2018 Scale-Effect Considerations for Shear Strength Assessment of Coal Mine Spoil

One of the most debated subjects among mine geotechnical practitioners is the degree to which shear strength measurements obtained from conventional geotechnical laboratory equipment can be confidently relied upon to predict the shearing behaviour of actual spoil dumps. The reluctance of practicing engineers to apply shear strength parameters determined from standard laboratory testing equipment relates to uncertainty regarding the significance of scale-effects. This is particularly relevant in current times as spoil dumps approach uncharted heights, and load limitations standard geotechnical testing equipment often results in the extrapolation of Mohr-failure envelopes well beyond their intended stress range. There are two scale-effects of note: The first relates to the degree to which the grading of a spoil sample must be down-scaled to comply with device capacity, such that the influence of prototype-sized particles on shear strength is not anomalous. The second relates to the normal stress limits of the test apparatus; and whether the failure envelope developed from measured strengths can be reliably extrapolated out to the “much higher” stress ranges to simulate field conditions. Scale effects on shear strength have been studied in the literature for soils and rockfills, however, the influence of scale on the shear strength of coal-measures spoil specifically has received little attention. This paper presents the results of an experimental study into the significance of scale effects by comparing shear strength data for a silica sand, considered to be immune to scale effects, with an Australian coal mine spoil tested in standard-sized direct shear machines (100mm, 300mm) and in a large direct shear machine (720mm) which was purpose-built to replicate a large-scale field condition for spoil dumps up to 400m in height. Spoils hear strength is shown to be strongly scale-dependent, both in terms of specimen size and magnitude of normal stress. Furthermore, it is demonstrated that adoption of shear strength measurements obtained from standard laboratory equipment will overestimate the available shear strength of coal mine spoil dumps with height-equivalent stresses ranging between 450kPa-4600kPa.