The Lesotho Highlands Development Authority (LHDA) has contracted consulting engineering and infrastructure advisory firm Zutari to provide the design and site supervision services for the construction of the Senqu Bridge, the Mabunyaneng Bridge and the Khubelu Bridge as part of the Lesotho Highlands Water Project (LHWP) Phase II.

The company expects to break ground on the project in the second quarter of 2022, with construction timelines of three years for the Senqu Bridge and two years for the Mabunyaneng and Khubelu bridges.

The Senqu, Mabunyaneng and Khubelu bridges, which are situated on the A1 national route, the main access road between Oxbow and Mokhotlong, are to be reconstructed to higher levels to avoid the flooding of the existing bridges and roads following the impoundment of the Polihali dam.

The project, located in the mountainous Lesotho Highlands, where temperatures can be as low as –20 ºC, poses a significant logistical challenge, owing to the remote nature of the site and extreme weather conditions.

The group undertook an elaborate preliminary design process to determine the most feasible alternatives that would overcome all the envisaged constraints.

Three-dimensional models of the bridges were created in Revit, which enabled the better visualisation of the structure and construction process, and the normal standard tender drawings were generated from these models. An interactive model of each bridge was created to allow the visualisation of the structure in its surroundings before and after dam impoundment.

The team also prepared a construction sequencing video to demonstrate the intended construction process and drones were used to create videos and surveys of the construction site to provide a record of the sites before construction, give the design team a better view of the construction area and show the sites to prospective tenderers.

The Senqu Bridge comprises a 825-m-long, incrementally launched box girder deck and an extradosed stay cable portion to allow for a 100 m 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,” says Zutari transportation services technical director Natie Wilson.

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 100 m 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 100 m span. The substructure consists of 90-m-high piers founded on spread footings.”

The 120 m Mabunyaneng Bridge and 270 m Khubelu Bridge superstructures consist of variable-depth prefabricated post-tensioned concrete beams, with in situ cast reinforced concrete top slabs.

The substructures will comprise 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.

“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 the start of construction,” says Wilson.

Local labour requirements will be coordinated through the 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. These individuals were fully integrated into the Zutari design team and hence gained invaluable experience,” Wilson continues.

Key members of the Zutari team are chief design and senior bridge design engineer Johan Kotzé, senior engineering geologist Salona Naidoo, senior road design engineer Francois van Reenen, White Life Consultants environmental specialist Tau Mahlelebe and occupational health and safety specialist Evert de Vries.