A fleet of rollers is being used to help construct the 39m high RCC dam and integral spillway which forms part of the Nam Theun 2 hydroelectric project
Nam Theun 2 (NT2) is the largest foreign investment project ever undertaken in Laos and is being developed as a venture with neighbouring Thailand, which will buy 95% of its electricity supplies, with the remainder being used domestically.
At a cost of US$1.25B, the project is also the world’s largest private sector cross-border power project. It is sponsored by Electricite de France (edf), which has 35%, Electricity Generating Public Company (EGCO) of Thailand, with 25%, the Lao government-owned Lao Holding State Enterprise (LHSE), 25%, and Thailand’s largest construction contractor, Italian-Thai Development (ITD), which has 15%. These organisations have formed a company, Nam Theun 2 Power Company (NTPC), which is constructing the project and will own and operate it over a 25-year concession period.
Located in the Lao central highlands, the site of NT2 at the Nam Theun river is particularly suitable for a hydroelectric project that will produce a significant head of water without a corresponding requirement for a high dam. The dam is therefore being built as a 39m high, 436m long concrete gravity structure that, along with 13 smaller earth saddle dams across various other valleys, will create a 450km2 reservoir, flooding the Nakai Plateau.
Some 70km to the southeast of the Nakai dam is the intake channel for the generating plant, located where the abrupt change from mountains to lowland floodplains provides a drop of 348m. From the turbine house the water passes into a 27km downstream channel and then into the Xe Bang Fai river.
The project includes a 130km, double circuit 500kV transmission line to the Thai border and about 70km of a 115kV transmission line to the regional Lao grid.
With NTPC contractually responsible for the project, the civil works for the main dam, spillway, saddle dams, headrace channel and roadworks are being undertaken by Italian Thai Development (ITD), to designs developed by klohn-crippen of Canada.
Klohn Crippen, who was involved from the beginning in evaluating the project layout, recommended a roller compacted concrete (RCC) dam and integral spillway rather than a concrete face rockfill dam and separate spillway because such a design uses less volume of material and is consequently faster and more economical, given the appropriate conditions.
Continuous placement of the RCC also adds to the speed of the work. Not only was this considered important for the economics of the project, it is of practical benefit given the flooding that occurs during the wet season and at other times of the year.
The dam is therefore using a continuous supply of a stiff, zero-slump concrete mixture with the consistency of damp gravel comprised of local aggregates, Portland cement and water. The mixture is produced in silos on the downstream side of the dam and trucked the 200m to the dam crest, where it is placed in layers of 30cm in thickness and compacted using four rollers from Dynapac.
Immediately after the machines have completed the compaction, taking between two and four passes, water is applied as a fine mist to cure the concrete.
Dynapac’s distributor in Thailand, the MEC Group, has supplied the Dynapac rollers. MEC director Anucha Burapachaisri said that a combination of two large articulated tandem rollers, a CC422 and a CC421, and two small CC102 steel-and-rubber drum units, is being deployed on the dam crest.
‘ITD wanted the flexibility that the different sizes offer,’ says Burapachaisri. ‘The large machines of course do the bulk of the work, but the CC102’s are here being used to complement them by compacting cross-joints and in the restricted corner spaces.’
Because of the size of the dam crest, and the fact that a considerable amount of other equipment such as a bulldozer and cement trucks are usually also present, the RCC is laid in areas of around 150m2 at a time. Working in this way, one complete 30cm thick layer of RCC is laid on the crest from abutment to abutment during one eight-hour shift.
The Dynapac rollers are also frequently deployed to keep the access ramp and the earth roadways around the site in a good state of repair.
Flooding in this area is a feature of the rainy season, which is at its height in the period July to September, but heavy rains are frequent throughout much of the year and keeping the access roads open and functioning is an ongoing operation.
There is a total 145km of new and upgraded roads and bridges associated with the project, not only for access to the site but also planned as an important part of the development program for this deeply rural part of Laos, with its scattered villages.
Two Dynapac CA252D compactors are being used to grade, compact and maintain the earth surface of the roads around the saddle dams. These are smooth-drum machines, able to work in the mix of clay and loam that constitutes most of the soil in the heavily forested hills. As much of their work is maintenance, the Dynapacs are kept rolling over the surface, and will eventually be used for compacting the asphalt surface that will be laid once the project is complete.
A third CA252D is working at Thakhek, the border town with Thailand on the banks of the Mekong river, where a substation transformer is to be located. The roads here, where they meet the new road that connects to the site, are being upgraded, and the CA252D is being used to compact the asphalt surface, passing four times to achieve the necessary compaction.
Preliminary work on NT2 got fully underway in the middle of 2005, and completion of the entire project is due in late 2009.
Portable Air provides ‘fair face’
Italian-Thai Development is also using an Atlas Copco
XRHS 385 oil-injected rotary screw compressor to power the abrasive equipment that it ensuring a smooth facing to the downstream side of the RCC structure; providing a more aesthetically pleasing surface.
The compressor is providing an actual free air delivery of 22.3m3/min working at an effective pressure of 20 bar.
The unit was supplied to ITD by Atlas Copco (Thailand) Ltd, whose manager for compressors and generators Adisorn Moleesaide said that the need was for continuous operation in a remote site where only routine spares could be stored, and where any serious breakdown would inevitably mean a long delay, the Thai border being a distance of 140km of dirt road, frequently made almost impassable by rainfall.
Featuring two large fuel tanks each with a capacity of 544 litres ensures extended operational duties. The compressor also includes a spillage-free frame and a silenced canopy meeting reduced noise levels of 74 dB(A) at 7m.
Work has been carried out with a number of grinding-and-polishing handtools, with the abutments being the first stage. The compressor has been placed against the downstream face of the dam, and works on a continuous basis during two eight-hour shifts.
Work progresses on the Nam Theun 2 project RCC dam A fleet of rollers are being used to construct the 39m high dam Dam View of the project site Project site Continuous placement of the RCC adds speed to the work RCC Author Info:
For further information on the Dynapac machines, contact: Jan Olofsson, Dynapac AB. Tel: + 46 (0) 40208872. email: firstname.lastname@example.org