Empirical methods of subsidence prediction—a case study from Illinois

Abstract

Subsidence profiles above two adjacent panels in Illinois are compared with profiles predicting subsidence behaviour obtained using the (i) National Coal Board (NCB) [1] method, (ii) the profile function method and (iii) the influence function method. The NCB method predicts the maximum subsidence values at the centre of the troughs accurately but the overall shapes of these predicted profiles do not match the profiles from the measured data well. Consistent values for the angles of draw were measured by these angles were different in the transverse (average values 43°) and in the longitudinal (average value 17.5°) directions. Time dependent subsidence effects are shown to be small but measurable. These displacements continue at a linear rate for at least a 12 month period. A comparison of measured horizontal distances, interpreted as horizontal strain, and the NCB predictions for strain shows that the peak measured strains are greater by a factor of about four than the predicted strains. The relationship between surface curvature and strain is investigated. Problems associated with calculation of surface curvatures from vertical displacement data are highlighted and a recommendation is made for future studies to consider direct measurement of this parameter. Surface curvatures above a moving face are found to be about three times less than the curvatures at the stationary end of the panel. On the other hand, a hyperbolic tangent profile function is shown to serve as an accurate predictive tool for subsidence behaviour in two adjacent longwall panels at Old Ben Number 24 mine in Illinois. This function predicts not only the vertical displacements but also the surface curvatures above both panels. Influence functions are shown to be more problematic, although potentially more flexible, in their application than the profile functions.