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Topsoil and Overburden Removal

The soil profile is made up of a number of layers, including the topsoil and overburden layers. The topsoil layer (0-15cm) contains a large store of seed and nutrients that are vital to the success of the future mine rehabilitation.  The overburden layer is 30-100 cm of gravely sub-soil material sitting above the caprock.  Before mining can begin, the topsoil and overburden are removed separately.  The topsoil is directly returned to areas being rehabilitated.  The overburden is stockpiled and is returned in the mine pit when mining is completed.  Overburden contains few seeds and nutrients so stockpiling does not significantly affect its properties. 
The removal of topsoil and overburden occurs from November to April when the soil is dry.  Dieback and dieback-free topsoil and overburden are removed, transported and stored separately – this is critical because we have an objective of not increasing the area of dieback forest in and around our mines.
The direct return process involves the removal of topsoil from an area that is being prepared for mining to an area that has been mined and is ready to be rehabilitated, thus the soil is not stockpiled. Direct return of topsoil reduces double handling of soil and reduces the area of forest cleared for stockpiles.  The value of using direct return of topsoil for increasing plant diversity in rehabilitation has been determined by Alcoa’s environmental research scientists.  Over 70% of the plant species recorded in our rehabilitated mine pits comes from the topsoil. One winter of stockpiling reduces the seed from the topsoil to 15% of its original amount.  To gain the maximum benefit from the seed in the topsoil it is best to carry out clearing, soil stripping, soil return and ripping in quick succession during the dry summer months when the maximum amount of seed is in the soil. It is important to avoid delays in the handling of the soil, particularly during the wet winter period. Direct return also preserves important nutrients, organic matter and microbes in the soil which are lost during stockpiling.
The standard method for soil removal and return is with a scraper. Trucks and loaders can be used to return soil from stockpiles, in which case dozers and graders are used to level the soil across the landscaped surface.

Scrapers stripping the first 5cm of soil

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Scrapers stripping the first 5cm of soil, topsoil, ready for Overburden removal

The second soil layer.

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The second soil layer, overburden is stripped from the top of the caprock layer after topsoil removal. Here, the scraper removes the overburden, revealing the hard layer of caprock below.