A Century In the Making Tunneling Into History
 Cliff Schexnayder / ENR
A shielded TBM would be trapped, experts warned.
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Even though the 5-meter-dia, unshielded Robbins gripper TBM is tunneling through one of the lowest reaches of the Andes, it is still one of the deepest tunneling projects in the world. The Gotthard Base tunnel in Switzerland, with 7,588.6 ft of overburden, is the deepest.
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Related Link: There are big differences between the two projects, but a key one is that the Gotthard project’s four Herrenknecht gripper TBMs are shielded and the $14-million Pacha Mama is not. Experts who analyzed the plan for design-build contractor La Concesionaria Trasvase Olmos S.A., which is comprised solely of the Brazilian engineering and construction giant Odebrecht, judged that extreme pressure and many microfissures and faults the TBM will encounter inside the mountain would make the rock swell and shift enough to trap a shielded machine. One of the Gotthard machines did become wedged in 2005. It took five months for work crews to free it.
 Cliff Schexnayder / ENR
Bracing helps counter deformation.
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Other deep tunnels bored by Odebrecht in the Andes—notably the San Francisco tunnel project in Baños, Ecuador—have used double-shielded TBMs to protect the machine from possible cave-ins. That wasn’t an option for this project, according to project leader Biaggo Carollo.
“Double shield is to help protect the TBM but in this situation it could cause it to become stuck,” he says. “With the type of pressures we are dealing with...we would lose the machine.”
Odebrecht engineers estimate a double-shield TBM would be able to bore only 12 meters before the pressures of the rock would immobilize it. The unshielded TBM, by contrast, with its 35 17-in.-dia disc cutters, can continue to move forward regardless of the deformation behind the cutter head. It also has the ability to overbore the tunnel diameter by 50 mm, if necessary.
“Going in we wanted to protect ourselves from the risk,” says project director Erlon Arfelli, explaining the choice of an unshielded machine. But he adds, “In truth, no one knew exactly what a job like this would involve.”
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 Odebrecht
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 Odebrecht
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 Cliff Schexnayder / ENR
Building access road to tunnel head required steep retaining walls.
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An Old Dream
The tunnel is part of a 20-year, $190-million effort to accomplish a plan that dates back to the early 1900s. The Andes are an immense barrier that prevents rain from moving from the jungle regions of northeast Peru and Brazil toward the dry coast. The vision of conveying water from the Atlantic side of the mountains to the fertile but dry Pacific side has enticed Peruvian leaders from one administration to the next. It was only after the government of Alejandro Toledo relaunched the project on the basis of a design-build-operate, public-private partnership concession, and enlarged it with a hydroelectric component in 2001, that the current, promising effort took shape.
Odebrecht won the concession in July 2004. The project became the first in Peru financed through a PPP-style system, with federal support of $77 million.
When complete, the tunnel will be part of a system to annually transfer more than 2 billion cubic meters of water from the wet to the dry side of the mountains. The project includes damming the Huancabamba River near the village of San Felipe with the 43-meter-high, roller-compacted concrete Limón Dam to form an impoundment reservoir, Water will then be diverted through the mountains to the usually dry Olmos River on the Pacific side. The first phase, which involves construction of the dam, excavating the tunnel, moving a gas pipeline and building an access road, is under way.
Water should begin flowing by March of 2010. It will irrigate more than 56,000 hectares of scrubland along the Pacific Coast north of the town of Chiclayo.
Later phases will include construction of at least two more tunnels and two more dams, as well as an extensive canal system on the Pacific side to distribute the water on the coast. Two hydroelectric generation facilities capable of generating 600 MW are also planned.
Pacha Mama, which means “Mother Earth” in the local Quechan language, began work in February 2007. Despite delays in December due to bursting rock, and...
... another three-week delay on the Atlantic side caused by heavy rains and flooding that cut power and washed out the access road, the TBM is still on schedule to complete the 14 km it is projected to dig by March 2009. It is advancing roughly 22 m per day. The other 6.2 kilometers were either dug previously or are to be excavated by drill-and-blast methods.
The machine is operated in two 12-hour shifts with a 4- to 6-hour downtime between shifts for maintenance and probe drilling.
 Cliff Schexnayder / ENR
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 Cliff Schexnayder / ENR
Operators use a laser target on the side of the TBM to stay on track. Air conditioners on the machine and in the tunnel bring temperatures to a tolerable 84ºF.
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Playing It Safe
A stuck machine is a problem Odebrecht hopes to avoid, but there is a lack of geologic data because of the route’s depth, says production manager Winston Lewis. “We don’t use core drilling because the depth of the works are simply too extreme,” Lewis says. “The tunnel will be bored through hard rock that has good quality and it is stable, so we believe probe drilling is sufficient.”
Surveys suggest the TBM will mostly encounter quartz porphyry, andesite and tuff. There are more than 400 known faults along the the tunnel, but only two are considered major.
“There are not large fissures to contend with, but there is a pretty large number of microfissures,” Lewis said. While the fractures are a concern, they aren’t as critical as similar fissures might be in other places due to the local conditions, Lewis said. “When we do encounter a major fracture, it is much more stable because it is not holding much water,” he explained. “A dry fracture doesn’t have the same amount of pressure to threaten the TBM.”
 Cliff Schexnayder / ENR
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 Cliff Schexnayder / ENR
Roller-compacted-concrete Limón dam on Huancabamba River will impound 44 million cu m of water for diversion.
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Lewis says crews are installing rock support behind the head to ensure the tunnel’s stability throughout construction, as well as the life of the project. The TBM is equipped to install rockbolts, ring beams and wire mesh to secure the cut as it progresses, as well as shotcrete segments for the floor of the tunnel.
The depth brings another problem though. Ambient air temperature in the tunnel is 130ºF, not a significant problem for the machine, but one for the workers. To bring the internal temperature down to 84ºF, the contractor has installed two cooling systems on the TBM and has two more stationary cooling systems inside the tunnel.
The number of units required is partly due to the altitude. The tunnel’s east entrance is 1,085 meters above sea level. The thinner air reduces the heat-transfer efficiency of the cooling equipment, making it necessary to use twice as many cooling units as would be required for the same job at lower elevations.
