The Sartorius Von Bach Dam (Von Bach Dam), situated east of Okahandja in Otjozondjupa Region, has recently received a new bituminous seal layer after approximately 50 years of service. The previous asphalt seal had become brittle, due to loss of volatiles from the bitumen, which typically leads to cracking on the surface of the asphalt.
Knight Piésold Consulting (KP) was appointed to perform an inspection of the old asphalt seal early in 2016. Results of core drill samples and a visual inspection led to a conclusive outcome; the asphalt seal needed to be replaced as soon as possible to avoid a probable uncontrolled loss of water should the dam fill up again.
Construction of the Von Bach Dam commenced in 1967 and was completed in 1970. The dam structure is referred to as a concrete faced rockfill dam (CFRD) of which the watertight lining on the upstream side is not a conventional concrete seal, but a specially designed hydraulic asphaltic seal. This type of sealing mechanism is often used in Europe and North America for dams as well as watertight membranes for landfill sites when leachate is not permitted to enter into the groundwater, but must be collected and treated. The advantages of an asphalt seal is its ability to flex and move with settlement of the supporting embankment structure, opposed to rigid concrete face dams, of which several have failed over the last decade due to the inability of the concrete to flex and compress along the lower upstream face.
A three-year draught in Namibia reduced water levels in some dams, including the Von Bach Dam, to well below 10%. These low water levels, last seen in 1996, provided an ideal window of opportunity to maximize the area of refurbishment for the exposed asphalt seal. It was recommended to fast-track the appointment of a contractor and repair the membrane before any possible significant inflow occurred into the dam from the approaching rainy season.
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