By Bobby Jarratt
A six-storey residence at 145 Kloof Road with its three-level basement at the foot of Lions Head in Clifton showcases the extent of JG Afrika’s civil- and structural-engineering capabilities
JG Afrika and geotechnical subcontractor, Franki Africa, designed a method to construct the basement in an extremely sensitive environment that ultimately incorporated the final state of the structure.
The close proximity of existing properties to the building, as well as boundary restrictions, precluded the use of conventional soil anchoring systems.
In addition to mitigating the slightest ground movements during construction, the system had to perform in notoriously complex ground conditions. This 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. In addition, the area features a perched water-table during the rainy winter season.
Solutions to boundary restrictions and ground conditions
The solution was designed in only three months so as not to delay Franki Africa, which had already been appointed by the time that the professional team understood the true extent of the risk associated with crossing the site boundary using traditional methods.
We designed a reinforced-concrete (RC) box system, with tension anchors rooted within the property boundaries to resist horizontal forces and conventional vertical piles that intercepted the slip-circle plane. Importantly, meticulous attention would have to be paid to the co-ordination of the construction sequence to ensure stabilisation of the back and side faces.
Work commenced with the establishment of a 3 m-wide platform at second terrace level from where contracting teams would start building the RC box.
A 13 m-high rear slope was excavated at 60°, and then temporarily stabilised using soil anchors and covered with mesh and sprayed with gunite. The installation of the temporary anchors was carefully monitored to ensure that none crossed the property boundary.
Drilling rigs were lifted and placed onto the 14m-high platform to insert the 300 mm-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 25 m 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, followed by the excavation of the rear face in controlled 3 m widths to mitigate the chance of slope failure.
A retaining wall was then built up to the total 5 m retained height sequentially to ensure stability.
Having now achieved a stabilised construction sequence, work on the basement at the lower terrace level commenced by excavating in 3 m heights and installing a conventional soil anchoring system along the rear face, as this was well within the property boundary.
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. Steel propping members were positioned to coincide with the final state floor levels which would replace the propping once constructed.
With the side walls being 13 m high, lateral loads on the props were substantial. Some 20 tonnes of tubular structural steel were sequentially assembled while excavation took place, to create a 3D lattice girder system to span the 22 m gap. This enabled some 3 000 m³ of material to be safely removed.
The girders were gradually dismantled as lateral loads were transferred to newly constructed concrete floor slabs at each level.
Minimal ground movements were detected during the entire two-year-long build. This is proof of a very successful technical solution to the challenges set before the design and construction teams.
JG Afrika’s senior engineer, Bobby Jarratt, was involved in the project right from the early design stages in 2014. He worked alongside Tim Davidson, a technical director at JG Afrika, who led the firm’s team on this project.
The project won two SAICE 2017 Awards: