Armand van Wijck, TunnelTalk

It is planned as one of the largest and boldest infrastructure projects in South America to date: a privately-funded US$3.5 billion 52.5km TBM-bored twin-tube rail tunnel between Argentina and Chile, across the mighty Andes. Under the leadership of the Argentine investment company Corporación América, the international consortium is advancing towards an international request for qualifiers to take part in the one of the largest and most challenging construction projects ever: the Bioceánico Aconcagua. TunnelTalk reporter Armand van Wijck speaks to Project Director Nicolás Posse.

Fig 1. Alignment of the 52km trans-Andean link

Some 45 million tonnes of cargo travels annually between the South American east and west coast at the latitude that connects Buenos Aires in the east and Santiago in the west, but the barrier presented by the mighty Andes Mountains ensures that much of it is transported via expensive and time-consuming shipping routes. The only road connection - the high-altitude Cristo Redentor Pass - is inadequate for the task, but, crucially, is considered to be the best natural location for a supplementary Trans-Andean crossing because it is exactly at this latitude that there exists an industrial corridor of converging trade routes to the east of the Andes.

"The Andes are such an obstacle that cargo coming from the Argentine province of Mendoza - which lies along the border with Chile - is sometimes transported via ship from the Argentine coast to Chile," points out Corporación América Project Director Nicolás Posse. "It is a journey that easily takes several weeks."

Analysis of trade figures shows that some 83% of cargo transported across the Southern Cone does so by ship - with two-thirds of the rest, some 4.9 million tonne each year, crossing the Andes via the Cristo Redentor Pass, a steep mountain road with an altitude of 3,200m that includes a 3km-long tunnel.

"It is the best option by land, but this has become a major bottleneck," said Posse. "The route is also closed 45 to 60 days per year by heavy snowfall." The road, with its 26 hairpin bends and steep inclines, cannot be widened. There was a fragile passenger railroad that crossed the Andes around the desired latitude, but the Ferrocarril Trasandino Central was closed in the early 1980s and it is not considered feasible to upgrade and reopen it as a cargo railway.

In the meantime, trade between South America and emerging Asian countries, including China and India, is increasing rapidly. Corporación América projects that by 2040, 70 million tonne per year will be transported to opposite sides of the Southern American continent. An improved land connection between Argentina and Chile seems inevitable.

In 2008, Corporación America proposed an all-weather freight railway line to the Governments of both Argentina and Chile. The Bioceánico Aconcagua has a total track length of 205km and includes a 52.5km-long bored twin-tube base tunnel. It connects directly on both sides of the Andes with the existing rail and highway infrastructure. When complete, the first tube of the proposed truck-on-train freight forwarding system would transport 24 million tonne of cargo per year, increasing to 77 million tonne per year when both tubes are in operation. Some 14 alignments of varying length and gradients were seriously studied, including one that featured a "helter-skelter" declining circular tunnel design to ease gradient over a short distance. In addition to the main base tunnel a further 20km of underground alignments split into 33 different tunnels are also incorporated into the project's overall design.

Project goals of the Aconcagua link

The selected alignment (Fig 1) is named after the peak of Aconcagua, which, at 6,960m, is the highest mountain in the Western Hemisphere. The tunnel itself will be excavated just south of the mountain, beneath the Cristo Redentor Pass (Fig 2). According to the joint venture - which consists of Empresas Navieras SA from Chile, Mitsubishi Corporation of Japan, Geodata, Italy and Contreras Hermanos of Argentina - the proposed rail line would reduce travel times through the Andes from 12hr to 4hr, saving tens to several hundreds of dollars for each tonne of cargo.

The Governments of Argentina and Chile formed a Bi-national Commission to evaluate the project, call for an international tender, and regulate the future concession. A preliminary environmental impact assessment was completed on both sides of the tunnel and railroad alignment. Just last month (October 2012) the Commission completed its review of the feasibility study and is now requesting an extension of the original plan. Soon it will submit a recommendation to the Governments of both Chile and Argentina and then prepare the call for expressions of interest and release an international tender for the concession. "Next year they will start the call for tenders and by the end of 2013 we hope to start first excavations," said Posse.

It is a difficult task ahead. The region's geology consists of a number of major fault zones, with overburdens rising in places to more than 1,600m above the tunnel alignment. "Furthermore, we have to deal with possible hazards, including volcanic intrusions, slaking and high-temperature thermal waters," said Posse. Designers have carefully modelled the tunnel and its surroundings in 30 homogeneous segments. For each reach they have determined the associated risks and possible solutions. Double shield TBMs with a precast segmental lining are anticipated as the weapons of choice. "Typical countermeasures will be altering the speed of the TBMs, the use of advanced probe drilling and precast segmental lining and tail void grouting," said Posse.

Construction of the Bioceánico Aconcagua base tunnel shares major similarities with the Gotthard Base Tunnel in Europe, where similar difficult rock behaviour and main beam TBMs dominated the scene. "I visited the Gotthard project myself," said Posse. "We have a more aggressive approach in mind, using four 10m diameter TBMs for each tube to help speed delivery of the project. Two TBMs will start boring steep access adits to the main tunnel alignment and at about 17.5km from the tunnel portals. At the same time, a third TBM will start from the Chilean entrance, located in Saladillo at 1,536m above sea level.

Upon completion of the two access adits, excavation will begin from each portal to complete the first tunnel in its entirety. Only then will attention be turned to excavation of the second tunnel. At the same time, drill+blast connections to technical caverns and access tunnels to underground stations will be completed. The final front of excavation is that of the 2,393m-high Punta de Vacas entrance in Argentina, which will complete the last section of the first tube. Once the excavations of these various sections are finished, the final lining in drill+blast excavations will be completed.

Fig 3. Three-phase approach to construction

Normally the rarefied air at such altitudes can be problematic for the ventilation systems during operation, but not this time, according to Posse. "The height difference between the two tunnel portals makes for a 2% gradient. This gives a chimney effect. There will always be a wind blowing through the tunnel. Furthermore we will only use electric trains, keeping air pollution to a minimum."

The maximum internal diameter of each tube is determined at 8.5m, enough for a single train track. "We expect the first tunnel to be completed in ten years," explained Posse. "Lorries can transfer their cargo to trains, or ride on them. We will operate two types of 750m-long trains. The first type has room for 30 trucks, the second is a freight train with a cargo capacity of 1,500 tonne." In stage two, parallel railways to and from the tunnel, on both sides, will be constructed, increasing capacity further. The second tube will be bored during the third and last stage, after which passenger trains will be added to the service (Fig 3).

Trucks will board trains at transportation hubs

Despite having just one tube during the first two stages, the JV isn't too worried about events that might force the tunnel's closure: like a stuck train. "Tunnel equipment, railway system and rolling stock are designed in order to prevent any possible risk," said Posse. "The trains will have three locomotives, any of which can continue at 8km/hr for 15 minutes during a fire, long enough to ensure a safe exit. We will also install heat detectors at the tunnel portals for both the cargo and rolling stock." Posse further added that Corporación América is working on an alliance programme with the Massachusetts Institute of Technology (MIT), to develop a system based on nanotechnology that can detect smoke minutes before a conventional detector.would be able to.

The $US3.5 billion Bioceánico Aconcagua is a fully private initiative. The Corporación América JV will operate the tunnel, earning its return on investment via tolls. "We believe the location of the link is so strategic, and the volume potential so great, that public funds from the Governments will now be needed. But the Bi-national Commission will only provide the loan guarantees, not taxpayer money directly."

References

Andes link a priority for Chile-Argentina-Brazil - TunnelTalk, December 2011
Final finish for Gotthard Baseline excavation - TunnelTalk, March 2011
Reflections on the UK Channel Tunnel - TunnelTalk, December 2010

Bioceánico Aconcagua Corridor project website

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