Final breakthrough of the last Herrenknecht TBM into Majestic Station marks the end of tunnelling on the much-delayed Bangalore north-south underground line of Namma Metro Phase I.
Delivery of the twin-running 4,800m x 5.6m i.d. underground tunnels that form the central section of Bangalore’s 24.2km Green Line had been held up as a result of land access issues; the breakdown of a Seli machine midway through the first of two planned parallel drives for delivery of the northern section of the alignment; and extremely tough mixed-face conditions of hard granite and large boulders intermixed with soft soils. The geology, the high water table, and the presence of extremely frail infrastructure above the tunnel crown, proved extremely challenging for all three of the EPB machines, one manufactured by Seli, and two more from Herrenknecht, that were selected by the Transtonnelstroy/Coastal contractor JV to complete the job.
In the end the project owner, Bangalore Metro Rail Corporation (BMRC), turned to Robbins Field Service to assist its contractor JV by overhauling the Herrenknecht machines and then supervising the final 750m-long parallel drives out of Chikpet Station and into Majestic. The first machine achieved breakthrough in June (2016) after a drive lasting 15 months. On September 29 the Robbins team guided the sister machine into Majestic Station to wrap up mechanized tunnelling on the project. In the end it took nearly four years to complete the 1,600m of underground excavations south of Majestic; and just under three years to complete the 975m parallel tunnels to the north.
“We provided a team of over 60 staff including TBM operators, TBM technicians, ring builders, a grouting team, and more," said Jim Clark, Projects Manager for Robbins India. “We were also responsible for running surface installations and equipment such as the grout batching plant, gantry cranes and power supply. Contractor Coastal Projects Ltd. (CPL) provided a team of people including surveyors, QC engineers, and loco operators who reported directly to our site management team.”
The Robbins crew carried out tunneling operations while Chikpet Station was being constructed around them to mitigate delays incurred before they took over project operations in February 2015. The most difficult challenges faced included a low overburden and unconsolidated ground along the alignment, and the discovery of several uncharted wells directly along the alignment. Difficult ground frequently prevented proper pressurization during cutting tool replacement, requiring a grout solution to be pumped in to fill voids, and then left to cure. Initially the curing process took up to 36 hours, but with improved application methods this was reduced to 12 hours.
Another challenge involved the sensitive building structures along the tunnel path. Issues with surface vibration, explained Clark, required that cutterhead speed be limited to 1.8 revs/min during the day shift and 1.2 revs/min during the night shift. Despite the obstacles, the TBM advanced at rates of up to 50mm/min in highly weathered rock.
“This is an industry first, wherein a TBM manufacturer has utilized its in-house expertise and knowledge to take on this level of responsibility for a project,” said Clark, addressing the magnitude of the successful breakthroughs. “The fact that it was ‘a first’ and that we were successful in bringing this high-profile project back on track is a great achievement for The Robbins Company.”
It is now anticipated that Phase One of the Bangalore Metro will be opened in its entirety in 2017. The underground central section of the east-west Purple Line delivered by the CEC/Soma/CECI JV using two slurry machines manufactured by Hitachi Zosen, was completed in March 2014. One of the machines that successfully delivered 5,100m of tunnelling for that contract was subsequently used on the northern section of the north-south Green Line after it became clear that the broken down Seli machine would not be able to complete its parallel drive.