Construction of the 150MW Los Condores Hydroelectric Power Project in Chile has reached a key stage with breakthrough of the tunnel boring machine (TBM) on the main 3.3km long tunnel.

Project client and developer Enel used a 4.6m diameter Robbins TBM to overcome the challenging ground and complex hydrogeological conditions.

Cover above the tunnel, which was driven through the Andes mountains, was up to 450m with the geology varying from tuff, sandstone, breccia, and conglomerate with rock strength of up to 600MPa experienced. The crossover [multimode] XRE TBM was designed to cope with the variable ground conditions and had the capability to switch from hard rock configuration to earth pressure balance (EPB), if needed, however it was the ground water pressure and inflow rates that turned out to be the biggest challenge.

The challenging ground conditions have been linked to delays on getting the project underway with the facilities originally scheduled to open in 2018. Progress on the project was also reported to have been slow in the early stages as a result of design changes needed for the surge shaft called for by Chile’s environmental regulator.

The Robbins crossover machine that successfully overcame the issues featured a heavy duty, centrally mounted screw conveyor for the duration of the drive. The TBM remained in a hard rock configuration with muck chute installed, along with paddles, bucket lips, scrapers and disc cutters on the cutterhead. According to Robbins field service site manager for the project Omar Alvarez, 75% of the excavation was performed using the main drive gearboxes in high torque configuration (EPB or low speed mode). “We never physically changed the cutterhead or screw conveyor to EPB mode,” he said.

“When we started the excavation, we used dewatering hoses to reduce the water into the cutterhead during the excavation.”

However, water pressures rose whenever the TBM stopped. “During the segment ring installation, we stopped the water from draining through the rear shield drilling ports and we closed the screw conveyor rear gate,” explained Alvarez. “We reached more than 7bar in the cutterhead earth sensors.

“We bored in places with greater than 5,500litres/min inflow, making back-fill grout injection behind the concrete segments a challenge. We decided not to use the grout injection through the tail shield ports, but instead injected grout directly through the concrete segment with hoses. This approach was more flexible and reduced the need for reinjections.”

Maximum advance rates topped out at 605.8m in one month and 212.8m in one week.

Enel project engineer Riveros Puratic said: “Towards the end, [the TBM] exceeded expectations. Crossover TBMs are suitable for Andean geology of sedimentary and volcanic type, where there is a great range of rock strengths and hydrogeological conditions.”

The new hydroelectric scheme, which uses the existing Laguna del Maule reservoir, is expected to go into operation in autumn next year.

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This report caught my attention for a number of reasons. As a somewhat amateur railway historian, the two tunnels most well known for their connections with Isambard Kingdom Brunel – the Rotherhithe to Wapping tunnel and Box tunnel – have always held a fascination for me. The first certainly had what might be called “complex hydrogeological conditions” to be overcome by Marc and Isambard Brunel; and Box Tunnel required lining for long sections to counter rockfalls from its roof. The idea that Isambard tried to align the latter with his (or his elder sister’s) birthday has been shown to have been almost certainly a post hoc invention – an ‘urban (or rural) myth’.

On the factual side, in the 1970s I was involved with the survey for the Lesotho Highlands Water Project and with a Diamond Exploration Project in Lesotho. The former included core boring to find the interface between the basalt that overlay the sandstone in the region. This was to set the route for the tunnel that would take the water from the intended dam to the proposed hydro-electric power station. Here there were two simple types of rock to be bored through.

In the Diamond Exploration Project, I used a two-inch diamond-tipped core drill to investigate the properties of the ground where there were depressions that might have come from the erosion of relatively soft kimberlite in either the basalt or the sandstone country rock. The drill bits dealt easily with both hard and soft material, but had trouble with gravels which were churned around, rather than being drilled cleanly through. Instead of producing good cores, the gravels wore the bits away, with little entering the hollow core collection tube. The Chilean TBM successfully dealt with a variety of rock at a much greater scale.

One plea. Why is every problem, difficulty, concern now always a “challenge” or “challenging”? In three consecutive paragraphs in the report, there were: “challenging ground and complex hydrogeological conditions”: “ground water pressure and inflow rates that turned out to be the biggest challenge” and “The challenging ground conditions have been linked to delays on getting the project underway with the facilities originally scheduled to open in 2018”. The great lexicon of the English language is there to used, not for parts of it to gather dust. My English master would have had something to say – in red ink – had I used such repetition in a school essay. Variety makes for not only a more interesting life, but also for more interesting (less boring?) engineering reports – even those on tunnelling projects.

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