3.3.1 Mining subsidence
Subsidence is an inevitable hazard wherever underground mining is carried out.
The major factors affecting the extent of subsidence are seam thickness and its depth beneath the surface.
The amount of subsidence can be calculated roughly by using the formula:
where s is the amount of surface subsidence (in m), t is the thickness of the worked seam (in m), and d is the depth to worked seam (in m).
(a) Plot a graph on Figure 29 to investigate the subsidence produced at depth by mining a seam of 3 m thickness under the Selby area.
Hint: You will need a calculator to do this. You will have to work out, using Equation 2.1, the amount of subsidence s for a seam thickness of 3 m for a range of working depths. Use a range from 50 m to 750 m, at 100 m intervals, and then record your calculated values of s and d in a table. Then plot your values of s and d on Figure 29, and draw a curve through them.
(b) From your graph, what is the minimum depth at which a 3 m seam can be worked to produce less than the 0.99 m of surface subsidence demanded at Selby?
(a) Using Equation 1, you should have been able to calculate the values in Table 2. Your graph of s against d should be similar to Figure 30.
(b) The graph shows that a 3 m seam can be worked at depths of about 60 m and deeper if it is to produce less than the 0.99 m of surface subsidence demanded at Selby.
Roof collapse will often start within 24 hours of coal extraction, but the full effects are transmitted rather slowly upwards, eventually resulting in subsidence at the surface (Figure 31). It may be over 10 years before the surface is completely stable again. Vulnerable structures, such as conurbations, dams, viaducts, and historical buildings are protected by leaving coal unworked beneath them, but such protection may be extremely costly where it significantly affects the layout of the mine. The Selby mine highlighted a particular problem of subsidence by affecting the land surface in a low-lying area. At Selby, government-imposed restrictions minimized both the visual impact of mine site buildings and the effects of ground subsidence in areas particularly liable to flooding or subsidence damage. The mine operator also had to finance flood protection measures and the relocation of the main east coast railway line.