Adding capacity to the over-tasked wastewater system in Mexico City, an alignment through changing ground conditions is a likely candidate for Robbins’ Crossover TBM, Nicole Robinson reports.In the mountains northwest of Mexico City, the soft rock is self-supporting and very consolidated, a dream to mine. "Even the face is self-supporting," says Roberto Gonzalez, Robbins' general manager in Mexico. "You could use a normal backhoe and excavate like that. It's a beautiful ground to bore."But the alignment crosses valleys of tuff, faults and finishes with a stretch of soft ground with low cover. This is the scenario for Túnel Emisor Poniente II (TEP II), or the English translation of West Drainage Tunnel II.Conagua, Mexico's national water commission, is building the 5.9km-long tunnel with a 7m i.d. to reduce flooding in the area, and increase wastewater capacity. Across three municipalities, some 2.1 million people will benefit from the tunnel project.The contractor joint venture of Aldesa, Proacon and Recsa chose an 8.7m diameter, dual-mode type machine capable of "crossing over" between rock and EPB. With the August 2015 tunnel boring machine (TBM) launch on TEP II, manufacturer Robbins has supplied its first Crossover machine in Mexico.End gameRobbins draws comparisons to the Kargi Kizilirmak hydroelectric project in Central Turkey. The design of the TEP II machine was based largely on experience from past projects, and that TBM in particular. While initial reports on the Turkey project showed fractured hard rock, Robbins explains, within 80m 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.Robbins says these modifications proved instrumental to the design of its Crossover TBMs, including the TEP II machine.In Mexico, the contractor JV expects to convert the machine from hard rock to EPB mode due to changing ground conditions in the last kilometre of the alignment. "Initially the proposal was a hard rock machine but they found they have 800m of water-more EPM conditions-that's the reason we proposed a Crossover," explains Javier Alcala, job site engineer for Robbins on TEP II.The ground conditions at TEP II are complex, from competent to weathered volcanic rock to clay, and sand. The final 800m is also the portion of the alignment with the lowest cover, some 12m, and the most populated. This is one of the reasons for using a Crossover machine. The rest of the drive has between 50-60m of cover on average with some stretches up to 150m."We try not to convert unless it's completely necessary because you stop, you have to drain the screw conveyor inside the machine, and you have to make a lot of changes, for example on the cutterhead," Robbins' Gonzalez explains.As an open mode machine boring in rock, the TBM is equipped in the event of entering running ground, he says. "These closure doors are able to maintain the material in the cutting chamber. They're just a safety." In smaller valleys of tuff there is potentially some water, but it's unknown for now, he explains."For these cases we believe that these closure doors will be held to see what we have to do with the material, if we have to consolidate in the front."Tight fitAldesa's Castillo says one of the biggest accomplishments on the project so far has been organising the logistics in such a small work space-fewer than 10,000 sq m. The JV excavated a 30m deep launch shaft supported by 800mm-thick Milan walls (slurry walls), and used on-site first time assembly, he says, to start excavation as soon as possible.Once assembled by gantry crane, the machine bored 100m before adding back gantries. When completely assembled the machine has nine gantries for a total length of 1,030m.At the time Tunnels & Tunnelling visited the project, the crews were still adjusting to having the full machine in operation, and had only recently started using the continuous conveyors for muck.The TBM was mining through a transition zone between tuffs and dacites, and had excavated 435m by mid November 2015. At the time of publication the TBM has bored 1,417.5m, which equates to 945 rings. The best day has seen an advancement of 42.8m and the best week is 185.1m. Robbins' Mexico office reports the TBM has reached softer geology and is boring very well.Tunnelling is expected to finish within this year and a second lining of reinforced concrete will be installed following excavation to extend the life of the tunnel. "Once we arrive to the final bit, it's a very close curve of 400m radius," Alcala explains.The tunnel alignment ends along the rivers of San Javier and Xochimanga in Atizapan de Zaragoza.