2.7.1 Recognizing geological problems
The most profitable coal mines are those that possess unbroken, horizontal seams of constant thickness and quality. In mines where this is not so, profit levels will depend on the ability of the mine geologist to predict changes in the seam before they are encountered at the face.
Geological problems fall into two categories — gradual changes and sudden changes. Where a change is gradual, such as a seam thinning or splitting, data from boreholes in advance of the workings, supplemented by observations on working faces and development roads can be used to identify those areas where a seam becomes too thin or split to be worked.
Where the changes are sudden, for example with washouts or faults, other methods have to be employed. This may involve the detailed study of the strata exposed in roadways and in borehole sections in order to plot the paths of distributary channels. Forecasting fault positions is more difficult. In particular, unforeseen small faults with a displacement of less than 5 m may require major changes to the mine layout, threatening the profitability of the mine.
On an advance face, information about the geological conditions that lie ahead is limited to that which can be deduced from any nearby workings or boreholes. Therefore, an advance face is a high-risk layout, though this risk has now been mitigated to some degree by using geophysical techniques to 'see' horizontally into the rocks ahead. On a retreat face, the access roads expose any geological problems in the block of coal to be extracted, and the face can be worked back towards the main roadways without risking interruptions. Consequently, the average daily output from retreat faces is some 25% higher than that from advance faces.
Why do geological disturbances result in so many problems for underground mining, yet have very little effect upon surface mining?
Underground mining is inflexible and needs uniform geological conditions to maximize its potential. Such mines are incapable of negotiating any serious variations in the thickness of the seam. Surface mining is extremely flexible and can strip away all the non-coal rocks regardless of geological variations, leaving the coal to be extracted easily.
For sound geological, economic and safety reasons, surface working will always be preferred to underground mining, so long as the stripping ratio is favourable. Over 95% of all near-surface coal can be recovered in this way, and it is relatively straightforward to exclude all the valueless roof and floor sediments during production. By comparison, underground mining rarely recovers more than 70% of coal in thick seams to ensure exclusion of roof and floor rock. Thin seams, even within the mined sequence, cannot be economically extracted. Areas where the seam is disturbed geologically may not be workable, and support pillars may need to be left to prevent roof collapse and subsequent damage to underground roadways or surface buildings.
When a coalface first enters a new area of reserves, it may encounter a variety of geological hazards, which may halt production. Which hazards may be described as gradual changes, and which are sudden changes? What would be the effect on production of each of these two classes of hazards?
Gradual changes include seam thinning and splitting, whereas sudden changes include faulting and washouts. Gradual changes result in the deterioration in the quality of coal produced because more impurities are extracted as the seam thins. Sudden changes result in the face being halted because the seam is suddenly absent.