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Robbins TBM completes Mexico City drive Apr 2012
Desiree Willis, Technical Writer, The Robbins Company
After two years of technical challenges, variable ground conditions, tight urban alignments, and a walkthrough stop-start excavation schedule through seven cut-and-cover stations, a giant Robbins EPB has emerged from its final drive on Mexico City's latest metro extension.
7.7km Line 12 tunnel alignment (red)

7.7km Line 12 tunnel alignment (red)

Mexico's largest ever TBM, at 10.2m (33.5ft) diameter, completed its successful 7.7km (4.8 mile) tunneling run for a consortium of ICA, Carso, and Alstom, last month.
Ground conditions were highly variable, including watery clays, cobbles, and large boulders. The new metro Line 12 will be the first in a decade for Mexico City, a rapidly growing metropolis of over 20 million people.
One of the more unique features of the drive was the need to pass through seven cut-and-cover station sites, which necessitated lengthy stoppages to move the launch site and the TBM forward.
Distances between the stations ranged from as much as 1,800m to as little as 400m, and 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.
10.2m diameter TBM completed seven drives

10.2m diameter TBM completed seven drives

During each intermediate breakthrough the machine spent 1-2 months in the station in order to rebuild launch structures, carry out machine maintenance and cutterhead inspections, walk it through the station, and transport the supplies to the next site.
The EPB was supported on a concrete cradle and walked forward using a thrust frame and pushed off 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.
Much of the tunnel is under very low cover of 7.5m (25 ft), which required careful monitoring of surface subsidence. The project's city center location put the alignment in close proximity to a number of structures, and the route passed within 1.5m (4.9 ft) of a 4m (13 ft) diameter collector sewer, within 2m (6.6 ft) of building foundations, and just 3.5m (11.5 ft) below the metro's active Lines 2 and 3. At one point, the tunnel passed between two supports of an existing freeway bridge, with about 6m (19.7 ft) distance between the TBM and bridge pile foundations. Real-time settlement monitoring had to be rigorous throughout the project, and the crew had to remain diligent in maintaining earth pressure during excavation. TBM elements including a two-liquid back-filling system, with rapidly hardening cement also aiding in settlement reduction.
  • On site in October 2010

  • 1-2 months maintenance at each station

"Settlement stayed within the limits of between 2cm-5cm (0.8in-2in) throughout the bore," said Ismail Benamar, ICA Tunnel Manager.
The complexities of the densely urban location have been a hallmark of the project from the start, when the machine underwent Onsite First Time Assembly (OFTA) from a tight launch shaft measuring just 34m x 14m x 17m. Due to the small launch pit, the machine bored the first 70m (230 ft) of tunnel using umbilical cables connected to back-up gantries on the surface. Gantries were then lowered into the shaft successively as the machine bored forward.
The machine was launched from the small shaft in February 2010 and proceeded to break through into seven cut-and-cover station sites ranging from 150m-190m (490ft-620ft) in length. Despite the numerous intermediate stations, and the time required to walk through each station, advance rates topped out at 135m (443ft) per week, and averaged 400m (1,300 ft) per month.
  • Launch from tight shaft in February 2010

  • Lowering the cutterhead into position

Custom EPB features aided in the efficient excavation, and included a two-stage screw conveyor with an initial ribbon-type screw to allow the passage of boulders up to 800mm (2.5ft) in diameter. Active articulation allowed the machine to negotiate tight curves down to 250m (820 ft) in radius with no segment deformation.
When complete the 25.4 km (15.8 mile) Line 12 of the Mexico City Metro will be the longest in the system. The Mexican Federal District predicts that the new line will carry an average of 367,000 passengers daily, making it the fourth busiest commuter rail route in the capital.
References
Onsite assembly in Mexico for Robbins EPBM - TunnelTalk, January 2010
Mexico's largest EPB to be supplied by Robbins - TunnelTalk, October 2008

           

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