Robbins launches the first of its new generation Crossover TBMs in North America for excavation of the Túnel Emisor Poniente II (TEP II) in Mexico City.
Assembled using OFTA (Onsite First Time Assembly), the 8.7m (28.5 ft) dual-mode machine is an XRE TBM optimized for hard rock conditions (R) but capable of switching (X) to EPB mode (E).
Variable ground conditions – which are expected to require the switch over to EPB mode to cope with the softer ground expected towards the end of the 5.9km drive – was the main reason for the contractor’s choice of machine. The tunnel consists of sections ranging from fairly competent to weathered volcanic rock, soft sands, and clays.
While standard TBMs tend to lose efficiency in these kinds of variable conditions, the Crossover TBM is equipped with special design features to tackle mixed ground. These field-tested features include a single-direction cutterhead for more efficient excavation in abrasive ground, and multi-speed gearboxes. The specialized gearboxes also provide added torque at low RPM in fault zones and soft ground, as well as high a high RPM for excavation through rock.
“In my opinion, the best part about the design of this TBM is the cutterhead; it is very robust,” said Sebastián Gallego Murillo, TEP II Production Manager for PROACON, which is part of the joint venture contractor team that also comprises ALDESA and RECSA. One of the greatest challenges for the machine and the JV will come towards the end of the tunnel drive, said Gallego. “At this time we expect to convert from hard rock to EPB mode as a result of the soils in this area, and we will need to change out the cutters and modify the cutterhead.”
The design of the TEP II machine is based largely on experience from past Robbins projects, in particular the Kargi Kizilirmak Hydroelectric Project in Central Turkey. At Kargi, Robbins supplied a 9.84m (32.3ft) diameter Double Shield TBM, based on initial geological surveys indicating fractured hard rock. Within 80m (260ft) of launch, the geology became substantially more difficult than expected, consisting of blocky rock, sand, clays and water-bearing zones. The machine required multiple bypass tunnels and major modifications before it could resume excavation, and in the end a six-month contract extension had to be negotiated and the excavation schedule changed to allow for a reverse drill+blast heading.
Modifications to that machine included a custom-built canopy drill and positioner for enhanced drilling and ground consolidation, gear reducers to adjust torque and RPM to changing ground conditions, and short stroke thrust jacks to double total thrust capabilities. After the modifications, advance rates increased dramatically in the difficult ground conditions and soared to 723m (2,370 ft) in one month as conditions improved. The modifications proved instrumental to the design of Crossover XRE (Rock/EPB) TBM, including the TEP II machine.
The benefits of the project are worth the potential challenges. “This tunnel will reduce flooding in the west and northwest areas of the Valley of Mexico, and increase wastewater capacity. It will benefit three municipalities that are home to 2.1 million people,” said Gallego. Twenty years in the making, the TEP II tunnel will eradicate chronic flooding in the sensitive municipalities of Tlanepantla, Atizapan de Zaragoza and Cuautitlan Izacalli, which have historically been affected with overflows as high as 2m (6.6ft) during the rainy season.
Robbins first Crossover TBM has been successful in excavating a steeply inclined coalmine access tunnel in Australia.