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Due to the uneven nature of the caprock, the scrapers are unable to remove all of the overburden layer.  Scrapers leave behind the overburden material in the concave pockets of the caprock.
It is important to remove this overburden (called secondary overburden) because analysis has shown it to be high in chemicals that greatly decrease the efficiency of the alumina refining process.  Removing this overburden saves the refineries millions of dollars each year.
The removal of this secondary overburden is known as SOBR (Secondary Overburden Removal).  SOBR follows the removal of the overburden by scrapers and is accomplished using small, tracked excavators.  The excavators create windrows of the secondary overburden material.  Scrapers are then able to collect and transport this material to the overburden stockpile.
The segregation of dieback and dieback-free overburden is closely monitored during the SOBR phase because research has shown that secondary overburden is highly infected with spores of the dieback pathogen.
The SOBR operation is complete when most of the caprock has been exposed and no pockets of secondary overburden remain.  This leaves a consistently clean brown coloured caprock with gravel particles tending to larger nodules rather than small spherical pisolites. In areas where there is no cap rock present or the cap rock has a smooth sheet like appearance (little, no depressions), scrapers can perform both the primary and secondary overburden removal operations. 

The final product after SOBR and before backfillin

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The final product after SOBR and before backfilling.

Soil Profile

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A typical representation of the soil profile found in the Darling Ranges. This diagram shows where Alcoa’s pit floor would occur.