Paterson, B.R. 1981 The effects of engineering geological factors on batter stability : Ohai no. 16 opencast coal mine. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 358 25 p.
Abstract: At the Ohai coalfield the No. 16 opencast pit is being developed to provide additional coal during the winter peak demand. To expose the main coal seam (Morley No. 2) the opencast mine is being developed down dip, and an increasing amount of overburden is being removed by heavy earthmoving machines. As well as the problem of increasing batter heights (30 m to 100m), opencast development will extend into areas previously mined using underground techniques. A batter stability assessment therefore involves not only undisturbed but also possibly relaxed material as well as underground workings. In this report the affects of geological factors on better stability are outlined, including the distribution and physical properties of the overburden lithologies, and the extent of faulting. Surface drilling supported by existing batter exposures confirm that the overburden will consist predominantly of locally cemented, medium to coarse-grained sandstone, and massive siltstone and mudstone interspersed with minor coal seams. Minor rock defects, eg joints, are poorly developed so that batter failures will be controlled by overburden strength parameters. Because of the possible occurrence of geological faults and the detrimental effect they have on the stability of other open pits, it is likely they will have a significant local effect on batter stability where they are encountered. The effects of groundwater, particularly in combination with relaxed overburden are likely to play an important role in batter stability. As a consequence a groundwater monitoring programme is in progress, and a proposal has been made to excavate a trial batter so that the behaviour of ground disturbed by previous mining can be assessed.
