Specialist geotechnical contractor Franki Africa has completed construction of a 175-m-long quay wall with a draft of 12.3 m for marine engineering company Dormac’s new floating dock at its marine works facility in Durban.
Franki KwaZulu-Natal branch manager Paul Pearce highlights that geotechnical work in the marine environment demands creative solutions to complex challenges. “Our quay wall proposal of continuous flight auger piles and jet grout columns gave us the advantage we needed to secure the contract.”
The existing site consisted of a rock revetment, a dilapidated sheet pile quay wall and a functioning slipway with associated grillage used for ship repair. Pearce says, although a sheet pile or diaphragm wall structure had been envisaged during the 15 years of feasibility studies, Franki’s alternative proved more economical and less time consuming.
“Initial delays in awarding the contract meant that the time available for construction had to be reduced to meet the delivery date of the floating dock, which had already been fixed. “However, shipping is renowned for tight schedules and high operational costs and this project was subject to the same pressures in order to meet the schedules of the future users of the dock. “Dormac offered Franki an incentive to complete the works early and we took up the challenge.”
Pearce highlights that, although the project involved one structure, it required many highly specialised geotechnical skills for a successful outcome. One of these skills was specialised jet grouting using a rig from Franki owner engineering firm Keller’s German division.
“The cooperation between Franki and Keller Germany was most helpful and confirms the supreme advantage that we have.”
He adds that the jet grouting was a highly technical and complicated operation requiring top-end management, operators and support. “After a three-week training and test phase, the jet grout operations began and, after some initial hiccups with Franki Cape Town coming to the rescue with their experience in jet grouting, we were able to source the additional equipment from Europe and continue with a productive and successful operation.”
There were other significant challenges on this job – the first, as expected, being to work on the edge of Durban harbour and being exposed to unpredictable tidal conditions. The Franki crew worked around the clock to accommodate tidal movement and, operating when they could, laid down a high-spec working platform for the piling rigs.
The second challenge from a piling perspective was the positioning and tolerance of the piles in relation to the jet grout columns. Pearce explains that it was critical to get the positioning accurate to ensure a vertical face to the quay wall. Franki’s works department, thus, manufactured custom-built guides and frames to assist it in positioning the piles in accordance with the fine tolerances, which worked perfectly, he adds.
A third piling challenge was to increase the piling production rates to meet Dormac’s request for Franki to complete the contract two-and-a-half months earlier than initially agreed to. “We decided to buy new, more efficient equipment specifically for the Dormac contract. “The end result was that we were able to sufficiently increase our production.”
Pearce adds that a challenging yet successful aspect of the job was Franki’s construction of the 3-m-deep reinforced capping beam incorporating the tie bars and the hanging of the precast fender panels, creating an aesthetically pleasing and erosion-resistant face to the quay wall.