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.
Some 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 Government: Department of Roads and Public Works, is situated in the Saldanha Bay municipal area 125km 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 means of basing out an enlarged base below the temporary installed casings. These piles were expected to be around 14m 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 1.0 m below natural ground level, followed by up to 3-4m 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 varying 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 alternative 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 profile. 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 structure. 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-effective solution,” Stoll says. “The Green fields Link Interchange is certainly 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 southern Africa for the past 70 years (and is still today one of the most popular pile-types) proved their worth in the solution.
The main feature of the Franki Pile is the enlarged base formed at the toe of the pile. In forming this base, the end-bearing area is increased significantly and the displacement achieved when expelling the plug and forming the enlarged base compacts and preloads the soil surrounding the base. Thus, the end-bearing of a Franki Pile in sands develops at much lower base deflections than that of a bored pile.
Other important advantages of the Frank Pile include: it is often a very economical system; it has an extensive range of pile sizes; it has an exceptional load/deflection performance; noise levels are relatively low; it has excellent tension load capacity.
The CFA piling system is also a fast and economical one, which has no vibration and limited noise levels. Some of its other attributes include: high production levels in suitable soil conditions; economical in suitable soil profiles.
“The fact is there are some limiting considerations to be taken into account with the CFA system reducing its popularity compared with driven piles. The conditions in the Greenfields Link Interchange, however, made the system a very successful choice,” Stoll says.
The key to the success of the piling project was the implementation of Franki’s alternative plan. “It took teamwork to get this right,” says Stoll, “and I would like to pay special tribute to Ross Dold, Nabeel Omar and Wilhelm Wessels on site for Aecom, together with Abie Newmark and Alexi Maravelias from the Aecom office, Harry Viljoen and Mish-al Booley from the Department of Transport and Public Works, while Stefan Herbst and Willie Broekman of WBHO Construction assisted us with the implementation. This was indeed a great team effort,” he says.
Stoll emphasised that no project is too big, or too small, or too complex for Franki. In terms of South Africa and Africa in general we are geared up like no other geotechnical company in the world. We have permanent offices and yards in South Africa, Mauritius, Mozambique, Tanzania, Zambia, Kenya, Ghana and Angola. We are also registered in Uganda, Botswana, Swaziland, Lesotho, Seychelles and Namibia. Our French speaking colleagues are present in Morocco, Cote d’Ivoire and Algeria and our middle eastern colleagues are present in Egypt.
“Add to this the depth of global experience that the Keller group has it becomes clear that Franki’s ability to provide a world class service in this country and across the continent is second to none,” he concluded.
Published in Construction World Magazine – September 2018