Challenging mixed ground overcome in Bangalore 14 Jun 2016

Amey Yong, Technical Writer, The Robbins Company

Final breakthrough on the 4.8km north-south line of the Bangalore Metro is just two months away following completion of mechanized excavation of the penultimate drive into the south shaft of Majestic Station.

Coastal and Robbins breakthrough celebrations in Bangalore
Coastal and Robbins breakthrough celebrations in Bangalore

On June 8 the first of two 6.4m diameter mixed-face Herrenknecht EPB machines (TBM Kaveri) broke through on its final 750m drive from Chickpet following a tortuous 15 months through extremely difficult ground. Sister machine TBM Krishna, launched in December 2015, and excavating the parallel tunnel, is expected to complete final project breakthrough in August.

In February 2015 Robbins India was asked by project owner Bangalore Metro Rail Corporation (BMRC) to take over the operation and maintenance of two competitor-owned EPBMs. As the north-south Bangalore Phase 1 Metro tunnels were on the critical path, BMRC approached Robbins to step in to assist the contractor, Coastal Projects, in completing the drives and keeping overruns to a minimum. A team of 70 field service personnel refurbished and modified the existing equipment, both while in the tunnel, and ahead of relaunch from Chickpet station. The on-site team supervised all aspects of TBM excavation, segmental lining, mucking out and grout plant operations for the final two drives.

“I am particularly proud of being involved in this project, and to support BMRC with the completion of this tunnel drive,” said Jim Clark, Projects Manager with Robbins India. “This is also the first time The Robbins Company has been responsible for the complete scope of operations on a project.”

He added: “One of the biggest challenges we faced was carrying out cutterhead interventions under compressed air in mixed ground conditions that consisted of varying grades of granite in the lower half of the face, and loose, unconsolidated material in the upper section.”

6.4m Herrenknecht EPBM faced difficult mixed-ground conditions
6.4m Herrenknecht EPBM faced difficult mixed-ground conditions

Clark said that on numerous occasions, compressed air would percolate through to the surface, forcing crews to abort cutterhead interventions. They eventually found a solution by pumping a weak-mix grout though the mixing chamber, and into the geology surrounding the tunnel face. The weak mix grout stabilized the ground sufficiently so the cutterhead interventions could be completed.

The Robbins crew carried out tunneling operations while the station was being constructed around them to mitigate delays incurred before they took over project operations. The project’s most difficult challenges included a low overburden and consolidated ground along the alignment, and the discovery of several unmapped wells directly along the alignment. In addition, the majority of the tunnel was bored directly beneath buildings constructed upon what would today be considered substandard foundations.

Once TBM Krishna breaks through in August, the 4.8km of North-South tunnels will be connected. It is being completed under a tripartite agreement between Robbins India, Coastal Projects Limited, the lead contractor, and project owner, Bangalore Metro Rail Corporation Limited. It is anticipated that Phase One will be open in its entirety by November 2016.

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