Tuesday, 11 July 2006 - 4:20 PM

Rehabilitating Hardsetting Subsoils in a Reconstructed Mined Landscape.

Katharine L. Brown, Christoph Hinz, and Robert Gilkes. School of Earth and Geographical Sciences The University of Western Australia, 35 Stirling Highway, Crawley 6009, Perth, Australia

On the coastal plain of south-west Western Australia mineral sands are mined from ancient strandlines buried several metres below the current ground level. Soil material above the ore is stripped and stockpiled for replacement once mining has ceased. Hardsetting of subsoils constructed from the stockpiles is a consequence of the bulk handling of the soil and is thought to be caused by the rearrangement of soil particles during disturbance. A field experiment to investigate the reduced porosity, increased density and increased soil strength of constructed soils showed that reduced root penetration and water availability are limiting factors for pasture productivity. Our research was driven by two objectives: (i) examining the relationship between strength, particle size and moisture content of the hardsetting and non-hardsetting subsoils and (ii) determining the effects of sand and organic amendments on subsoil strength. The physical properties of mine soil materials including tailings sand, native and constructed subsoil materials, clay overburden and slimes were investigated. Representative, sieved samples of each soil material were packed into small cores as controls. The method was repeated after mixing either 2 or 4% organic matter or 10 or 25% tailings sand with each sample. The cores were saturated prior to equilibration at matric potentials of -10, -33, -300 and -1500 kPa. The bulk density and soil water characteristic of each packed sample was determined in the laboratory. The penetration resistance was tested as a measure of soil strength for each sample using a 6 mm flat tip probe. For penetration resistance values of 1 to 2 MPa and above root growth is expected to be limited and in soils with values greater than 3 MPa root growth may be restricted altogether. The particle size distribution of a representative sample of each soil material was also determined and was of particular interest as size is considered the most important particle property contributing to soil particle rearrangement. In all samples the penetration resistance increased as matric potential decreased. All soil materials except tailings sand and the native subsoil had penetration resistances above a lower limit of 1 MPa at matric potentials of -33, -300 and -1500 kPa. The penetration resistance of the constructed subsoil and clay dominated materials rose to extreme values of between 6 and 11 MPa at wilting point (-1500 kPa). Regardless of the soil material a 2% increase in organic matter resulted in an increase in penetration resistance and in the samples with 4% organic matter the penetration resistance, although slightly lower, remained above that of the controls. The results indicate the severity of hardsetting is moisture and particle size dependent and that testing the penetration resistance of packed soil materials prior to soil construction may be a useful indicator of soil strength. Further research on the processes affecting particle rearrangement and the ability of soil structure to recover from degradation due to disturbance is required.

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