The 18 km alignment requiring investigation and management was mostly in areas of cut and included the consideration of Bassendean Sands associated with drainage basins. The project required approximately 2 cubic metres of fill material, consequently in order to reduce the strain on the local materials market, opportunities within the construction footprint were identified by the engineering team to increase the depth and size of the drainage basins. Two challenges were associated with the solution identified, the first was an increase in clearing of native vegetation and the second was associated with the presence of shallow coffee rock and Bassendean sands.
The team conducted an intrusive investigation of each basin to better understand the nature of soil acidification and buffering capacity. The Bassendean sands had already been subject to years of oxidation. The basins base was designed to be a minimum of 0.5 m above the historical groundwater elevations. The sands also were identified to have either a low or negligible buffering capacity. Despite all of these characteristics the environmental team investigated various species options to landscape and rehabilitate the basins. It was proposed to compact and dust the basins with lime to reduce the infiltration rates and to mitigate acidity during the first flush which would follow an initial rain event.
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