Dulce Simoes Jet Grouting, Marine Works, Piling

Franki Africa (Franki), part of the Keller group, has developed a reputation throughout Africa for its innovative and cost-effective solutions for geotechnical work in the marine environment. 

 One such project was the recently completed construction of a 175m-long quay wall with a draft of -12.3m CD  for marine engineering company Dormac’s new floating dock at their marine works facility at their Bayhead, Belfast Rd site in Durban.

 Franki’s Paul Pearce says that geotechnical work in the marine environment, more than anything, demands creative solutions to complex challenges and this is what won Franki the Dormac contract. “Our quay wall proposal of CFA piles and jet grout columns gave us the advantage we needed to secure the contract,” he says.

 The existing site consisted of a rock revetment, dilapidated sheetpile quay wall and a functioning slipway with associated grillage used for ship repair.  Pearce says that although a sheetpile 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 award meant that the time available for construction had to be reduced as the delivery date of the floating dock was already fixed,” says Pearce. “But 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.”

 On the question of the technical work, Pearce says that although this was 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 Keller’s German division. “In fact the cooperation between Franki and Keller Germany was most helpful,” says Pearce, “and confirms the supreme advantage that we have being part of the Keller network and being able to tap into the best technology and equipment in our industry.”

  He adds that the jet grouting was a highly technical and complicated operation requiring top-end management, operators and support. “After a 3-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,” Pearce says.

 There were other significant challenges on this job with the first, as expected, being to work on the edge of Durban harbour and being exposed to tidal conditions and to the ‘moods’ of the ocean. The Franki crew worked around the clock to accommodate tidal movement and, operating when they could, they laid down a high-spec working platform for the piling rigs. “This was an exacting job to say the least,” says Pearce, “but when it was complete we were able to work without the risk of tidal inundation and one of the biggest risks to our operation allayed.”

 The second challenge from a piling perspective was the positioning and tolerance of the piles in relation to the jet grout columns. “It was critical to get the positioning spot on to ensure a vertical face to the quay wall,” Pearce says. “Our works department came up with a very innovative idea in the manufacture of custom built guides and frames to assist us in positioning the piles in accordance with the fine tolerances and this worked perfectly.”

 Piling challenge number three – to increase the piling production rates – was as a result of Dormac’s request for Franki to complete the contract two and a half months earlier than initially agreed to. “This required some fleet-footed action and we decided to purchase new, more efficient equipment specifically for the Dormac contract. Our Plant division put in a sterling effort with the new machinery, going way beyond the call of duty to fine-tune the machines and to train the operators. The end result was that we were able to sufficiently increase our production,” Pearce says.

 The quantities tell the geotechnical story: 171 no. x 900mm diameter CFA piles 24m deep; 171 no. x 1 200mm diameter jet grout columns 16m deep; 65 no. 63mm steel tie bars, which had to be fixed to various deadman anchors incorporating pairs of 900mm x 10m CFA piles, existing slipway retaining walls and existing reinforced concrete slabs; 180m long x 5m high quay wall capping beam.

 Other works included the dredging of approximately 100 000m³ of in-situ harbour beds, a 30m x 5.5m high reinforced concrete caisson enclosure to close in the existing slipway, earthworks, 7 500m² paving, various buildings, bollards, fenders and quay-side services and last, but not least, the installation of 2 no. 1 750mm x 32m steel tube dolphin piles as mooring structures for the dock.

 What Franki considered “specialist” works were sub-contracted – this included the dredging, the manufacture of the precast quay fender panels and the precast caissons, as well as the earthworks and paving.

 “Overall, the utilisation of existing structures, while contributing to our competitiveness, certainly added to the complexity of construction,” says Pearce.

 He adds that a challenging yet successful aspect of the job was Franki’s construction of the 3m-deep reinforced capping beam incorporating the tie bars and the hanging of the pre-cast fender panels creating an aesthetically pleasing and erosion-resistant face to the quay wall.

 “This was a challenging project but we are proud of our exceptional ability in marine conditions – a skill that we have acquired throughout the company over many years. The team at Dormac gave their all under challenging, unfamiliar conditions and were rewarded when the official naming of the dock took place on 2nd September 2016,” says Pearce.

Published in the SAICE Civil Engineering Magazine – November 2016

Aerial View – Dormac