Tunney Ridge Ext 12 In Elandsfontein, Germiston

Variable geology and sloping land presented an exceptional set of challenges in the construction of a single-level post-tensioned surface bed for an earth-moving equipment warehouse and distribution centre. Two exceptionally high concrete retaining block (CRB) walls were specified by the geotechnical engineer to secure a bulk-fill terrace which supports the surface bed.

The lower wall, 450m long and topping 13m, was built to face off and secure the bulk-fill terrace above. The upper wall, 436m long and 15m high, stabilises the embankment which rises above the building platform. It was a cut-and-fill operation which involved blasting and excavating large quantities of rock.

CRB walls retain their modular structure which makes them sufficiently flexible to accommodate ground movement. Solid interlocking blocks were used to build the walls in a closed-face configuration for the first 3m-5m and in open-face thereafter.

The bulk earthworks platform was battered at 45˚on the lower wall which was built at an angle of 70˚. The embankment of the lower wall was benched to avoid the creation of a preferential failure plane and geosynthetic reinforcing material was used at a ratio of 70% to wall height at every third layer.

A 70% to wall height ratio erred very slightly on the conservative side, but it provided a relatively inexpensive means of further safeguarding the stability of the wall. And any potential failure through the backfill would be prevented by the geogrid’s tensile resistance.

The upper wall was a cut section which consisted mostly of rock from several different geologies topped by ± two metres of soil. Although global instability was not an issue, numerous wedges (jointing) in the rock face meant that over time pieces of rock would dislodge and fall.

The upper wall was built at an angle of 75˚ using a minimum of 1m geofabric and stabilised fill compacted at 150mm layers. Above the rock a more traditional geogrid installation of 50% to 60% wall height was applied. Because cement-stabilised fill was used on the cut face the drainage system was designed to avoid the build-up of hydrostatic pressure. Band drains - a geosynthetic wrapped membrane placed against the face - were used to allow ground water to flow down the face into a wick drain at the base of the wall.

An equally challenging aspect of the project was casting the post-tensioned slab on a platform underlain by one third rock and two thirds compressible clay. To equalise the settlement variance between the two materials, 1 600 micro piles were installed in the clay section.

Varying in depth between 8m-28m and 141mm in diameter, the piles mitigated most of the settlement. The balance was taken up in the post-tensioned slab. Prior to that, a seismic survey had established the depth of the rock surface so that the piles could be sunk to within approximately 2m above it. Moreover, rock to a depth of 1.5m-2m was removed and back-filled to allow some settlement in the rock section, further normalising the settlement across the platform.