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Cutting edge tech for tricky Lesotho bridge builds

Home Infrastructure Construction & Civils Cutting edge tech for tricky Lesotho bridge builds

CONSULTING engineering and infrastructure advisory firm Zutari has been awarded a contract by the Lesotho Highlands Development Authority (LHDA) for design and site supervision services for the  Senqu, Mabunyaneng snd Khubelu bridges, all part of the Lesotho Highlands Water Project (LHWP) Phase II.

That’s according to Natie Wilson, Zutari Technical Director, Transportation Services, who said as part of a joint venture, Zutari is also involved with the Polihali Diversion Tunnels project and the Polihali Transfer Tunnel project.

The three bridges are situated on the A1 national route, which is the main access road between Oxbow and Mokhokong. The impoundment of the Polihali Dam will result in the flooding of the existing bridges and road in these locations, and access to Mokhotlong will be lost unless the bridges are reconstructed at higher levels.

The project is located in the mountainous Lesotho Highlands, where minimum temperatures of -20˚C have been recorded. Wilson said the remote nature of the site, coupled with extreme weather conditions, pose a significant logistical challenge.

An elaborate preliminary design process was followed to determine the most feasible alternatives that would meet all the required constraints that influence these bridges. One of the most important constraints considered is the influence of the structures on the local communities that will be affected by the dam development.

Senqu Bridge consists of an 825m-long, incrementally launched box girder deck and an extradosed stay cable portion to allow for a 100m centre span. One of the main construction constraints for this bridge is that construction must be completed in time for the impoundment of the dam.

The construction time could be reduced by constructing the bridge from both sides. In addition, flooding of the Senqu River might delay construction when a pier is constructed in the centre of the river.

To accommodate these two constraints in the design, the normal practice of a temporary steel launching nose was removed and replaced by supporting the front portion of the deck by means of cable stays. This pier in the centre of the river was removed, resulting in a 100m centre span above the Senqu River.

This span would be formed by stitching together the two decks launched from either side of the river, meeting in the centre of the 100m span. The substructure consists of 90m-high piers founded on spread footings.

The Mabunyaneng Bridge (120m) and Khubelu Bridge (270m) superstructures consist of variable-depth prefabricated post-tensioned concrete beams, with in-situ cast reinforced concrete top slabs. The substructures will consist of reinforced concrete wall-type piers and closed-wall abutments with wing walls. The design of these two bridges was carried out by Leporogo Specialist Engineers.

Three-dimensional models of the bridges were created in Revit, and the normal standard tender drawings, as required by the construction industry, were generated from these models.

The 3D models were used to better visualise the structure and construction process for the client and prospective tenderers. For this purpose, an interactive model of the bridge was created to allow the client to visualise the structure in its surroundings before and after dam impoundment.

Drones

A construction sequencing video was also prepared to demonstrate the intended construction process to the client and potential tenderers. In addition, drones were used to create videos and surveys of the construction site.

These provide the client with a record of the sites before construction, give the design team a better view of the construction area, and also show the sites to prospective tenderers.

These innovations proved their worth during COVID-19, providing prospective tenders with the required site and construction information without a site visit.

An innovative virtual tender briefing was carried out in March 2021 for Senqu Bridge, with the tender closing in July. It is anticipated that ground should be broken in Q2 2022. The duration of construction is estimated at three years for Senqu Bridge and two years for the Mabunyaneng and Khubelu bridges respectively.

During construction supervision, Zutari will provide a range of staff, many of whom will relocate to site for the duration of the project. All prevailing COVID-19 regulations will be adhered to, and a site-specific health and safety management plan will be compiled by the successful tenderer and reviewed by Zutari prior to commencement of construction.

Local labour requirements are part and parcel of the tender philosophy and will be coordinated via the LHWP implementing authority, Lesotho Highlands Development Authority (LHDA). During the tender design stage, skills development was overseen by the Zutari team, with senior staff members working closely with and imparting their knowledge and experience to junior staff members.

In addition, two young professionals were seconded by LHDA to Zutari. Wislon said they were fully integrated into the Zutari design team and gained invaluable experience.

 

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