Featured Project

Von Bach Dam Asphaltic Seal Replacement

The main water supply dam of Windhoek, the capital of Namibia, had significant cracking in its asphaltic seal membrane, requiring replacement of the asphalt seal or face uncontrolled leakage and subsequent dam failure.

The Sartorius Von Bach Dam (Von Bach Dam) is an asphaltic-faced rockfill dam, of which there are not many in Africa. Experience in asphaltic-faced rockfill dams, particularly for large dams, is not available in Southern Africa; therefore, expertise was sourced internationally.

After a three-year drought, the Von Bach Dam storage volume reached 10%, its lowest water level in 20 years, thus providing an ideal window of opportunity to rehabilitate the deteriorated asphaltic seal. A safety inspection indicated crack penetration through the old seal layer, endangering the storage ability of the most important dam in Namibia.

Due to time constraints from the approaching rainy season, a fast-track approach was recommended by Knight Piésold and implemented by the client. Within two weeks, three quotes were received and after a process of negotiation, which included the estimated time of completion, a contractor was appointed.

Specialized machinery had to be demobilized from sites in Europe and shipped to Namibia. Assistance from the CEO of the Namibia Water Corporation Ltd. (NamWater) was provided in the form of letters to the Ministry of Home Affairs for issuing work visas for operators and specialized labour, to the Ministry of Finance for rapid customs clearance, and to the Roads Authority for rapid approval for bulk road transport of abnormal-load machines. 

During the importing process, an inactive asphalt batching plant was revived by a subcontractor of the main contractor. Trail batches of dense asphaltic concrete were mixed and tested for application. Failure to reach less than 2% void content in the asphalt required changes in the raw material until the design specifications were reached.

Once established on site, less than a month was available to mill off the old asphalt seal, prepare the joints between the new layer and the existing concrete structures, and place a new drainage and dense asphaltic seal.

The contractor worked seven days a week, utilizing daylight only for problematic quality control in artificial light. The contractor finished the work by mid-December; two weeks before the first significant summer rains started filling the dam.

Knight Piésold’s Role
  • Knight Piésold was appointed to perform an inspection of the dam’s old asphalt seal. Core drill samples and a visual inspection indicated the asphalt seal needed to be replaced as soon as possible to avoid a probable failure.
  • The unique type of seal structure demanded a best-practice approach during the design phase since design guidelines on hydraulic asphalt seals are based on an empirical approach. Experts in construction and rehabilitation of asphaltic membranes were consulted and several configurations of asphaltic membrane seals were considered.
  • The engineer reviewed design guidelines for asphaltic seals regarding densities and particularly the drainage structure required for these seals. A suitable option was selected for the Von Bach Dam and recommended to the client.
  • Knight Piésold recommended replacement of the asphaltic seal, negotiated with the client and the contractor to reach a point where all three worked together to complete the project on time and within budget, for the greater good of the most densely populated area in Namibia.

Award

2018, Consulting Engineers South Africa, Aon Engineering Excellence Awards, Commendation, Best International Project Category, Von Bach Dam Asphaltic Seal Replacement Project