In 1980, the Electricity Trust of South Australia (now NRG Flinders Pty Ltd) excavated a trial pit at Bowmans, north of Adelaide, to test coal at depth and evaluate its viability for power generation.
The conical pit was 65m deep, and 250m across. Testing was carried out over 20 years, during which time the natural water table dropped by 20m. Wind generated wave action and poor soil structure caused severe erosion and scouring of the banks. The resultant 10-15m exposed pit walls created a safety hazard.
The three-stage rehabilitation process was carried out during 1997-2000. Stage 1 included major earthworks to batter down the exposed pit walls to a 1 in 4 slope, based on assessment of soil characteristics and local factors. The entire circumference was reshaped, with about 270,000 cubic metres of overburden dozed into the pit. However, this resulted in a water level displacement of only 4m.
While further survey work was conducted to quantify the amount of material required to displace the appropriate amount of water, there was still uncertainty about the state of the pit floor and walls below the waterline.
Consultation with Maptek staff raised the idea of generating a computerised model of the pit from a hydrographic survey to help identify the contours of the pit interior.
Conventional GPS survey data of surface topography was merged with depth soundings from bathymetry to create a model of the existing physical surface.
From this information, the total fill volume was estimated. This volume was greater than the material available, so an alternate rehabilitation strategy was adopted.
Not only was the VULCAN model critical for definition of cut and fill volumes, it was beneficial in determining payments to contractors. VULCAN provided a dynamic tool for visualising and understanding the erosion problem, and explaining the remediation process to stakeholders.
Once the modelling process was complete the earthworks could resume. Top-soils were dozed into short-term stockpiles for future use in restoring the site.
Revegetation focused on re-establishing species native to the region using local seed collection, and contouring the slopes for planting.
The final stage was to prevent erosion recurring, especially on the newly dozed slopes at water level. Coastal erosion experts suggested shaping the waterline into beach-like profiles to dissipate wave action, and to further protect the banks by installing collapsible fabric block mattresses.
The fabric contains compartments into which concrete is poured in-situ. The block mattresses, 4m in diameter, were placed 20cm above the existing water level. Over the seasons the material collapses and nature works to smooth the angles.
The rehabilitation techniques used in the Bowmans pit have potential for many mining scenarios, particularly in areas with unstable soils. Applications include ash and tailings dams, and settling ponds. The cost-effective $600,000 solution was significantly cheaper and quicker than filling the entire pit. VULCAN played a vital role in this innovative solution to a complex engineering and ecological task. The project won an environmental excellence award in the South Australian Minerals and Resources Industry Awards.
* Article from Maptek's Forge Newsletter, No 3 of 2002, republished with permission.
