In order to maintain its global competitiveness relative to other international convention venues, the Cape Town International Convention Centre (CTICC), which has reached its capacity, needed to make structural enhancements. To this end the approved extension plans were announced in 2013, with Franki Africa being contracted to implement a range of geotechnical works in various phases.
In February 2015, the CTICC East contract was awarded to Franki Africa by Aveng Grinaker-LTA Building Cape, following on from the basement lateral support for the first phase of the CTICC extension which had been successfully completed in 2014, with WBHO as the Main Contractor.
The CTICC East contract consists of lateral support in the form of Continuous Flight Auger (CFA) piles with gunited arches for the underpass, connecting to the existing CTICC, together with piling for the access ramp from the basement, which required piled lateral support and anchors.
However, Franki Cape Town’s Anton Stoll points out that conventional lateral support could not be installed everywhere due to the highly sensitive service lines feeding the city centre and overpass headroom restrictions. “In order to create the necessary lateral support under this major servitude of cables, a jet grouting gravity wall was considered to be the safest solution, with the lowest risk of damaging these vital services,” he says.
He adds that the existence of all the electrical cables, water ducts and optic fibres feeding Cape Town posed the greatest challenge on this project. “These lines were so closely laid that piling between them for lateral support was just not possible. However, jet grouting allows the installation of soil cement columns (with diameters of up to 3.5m) through a small diameter borehole of less than 200mm, which makes this technology exceptional for working around utilities without having to disrupt their operations,” he says.
With the aid of high pressure cutting jets, the soil around the borehole is eroded, rearranged and mixed with the grout. The columns can then be arranged and combined in any way to form each type of element. All service lines had to be identified and completely exposed beforehand, and to prevent possible damage during the installation of the jet grouting columns, they were wrapped and encased in wooden boxes.
Franki Africa recommended that temporary 165mm diameter ducts be installed in order to guide the jet grouting process and protect the service lines during drilling and jetting. These sleeves were placed at a minimum depth of 500 mm below the service lines.
Thereafter, the exposed lines were backfilled and compacted up to the existing ground level to create access for the jet grout rig. After all the preparatory work, the installation of the gravity wall could start with the execution of three test columns to verify the pre-determined jetting parameters.
“Thanks to the additional support from Ralf Junker, a highly-experienced supervisor from Keller Germany, we were able to complete the trial phase and prove the maximum required column diameter of 2600mm, after which the actual execution of the gravity wall could begin,” Stoll says.
The drilling rod was guided within the sleeves and overlapping columns were plotted and set out to create a mass gravity wall below the service lines. During the process, a soil-cement mix (sludge) was partly flushed out to the top of the borehole through the annular space between the jet grouting rods.
Stoll says that pumping the sludge to a temporary storage facility and then disposing of it always poses a challenge, especially when working in areas with restricted space. “Sludge management is often crucial for a successful jet grouting site, as it can dictate the production rate. This also applied to the CTICC project. However, after two weeks we managed to complete the two gravity walls, on which a slab could be cast, as well as spanning the walls and forming the roof of the exit and entry ramps on which the service lines could be supported,” he concluded.
The entire contract consists of piling, jet grouting and lateral support, which include oscillator piles for future developments, CFA soldier pile walls, a Berlin Wall for temporary support and bored piles for cranes.