Publication: Who's Who Engineering Namibia
Issue: 2016/17 Edition, November 2016
Author: Dr. Gert Cloete
The Von Bach Dam, situated outside Okahandja in Otjozondjupa Region, was built in 1970. Unlike Avis Dam in Windhoek, which is a concrete-faced rockfill dam (CFRD), the dam at Okahandja is an asphalt-faced rockfill dam (AFRD), a design that provides a more flexible sealing membrane. Other AFRDs are the Hardap Dam in Namibia and five similar dams found elsewhere in Africa. Over the past 10 years, three large CFRD dams have failed — one in Lesotho and two in Brazil — due to compressive forces in the rigid concrete membrane; an AFRD construction would likely have been able to absorb these forces without failing.
As the CFRD construction technique is still widely used in South Africa, it has recently been adjusted to accommodate the type of movements that had caused these failures. AFRD structures are more readily found in Europe and North America, not only as dams but also as landfill sites, where leachate is not permitted to filter into the groundwater. Ultimately, the choice between CFRD and AFRD dam types depends on the availability of suitable local building materials.
The Von Bach Dam is an embankment dam, formed of locally sourced river rubble that is compacted in layers, providing side slopes with a suitable factor of safety against slope failure. The downstream face is comprised of graded rubble and filter layers to prevent erosion, and rockfill on the upstream side is graded and prepared with a porous bituminous trimming layer, approximately 150 mm thick.
At the Von Bach Dam, the trimming layer is covered with a 50 mm-thick asphalt drainage layer and two dense asphalt concrete (DAC) layers, each 50 mm thick. The DAC layers provide a watertight barrier for the dam. A 4 to 5 mm thick, sacrificial bitumen/mastic layer was spread on top of the DAC layers to protect them against UV radiation. UV radiation reduces flexibility in the DAC, thus making it harder and more prone to cracking. Frequent maintenance of the mastic layer increases the lifespan of the DAC and also reduces the need for maintenance on the DAC.
At the Von Bach Dam, an uncontrolled flow of the mastic layer over the years has exposed parts of the DAC layer to the elements and UV radiation. Degradation of the exposed asphalt followed and cracking occurred mostly along the upper slope area. Core drill tests found that the DAC along the lower part of the embankment, which is mostly covered by water, was in a better condition and suitable for several more decades of operation.
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