A multi-tiered concrete encapsulation facility at Holfontein required closure and capping. The facility consists of a structure in which barrels of highly hazardous liquid and solid waste have been encased in reinforced concrete. No standards exist for the closure and rehabilitation of encapsulation cells in the Department of Water Affairs (DWAF) Minimum Requirements specification. In determining the possible requirements of a closure capping design Jones & Wagener found the long-term integrity of typical landfill capping designs to be unsuitable for this specific case. Most capping designs aim to limit the ingress of moisture in the form of rainfall. However in this particular case, exposure to CO2, O2 and water would result in the deterioration of the encapsulating concrete through depassivation of the reinforcing steel, resulting in corrosion and eventual spalling of the concrete. Hence the selected capping design has to be not only “waterproof” but “gasproof” as well. The selected capping components are summarised as follows (from the bottom to the top):
 
• A minimum 300mm thick clay layer, constructed as two 150mm compacted layers. This together with the HDPE liner provides the moisture barrier component of the capping system.
• A 1.5mm double-textured HDPE geomembrane liner. This liner was welded to the existing basal liner so as to form an airtight liner over the whole area.
• A 500mm thick topsoil layer. This layer surcharges the HDPE to ensure intimate contact and a composite action with the clay liner below. It also serves to protect the HDPE geomembrane and allows for the establishment of vegetation and thus the long-term erosion protection of the capping layers.
 
Concern over the possibility of a veneer slip failure at the interface of the soil layer and the HDPE geomembrane led to a geogrid being installed at the interface of these two layers. Stability was considered for two load cases i.e. the long-term case and the construction phase case. A PET geogrid that mobilised high strength at low strain was selected to minimise movement of the topsoil layer. Factors such as creep, environmental damage, installation damage, sufficient anchorage and seepage were all considered in the design and construction sequence.

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