine at Riccall closed late in 2004, some five years short of that originally envisaged for the complex.