One look around and you’ll notice that nothing in Mexico City is static: the flow of buses, bicycles and cars is non-stop while pedestrians crowd the sidewalks. Not even the ground is constant — in this sinking metropolis built on lake clays the earth settles about 60mm lower per year. Building a tunnel in this uniquely dynamic environment is no easy task, but the Mexican Federal District was undaunted by the challenge.

In 2007, the project owner laid the groundwork for the country’s first new metro line in a decade: the 25.4km long Mexico City Metro Line 12, to be completed in 2012. The new route, also called the ‘Gold Line’, would be the longest in the metro and include 7.7km of mixed ground TBM tunnelling with eight underground stations.

Everything about the underground portion of the project was designed around the concepts of limited space and potentially unlimited movement. "The stations were built into bedrock, while the tunnel lining itself is quite rigid. Special structures connecting the station with the lining allow long-term ability to absorb some of the settlement by offering some give," says Ismail Benamar, tunnel manager for the majority of TBM tunnelling with contractor Ingenieros Civiles Asociados (ICA). The dynamic project has been well planned from TBM assembly through its completion on 1 March 2012, making it a standard for urban tunnelling in complex conditions.

Parting the crowd
At more than four hours per day on average, commuters from the southern neighborhood of Tlahuac often spend much of their time in buses and cars travelling to and from work in the downtown area of Mexico City. That will all change later this year, when Line 12 will open between Tlahuac and Mixcoac areas, cutting commute times by more than 2.5 hours. Up to 437,000 daily passengers are expected to ride the rails, making it the fourth busiest line in the system.

To speed the project schedule, construction of the line was divided into two phases: above ground and below ground. The metro line goes below ground where the geology is less rocky. Opting for TBM tunnelling had the advantage of minimising disruption on surface streets. Combining TBM tunnelling, cut and cover, and elevated track, minimised tunnel construction time allowing the project to stay within budget. In 2009, the contract was awarded to the ICA consortium, consisting of ICA, Carso, and Alstom for construction, and for tunnelling with a 10.2m diameter Robbins EPBM.

Evolving geology
Tunnelling is located in the complex geological strata of the Valley of Mexico, which was founded on what was once an island in the middle of Lake Mexico, then subsequently drained. Geotechnical investigations of the metro tunnel area showed an abundance of lake clays, interspersed with sections of sand, gravel, and boulders up to 800mm in diameter. Long dormant and eroded volcanoes are buried throughout the area, where they have deposited volcanic rock and fields of boulders in the lakebed.

The geological profile for the Line 12 project indicated a large variation in the conditions along the tunnel length. The initial portion of the excavation was relatively homogeneous, consisting primarily of sensitive, watery lake clays. In particular, the first half of the tunnel consisted of very soupy clays with up to 75 per cent water content.

"We decided on an EPBM after analysis of the geotechnical data, which showed that soft clays were predominant," said Benamar.

The final portion was more heterogeneous, and included several sections of compacted sand, pebbles, boulders, and clay. A high percentage of boulders was expected, with diameters up to 800mm.

The variation in conditions required initial muck removal using a sludge pump for the clay, which was changed out with muck cars after passing through Eje Central Station as the ground became harder.

EPBMs are capable of handling the mixture of soft ground with large boulders, and the design required several specialised features. These included a two-stage screw conveyor setup with an initial 1,200mm diameter ribbon-type screw conveyor.

Large pieces of material travelled up the center of the screw to exit out of the boulder collecting gate, while more fluid muck continued on to the secondary shaft-type screw conveyor for conventional removal by machine belt conveyor.

Conditioning of the variable ground was dealt with using additives injected through six independent ports in the cutterhead. The independent lines consolidated the flow of muck and reduced the risk of clogging, which can lead to uneven wear of the cutterhead and cutting tools. While the watery clays did not need additives other than water from the sludge pump, the cobbles and boulders encountered later required the use of foam additive. The foam, consisting of water, surfactant, and additive, aided in maintaining earth pressure and reduced the required cutterhead torque.

Big TBM assembly, small space
Prior to launch of the machine in February 2010, a big problem needed to be tackled: how to assemble the country’s largest TBM in a jobsite the width of a city street, in a launch shaft only 14m wide and 34m long.

Because of the tight schedule and large machine diameter, Onsite First Time Assembly (OFTA) was used to initially assemble the TBM on location, rather than in a manufacturing facility. It was estimated that the assembly saved four to five months on the overall delivery schedule.

"OFTA has the benefit of no pre-assembly – everything was delivered directly to the site and assembled here. The assembly went very smooth, and it was a little over three months before we started to turn the cutterhead and push the machine forward," says Ron Jelinek, Robbins field service technician.

Critical subsystems, such as the electrical and ventilation systems, were tested before being shipped to the jobsite. Multiple dimensional checks ensured precision components and proper fit up. These measures include inspection of all sub-suppliers, who had to use a template when manufacturing components.

Components were lowered into the 17m deep launch shaft for assembly inside a concrete cradle. Assembly began with ‘inner core’ components including the cutterhead support and screw conveyor. Then the upper and lower halves of both the front and rear shields were lined up with welding ports in the cradle, used as a space for crew members to weld the pieces together. The front and rear shields were connected by articulation cylinders for active articulation in curves. Machine components were not assembled directly on the concrete cradle, but on two rails at 60-degree angles. The rails were then used to push the machine to the tunnel face during startup.

Designing for movement
The project’s location at the city center meant that from the start, sensitive structures were close by. The tunnel’s low cover of between seven and 14m due to the depth of the stations, and the passage of the machine within 12m of a 16th century church, required an intensive monitoring program from TBM launch. "We have a real-time monitoring program to detect displacements and pore pressure on the surface, underground, inside the tunnel, and in the most critical structures next to the tunnelling line," says Benamar.

During tunnelling the TBM also passed within 1.5m of a 4m-diameter collector sewer, within 2m of building foundations, and just 3.5m below the metro’s active Lines Two and Three. At one point, the tunnel also passed between two supports of an existing freeway bridge, with about 4m of distance between the TBM and bridge pile foundations. High pressure water pipes responsible for 25 per cent of the city’s supply ran parallel to the tunnel for 800m with about 6m of separation.

All were navigated successfully with a combination of monitoring and effective tunnelling — no jet grouting was used around the structures.

"The settlement stayed within the 20 to 50mm limits throughout tunnelling, keeping in mind that annual settlement in the city is 60mm per year. Our surveyors had to update their data points every two to three months because they kept moving due to citywide sinking. We placed some reference points in the bedrock to allow for correction," explained Benamar.

The risk of surface subsidence and vibration was controlled during excavation by regulating the rate of advance and controlling earth pressure at the front of the machine, as well as the backfill grouting pressure. The contractor was able to decrease the machine’s rate of advance using variable frequency drives in sections close to sensitive structures. The EPB cutterhead rotation was also kept low throughout the excavation, at a maximum of about 1.5rpm.

As the machine advanced, the tunnel was lined with 400mm thick universal concrete segments in a 7+1 arrangement. A two-liquid back-filling system was used to quickly stabilise the annular space between the tail shield and concrete segments. The liquid mixture consisted of water and bentonite cement plus an accelerant, which were combined in the tail shield to harden rapidly after injection.

Segments and stations
The segments and stations themselves were designed to withstand the expected long-term settlement of the city’s soils. "The lining is quite rigid because the stations are fixed and the diaphragm walls are driven into the bedrock," said Benamar. The first half of the tunnel in soft clays is expected to experience greater settlement, so the rebar-strengthened segments in this section are more heavily reinforced and a secondary concrete lining will be added in the coming months.

The custom segments were manufactured 24 hours a day by ICA about 30km from the jobsite, then trucked in, as the small stations only had the capacity to hold about 1.5 days worth of segments.

Most stations were built 2 to 3m below street level, and were excavated as box culverts using diaphragm slurry walls.

Windows were cut into the diaphragm walls as the entry and exit portals for these machines, and material around the windows was replaced with cemented ground for greater stability.

Breakthrough, times seven
One of the most unique aspects of the project was the multiple intermediate breakthroughs as the machine entered and exited seven cut and cover station sites (plus the launching site for a total of eight stations). Distances between the stations ranged from as much as 1,800m to as little as 400m, while the stations themselves were between 150 and 190m in length. Each time the launch site was moved forward, storage areas were moved forward to that station site as well, including settlement ponds for the sludge pump while it was used.

"Every time, the machine spent about one to two months in the station in order to rebuild launch structures, do maintenance on the machine such as cutterhead inspection, walk it through the station, and transport the supplies to the next site," said Andrei Olivares, project manager for Robbins Mexico. The EPBM was supported on a concrete cradle and walked forward using a thrust frame and pushed off of free-standing rings between the tail shield and thrust frame. Entrance and exit seals in the diaphragm windows were used to avoid any voids or settlement as the machine moved from station to station.

Olivares was pleased with the TBM performance and the maintenance of tunnel alignment during excavation: "The TBM guidance system tracked through curves, and the active articulation allowed us to negotiate curves as small as 250m in radius. The overall alignment of the tunnel finished up perfectly."

Despite the numerous planned stoppages, the TBM achieved good advance rates of up to 135m per week. By the time of the machine’s final arrival into Mixcoac station on 1 March, the TBM was averaging 400m per month.

As of March 2012, construction is wrapping up on the secondary lining, and the entire Line 12 is scheduled to open by the end of this year.