A 5.74m diameter Terratec double shield TBM driving the 11.5km long tunnel for the Xe-Pian-Xe-Namnoy hydro drive in Laos passes half way, notching up significantly high advance rates along the way.
Twelve months after assembly and positioning through the 500m-long launch adit, the TBM has reached the 50% milestone, recording a total distance of 6,442m as of 31 July (2016). The drive is being managed by Seli Overseas as tunnelling contractor to the main contractor, SK Engineering and Construction.
In the last three months, the TBM has recorded advance rates of 806m, 824m and 1,004m meters, which machine manufacturer Terratec believes to be a record for South East Asia for a mid-sized TBM.
A number of different geological formations have been encountered along the drive path to date: mainly mudstone, sandstone and siltstone with an average UCS of 80MPa, but with peaks of up to 200MPa. The TBM has traversed numerous fracture zones and three geological faults, with contractor and manufacturer working together to overcome the challenges presented by the difficult geology.
The remaining 5km of mechanized excavation are expected to be completed by early 2017, at which point Seli Overseas will hand the tunnel over to SK Engineering and Construction, the main contractor, to complete the project – which also includes 1.9km of excavation by conventional means.
TBM operations are under way for the Xe-Pian Xe-Namnoy Hydropower Project in Laos. The 5.74m diameter Double Shield hard rock machine, manufactured in China by Australian manufacturer Terratec, will complete 11.8km of the 13.7km upper headrace tunnel for tunnelling specialist Seli Overseas, working under sub contract to South Korean turnkey contractor SK Engineering & Construction.
To witness the important milestone, a colourful ceremony was arranged and attended by officials from the client, Xe-Pian Xe-Namnoy Power Company, SK Engineering & Construction, Seli Overseas and Terratec.
After successful factory testing in December (2014), the machine was dismantled, transported to the remote site, and reassembled within approximately three months.
Following completion of site testing the TBM will now be advanced through 500m of tunnel adit excavated earlier by drill+blast. Once the TBM reaches the tunnel face, it will commence excavation and installation of the fully segmentally lined tunnel.
Geology consists primarily of two formations: the Tholam Formation consisting of mudstones and siltstones, and the Champa Formation including siltstones, sandstones and conglomerates. While available geological and tectonic maps show no major weak zones, faults are anticipated with the possibility of some water ingress. Seli Overseas reports that the tunnel is expected to cross several fault zones, some of which will be more than 20m wide.
Seli Overseas is preparing to launch a Terratec manufactured TBM to excavate most of the 13.7km long upper headrace tunnel for the Xe-Pian Xe-Namnoy (XPXN) hydropower scheme in the Lao People’s Democratic Republic (PDR). Working on subcontract to South Korean turnkey contractor SK Engineering & Construction, the Italian tunnelling specialist anticipates launch of the 5.74m diameter double shield TBM by late March/early April.
The Terratec TBM will be launched just uphill of the surge shaft location and will bore the 5m i.d. tunnel upstream for approximately 11.5km towards the upper section of the headrace that is being constructed by drill+blast.
The XPXN hydro project is located in the south west of the country on the Bolaven Plateau, near the border with Thailand. It will take water from the top of the plateau to a power plant via concrete-lined conveyance tunnels. Gross head across the XPXN scheme is more than 630m.
Layout of the main underground works, from upstream to downstream, and as given by the developer, Xe-Pian Xe-Namnoy Power Company Limited (PNPC), is:
Penstocks will link the high-pressure headrace to the powerhouse. Other key structures on the scheme include dams and a surface powerhouse.
Geology along the upper headrace comprises mostly mudstone and siltstone with about 20% of the length in sandstone of typically 70MPa-150MPa UCS. The sandstones are moderate to very strong rocks, and contain large amounts of quartz, according to subcontractor Seli Overseas. They are well cemented and therefore have a very low permeability, and form near vertical cliffs in the escarpment of the plateau. While available geological and tectonic maps show no major weak zones, faults are anticipated with the possibility of some water ingress. Seli Overseas reports that the tunnel is expected to cross several fault zones, some of which will be more than 20m wide.
The cutterhead of the Terratec hard rock shield is fitted with 17in disc cutters and four bucket openings. Its main drive power is 2,000kW (eight motors x 250kW), its torque is up to about 8,000kNm, and maximum rotation speed of the cutterhead is 7 rev/min. Terratec said the shield has high torque to help cope with fractured zones along the alignment of the low pressure headrace. The TBM can also advance in single shield mode, and has a capability for high-pressure emergency thrust.
Precast concrete segments of 250mm thick and 1.5m wide will create the 5.5m o.d. hexagonal segmental lining for the TBM-bored section of the headrace.
Terratec produced components for the TBM in Australia and Japan and assembled the machine in its facilities in China. Factory acceptance was announced in early December and the TBM is under transport to site.
With the TBM to bore about 84% of the upper headrace, the remaining 2.2km stretch of the low pressure tunnel is being blasted and shotcrete-lined. A drill+blast heading near the surge shaft has also provided access and the launch chamber for the TBM. Most drill+blast on the low pressure tunnel will excavate the 1.9km section from the intake. With a start in early April, the TBM bore is anticipated to be complete by about mid-2017.
Turnkey contractor SK Engineering & Construction is one of four partners working as PNPC to develop the 410MW hydro scheme, which is to be completed by 2018. The other JV partners in the 32-year Korean-Thai-Lao concession development company are Korea Western Power, Ratchaburi Electricity Generating Holding PCL and Lao Holding State Enterprise.
PNPC signed the concession agreement in 2012 following initial investigations in the 1990s and various pushes to develop the scheme. A key factor in reaching the concession stage was the tariff deals with Thai and Laos energy utilities and the negotiation of the take-off deal or power purchase agreement (PPA) in 2013. In total, the scheme is expected to generate at least 1,800GWh/year with most of the output being exported across the border to Thailand.