Working in the wilderness

Electricity generated from hydroelectric schemes is Lao's main export. The small south-east Asian country has several large rivers that drain its mountainous eastern and northern regions into the Mekong River, which forms the border with neighbouring Thailand. This geography provides the key combination of high flows and big vertical drops, which are the crucial ingredients for hydroelectric schemes.

Lao's first hydroelectric scheme was the 110 MW Nam Ngum 1 (NN1) project, which was completed in 1971 and includes a 75 m high dam. Electricity from NN1 is fed into Lao's national grid for domestic use, but plans are now underway for more schemes further up the Nam (River) Ngum, and most of the power from these will be exported to neighbouring Thailand.

As many as four more dams are under consideration at various locations upstream from NN1, but the first to get the go-ahead is Nam Ngum 2 (NN2). The consortium building the US$ 530 million scheme is called South East Asian Energy (SEAN), and following completion of the work, SEAN will operate the facility on a 25 year concession. The largest shareholder is CH Karnchang, the main contractor, which holds 28.5%. The Government of Lao owns 25%, as does Ratchburi Electricity Generating Holding, a Thai energy company, while four other companies hold small stakes.

Access

One of the biggest logistical challenges on NN2 is site access. Downstream the NN1 dam, which is about 90 km north of the capital Vientiane, is served by reasonable – which is to say paved – roads. But beyond NN1 site access to the new project starts to get interesting!

There is no road around the lip of the NN1 reservoir, so everything and everyone involved in NN2 has to get there by boat. It is an unusual method of site access, but according to Weera Soddayasai, a site agent on NN2 with CH Karnchang, it is the most practical approach given the terrain.

“The plan for this project is to bring everything in by barge. The road route is 50 km and it will take at least two years to build, but bringing things across the reservoir takes only two hours,” he said.

It is certainly a pragmatic approach but it is not without its problems. When NN1 was built the 450 km² area to be flooded was not cleared of trees and vegetation. Even though this was 35 years ago, the dead tree trunks still break the surface of the reservoir and make navigation hazardous, particularly when water levels are low in the dry season.

Low water levels can also mean there is not enough draft – particularly in the narrow channel close to NN2 – for the barges bringing heavy equipment to the site. Deliveries therefore have to be scheduled away from the height of the dry season, when drawdown and evaporation take the reservoir to its cyclical low point.

Outline

The project involves driving two 1.3 km long, 13 m diameter diversion tunnels through the valley wall on the eastern bank of the Nam Ngum. Once these are complete, a cofferdam will be built in the channel, allowing construction of the main 181 m high concrete-faced rockfill dam to get underway.

The hydroelectric portion of the scheme will comprise a 700 m long headrace tunnel, leading to three 150 m long pressure shafts, inclined at 52°. These will each lead to 500 m long, 5 m diameter steel-lined pressure tunnels, which will feed the three 205 kW turbines.

In addition to these, there will be a 40 m deep, 14 m diameter vertical surge shaft from the hilltop above the inlet down to the headrace tunnel to protect the system against dangerously high water pressures.

In addition to all the underground construction, NN2 requires a formidable amount of surface rock excavation. Locating the powerhouse in the steep-sided valley, along with the excavation for the dam and spillway will entail 7 million m³ of rock excavation. In addition to this 8 million m³ of rock will be quarried for the construction work for a total of 15million m³.

As well as the difficulties of site access, the construction team has to contend with Lao's tropical climate. Temperatures in the dry season regularly exceed 35°C, while the rainy season, from May to September, is characterised by extremely heavy thunderstorms that bring work to a halt.

Timing activities around the seasons is crucial to the success of the scheme. CH Karnchang has already hit problems due to administrative hold-ups, as Mr Soddyasai explained.

“We have already lost one dry season. This project has had many setbacks, including some from the Lao Government – there are lots of organisations and Ministries that wanted to check the contract, and it was only finally signed in March,” he said.

Continuing the theme he said, “The dam construction depends on the seasons. If we started construction in the wet season, the cofferdam will be much more difficult to construct and without that we can't build the main dam,”

Progress

Construction of NN2 is in its early phases, and the main focus at the moment is on the diversion tunnels. Work got underway in December last year, and CH Karnchang has sub-contracted the tunnelling work to another Thai company, Right Tunnelling (RT).

RT started work on site with excavation of the diversion tunnel portals. When iC visited the site in early April, the no.2 tunnel had progressed about 50 m, while preparation of no. 1 tunnel's portal was underway.

The company has some 4 km of tunnels to drive over the course of the scheme, and is using the drill & blast method to work its way through the valley's siltstone and sandstone geology. RT is procuring eight used Atlas Copco tunnelling boomers for the work, which it has sourced from various countries in Asia.

Although even at the peak of activity there will only be four working faces, the company expects to have six two-boom and two three-boom rigs on site. Four of the boomers will be used for drilling, with a further two installing rock bolts and the other two on standby in case of breakdowns. Sursak Seekhiew, RT project manager at NN2 said, “We have had a good experience with Atlas Copco on a previous project and our team is familiar with the machines and how to maintain them.”

Only the top 8 m of the tunnels' cross-section will be excavated with the boomers. The lower 5 m will be drilled and blasted by Atlas Copco hydraulic crawler drills – the so-called 'benching' method of tunnel excavation.

RT plans to use eight 35 and 40 ton (31.8 and 26.4 tonne) Cat articulated dump trucks to haul the spoil, and has ordered four lots of 4 m long tunnel formwork from Cifa to cast the concrete tunnel lining in-situ. According to Mr Seekhiew the rock conditions are very poor in places, and in the worst spots steel arches will supplement rock bolts and shotcrete.

The rock conditions also affect the length of rounds that can be drilled and blasted. Under good conditions the engineering design allows each 136-hole round to be 4 m deep, but this is progressively reduced down to 1.5 m in the worst conditions. Working in two shifts, RT is aiming to advance an average of 4.2 m per day overall across the four faces.

“In the dry season we aim for three rounds every two days,” said Mr Seekhiew. However, when iC visited the site in April the tunnelling work was yet to find its rhythm and only one round was being completed per day. The key problem had been with bolting and shotcrete work, and RT was in the process of moving to a shotcrete incorporating steel fibres and Atlas Copco's Swellex rock bolts. These hollow bolts are installed and then 'inflated' with pressurised water so they swell in the cavity to stabilise the rock.

So although the project is in its early stages, the pressure is already on for RT. “Time is critical now. We need to finish the diversion by the end of the next dry season so the river can be closed and the dam can start,” said Mr Seekhiew.

“The rainy season doesn't stop us because we're underground, but we have to pay more attention to things like pumping,” he added.

Schedule

Parallel to all the tunnelling, work got underway in April on the surface rock excavation. A total of 15 million m³ of material must be drilled and blasted to provide aggregate for the work and to carve out the sites for the dam, spillway and turbine house in the steep valley. The dam itself will be a 118 m high rockfill/concrete-faced structure with a crest length of about 175 m. The area flooded by the dam will be about 86 km².

Work started on the site in December last year, and most of the work is expected to be completed by the end of 2008, with the first electricity due to be generated in March 2009. The first milestone that needs to be achieved is the completion of the diversion tunnels, which will allow work on the dam to start, and this is due in mid-2007.