“We are proud of having successfully finished the excavation, despite all the adversities we faced, such as large inflows of water, hydraulic loads and constant changes in geology. We solved these by adapting the excavation mode according to each type of geology found,” said Hector Arturo Carrillo, machinery manager for Lot 4 contractor Carso Infraestructura y Construcción (CARSO).
The whole operation achieved a project record of 30m in one day, and a high of 528m in one month. It’s a result that, Carillo says, has much to do with the continuous conveyor system being used for muck removal.
Carillo adds: “It should be noted that our advance rates were achieved thanks to the great Robbins conveyor design. The tunnel conveyor was composed with elements such as the booster, vertical belt, curve idlers, and advancing tailpiece, as well as elements on the surface.”
The breakthrough was the latest milestone for an urgently needed wastewater project that spanned some of the most difficult geology ever encountered by EPBs. The 10.2km long Lot 4, running from Shaft 17 to Shaft 13 at depths of up to 85m included sections of basalt rock interspersed with permeable sands with high water pressure. “Our machines had to go through the worst geology, but they were designed for it,” said Roberto Gonzalez Ramirez, general manager for Robbins Mexico, of the three Robbins EPBs and continuous conveyor systems used on Lots 3, 4, and 5 of the project.
A spokesperson for the manufacturer said, “All of the machines were designed for water pressures from 4 to 6 bars, with mixed ground, back-loading cutterheads to tackle variable ground conditions. High pressure, tungsten carbide knife bits could be interchanged with 17-inch diameter carbide knife bits could be interchanged with 17-inch diameter carbide disc cutters depending on the geology. Other features included man locks and material locks designed to withstand pressures up to 7 bar, a redesigned bulkhead, and Hardox plates to reinforce the screw conveyors as well as removable wear plates to further strengthen each screw conveyor flight. The rotary union joint was redesigned to improve cutter change times during cutterhead interventions, while a new scraper design offered more impact resistance in mixed ground conditions with rock.”
The Lot 4 TBM was assembled in the launch shaft 17 and commissioned in August 2012, with the bridge and all the back-up gantries at the surface. Two months later in October 2012, after advancing 150m (490 ft), the machine and its back-up were completely assembled in the tunnel. One month later, the continuous conveyor system was installed and running.
After 405m of excavation, the presence of rocks, scrapers, parts of the mixing bars and other wear materials in the excavated muck prompted a cutterhead inspection. With high pressure up to 3.5 bars, it was determined that a hyperbaric intervention was necessary, and on June 2nd, 2013 the first hyperbaric intervention through an EPB in a tunnel was performed in Mexico. After about 50 hyperbaric interventions the remainder of the project’s interventions were done in open air.
“The interventions carried out in atmospheric mode were the biggest challenge. The great influx of water tested the limits, because we were excavating on a decline. In all of these interventions we had to implement a double pumping system, at both the TBM and the shaft,” said Carrillo. “Despite the challenges of pumping water at volumes up to 180l (48 gal) per second and cleaning fines from the tunnel each time the operation was performed, atmospheric interventions were still lower in cost and quicker than those done at hyperbaric pressure.”
