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 fi nishes 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 fl ooding in the area, and increase wastewater capacity. Across three municipalities, some 2.1 million people will benefi t 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 TBM launch on TEP II, manufacturer Robbins has supplied its fi rst Crossover machine in Mexico.

END GAME

Robbins 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. Initial reports on the Turkey project showed fractured hard rock. Robbins explains, within 80m of launch the geology became substantially more diffi cult than expected, consisting of blocky rock, sand, clays and water-bearing zones. The machine required multiple bypass tunnels and major modifi cations before it could resume excavation.

Modifi cations 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. Robbins says these modifi cations 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 kilometer 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 geological profi le of the project goes through six different types of lithology, and among them are the hard rocks, such as dacites, and tuffs with low overburden," says Aldesa’s Enrique del Castillo.

"Besides going through several different ground conditions, the most important will be going through the Barrientos fault, as well as preparing the machine for going through the tuff that is found in the 800m at the end of the project," Castillo says of the project’s geological challenges.

The fi nal 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, further outlining the changeover: First, there are removable plates that increase the percentage of the cutterhead that is open; 20in disc cutters are swapped out for 9in soft ground tools; a rotary union is installed to supply additives and foam at the cutterhead; and then the screw conveyor is extended at the bottom.

While boring in rock the machine has a single direction cutterhead, and the capability to be bi-directional once in soft ground.

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." It’s not expected to be necessary in first portion of the alignment where most of the ground is andecite. 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."

The machine has two types of drills, shorter drills to inject an umbrella in the front, and a rearmounted drill to probe the material in the front, and inject grout if necessary.

Another feature is multi-speed gear boxes. "On this EPB you will need more torque, less sped from the cutterhead so in the end we have a two-stage gear reducers: the gear reducers for rock, in which the cutterhead is able to go almost to 6.5 revolutions per min, and then we can engage the other reducer to be able to have a higher torque and less speed." Gonzalez says. "For example we are going to be rotating at 2.5 revolutions, more or less, with a lot more torque."

TUNNELLING

Aldesa’s Castillo says one of the biggest accomplishments on the project thus far has been organising the logistics in such a small work space-fewer than 10,000m2. 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 of 1,030m length.

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. Project estimates call for an average of 15 rings per day. 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, and from there the TBM will be extracted from the shaft following the curve. "The Robbins conveyors have that advantage because they have these structures that can move according to the curve," he says. "They are not fixed. It’s easier to control the curve of the conveyor system."

The JV is using continuous conveyors for spoil removal, with structure and cassettes supplied by Moldequipo Internacional. Presforza, located in Puebla, Mexico, is manufacturing the steel-reinforced segments for the initial lining.