Iconic dams |
Above: Wolwedans dam was the second RCC arch gravity dam to be completed in South Africa in 1990
The Knellpoort Dam has a downstream slope of 0.6:1 (H: V), an overall crest length of 200m, incorporating a 60 m long uncontrolled spillway, and containing 45,000m3
of RCC.
Wolwedans Dam has a steeper downstream slope of 0.5: 1 (H: V), a crest length of 270m, incorporating a 77m long uncontrolled spillway, containing 203,000m3
of RCC. Both dams were provided with groutable crack inducers.
RMC in South Africa Rubble Masonry Concrete (RMC) technology for dams blends ancient manual construction methods with state-of-the-art design techniques to create a dam type that history missed, the RMC arch dam. Developing a concept first implemented in neighbouring Zimbabwe in the mid-eighties, new generation RMC arch dams were introduced in South Africa in 1995. Since then, the obvious benefits of this technology have been confirmed by more than ten successfully completed structures.
RMC dam construction has been demonstrated to be an efficient, cost-effective method for the labour-based construction of small and medium-sized dams. This technology is likely to become increasingly prevalent where there is a need for job creation and skills development. In view of the fact that the design approach applied to date has been developed on largely empirical grounds and construction has included some fairly ambitious dam structures, there is a need for a more analytical and all-embracing approach to RMC dam design. This is particularly important
38 | April 2024 |
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to allow dams of this type to be applied in the diversity of environments inherent to South Africa and using a fairly broad spectrum of construction material types. As a broad definition, RMC is a matrix comprising large stones, or plums, in a mortar binder. To minimise cost and optimise structural properties, it is necessary to ensure the correct rock or stone content. While a rock component content of 65-70% is achievable, 55% rock with 45% mortar is realistic. The mortar used will generally comprise one part cement and 4-6 parts sand (by volume), giving typical 28-day compressive strengths for the RMC of the order of 14 down to 9MPa. Rock used will vary in size from approximately 50mm maximum dimension to 300mm, the largest dimension being dependent on the thickness
of the member under construction and the obvious restrictions of manageable weight. By coincidence, the first two RMC dams to be constructed in South Africa were initiated independently but virtually in parallel in 1995. Bakubung Dam in Pilansberg National Park is a 14.5m high RMC multiple arch buttress dam, while the Maritsance Dam near Bushbuck Ridge is an 18m high single curvature arch dam with an embankment flank. Both dams were completed in early 1996. With a height of 30m, Mndwaka Dam is the highest multiple arch-buttress RMC dam constructed in South Africa, with an RMC volume of approximately 30,000 m3
. The
highest RMC dam is located in Zimbabwe, namely Lucilia Poort Dam, a single curvature arch dam 42m high.
Finite element analysis - Yusufeli dam A great blog by Ryan Cassells of ARQ Consulting Engineers (https://www.midasgeotech. com/blog/concrete-arch-dam-yusufeli) discusses the design of the dam, and details the finite element model for analysis created using midas FEA NX, incorporating a mesh strategy to ensure accurate results. The foundation model considers different stiffness parameters and asymmetrical configurations. Analysis results include displacements, stresses, and strains, with plots showing radial displacement, vertical stress, horizontal stress, and principal stress contours. The dam is shown to be in compression, with negligible tensile stresses. A sensitivity analysis evaluates the structure’s response to changes in cushion stiffness. The blog highlights how designing a concrete arch dam on a layered rock foundation is complex, requiring careful consideration of foundation conditions. The finite element model accurately represents the varying stiffness of the foundation, providing confidence in the design calculations.
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