MANY DISUSED pits and quarries continue to be used for landfill sites. Total containment of these facilities is normally required to prevent migration of landfill gases and leachates.
But while design of horizontal basal containment and capping is often relatively straightforward, the lining of the side slopes can be more challenging.
There are a variety of sealing systems available for steep slopes, including low permeability mineral liners and geomembranes.
When geomembranes are used, face preparation is typically needed to provide a smooth planar surface suitable for the geomembrane.
Other design considerations include the stability of the sidewall lining system and overall stability of the waste mass along with liner stresses and liner protection.
Weeks developed a method of face preparation for steep-sided slopes of landfill sites in disused quarries in 1994 that is still being used today.
The shuttering system of polystyrene panels was developed with polystyrene product manufacturer Cordek and landfill operator Greenways.
After testing on Greenways' Borough Green site in Kent, it was first used at Albury sandpit and landfill near Guildford in Surrey (Ground Engineering November 1994). The system is commercially available through Cordek as Tipform.
The geosynthetic lining system may vary depending upon site specific requirements, but typically comprises an HDPE geomembrane protected by a non-woven needle punched geotextile made of polypropylene.
Geonet can be placed between the geomembrane and geotextile to provide a high permeability curtain for leachate drainage and/or gas venting. Geocomposites performing the dual function of drainage and liner protection are also used.
Waste settlement can cause downdrag, straining the liner and inducing high stress within the face preparation system. Differences in the frictional properties of the various components of the geosynthetic lining system can reduce these effects and can be achieved by using a mono-rough geomembrane, with the rough surface placed against the prepared quarry face.
As the frictional resistance of this surface exceeds the interface friction between the smooth side of the geomembrane and the needle punched geotextile (or geonet, if present), waste settlement results in slippage on the geomembrane/geotextile interface.
Providing the design makes allowance for this movement, high tensile strains on the geomembrane are avoided, and downdrag on the lining and face preparation system reduced. The presence of a low friction interface within a sidewall lining system should be recognised when assessing the overall stability of the waste mass.