Construction World September 2018

COVER STORY

INNOVATIVE PILING SOLUTION from Franki in Saldanha Bay

S ome of this infrastructure includes roads and bridges such as the Greenfields Link Interchange project, awarded to WBHO Construction in late 2017 and who subsequently awarded the geotechnical piling contract to Keller’s Franki Africa. The project, initiated by the Western Cape Department of Trans- port and Public Works, is situated in the Saldanha Bay municipal area, 125 km north of Cape Town. Franki Senior Contracts Manager, Anton Stoll, explains that at tender stage the pile founding solution was stipulated as ‘predrilled, based temporary cased auger piles’ for all three bridges, founding in the greenish, grey mottled orange and brown dense sand, by The old adage that building roads and bridges is fundamental to the growth of an economy has certainly been heeded by the Saldanha IDZ (Industrial Development Zone), which, in contrast to many areas in South Africa, has been undertaking significant infrastructural development with the funds they have received from the Western Cape government for that specific purpose.

means of basing out an enlarged base below the temporary installed casings. These piles were expected to be around 14 m in length from underside of pile cap. “From the outset we were not quite sure that the tender solution was the correct one for the circumstances,” says Stoll. “The fact is the West Coast area around Saldanha Bay is known for its difficult geology and founding conditions for many a structure, including, of course, this particular bridge interchange. He adds that the geology of the site consists of windblown sands for around 0,5 to 1 m below natural ground level, followed by up to 3-4 m of strongly cemented hardpan calcrete (pedogenic), then varying layers of loose, silty, clayey sand and calcrete lenses, followed by a greenish, grey mottled orange and brown dense sand. This vary- ing profile continued to depth and a high water table was present. The interchange has three bridge structures and, while the largest structure was clear of any obstructions, the other two had a water main in close proximity to the piles and pile cap, raising a concern over vibration during pile installation further complicating things from a geotechnical perspective. The ultimate outcome was that Franki decided that the site conditions suited a different pile type and installation methodology with the varying soils, the very high water pressure and an aquifer which became evident in close proximity to where the piling would be. “The water had further softened the dense sand layer in places, making the formation of the enlarged base difficult due to large volumes of basing material required to improve the silty sands,” says Stoll. “As more skips were added, the basing appeared to soften rather than densify the layer. At times, plumbs were added without effect, ultimately resulting in the plug suddenly being expelled by the 6-ton hammer from the temporary casings allowing water into the casing. After a number of attempts, it was proposed that an alter- native piling solution would be more suitable to the site conditions, geology and pipeline obstructions.” After discussion with the engineers and main contractor, Franki proposed that the two bridges with the existing water pipeline obstruction would be more suitable to CFA (Continuous Flight Auger) piles while the main bridge could be found on DCIS (Driven Cast-in- Situ) Franki piles founded at a dense layer, higher up in the soil pro- file. This required a variety of piling rigs, service cranes and technical skills, all available within the Franki group. Stoll says that by this time, the programme had become critical and the merits of the alternatives were debated, approved and immediately implemented with non-working test piles at each struc- ture. Predrilling remained in order to penetrate the hardpan calcrete layer prior to installing the piles. The test pile results proved the alternative methodology was correct and working piles were installed accordingly. Franki has developed a reputation far and wide for finding the right solution for the job at hand. “Our experience in South Africa and in many parts of Africa in general over the years, gives us an unmatched understanding of the geology and of what is required relative to it to ensure that we give our clients the most cost-effec- tive solution,” Stoll says. “The Green Fields Link Interchange is cer- tainly not the first time that we have successfully offered a solution different from that of the tender proposal.” Of course, knowledge is one thing but having the right hardware for the job is as important. In this case the CFA piles and the world- renowned Franki Pile, which has been used extensively throughout

Installing ‘Based Temporary Cased Auger Trial Pile’ at Greenfields Link Bridge site.

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CONSTRUCTION WORLD SEPTEMBER 2018 AUGUST 2018