After 35 gruelling months, the Seli EPBM that launched from the North Ramp of the north-south Purple Line in Bangalore in June 2013 is through.
Crews from the Coastal/Transtonnelstroy (TTS) joint venture contractor team and representatives from the owner, Bangalore Metro Rail Corportation (BMRC) joined in the celebrations as the 6.5m diameter machine finally reached the end of its short 970m drive into the shaft at Majestic station.
Much of the delay has been caused by difficult mixed face ground conditions along an alignment that also featured sections of hard rock – factors that ultimately resulted in a broken cutterhead at 352m into the drive. Speaking to TunnelTalk from Bangalore, Coastal Director of Operations Sharad Kumar spoke candidly about the difficulties the contractor team had faced.
“Geological strata in the initial section of the tunnel consisted of loosely jointed rock,” said Kumar. Progress was painfully slow for the first 12 months – just 350m – and eventually the wear on the machine became too great.
“Due to severe jointing, rock chunks were continually coming loose from the face, but the cutterhead would keep on rotating without the material being crushed. This created an imbalance in the machine and ultimately the cutterhead became overexposed to high torque, resulting in high wear and tear. In the process, the cutting tools and cutter mounts, as well as the cutterhead itself, were damaged.”
With the machine trapped underground there was no alternative but to excavate a recovery shaft ahead of the face. “Constructing this shaft was our greatest challenge, given its location adjacent to the existing railway line and in a highly congested residential area,” said Kumar.
Recovering the machine and shipping in a new cutterhead manufactured by Palmieri of Italy took a further 12 months out of a schedule that was already under serious pressure. At contract signing in 2011 the whole of the 5,000m-long underground UG1 contract was to have been completed by 2014. Even before the breakdown occurred, however, a decision had been made to utilise one of the slurry TBMs that had successfully completed the east-west Green Line UG2 contract to excavate the North Ramp-Majestic downline. The initial UG1 strategy had been for the Seli machine to complete both the upline and the downline.
Kumar went on: “Apart from the cutterhead, we undertook a major refurbishment of the machine, including repair of the screw conveyor system and other ancillary systems.”
The machine was finally restarted for the remaining 620m of the drive in July 2015, but the challenges continued. Kumar explained: “Since restart, we have encountered both full face soil and mixed face conditions, as well as a challenging 46m stretch of solid rock of up to 120 MPa. During this rock section, frequent interventions for performing cutting tool replacements were necessary – one for every 3.3m of advance on average.”
With the other sections still requiring interventions at intervals of 27m, a total of 35 forced tool-change and maintenance stops were performed following restart – 33 of them in hyperbaric conditions, and only two in free air.
Kumar told TunnelTalk that a total of 190 cutters were changed following the restart of the machine. “The rock stretch accounted for only 7.5% of the drive but required the changeout of 59 cutters. The remaining 92.5% of the drive through soil/mixed strata, required the changing of 131 cutters.”
Successful breakthrough by the Seli TBM into Majestic station from the north means that the TTS JV is near to completion of all eight drives of the underground section of the north-south line. The two Herrenknecht EPBMs procured to complete the six drives between the South Ramp and Majestic station – which are themselves seriously behind schedule – are now also close to breakthrough.
“One of the machines has completed 690m and is now just 60m from breakthrough, while the parallel drive is 570m complete and is 180m from breakthrough,” said Kumar.