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Consulting engineering and environmental consulting firm JG Afrika has been honoured by the South African Institution of Civil Engineering (SAICE). The company received a number of accolades at the recent SAICE Awards, including an award for a waste-to-energy plant and a residential development.

145 KloofThe new six-storey residence at 145 Kloof Road at the foot of Lions Head in Clifton is a major civil- and structural-engineering feat, and the extent of innovation that was applied by JG Afrika’s structural engineers to overcome extremely complex technical challenges on this development earned it the honours it received. The panel of judges was impressed with the unique designs and intricate construction processes involved in developing a three-level basement for the structure in an extremely sensitive area, they said when conferring the award.

Working closely with geotechnical sub-contractor Franki Africa, JG Afrika designed an alternative to conventional ground stabilisation systems during construction which ultimately incorporated the structure’s final state. The close proximity of existing properties to the building’s boundary eliminated the possibility of conventional soil anchoring systems, and the design needed to eliminate the slightest ground movements during construction to avoid damage to neighbouring structures.

This system also had to perform in notoriously complex ground conditions, which included weathered residual soils, soft to medium rock at depths of between 12 m and 15 m, as well as large intermittent boulders, before reaching the granite bedrock. The area is also characterised by a perched water-table during the rainy winter season, which added to the complexity of the build.

A number of solutions were proposed, but three potential systems were the final options. These were constructing a conventional cantilever retaining wall, a propped retaining wall using side-wall buttresses, or an anchored reinforced-concrete (RC) box system. A conventional cantilevering wall did not provide adequate stability during excavation, nor did it safely transmit sliding forces to the basement level, while a retaining wall using side wall buttresses failed to address potential slip-circle failure and required very high levels of accuracy during construction.

A winning solution

The construction of an RC box system was deemed the best solution for the project. This system included tension anchors rooted within the property boundaries that would resist horizontal forces and conventional vertical piles that intercepted the slip-circle plane.

Construction of the structure was divided into four phases, starting with the establishment of a small working platform at second terrace level where a 13m high rear slope was excavated at 60°. This was temporarily stabilised using soil anchors, covered with mesh and sprayed with gunite, and provided a secure 3m wide platform from where the contracting teams could commence building the RC box.

Drilling rigs were lifted and placed onto the 14m high platform to insert the 300mm diameter rotary percussion soldier piles and tension ground anchors at 10° to the vertical, to form the temporary side walls of the system. Within the platform area, vertical piles of up to 25m in length and tension anchors at 60° to the horizontal were then socketed into hard rock to resist slip circle failure and the horizontal sliding forces generated from the eventual rear vertical retaining wall.

A 500 mm-thick foundation slab was then constructed and the excavation of the rear face followed in controlled 3m widths to mitigate the chance of slope failure. The retaining wall was then built up to the total 5m retained height sequentially to ensure stability.

A structural steel propping system was placed to shore the side walls during excavation as these faces were still in close proximity to the property boundary. Approximately 20 tonnes of temporary structural steel props were installed. Spanning 22m, these were fabricated in sections and assembled on site by the contracting teams. The steel props were removed once the concrete slabs, tied into the edge beams located just below the propping positions, had sufficiently cured.

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