INTRODUCTION
Keller’s Franki Africa has a reputation for being able to deliver costeffective geotechnical solutions using a wide range of appropriate technologies in a host of different, and often challenging, ground conditions. “We have worked in southern Africa for many decades and have a profound understanding of the different soil conditions and the optimal geotechnical solutions for them,” says Franki’s Trenchless Technology Manager, Byron Field. He adds that this knowledge enables the company to be proactive in solving problems that, on the face of it, sometimes seem insoluble. The Drakenstein Municipality’s bulk sewer pipeline is an excellent example of this.

THE CHALLENGE
The path of the sewer pipeline (in the town of Paarl in the Western Cape) includes a stretch of approximately 105 m across Arboretum Road and the N1 highway, followed by a section of around 110 m which runs parallel to the Boschenmeer Golf Estate boundary wall at a depth of –6 and –8 m. According to Field, the main challenges were the relatively unstable ground conditions, which comprised sands from 0 to –4 m deep, with loose cobbles and boulders from –4 m to –8 m, and a very high water table. SANRAL also had strict wayleave conditions prohibiting the Drakenstein Municipality from conducting work beneath the N1 unless they were able to prove that every conceivable precaution had been taken to protect the highway and to ensure uninterrupted use. It was obvious that a trenchless methodology, like pipe jacking, would be required for the new sewer to run under the roads without interrupting traffic. The depth of the pipeline and its proximity to the Boschenmeer Golf Estate boundary wall also made open excavation impractical. When the ground conditions were analysed, a new challenge was encountered! Field explains: “Firstly, the level of the sewer passed directly through the cobble layer between –4 to –8 m deep, and when pipe jacking is performed through this type of ground it is virtually impossible to prevent collapse of the cobbles during excavation. Secondly, the high water table tended to draw fines from the surrounding ground towards the jacking shield. Both of these conditions could have led to over-excavation resulting in ground level settlement.”

FRANKI’S PROACTIVE PROPOSAL
Franki approached the Drakenstein Municipality with a proposal to treat the ground beneath Arboretum Road and the N1, as well as alongside the Boschenmeer Golf Estate boundary wall. The proposal entailed jet grouting – which involves the mixing and partial replacement of the in-situ soil with cement slurry – to consolidate the in-situ ground condition along the sewer centreline and between the depths of –4 to –9 m, and to then install a pipe jack through the treated ground. “The treatment of the ground would prevent collapse of the sand and cobbles during pipe jack excavation and would reduce the ingress of water to manageable levels,” Field says. He adds that jet grouting was Franki’s preferred method of treatment, as high-pressure jetting can be used to consolidate insitu ground at exact levels, and can provide up to 2.5 m diameter columns with only an 80 mm drill stem. The municipality’s design team included the proposed solution in the tender document for this phase of the works and, in August 2016, Franki was appointed by the main contractor, Vakala Construction, to carry out the specialist geotechnical work.

THE RESULT
Field says that the jet grouting went according to plan and was carried out with zero impact on traffic. “In addition, once the jet grouting had been completed and the site cleared, there was no remaining evidence at ground level that the ground beneath had been treated. “The entire pipe jacking operation went smoothly, with the ground treatment working better than even our highest expectations.” The sub-contract work was completed by Franki on time (February 2017) and within budget.

FRANKI – MORE THAN JUST PILES
Franki is renowned for its geotechnical solutions using an array of different piles, including driven tube piles, precast piles, augerpiles, full displacement screwpiles, rotapiles, micropiles, the famous Frankipile (driven cast-in-situ pile) and many more. It is also well-known for its soil improvement systems, including dynamic compaction, deep soil mixing, accelerated consolidation, and of course jet grouting as discussed above. Franki’s skills in trenchless technology are just as impressive. For more than 30 years it has successfully been providing pipe jacking and other trenchless technologies – augering, thrust boring and large-diameter case boring – to a wide range of clients in southern Africa. Trenchless technology is a ‘family’ of methods, materials and equipment capable of being used for the installation, replacement or rehabilitation of existing underground infrastructure with minimal disruption to surface traffic, business and other activities. It is, therefore, often the most cost-effective solution. Pipe jacking, an integral part of this ‘family’, is a technique for installing underground pipelines, ducts and culverts. Powerful hydraulic jacks are used to push specially designed pipes through the ground behind a shield at the same time as excavation is taking place within the shield. The method provides a flexible, structural, watertight, finished pipeline as the tunnel is excavated.

CONCLUSION
By being part of the Keller Group, Franki’s leadership in the geotechnical space in southern Africa has been significantly enhanced. Keller is the world’s largest independent geotechnical engineering contractor, offering Franki significant advantages, such as access to a wide range of innovative technologies, stateof-the-art machinery and a wealth of geotechnical intellectual property and experience.

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Civil Engineering April 2017

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