Namma Metro, meaning “Our Metro”, has been highly anticipated by the city of Bangalore’s 8.5 million residents since it was first approved in 2010. In phase one, owner Bangalore Metro Rail Corporation (BMRCL) – a joint venture of the Indian Government and the Government of Karnataka – slated 42.3km of rail in two lines. Of that, 8.8km is underground with the remainder elevated or at grade.
Two slurry TBMs were procured for the east-west line work, while three EPBMs were procured for the critical north-south line, a full 24.2km route through the heart of the city with a total of 24 stations. The two EPBMs “Krishna” and “Kaveri”, originally manufactured by Herrenknecht, were launched from the south ramp in October and November of 2012, and were slated to bore three sections of tunnel each, totaling 1,550m. While the first 400m-long drive from South Ramp to City Market station went well, the TBMs encountered severe ground conditions on the second, 432m-long drive from City Market to Chickpet Station.
The drives took 12 and 22 months, respectively, and were hampered by a mixed face comprising hard granite and soil, high groundwater levels, and tunnelling near fragile, historic building foundations in some cases hundreds of years old. Contractor Coastal Projects (CPL) has also said that the TBMs in this section encountered large boulders as well as reinforced blocks of concrete that damaged the TBM cutterheads. This, combined with regular cutterhead interventions and an inability to grout unstable areas from the surface due to congested residential areas, resulted in reduced advance rates.
It was at this point that contractor CPL and owner BMRCL approached Robbins and asked it to take over the operations of the TBMs as the critical path tunnels needed to be brought back up to speed. The last 750m drive between Chickpet and Majestic stations was all that stood in the way of opening a substantial section of Namma Metro’s Phase One. After obtaining agreement from the project owner and the contractor, Robbins took over the responsibility for all aspects of the underground operations.
“We provided a team of over 60 staff including TBM operators, TBM technicians, ring builders, a grouting team, and more. We were also responsible for running surface installations and equipment such as the grout batching plant, gantry cranes and power supply. Contractor CPL provided a team of people including surveyors, QC engineers, and loco operators who reported directly to our site management team,” explains Jim Clark, projects manager for Robbins India.
The Robbins crew carried out refurbishment of the two TBMs. In particular TBM “Krishna” underwent 112 days of repairs and testing. The refurbishment, and subsequent assembly and launch of the two machines, was carried out even as the Chickpet station was being constructed in order to mitigate any further delays. “Apart from the tunnelling operations we had to operate out of a station box where many civil engineering activities were taking place at the same time.
“Great care had to be exercised in coordinating train movements bringing spoil from the TBM and lifting spoils to the surface, and also the lowering down of supplies (i.e., segments, pipes, rails, etc.),” says John Simm, Robbins field service site manager.
The two TBMs were re-launched in 2015 on their last drive—in March for TBM “Kaveri” and in December for TBM “Krishna.”
Agauntlet of challenges
While TBM “Kaveri” launched first, it wasn’t immune to challenging conditions. “Robbins had a geologist onsite, conducting face mapping for the duration of the project,” explains Clark. The initial 160m of the drive was found to consist of residual soil, gradually transitioning into a mixed face of soil and highly weathered granite over the following 100m before finally becoming a full face of fresh granite in the last 50m. The zone of transition was particularly difficult. “During the first drive, we had difficult geology in the face for large parts of the tunnel. We had competent rock in the face but just above the cutterhead was residual soil–this soil would occasionally fall due to the vibrations when boring the stronger rock in the majority of the face, and as a result of these conditions, it was not possible to maintain hyperbaric air pressure during cutterhead interventions,” says Simm.
The problem was compounded by the presence of sensitive building foundations overhead that required minimal settlement. This problem was overcome by pumping a weakmix grout solution into the ground surrounding the TBM. The solution permeated into existing voids and effectively prevented air from percolating through to the surface. A period of approximately 36 hours was initially required for curing of the grout solution but on-site trials with various additives enabled the standing time to be reduced to 12 hours.
“We inherited a process from the contractor that involved pumping a very weak mix grout into the cutterhead chamber, which involved a 32-hour curing period. Because of its fluidity this didn’t always work and the process had to be repeated if air losses continued. As this was counterproductive to the project timelines, Robbins carried out trials using sodium silicate and other additives. However, there were concerns about trapping the shields during longer cutterhead
interventions, and eventually we found a grout/bentonite mix that resulted in a 12-hour curing time and was successful in every intervention,” says PN Madhan, engineering geologist for Robbins India.
Difficult ground was not the only obstacle, however. “One of the biggest challenges faced on the project was the existence of several uncharted wells along the alignment of the tunnels,” says Clark.
In cases where well locations were known, the wells were back-filled with lean-mix concrete. This proved to be successful in most cases; however, some leakages of ground conditioning foam and air pressure were recorded in the vicinity of some of the back-filled wells. For known well locations interventions were carried out and all worn cutter tools replaced in advance of reaching the locations. The same procedure could not be done for the uncharted wells though.
“Several of the uncharted wells were directly along the tunnel alignment and the first sign of their existence was the presence of old pottery and bottles appearing through the screw conveyor discharge,” says Clark. “It wasn't possible to implement mitigation measures for boring through the uncharted wells as we encountered them unexpectedly. By the time the excavated material had become a component of the muck in the mixing chamber and eventually came through the screw, we had bored through the well.
“It was decided that the best course of action was to continue boring and maintain face pressure.” Clark also notes that the voids that made hyperbaric changes more difficult seemed to occur around the wells, whether charted or uncharted. “We concluded that, over decades, water flowing into the wells from the surrounding geology has carried fines into the wells, resulting in voids.”
Excavation under sensitive building foundations presented a further obstacle. “One issue that was encountered was surface vibration while boring through rock stretches beneath residential buildings. Although peak particle velocity (PPV) values were only around 1.5mm per minute there were the usual issues regarding human response to tunnelling-induced vibrations. It was decided that the maximum cutterhead speed during the day shift would be limited to 1.8 RPM and this was reduced on the night to 1.2 RPM,” says Clark.
Twin successes
Despite the challenges, the TBMs were able to achieve advance rates of 50mm per minute in highly weathered rock and 22mm per minute in sections of competent hard rock. TBM “Kaveri” completed its final breakthrough in June. The second TBM “Krishna” had the advantage of known geology and charted wells, and completed its excavation in about nine months on 23 September. “Logistically our environment was changing on a daily basis, so to keep up good levels of TBM production was a testament to our team in overcoming the daily challenges,” says Simm. The success of the TBMs was dependent on good cooperation from all those involved. For Clark, involvement in all components of the underground construction was a point of pride. Cleanup and final commissioning of the tunnel will be completed in 2017, and is the last obstacle before BMRCL can open the Malleswaram-Majestic link. Now that tunnelling is complete through the Majestic Station, the North and South runs of the Namma Metro will be connected–a line that, once in service, will carry an estimated 40,000 passengers daily. It is anticipated that Phase One of the metro will be opened in its entirety in 2017.
