When the wind blows1 December 2002
The Midlands dam in Mauritius has been completed on-schedule, although the site is in an area prone to cyclones
THE brief given to formwork supplier Doka at the Midlands dam in Mauritius was superficially similar to one for many sites: use only as much formwork as strictly necessary. At many hydro sites the reason is that they are inaccessible. At Midlands the reason was that the site is in an area prone to cyclones, and the risk and cost involved in securing formwork against a cyclone had to be minimised.
Work started on Midlands in January 2000. The 2450m-long earthfill dam was designed by lahmeyer International and the project cost - some US$60M - has been funded by the Government of Mauritius, the Kuwait Fund for Arab Economic Development and the Arab Bank for Economic Development in Africa. It has a height of 38.4m, a length of 88m, a base width of 26.5m and a width at the crest of 4m. The main contractor for construction work was a joint venture of DTP Terassement - Bouygues TP. By late December 2002 the dam is due to be completed and water impoundment is already beginning.
The immediate function of the Midlands dam is water supply and irrigation: it will provide irrigation for 1900ha in the Northern Plains project and 2850ha elsewhere and is expected to satisfy all the water demand in the north of the country. As well as cyclones, the area suffers high rainfall - up to 4000mm yearly - so the design includes a concrete cutoff wall and an asphalt surface seal, both used for the first time in the country.
The potential for power generation has been assessed, and although no power house is currently included, a penstock has been incorporated inside the spillway.
The spillway is a major part of the plant design, rising up to 30m above the foundation level and comprising four spans each 12.5m wide. The central pier is 6m wide and the two other piers are each 1.5 m wide.
The spillway will absorb some 50000m3 of concrete. Formwork for the concrete pour was supplied by Austria-based Doka. The company explains that at this site the issue was not, as so often with hydro power, the remoteness of the site. On the contrary, says Doka's Heinrich Ecker, 'there was no problem getting there as there are roads existing. The issue was the weather conditions - they have regular cyclones there.' As a result, he said: 'Material for the site had to be provided 'just in time'. We did not want to store too much there as it has to be secured.' Careful assessment of the formwork reduced the requirement to only 1750m2. As well as simplifying the 'just in time' delivery this also reduced transportation costs.
The contractor used Doka Top 50 large-area formwork in lengths of 3m, 4m, 5m and 6m. These provided a uniform spacing for the supporting construction frames used on the starter blocks, and for the bracket and anchor spacings in the climbed pouting sections. All the formwork was designed for block heights of 3m and a concrete pressure of 35kN/m2. The supporting construction frames for the foundation blocks were also fitted with Doka dam formwork D22 components. This made it possible to continue raising the sidewalls and the centre pier with dam formwork D22 without through-ties.
The buttress walls of the dam were poured with through-ties, using Doka climbing formwork K as a scaffold unit.
With the formwork in place 'it was also necessary to secure it against potential high winds but this is standard on Doka systems,' Ecker said. 'Doka personnel were on-site when work started and they trained local staff on how to use the formwork in the right way.'