The tiny town known as Tugela Ferry (named after the ferry that used to connect the two halves of the town) in the local Municipality of Msinga, part of the Umzinyathi District Municipality in central Kwa-Zulu Natal is part of one of South Africa’s most impoverished areas.
People struggle to make a living in Tugela Ferry and the harsh conditions were, for many years, exacerbated by the town being split by the Tugela River and connected only by an inadequate single-lane bridge, the Tugela Ferry Bridge, which has significantly slowed the local economy by hampering vehicular movement and has been a threat to the lives of the multitude of pedestrians that have had to share the bridge with cars, tractors, bakkies, trucks and more.
Franki’s Geotechnical Solution
The KZN Department of Transport therefore embarked on a project to widen the bridge to two lanes by constructing a new, reinforced concrete bridge adjacent to the existing steel structure. “This was a good solution,” says Paul Pearce Franki Africa KZN branch manager, “as it meant that use of the existing bridge has not been interrupted during construction.”
Pearce says that the Tugela Ferry area has historically been a political hot-spot and, as a result, the award of the main contract was a lengthy process which involved extensive consultation with all the relevant local parties on their involvement in the project. Ultimately, in June 2016, Franki was appointed specialist sub-contractor for the piling works for the new bridge.
“The prolonged award of the contract and subsequent further consultation after our appointment, crucially meant that the works started towards the beginning of the wet season rather than, as had been expected, at the beginning of the dry season,” Pearce says.
This turned out to be one of the central challenges of the project. “The timing of the commencement of the works has caused major access problems for main contractor, Group Five, who have had to contend with the mighty Tugela River regularly flooding the access causeway during the wet season – even during the current “drought” in South Africa!” says Pearce. “The access causeway has been washed away several times with equipment having to be moved off position each time in anticipation. Fortunately, there have been no injuries and no major equipment has been lost because of these precautionary measures.”
Franki’s scope of work includes the installation of 48 no. 1,100mm diameter permanently cased oscillator piles. The bridge deck is approximately 165m long and piles are required to support both the north and south abutment and all 5 no. intermediate piers with 6 no. piles per pier raked at 1 in 6 and 6 no. vertical and 3 no. raked piles per abutment.
The geotechnical data available indicated typical river bed conditions with gravels, sands and boulders up to 1.5m in diameter in layers above soft to medium rock overlying hard to very hard rock. Fractures in the rock resulted in the tender design requiring 1.4m rock sockets and dowelling into the rock below every pile.
“Historically, river bridge piles requiring sockets are slow to construct using driven or screwed-in casings and “smash and grab” techniques to form piles and sockets in rock. The slow pace not only increases costs but also increases the risks associated with working in a river. We therefore opted for oscillating casings down to rock level and then using a cluster drill – a multi-headed percussion hammer – for the rock drilling. This solution has reduced construction time by over 50%,” Pearce says.
He adds that the increased performance of the cluster drill has also enabled Franki to successfully use a longer socket length in lieu of the dowels, saving further time and cost.
Both the oscillator and cluster drill are mounted on Franki’s Bauer BG28 piling rig, which has an operating weight of about 90 tons and is ideally suited for the Tugela Ferry piling solution.
“We owe a lot to the BG28 which has enabled Franki to implement many cost-efficient solutions to a wide range of clients since it first arrived in this country from Germany in 2012,” Pearce says.
Another time-saving solution used by Franki was the construction of a movable ramp to create the required 1 in 6 rake, meaning that it is not necessary to modify the platform earthworks for each pile. “With all six pier piles raking in different directions this represents a major cost and time saving solution,” Pearce says.
Dealing with the Unexpected
The necessary raised platform and variable rock levels, resulted in suitable founding rock only being encountered at depths of over 25m for some piles – 10m deeper than the average expected – but piling production has still met expectations. “The BG28 has comfortably dealt with deep boulder layers and, as expected, the cluster drill has formed sockets in vastly reduced time,” Pearce says.
It was not all plain sailing, however, and one operational disadvantage has been that the hard boulders and rock have caused extreme wear on cutting teeth and bits, which have required regular maintenance and replacement.
Another disadvantage has been that when forming the socket with the cluster drill, the rock in which the socket is created is reduced to small chips and dust. This means that the rock quality is more difficult to assess post-drilling and a thorough geotechnical investigation is essential prior to construction. “As with most geotechnical works, the cost of a thorough geotechnical investigation is easily recovered during construction through design and construction cost savings, for the ultimate client” Pearce says.
With several lengthy work stoppages due to flooding of the access causeway, the piling is expected to be completed in mid-July 2017 after which the main contractor will complete the abutments and bridge piers and place the precast beams supporting the new deck providing a safe, dual lane link between the north and south banks of the Tugela River and helping to uplift Tugela Ferry and the entire surrounding area.
Franki Africa is part of the Keller group, the largest independent geotechnical contractor in the world.
Article published in the Construction World Magazine – July 2017