Laos headrace finishes early despite challenges 30 Mar 2017

Seli Overseas has completed the 11.5km long TBM drive for the Xe-Pian Xe-Namnoy (XPXN) hydropower scheme in Laos more than three months early and despite initial challenges for the programme.

During the last year, average progress achieved by the 5.74m diameter Terratec double-shield TBM was near 680m/month, said Dario Vizzino, Project Director for Seli Overseas from the Laos jungle site when contacted by TunnelTalk following the 18 March breakthrough.

From formal TBM launch in early 2015, the monthly average is about 500m. This included a slow start due to power supply challenges and a stop in January 2016 for cutterhead enhancements to deal with harder than expected sandstone.

Team completes 11.4km headrace drive three months early

The initial plan for tunnelling anticipated a TBM boring programme of about 800 days to a completion by mid-2017. Seli Overseas has actually finished by three months ahead of that schedule, said Vizzino and despite the early challenges.

The Terratec-supplied TBM progressed upstream from near the surge shaft on the headrace alignment with a total of 7,636 rings of precast concrete segments to line the 11.5km x 5m i.d. TBM bored section of the 13.7km long headrace. Drill+blast advanced the 2.2km balance of the headrace from the upstream end.

Disassembly of the TBM is underway in the reception chamber at the junction point with the drill+blast heading following which some final grouting tasks remain in the bored tunnel section and final tunnel cleaning during the next two months and before handover of the complete contract, said Vizzino.

Construction challenges and success

Seli Overseas is subcontractor for the TBM tunnelling for the main, turnkey contractor SK Engineering & Construction of South Korea. SK is also a partner in the project development company, which has a 32-year concession to construct and operate the 410MW project.

XPXN is located deep in the jungle, in the south west of the country, and the location provided significant logistical challenges. Vizzino told TunnelTalk that good planning was critical for avoiding different kinds of delays. “Effectiveness of this planning was an important contribution to the tunnelling progress and early finish,” he said. A significant challenge was the limited availability of skilled and qualified local workers, which led to more use of expatriates, he added.

Breakthrough celebrations for the team and the Terratec TBM

Issues with the quality of the power supply from the local grid were eventually overcome when the project owner installed diesel generators on site to provide the right quality of power needed with continuous availability, Vizzino explained. As a result, while the TBM was formally launched in March 2015, the effective commencement of excavation was in August-September 2015, he added.

Geological challenges centred on an expectation of boring through mostly mudstone and siltstone with less than a quarter of the bore through harder sandstone and through typical rock strengths of 70MPa-150MPa.

The cutterhead was fitted with 39 x 17in back loading disc cutters, four bucket openings, and had main drive power of 2000kW (8 motors x 250kW) to generate a torque of up to about 8,000kNm and maximum rotation of 7 rev/min.

“Sandstone was expected to be encountered in the second half of the tunnel and it should have been of medium strength,” said Vizzino. “Instead, more than 2km of very hard sandstone was met in the first part of the TBM-drive. With the shield operating at about the limit of its thrust, it was decided after a few months to stop and reinforce the cutterhead to avoid serious damage. This upgrade work was completed in January 2016 and from then on the TBM bored continuously.”

Headrace is lined with hegaxongal segment rings

The monthly progress jumped from about 400m/month before the upgrade works to mostly exceeding 600m/month afterwards and achieving more than 800m/month across four consecutive months. The best month was 1,004m in July 2016, and building almost 670 rings of lining.

The precast segmental lining is assembled as 1.5m long, 5.5m o.d. hexagonal rings of four 250mm thick segments and pea-gravel and grouted backfilling the annulus. The TBM back-up had a hydraulic lift to raise and empty gravel cars and a double-capacity pumping system to support high production rates.

Except for the different distribution of the expected rock types and quality, the geology presented other particular challenges. High groundwater inflows and potential difficulties with faults had been anticipated but neither was of significant issue.

The Terratec TBM and the company’s support on site was praised by Vizzino. “The tunnelling experience has been very good,” he said. “Terratec deserves special appreciation for its support and the attitude of all its staff.”

Other underground works on the project include excavation of the surge shaft, a 55m high vertical high pressure shaft and a 1,300m long steel-lined lower pressure tunnel to the over a gross hydraulic head of 630m to the power plant built on the surface.

The concession for XPXN was awarded to the developer in 2012. The successful TBM tunnelling is a key contribution to the turnkey contractor achieving the scheduled completion of the project by 2018. Most of the output will be exported from Lao People's Democratic Republic to Thailand.

Loas is developing other hydroelectric power potential and TBM tunnelling was also used successfully on the Theun Hinboun hydro project on the country's Nam Theun River which will also supply electricity to Thailand.