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“The mechanics of coal mining waste rock”

By: Stephen Fityus
Professor in Geotechnical Engineering
Assistant Dean Research for the Faculty of Engineering and Built Environment
School of Engineering
The University of Newcastle

There is a trend in modern open pit mining practice to mine coal seams at increasingly deeper levels, with predictions that the spoil dumps may soon exceed 400m in height. Testing of waste rocks is not routinely undertaken, but even when it is, it is done using inappropriately-sized equipment which requires samples to be modified to particle sizes which are not representative of real mine spoil. Reliable shear strength parameter estimates based on back-calculation and testing which have served the industry well for spoil piles up to around 150m high, but there is unacceptable uncertainty as to whether these can be extrapolated to situations with much higher stresses. This talk will present the outcomes of an industry-funded research program to determine the trends in shear strength under the stress conditions of very high spoil piles, through the design and construction of a very large, high capacity direct shear machine. The results of a testing program of 50 tests conducted on four different spoil types will be presented. The envelope of shear strength is shown to be piecewise linear, but overall, it is shown that the trends used reliably for the moderate stress range for rocky spoils can be reliably extrapolated into the high stress range. The applicability of saturated and unsaturated design shear strength values is considered, and their specification in the design of open pit slopes is discussed.

Dr Stephen Fityus has undergraduate degrees in Geology and Geotechnical Engineering. He is Professor of Geotechnical Engineering in the School of Engineering at the University of Newcastle, Australia, and a principal researcher in the Priority Research Centre for Geotechnical and Materials Modelling. He currently carries the role of Assistant Dean for Research for the Faculty of Engineering and the Built Environment. His research spans a wide variety of interests including the origin and characteristics of expansive clay soils, mining geotechnics of Permian coal measures and mine wastes, the engineering geology of the Sydney Basin, slope stability in bedded sedimentary strata, landform evolution in Tertiary basalts and the engineering characteristics in Quaternary sediments. He teaches engineering geology, soil mechanics and geotechnical engineering design. He is currently a member of the National Committee of the Australian Geomechanics Society and the committee for the drafting of the Australian Standard for Site Investigations.

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