Structural geological factors in open pit coal mine design, with special reference to thrusting: case study from the Ffyndaff sites in the South Wales Coalfield

Abstract

A review of the shear strength characteristics of Coal Measures lithologies suggests that major structurally controlled discontinuities in mudrock or coal are likely to show near-residual shear strength values, with minimal cohesion and a friction angle of approximately 12°. The principal structurally controlled discontinuities are those associated with tilting, folding and faulting. The former result from bedding plane slip during flexural folding that smoothes the bedding planes and reduces cohesion. The latter are produced by fault movements that weaken the rock and produce a structural anisotropy in the fault surface. Both are likely to confine the movement of water through the rock, enhancing weathering and further weakening the surface. In open pit design, the stability of the highwall is discussed in relation to the orientation of the various types of structurally controlled discontinuity. A case study from Ffyndaff Opencast Coal Site in the South Wales Coalfield is analysed, and shows that the major normal faults that cut the site intersect the highwall with a stable orientation. Regional dip in the site also has a stable orientation, but the site is affected by several major north-verging thrusts that have been investigated by computer analysis of borehole and coal extraction data. The results indicate that the thrusts have a ramp-flat geometry and are likely to imbricate upwards from a bed-parallel detachment in a coal seam. The thrusts have generally developed in a piggy-back sequence, although at least one formed as a break-back thrust. The thrust displacement (slip separation) variation along the thrusts is described and suggests a normal displacement gradient of 0.16-0.18. These geometrical characteristics are used to interpret the thrust structure of a potential site from exploratory borehole logs, where the predicted geometry limits the location of the bounding highwall.