The ground conditions consisted of slightly weathered rock overlain by alluvial soils. The rock showed signs of cracks and fissures which presented the issue of subsurface flow of liquids from the tailings dam into the groundwater and Surprise Creek. The alluvial overburden was to be excavated post construction to expose the weathered rock, creating an interception trench for the tailings contaminants to be diverted elsewhere onsite.
Menard Oceania were tasked with the construction of two grout curtains to seal the fissured rock and mitigate the seepage from the tailings dam. This was achieved by drilling inclined holes at 10m centres to depths up to 30m, high pressure grout was then introduced to each bore at 5m intervals, penetrating the fissures and creating an impermeable ‘grout curtain’. Further bores were opened at 5m and 2.5m spacings along the curtain alignment to ensure the high-pressure grout had impregnated the rock mass sufficiently. The project was completed within a four-week timeframe during the wet-season, well within predicted estimates, to high quality standards and within budget.
The primary objective of the overall project was to mitigate the risk of leachate from the tailings dam entering the groundwater and ultimately the watercourse, Surprise Creek.