Four TBMs will drive half the total 66km tunnel alignment of the extensive US$700 million (2008 estimate) Alto Maipo hydro project under overburdens of up to 2,000m in the Chilean Andes, South America. The remaining excavations will be completed by drill+blast.
The Chilean division of Strabag (Austria) is awarded a US$490 million contract to excavate two thirds of the project's underground watercourse and powerhouse works in the valleys and mountains of the Colorado and Maipo river basins to the north, while a joint venture of Hochtief (Germany)/ CMC di Ravenna (Italy) is awarded the US$279 million contract to excavate 18.5km of southern tunnels in the Volcán and Yeso valleys. Hochtief's share is US$195 million.
Alfalfal II and Las Lajas main tunnels
Alignment traverses high-altitude Andes
Project scope also includes two underground powerhouses - the Alfafal II and Las Lajas plants - which will be located in caverns. Together the power stations are capable of generating a total output of 531MW of electricity.
The client is Alto Maipo Spa, a subsidiary of the Chile-based power company AES Gener and the USA-based AES Corporation. Design of the tunnels, shafts, adits, surge tanks and underground powerhouses is completed by Norconsult and Norplan under a separate contract awarded in 2010.
"TBMs represent 50% of the total tunnel excavation and the method is specified because of its better rate of advance than the drill+blast method," explained Vice President of Development, Michael Whittle, when TunnelTalk contacted him in the Chilean capital, Santiago.
Simplified Alto Maipo plan
"The diameter of the main headrace tunnel for the Alfalfal II is 4.5m while the 30m2 Las Lajas headrace tunnel will be excavated by drill+blast," he added.
Using TBMs means the project delivery date is just four-and-a-half years. Tunnelling contractors are already mobilising for advance works ahead of TBM procurement.
That process has not yet begun, said Whittle, but the geology of mainly volcanic rock will influence the contractor's choice of machine. Four different diameters are specified for the TBM-driven tunnels:
• TBM 1 (diameter 6.9m, open type) will excavate the majority of the 12.9km Las Lajas tailrace tunnel, working from the downstream, and is expected to excavate up to 10km.
• TBM 2 (3.4m diameter, open type) will excavate the Alfalfal II pressure shaft and surge shaft at an inclination of 45 degrees. It is expected to excavate 2km of alignment.
• TBM 3 (4.5m, open type) will excavate approximately two-thirds of the 14.5km Alfalfal II headrace tunnel, working from the downstream, for a total bored distance of up to 10km.
• TBM 4 (4.1m, open type) will excavate half of the 14km Volcan diversion tunnel, working from the downstream, before moving on to complete a 3km section of the Alfalfal II headrace tunnel.
Table 1: Main tunnel lengths and sections
Las Lajas tailrace tunnel
Las Lajas headrace tunnel 1
Las Lajas headrace tunnel 2
Alfafal II tailrace tunnel
Alfalfal II headrace tunnel
Volcan diversion tunnel
Alfalfal II access tunnel
Las Lajas access tunnel
Whittle told TunnelTalk that in addition to the four TBMs a Raise Boring Machine will be used to excavate the vertical pressure shaft and surge shaft of the Las Lajas tunnel system.
Strabag will procure three of the TBMs, with the Hochtief/ CMC di Ravenna JV responsible for procurement of the machine for the Volcán diversion tunnel. "Contract specification is that contractors must use new TBMs," said Whittle. "There will be 16 independent headings at the peak of working activities," he added.
The Alto Maipo project is located 50km southeast of the Chilean capital Santiago, in the high-altitude Maipo River basin of the municipal district of San José de Maipo, close to the Argentinean border, and with high Andean peaks of up to 6,500m.
Award to Strabag of two-thirds of the Alto Maipo project's main tunnelling and civil works represents yet another significant success in Chile for the Austrian company. Spokewoman Diana Klein said: "After being awarded a €100 million contract to excavate by drill+blast access tunnels and adits totalling 11.1km for the world's largest copper mine at Chuquicamata in July, Strabag is strengthening its position in the Chilean tunnelling market with this latest award at Alto Maipo."
Hochtief is currently involved in construction of the Cheves hydropower plant in neighbouring Peru, having recently completed work in 2011 on the La Confluencia hydropower project in Chile.
"With our know-how and experienced co-workers in tunnelling and power plant construction we make a major contribution to sustainable energy supplies in high-growth markets," said Executive Board member Ullrich Reinke.
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