In the heart of Mexico City, between a new university and rows of apartment buildings, sits a cordoned off 250 m long section of Ermita Iztapalapa Avenue, a main thoroughfare in the heart of Mexico City. The area functions as the assembly and launch site for the country’s largest ever TBM.

At 10.2 m in diameter, the Robbins EPB is the first machine of its kind to be fully assembled initially at the jobsite. The TBM will excavate sections of the Mexican Federal District’s first new metro line in ten years, travelling between the southern neighborhoods of Tláhuac and Mixcoac. Once complete, the 24 km long Line 12, known as the “Gold Line”, will cut commute times from more than three hours by bus to just 25 minutes.

The route of construction winds between existing utilities and structural foundations of the existing city buildings, travelling through some of the most difficult geology in the world.

Tunneling an Ancient Lakebed
Modern Mexico City was founded in 1524, on what was once an island in the middle of Lake Texcoco. The now-drained lakebed sits in the Valley of Mexico, consisting of geology so unique that beyond Mexico City it has only been observed in one known analog in areas of Japan. Buried, long extinct volcanoes in the area have deposited volcanic rock, such as tuff, along with gravels and boulders. Ground conditions along the route of the city’s new metro line consist of soft clay interspersed with sand, gravels, significant ground water, and large boulders up to 800 mm in diameter.

“We decided on an EPB after analysis of the geotechnical data, which showed that soft clays were predominant in the area,” says Ismail Benamar, Tunnel Manager for Ingenieros Civiles Asociados (ICA). ICA. EPB machines are capable of handling the mixture of soft ground with large boulders that is expected during tunneling.

The ground is not the only challenge—the project’s location at the city centrer is in close proximity to a number of structures. The planned route will pass within 1.5 m of a 4 m diameter collector sewer, within 2 m of building foundations, and just 3.5 m below the metro’s active lines 2 and 3. At one point, the tunnel will also pass between two supports of an existing freeway highway bridge, with about 6 m of distance between the TBM and bridge pile foundations.

Tunnel cover is shallow, ranging from just 7.5 m at the launch site to about 14 m between stations. “We have an extensive monitoring program using piezometers and other meters on the surface, underground, inside the tunnel, and in the most critical structures next to the tunneling line,” says Benamar. The risk of surface subsidence and vibration can also be controlled by regulating the rate of advance and controlling earth pressure at the front of the machine, as well as the backfill grouting pressure.

Downtown Machine Assembly
The machine build was accomplished in about ten weeks using Onsite First Time Assembly (OFTA). The Robbins-developed method has been shown to save time and money for the contractor, particularly for larger diameter machines.

The method allows TBMs to be initially assembled on location, rather than in a manufacturing facility. “With proper project management and fit up of components, OFTA can save about 70 – 80%of the time required for a similar assembly at a shop,” says Benamar.

“It’s like receiving many boxes of parts in the mail, and assembling an entire car from those components. It might sound difficult, but with the right quality control everything fits together very well and there is no need to call the manufacturer,” says an engineer at the jobsite.

Assembly began in late October 2009 using components manufactured in the U.S., Mexico, Japan and China. Back-up gantries were initially assembled at the surface prior to barge shipment of TBM components. Larger structures including the 133 metric tonne cutterhead support with main bearing were transported to the jobsite by truck.

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

Components were lowered into the 17 m deep launch shaft for assembly inside a 14 m wide by 34 m long 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 are 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 at startup.

Dynamic Design
The Robbins machine was engineered for changing ground conditions, with a soft ground cutterhead, variable frequency drives, convertible muck removal system, and two-stage, ribbon type screw conveyor to handle large boulders and high groundwater levels.

Adaptable Muck Removal
At the start of excavation, the EPB will utilize a sludge pump for muck removal. The sludge pump allows loose, clayey, and water bearing soils to be moved to the surface through a system of pipes. Linear force is applied with pistons to push water through the pipes and cycle the mixture to a settlement system at the surface.

“The system of settlement pools takes advantage of the length of the jobsite,” explains the engineer. Pools are arranged linearly down the jobsite and sit at different levels. Once sludge is pumped into the first pool, solids settle at the bottom and begin to fill the container. Excess water drains into a subsequent pool at a lower level, until muck fills this pool and the fluidized mixture flows into a third container. The action is much like that of a gradual waterfall. Once muck fills the pools, solids are scooped up with a pay loader. Water is then recycled back through tubes to the bulkhead, where it will be added at the face for increased ease of tunneling.

The final 30% of tunnel excavation is expected to consist of more compacted sands and gravels with large boulders. As sludge pumps cannot be used with material greater than about 8 inches 200mm in diameter, the muck removal system will be switched out for a belt conveyor and muck cars. The switchover to a belt conveyor can be done in as little as one hour, allowing spoils to empty into single-track muck cars for removal from the shaft.

Boulders will be disposed of through an initial 1,200 mm diameter ribbon-type screw conveyor. Large pieces of material travel up the center of the screw and exit out of the boulder collecting gate, while more fluidized muck continues on to the secondary shaft-type screw conveyor for conventional removal by machine belt conveyor.

Ground Consolidation
Additives will be employed as needed to condition the tunnel face through six independent injection ports. The independent lines consolidate the flow of muck and reduce the risk of clogging, which can lead to uneven wear of the cutterhead and cutting tools. “Since the first 70%of the tunnel is in soft clay, we are hoping no additives will be

needed besides water for the sludge pump. In the last 30%of tunneling, we may need to use foam to consolidate ground and maintain pressure at the front,” says Benamar.

The foam, consisting of water, surfactant, and additive, also reduces the required cutterhead torque as well as overall machine wear. If more watery material is encountered, the foam additive can be switched out with a bi-component consisting of polymer and epoxy. The bicomponent increases cohesiveness of the soil by promoting clumping.

Reducing Subsidence
Initial tunneling will begin under just 7.5 m of cover in watery soils at high risk of surface subsidence. The potential problem will be controlled in a number of ways. The contractor will decrease the machine’s rate of advance using variable frequency drives. For the remainder of the excavation, the EPB cutterhead rotation will be kept low (around 1.5 rpm at maximum), in stark contrast to the higher speeds (around 10+ rpm maximum) used in similar diameter hard rock TBM tunneling.

In hard rock, high rpm results in fast advance, while in soft ground high rotational speed can result in ground disturbance and surface settlement of non-self-supporting geology. In soft ground, the same result of high advance rates can instead be achieved by increasing the cutterhead torque and thrust, which increases the instantaneous rate of penetration.

As the machine advances, it will line the tunnel with 400cm thick universal concrete segments in a 7+1 arrangement. A two-liquid back-filling system will be used to quickly stabilize the annular space between the tail shield and concrete segments. The liquid mixture consists of cement plus an accelerant, which are combined in the tail shield and harden rapidly after injection. Because the two liquids are kept separate, high pressure concrete pumps, which can disrupt the surrounding geology, are not needed.

Carefully Coordinated Launch
Machine launch began in February 2010 following a commissioning ceremony. At startup the machine excavated using umbilical cables connected to back-up gantries at the surface. Due to the small shaft size, about 34 m long by 14 m wide, only the machine and bridge gantry were assembled at the shaft bottom. The machine then advanced forward approximately 355 m before the first backup gantry was lowered down. Successive gantries were lowered down until the machine had advanced forward 70 m, clearing room for the sixth and last gantry.

“The true benefit of the onsite assembly and coordinated launch is that it has initiated good cooperation between the manufacturer and customer. The teams have worked together to solve problems and keep the project on schedule,” says Andrei Olivares, Project Engineer for Robbins Mexico. A team of field service technicians will remain at the jobsite to operate and monitor the machine throughout the project. All tunneling is expected to be complete in 2011.


The Robbins EPB is the first of its kind to be assembled at the jobsite OFTA can save about 70-80% of the time required at a shop The country’s largest ever TBM Onsite assembly of the TBM