Project Overview

The Mexico City Metro is one of the world’s largest, with over 200 km (125 mi) of rail and nearly 4 million daily passengers. The 25.4 km (15.8 mi) long route passes through 22 new stations between Tlahuac and Mixcoac neighborhoods. The 7.7 km (4.8 mi) long tunnel represents the capital’s first new route in ten years, and will service thousands of passengers daily.

In 2007, the Mexican Federal District announced plans to build Line 12 of the Mexico City Metro. The ICA consortium signed a contract for a 10.2 m (33.5 ft) diameter Robbins EPBM, its back up system, and cutting tools. The TBM was the largest to ever bore in Mexico and was the first EPB TBM to be assembled at the jobsite using Onsite First Time Assembly (OFTA).

Geology

Geology on the metro line consisted of layers of clay, sand, and boulders up to 800 mm (30 in) in diameter, as the area is part of a drained lake bed. Ground conditions in Mexico City are very unique and thus required extensive vibration monitoring throughout the bore.

TBM

The giant machine utilized a specially designed two-stage, 1,200 mm (4 ft) diameter ribbon type screw conveyor followed by a shaft-type screw conveyor in order to handle the large boulders. For parts of the tunnel where the ground consisted of very soft clays with high water content, muck was removed using a sludge pump rather than conveyors or muck cars.  The machine also featured active articulation, used to prevent segment ring deformation on curves as small as 250 m (820 ft). The spoke-type cutterhead used tungsten carbide knife-edge bits to excavate the soft ground. Additives, as well as two-liquid back-filling, helped control ground subsidence. The two-liquid back-filling system consisted of cement and accelerant, which hardens rapidly and eliminates the need for high-pressure concrete pumps that can disturb the ground. As the machine advanced, it lined the tunnel with 40 cm thick universal concrete segments in a 7+1 arrangement.

The Robbins EPB was launched on February 15, 2010, after just eight weeks of assembly. The launch shaft, approximately 34 m long by 14 m wide by 17 m deep (112 x 46 x 56 ft), was located in one of the most densely urban areas of the city. Due to the small launch pit, the machine bored the first 70 m (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.

Tunnel Excavation

Much of the tunnel was under very low cover of 7.5 m (25 ft), which required careful monitoring of surface subsidence. The project’s location at the city center was in close proximity to a number of structures.  The tunnel route took the machine within 1.5 m (4.9 ft) of a 4 m (13 ft) diameter collector sewer, within 2.0 m (6.6 ft) of building foundations, and just 3.5 m (11.5 ft) below the metro’s active Lines 2 and 3.

Breakthrough

The machine reached the its first station during the last week of April 2010 after boring a total of 495 m (1,624 ft). There were several difficulties with the sludge line, but after engineers redesigned the system it worked exceptionally well. From there, the EPB continued on to six more stations, undergoing routine maintenance at each one. On March 1, 2012, the machine completed its successful tunneling run.

Line 12 of the Mexico City Metro is the longest in the system. The new line carries an average of 367,000 passengers each day, making it the fourth busiest commuter rail route in the capital.