News & Media

6 April 2010

SOUTH AFRICA'S FIRST GROUT-ENRICHED ROLLER COMPACTED CONCRETE DAM

The Bramhoek Dam forms the lower reservoir of Eskom’s Ingula pumped storage scheme in KwaZulu Natal, about 22 kilometres north-west of the town of Ladysmith. Though classified as a Category III dam, Bramhoek is 337 metres long and 37 metres high.

The Bramhoek Dam Joint Venture (BDJV), which is building both the lower Bramhoek and the upper Bedford dams, is a consortium comprising Concor Holdings, WBHO, Edwin Construction and Silver Rock, with Concor as the lead contractor. In order to accelerate construction and use new methodologies available and introduce economy measures, Concor Roads & Earthworks together with the project consultants decided to use 400 mm wide Grout Enriched Roller Compacted Concrete (GE-RCC) for the upstream and downstream faces.

“The use of RCC for dam construction is common practice in South Africa,” explains Mike Neumann, Bramhoek Dam supervisor for the BDJV. “The GE-RCC concept, however, is new, and initially, there were misgivings and some resistance to its implementation. Once the site team had mastered the process, the speed and simplicity of the operation quickly overcame these reservations. Consequently, I believe that even though the Bramhoek Dam is relatively small in comparison to the many big dams constructed in South Africa, the successful use of GE-RCC will convince others to use this facing material for future dam construction.”

The first challenge faced concerned optimisation of the mix design for the material. The RCC aggregates and other processed stone materials are mined in a quarry on site from the dolerite sill on which the dam itself is founded. “From the beginning we experienced problems with workability and we had to modify the mix five times before developing the most suitable product. We controlled placing and compaction of the RCC very tightly to limit the time from batching to final compaction to not more than 40 minutes.”

This resulted in a RCC which did not segregate, retained its moisture, thus remaining workable for longer, and compacted to a smooth finish with the vibratory roller, retaining a slightly spongy finish.

“The RCC is transported to the site in articulated dump trucks carrying about 10m3 RCC at a time,” Neumann continues. “Once the surface to be covered has been cleaned with a high-pressure air/water hose, the excess water is blown away and RCC is placed at the most distant point from the access. Dozers then level the RCC to an uncompacted thickness of about 360 mm.

“The RCC is then compacted by a 16 ton vibratory roller to within one metre of the upstream and downstream formwork. Grout with a water/cement ratio of 0.9 is then poured over the uncompacted RCC within a 400 mm wide strip from the facing formwork and allowed to seep in for roughly five minutes. The 400 mm wide strip of GE-RCC is then vibrated with immersion poker vibrators that are pushed into the underlying GE-RCC layer.” According to Neumann, trials had demonstrated that 10 litres of grout applied to the 400 mm width and a one metre running length were sufficient to ensure a dense GE-RCC facing concrete.

After vibration, the GE-RCC/RCC interface itself is compacted with a two ton vibratory roller and small walk-behind rollers up to the RCC which had already been compacted by the 16 ton vibratory roller.

“The compacted RCC surface is kept continually moist with hand-held hosepipes and sprinkler systems to provide uninterrupted hydration of the cement. Constant monitoring was essential, especially during the dry winter months and the windy period from July to September,” he says.

The process is conducted on a 24/6 basis, with successive layers being placed on hot joints where initial set has not take place, or warm joints where the initial set has taken place but the fresh RCC is still able to penetrate into the previously compacted layer.

“If RCC placing has stopped for some time so that penetration is inhibited, for instance after a weekend, the surface of the RCC is treated as a cold joint. The surface is then green-cut with a mechanical broom and kept wet.”

Neumann points out that the GE-RCC route eliminates the need for plant moving around the dam wall. “All you need is the pipeline on which you jack different hoses which one labourer uses. This speeds up the construction practices and reduces interference which in turn results in better cost-effectiveness and productivity.”