Main contractor Ingenieros Civiles Asociados has celebrated breakthrough on “one of the most difficult legs” of the 62km Emisor Oriente wastewater tunnel in Mexico City.
Lot 5 involved driving a 6km long section of tunnel through both hard rock and soft ground to complete the latest tunnel and was delivered using a Robbins earth pressure balance tunnel boring machine (TBM).
As well as variable ground conditions, which included abrasive basalts, the tunnel was also one of the deepest on the project with the TBM launched from a 150m deep shaft.
“I am proud of concluding the excavation successfully given that this is the section with the greatest depth on the project,” said Ingenieros Civiles Asociados equipment manager José Adolfo Méndez Colorado.
According to Méndez, the complexity of the ground conditions required the correct concentration and selection of polymers and this was a large challenge in itself. There were some issues with the screw conveyor conveying mixed ground and also in the articulation system, which were overcome with Robbins’ experience,” he said. “They also knew the correct operation of the continuous conveyor system, which kept us to expected performance.”
The TBM originally planned for Lot 5 was fast tracked to allow it to be used on another section of the project before being modified to cope with the higher groundwater pressures – up to 7bar – expected on this latest drive. Other modifications included addition of chromium carbide wear plates added to the screw conveyor and grizzly bars to the cutterhead along with heavy duty cutting tools.
The TBM was assembled in a 28m long chamber at the bottom of the shaft in August 2014 and launch of the machine took several months to complete. Tunnelling on another section of the tunnel is expected to be completed this spring, marking completion of the project which is designed to overcome flooding issues and problems with existing wastewater tunnels in Mexico City arising from subsidence as a result of drained lake beds and lowering of the water table in the city.