Paving the future for one of the most aesthetic infrastructure landmarks in Mauritius, the Frankipile team achieved another milestone with the successful completion of the foundations for a new iconic ravine bridge.
Located south of Port Louis, the A1M1 bridge – as it is commonly referred to – will effectively connect the areas of Chebel and Sorèze on either side of the Great River North West Valley and finally join the main A1 and M1 arterial roads.
The Road Development Authority of Mauritius (RDA) has been planning a bridge to link these two areas on the eastern and western side of the valley for some time in order to ease the growing vehicular traffic congestion in the area and provide a dedicated pedestrian walkway between Chebel and Sorèze. Finally, on the 11th of April 2018, the government of Mauritius and the RDA launched the construction of the A1 M1 link as part of the larger road decongestion programme.
Frankipile Mauritius International, a Keller Company, was appointed by the client to supply and install the specialised piled foundations to support the two piers of the bridge.
“The new bridge is based on a classic extradosed design and is the brainchild of Systra International Bridge Technologies,” said Frankipile Mauritius contract engineer and manager, Mirvesh Jugurnauth . “Boasting a bridge deck length of 330 m and supported by two piers towering in excess of 80 m from the river level, the bridge is set to become a national landmark.”
The bridge deck incorporates a dual lane asphalt surface for vehicles, a pedestrian walkway on each side and two observation decks, guaranteeing spectacular sunset views over the Indian Ocean.
Between a rock and a hard place
The comprehensive geotechnical report provided by ARQ Consulting Engineers and the position of the two piers, posed distinct logistical and technical challenges to the foundation solution.
Firstly, it was clear that the piles needed to be founded in the hard rock basalt some 18 to 22 m below the river bed level. Secondly, it was evident that the final pier positions were located in the largely inaccessible 100-m deep ravine, making it extremely complex for the heavy-duty piling equipment and materials to reach the site.
The underlying soil profile at each pier position revealed a combination of silty gravels, colluvium, clay, rock layers and boulders, common in river beds. The pile design called for a total of 40 piles per pier, and each pile was 1 080mm in diameter. The piles were designed to carry a working load of 8,85MN.
Through careful coordination and planning with the main contractor (made up of the Transinvest-General Construction Co-Bouygues TP- VSL Joint Venture), the experienced Franki team were able to mobilise the large track mounted piling equipment to the engineered platforms at the pier positions in a safe and carefully controlled manner. Franki used a combination of the powerful Bauer BG20 piling rig to handle the drilling and a Liebherr 845 crane to service the site, weighing in excess of 70 and 50 tons respectively. This robust combination was well suited to handle the installation of the 1 080mm Ø piles to a depth of 20m.
Belts and Braces
The Franki team were fully prepared for the anticipated ground conditions.
“To overcome the complex soil profile, a combination of rock augers, coring buckets and a chisel were used. Where thick rock layers were encountered, a cluster drill was used to penetrate these layers,” explains Mirvesh Jugurnauth.”The cluster drill incorporates numerous small down-the-hole (DTH) hammers and is specifically designed to penetrate hard rock. Having both methods available on site enabled the team to reach the required founding depth with relative ease, regardless of the ground conditions.”
This “Belts and Braces” approach ensured Franki was able to meet the approved programme and complete the project on time and within budget.
Due to the upper soil profile and the high water table, temporary steel casings were used to ensure the integrity of the pile shaft. The piles had to be cast underwater using a gravity fed tremie pipe system. The high slump, self-compacting concrete mix was transported to the pile positions using traditional readymix trucks.
In order to meet the tight timeline of the contract, the Franki team worked a double shift each day.
“All things considered, and with the strong commitment from the team, the project was plain sailing,” adds Yannis Mongelard, Frankipile country manager.
Osterberg cell (O-cell) testing
As a first for Mauritius, two O-cell tests were carried out to verify the design and construction of the foundation piles. O-cell tests require no kentledge or reaction piles and proved to be cost-effective for tests with very high test loads. These tests were carried out by Fugro Loadtest, to a maximum load of 17.65MN. The test piles performed extremely well, with settlement of 7mm at 14.5MN and 11mm at 21MN.
The platform at Pier 2 was very close to a steep cliff, which posed a safety risk due to potential rock fall. A sturdy catch fence was designed by ARQ and installed by the main contractor above the pier position to protect the works below.
In line with Franki’s stringent safety standards, the “Think Safe, Work Safe, Go Home Safe” mantra was always top of mind. The piling contract was completed with no lost time incidents recorded.
“This prestigious project emphasises Franki’s diverse technical expertise and capability to successfully overcome the unique piling challenges related to infrastructure of this nature,” comments Dr. Nicol Chang, Franki group technical director.
Written by Roger Feldmann, Business Development, Franki Africa
Published in the April 2020 edition of the SAICE Civil Engineering Magazine