A feasibility investigation for Phase 5 of the Luipaardsvlei General Waste Landfill Site (LFS) located within the jurisdiction of Mogale City Local Municipality was carried out in two phases.
The feasibility study’s aim is to establish the geotechnical and geohydrological constraints of the proposed northerly extension of some 3.12ha to the existing landfill site, the construction of two ring roads and a pollution control facility along the southern boundary which will form part of the Basic Impact Assessment Report of the existing landfill site of G:C:B class.
The investigation comprised a borehole – and spring census, test pitting, water and soil sampling and analyses, a geophysical survey, a search for mining plans and monitoring of on-site gas emissions. Three monitor boreholes were drilled and a thorough investigation was carried out in the pollution control dam area for clay liner material. A sampling of leachate at several collection points was carried out as well.
The site was subject to underground mining and the processed ore created large stockpiles of mine tailings and mine discard. At a later stage, the on-site and adjacent slimes dams and mine discard dumps were reclaimed en masse and shallower, auriferous reefs were mined by means of opencast mining. The mining activities created a complex infrastructure, affecting both surface and underground groundwater flow regimes. Following several mine closures from the 1970s onwards, the mine void which extends over the local C23D quaternary catchment has subsequently been ‘re-watered’.
The poor water quality of both surface and groundwater is attributed to earlier gold mining activities on and around the site and leachate being generated in the waste pile.
The fairly deep static water levels of the monitor boreholes in the centre and northern portions of the LFS ensure a fairly thick (>22m) attenuation zone, reducing the risk of groundwater pollution on this part of the site. The hydraulic gradient is fairly steep over this higher-lying section of the LFS, levelling off towards the southerly, low-lying portion of the terrain where a shallow water table (~1.8m) prevails. Note that these observations may change during high rainfall periods.
The centre line soil surveys of the two access roads recorded moist conditions and good quality pavement layer materials placed on fill. The access road in the lower southerly portion was partially eroded and wet underfoot, requiring rockfill and cover soils to make it passable.
The cover soils encountered in the northern portion of the LFS were found to be either unsuitable for capping and backfill applications or reflected inconsistent engineering characteristics with subsequent variations in compaction capabilities, permeability and settlement. The quartzite that underlies this portion of the site is soft excavatable in the top 0.5m of the rock profile followed by intermediate to hard excavatable bedrock.
The emissions of Methane, Hydro Sulphide and Carbon Dioxide are considered negligible and the capped refuse stockpile appears to be well-ventilated, preventing the build-up of volatile gasses.
The stability of the test pit excavations of the test pits excavated on the 6.2ha ‘Stage 3 of New Landfill Site’ (S3) and the ‘Waste to Energy Area’ (WEA) recorded stand-up times in excess of 24 hours. Slumping occurred within the wet sections of the test pits excavated in the south-western portion of the terrain on the PCD site and cognisance should be taken of the unsafe conditions of excavations deeper than 1.5m. The clayey subsoils exposed in the deeper sections of the test pits in the central and eastern portions of the PCD area recorded stand-up times in excess of 24 hours and no seepage was observed.
The current stormwater management system seems to be adequate and conforms to drainage requirements of landfill sites in general. However, continuous erosion of the drainage furrow alongside the westerly access road, has caused serious damage to the lower end of the road shoulder and a culvert was installed to divert the run-off onto the neighbouring property. Some concern has been expressed over the partially functional paddock system that was constructed on reclaimed areas that are present in the upper, north-eastern portion of the site as well as along the western and eastern boundaries, causing surface run-off to flow onto the lower end of the LFS, recharging the wetland with acid mine drainage and together with the occasional storm water run-off and the LFS leachate eventually flowing into Lancaster Dam, possibly also adding to the ‘re-watering’ of the mine void.
In addition to the new pollution control measurements to be implemented on the PCD site, it is recommended that the water emanating from the LFS be treated using chemicals or applying ‘passive treatment’ by means of wetlands, anoxic limestone drains and alkaline leach beds.
The extent of the AMD pollution plume has yet to be identified, but for all practical purposes, the eastern portion of the C23D quaternary catchment which is host to three gold mines will most probably be included, covering an estimated area of about 30km2.
Limited sources of subgrade class materials suitable for construction of a compacted clay liner are present on the S3 site. Due to a shallow water table on the western portion of PCD site, this potential source of clayey-sandy subsoils can only be exploited with difficulty. However, a tentative source of some 15,000m3 of seemingly suitable clay liner material is present in the central and eastern portions of the PCD site.
Slopes are not to exceed 1v:3h unless a slope stability analyses has been carried out. No harmful landfill gas, vapours and condensates that can adversely affect the clay or the geotextile layers of the geosynthetic liner were detected on site. It is envisaged that both a sacrificial compacted clay liner and a geosynthetic liner will be implemented in a composite manner. The site soils are corrosive and buried metallic objects should be protectively coated.