The closure and strain are assumed to be both driven by the in situ horizontal stress, and it is reasonable to assume that the compressive strains will reduce as the in situ stress reduces. Since the graph in Figure C6 has been based on data from Tower Colliery, where the in situ stress is particularly high, it is expected that the graph will generally be conservative and could overpredict strains by 100 per cent in some cases, particularly where the predicted levels of strain are low. The data spread in the graph shows the variations that have occurred in practice, and provides a guide to the potential range of strains that might occur in a particular case. Note: Observed closure across steepest section of valley (i.e. cumulative bay length differences within steepest section of valley [mm]) © Copyright, Waddington 2002 (Figure 12.3) Figure C6 Maximum compressive strain versus valley closure. From step 5, the closure is 131 mm. From Figure C6, the compressive strain for a 20-m-wide section is 6.4 mm/m. Therefore, the predicted compressive strain at point A in Figure C1 is 6.4 mm/m. Checking this value with the suggested threshold for compressive strain of 2–3 mm/m, the calculated value is greater than the threshold value; therefore, a swamp at point A would likely be damaged. The longwall panel configuration needs to be reassessed to lower the compressive strain. Reassessment of longwall configuration The data are as before, except that the longwall has been shortened so that the end of the longwall is now 450 m to the east of point A, as shown in Figure C7. Other configurations may also be applicable. The original longwall position is dashed. Figure C7 Revised site information. In this case, the information for step 1 and steps 3–4 are the same and the graphs have not been reproduced. However, this always needs to be checked. Step 1: Calculate valley closure due to mining Longwall 2 The closure is the same as for the original longwall layout at step 1 above—190 mm.
The closure and strain are assumed to be both driven by the in situ horizontal stress, and it is reasonable to assume that the compressive strains will reduce as the in situ stress reduces. Since the graph in Figure C6 has been based on data from Tower Colliery, where the in situ stress is particularly high, it is expected that the graph will generally be conservative and could overpredict strains by 100 per cent in some cases, particularly where the predicted levels of strain are low. The data spread in the graph shows the variations that have occurred in practice, and provides a guide to the potential range of strains that might occur in a particular case. Note: Observed closure across steepest section of valley (i.e. cumulative bay length differences within steepest section of valley [mm]) © Copyright, Waddington 2002 (Figure 12.3) Figure C6 Maximum compressive strain versus valley closure. From step 5, the closure is 131 mm. From Figure C6, the compressive strain for a 20-m-wide section is 6.4 mm/m. Therefore, the predicted compressive strain at point A in Figure C1 is 6.4 mm/m. Checking this value with the suggested threshold for compressive strain of 2–3 mm/m, the calculated value is greater than the threshold value; therefore, a swamp at point A would likely be damaged. The longwall panel configuration needs to be reassessed to lower the compressive strain. Reassessment of longwall configuration The data are as before, except that the longwall has been shortened so that the end of the longwall is now 450 m to the east of point A, as shown in Figure C7. Other configurations may also be applicable. The original longwall position is dashed. Figure C7 Revised site information. In this case, the information for step 1 and steps 3–4 are the same and the graphs have not been reproduced. However, this always needs to be checked. Step 1: Calculate valley closure due to mining Longwall 2 The closure is the same as for the original longwall layout at step 1 above—190 mm.