Robbins XRE TBM breaks through in the Andes

Working under 450m maximum overburden, the 4.6m-Ø machine has excavated a 3.3km-long tunnel for the Los Condores Hydroelectric Power Project, owned by power company Enel Chile. The machine faced varying conditions including tuff, sandstone, breccia and conglomerate with sections of high-pressure water inflows.

Crossover TBMs are suited to Andean sedimentary and volcanic-type geology, where rock strengths and hydrogeological conditions vary greatly. Maximum advance rates reached 605.8m in one month and 212.8m in one week.

The Crossover machine featured a heavy duty, centrally-mounted screw conveyor for the duration of the drive. The TBM remained in hard-rock configuration with muck chute installed, along with paddles, bucket lips, scrapers and disc cutters. However, explained Omar Alvarez, Robbins field service site manager at Los Condores: “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.”

A key challenge to the project were the high-pressure groundwater inflows. “When we started the excavation, we used dewatering hoses to reduce the water into the cutterhead during the excavation,” explained Alvarez – 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. We reached 7+ bar in the cutterhead earth sensors.”

“We bored in places with [more than] 5,500Lit (1,500gal)/minute, 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 re-injections,” said Alvarez.

Once brought online, the Los Condores HEPP will have an annual generating capacity of 150MW.