TMB emerges triumphant in Mexico City

Assembly on the TBM, which took place onsite in January 2010.

Assembly on the TBM, which took place onsite in January 2010.

Mexico’s largest tunnel boring machine, a 10.2 m diameter Robbins earth pressure balance, has successfully excavated 7.7 km of tunnel in highly variable ground including watery clays, cobbles, and large boulders.

The tunnel boring machine (TBM) has been tunnelling the metro Line 12, which will be the first new metro line in a decade for Mexico City, a rapidly growing metropolis of over 20 million people.

The tunnel route took the Robbins machine to within metres of a 16th century church, active sewer lines, building foundations, and other structures.

Real-time settlement monitoring was rigorous throughout the project, and the crew was diligent in maintaining earth pressure during excavation. TBM elements including a two-liquid back-filling system with rapidly hardening cement also aided in settlement reduction.

ICA Tunnel Manager Ismail Benamar said “Settlement stayed within the limits of between 2-5 cm throughout the bore.”

The complexities of the densely urban project location have been a hallmark of the project from the start, when the machine underwent onsite first time assembly (OFTA) from a shaft on a city street.

Robbins Field Service Technician Ron Jelinek said “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.”

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 150 to 190 m in length. During each hole through, the machine underwent routine maintenance and was re-launched. Despite the numerous intermediate stations and the time required to walk through each station, advance rates topped out at 135 m per week, and averaged 400 m per month.

Upon completion, the 25.4 km Line 12 of the Mexico City Metro is 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.

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