“Safety, constructability and cost were primary considerations on this project, but it was equally important to understand why the failure happened to ensure that any other areas of potential risk were adequately addressed in the designs,” Peter Blersch, Project Director, Aurecon.
Aurecon, with its extensive dam safety expertise on the African continent, was appointed to carry out a dam failure investigation, after a massive bursting failure of the two 8.0 metre-diameter reinforced concrete conduits, that form the outlet structure of the bottom outlet works at Massingir Dam, Mozambique. The uncontrolled discharge of approximately 1 000 cubic metres per second of water caused significant damage to the bottom outlet works and accompanying infrastructure. The investigation and design of the USD 50 million rehabilitation works started in the last quarter of 2008 with successful commissioning taking place in mid-February 2018.
Massingir Dam is a 48 metre-high, 4.8 kilometre-long embankment dam on the Rio dos Elefantes, a tributary of the Limpopo transboundary watercourse, in the Southern Gaza Province of Mozambique. It is located in an ecologically sensitive area due to its proximity to the Great Limpopo Transfrontier Park and has a significant socio-economic impact on the communities and businesses utilising water from the dam for irrigation, amongst other uses.
The Government of Mozambique, through the National Directorate of Water Resources Management - DNGRH (formerly DNA) and the South Regional Water Administration (ARA-Sul) appointed Aurecon to undertake the dam failure investigation, compile a detailed dam safety evaluation (DSE) and a feasibility study on different options for rehabilitation of the bottom outlet works, as well as detailed design and construction supervision of the rehabilitation works.
Aurecon’s investigation identified a unique failure mechanism and concluded that a minor construction defect in the form of a cold joint, combined with deficient reinforcement detailing, resulted in the main tensile reinforcement pulling out of the concrete on the curved inside faces. Aurecon’s design solution included a new control room using state-of-the-art technology, and the installation of 6.4 metre-diameter steel liners in the existing reinforced concrete outlet conduits, together with mass and heavily reinforced infill concreting and grouting. The two downstream radial control gates were rehabilitated and provided with new arms, together with the installation of entirely new hydraulic equipment. The originally designed hydraulic conduit was also replaced with hydropower offtakes.
Papers detailing the unique failure mechanism findings and rehabilitation solutions were presented at the International Commission on Large Dams (ICOLD), the South African National Committee on Large Dams (SANCOLD) and the Australian Committee on Large Dams (ANCOLD) conferences.
“We are proud of our contribution to reinstating the dam to full serviceability through safe design, and ensuring sustainable water supply to communities and the riverine ecology for many years to come,” says Peter Blersch.