The Mystery of Unsaturated Soils

Prof David Williams

Unsaturated soil mechanics continues to play poor relation to saturated soil mechanics, in part because of the dominance of conventional soil mechanics approaches involving saturated conditions, perhaps because the inclusion of another phase (air) adds considerable complexity, and because a soil at a given density is stronger, less compressible and less permeable (i.e. performs better) than the same soil in a saturated state. However, there are many applications that are unsaturated, not least in the mining field. Mining examples of unsaturated conditions include the wetting up and drain down of initially dry surface waste rock dumps; the irrigation and drain down of heap leach materials; the drain down, desiccation and rewetting of mine tailings; the dewatering of mineral products such as coal; the strength and compressibility of stored mine wastes; and the performance of geo‐covers placed on mine wastes by way of rehabilitation.

This presentation highlights the key unsaturated soil mechanics parameters involved, overviews the nature of mining and mineral processing wastes, and some mineral products, and discusses a number of the issues involved. Some applications of unsaturated soil mechanics addressing the strength, compressibility and permeability of mining and mineral processing wastes, and mineral products, are presented, together with data to highlight them.

Speaker biography

Professor David Williams is the Golder Professor of Geomechanics and the Director of the Geotechnical Engineering Centre within the School of Civil Engineering at The University of Queensland in Brisbane, Australia.  David obtained a BE with First Class Honours in Civil Engineering from Monash University and a PhD in Soil Mechanics from Cambridge University.  He worked for the Victorian State Road Authority and for Golder Associates before joining The University of Queensland.  He has an international reputation in the application of geomechanics principles to mine waste disposal and mined land rehabilitation, and associated issues.  He has made important contributions to the development and implementation of pumped co-disposal of coal mine washery wastes, store and release covers over potentially contaminating waste rock dumps in semi-arid climates, capping soft tailings, the hydrology of waste rock and tailings storage facilities, the settlement of high coal mine spoil piles, understanding problematic clay-rich coal mine tailings, and risk assessment and cost-effectiveness analysis applied to mining.

The Geotechnical Engineering Centre, which David initiated, is funded by Golder Associates, Rio Tinto, AngloGold Ashanti and BHP Billiton, with matching funding from The University of Queensland, to a total of $6 million over 5 years.  Under this Centre, academic staff numbers are increasing to seven, with supporting Postdocs, and unique Geotechnical Engineering Dual Major Programs for Civil and Mining Engineering students have commenced.  These programs have attracted very strong student enrolments and graduations will reach 70 per year.  Postgraduate research student numbers are 20, supported by research funding approaching $4 million.

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