Two large field in-situ direct shear tests were performed at the Leigh Creek Coalfield in South Australia during 1988–89 to determine strength and failure characteristics of bedding plane "weak seams" within low strength mudstone strata. The first test involved the shear jacking of a 14m × 11m × 1.8m rock slab, with a mass of 510t and a shearing surface of 150m2, over a distance of 0.6m. The second test involved a 21m × 13m × 2m slab, surcharged with concrete and sand, with a total mass of 1640t and a shearing surface of 270m2, over a distance of 1.1m. Site preparation, testing procedures and results are presented. Peak shearing stresses were mobilised after 10mm to 20mm displacement; residual conditions after 300mm displacement. The in-situ test results are compared with those derived by back analysis of major landslides at the mine and by conventional laboratory testing of rock cores and small, hand-excavated samples. Potential implications for pit slope stability and ground control monitoring are discussed.
Leigh Creek is one of the deepest open cut coal mines in Australia. At the time of the in-situ shear tests in 1988–89, pit workings were 50m to 90m deep and some mining options being evaluated at the time contemplated pit depths to 300m. Current pit depths in the terrace mining operation are around 200m. Figure 1: Second shear test site. Tsequence at Leigh Creek. These thick, but may be locally 200–300mm, and are typically infilled with silty clay, scattered fine rock gravel and sometimes gouge. Many have a lateral extent of several hundred metres. Slab-sliding failures of low wall slopes have occurred after the coal seams were mined, usually on slopes with strata dips in excess of 16o. Sliding occurs along the bedding plane "weak seams". Groundwater has compounded slope stability.