12/2012 – Wastewater facility gets facelift

Cut slope retaining at Wastewater Treatment Works

Installer: Decorton Retaining System

In 2009, The City of Cape Town initiated a project to rehabilitate three anaerobic digesters by constructing a regional sludge handling facility at the Cape Flats Wastewater Treatment Works. A segmental block retaining wall system was considered most suitable for this project, as insitu material consists of fine sand.

Work to be carried out includes partial demolition of the gas holding tank, construction of sludge bunkers, blending tanks, sludge holding and buffer tank, blended sludge pumpstation building, interconnecting pipelines and access roads.

Composite wall around a holding tank

Composite wall around a holding tank

Close-up of the retaining blocks

Close-up of the retaining blocks


During July 2010, Aurecon Consulting Engineers approached Klapmuts Concrete, a Cape Town based earth retaining block supplier, for assistance in designing retaining walls of up to 8m high to support earth fill and cut slopes of a new road leading up the sludge holding tanks installed at the upgraded site.

A total 5200 blocks were use

A total 5200 blocks were used

View from above the tank

View from above the tank


Problematic founding conditions and soft backfill materials required 2.3 meters wide reinforced concrete foundations and segmented back walls at 70 º inclinations, partially built in double skin formation and filled with Y16 reinforced concrete. All sand backfill was reinforced with 100kn geofabric every second layer.

A segmental block retaining wall system was considered most suitable for this project, as insitu material consists of fine sand and the client needed a closed face block system that prevents sand from leeching through the blocks. In total 5200 blocks were used.

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