Bangalore: Work on Namma Metro Phase 1 will be delayed at least by about two months as tunnel boring machines (TBMs) have to burrow through hard rock between Krishna Flour Mills, off Sampige Road Metro Station, and Majestic on the underground (UG) section of Namma Metro’s North-South corridor. The corridor connects Nagasandra in the North and Puttenahalli Cross in the South.
Godavari, an earth pressure balancing mechanism (EPBM) TBM has encountered hard rock after burrowing about 300 m from Krishna Flour Mills. It has to cut through about 75 m of hard rock before getting normal soil for the next 540 m even as burrowing of another tunnel from this direction is yet to start.
A TBM would burrow an average of six m a day under normal circumstances while through hard rock it could burrow just about 1.5 m, said BMRCL managing director Pradeep Singh Kharola. Burrowing gets delayed as the disks on the cutter head [the front portion of the TBM which excavates the earth] have to be replaced constantly. For every 1.5 m [to fit one concrete ring] at least 8 disks are to be changed while the cutter head has a total of 45 disks.
On the other hand, two EPBM TBMs — Kaveri and Krishna — are moving towards Chickpet UG station from K.R Market. They have already completed burrowing between Prof. P. Shivashankar Circle on K.R. Road and K.R. Market. While Kaveri has fixed about 185 rings, Krishna has installed about 150 rings. The K.R. Market-Chickpet stretch too has some rocky strata allowing fixing an average 2.5 rings a day as against 4 rings.
Another 45 and 75 days respectively are required for these machines to fix 300 rings in total on this stretch. Both these machines would then proceed from Chickpet UG station towards Majestic to fix another 450 rings [700 mts]. This would take another 6 months, Mr. Kharola said.
BMRCL is now planning to ask the contractor to introduce another TBM from Krishna Flour Mill side towards Majestic since only one machine is on the job at present. Another TBM would hasten the tunnelling process, the Managing Director said. However everything is lined up now and the corporation is confident of completing the tunnelling work on North-South Corridor by September-October and track and station works in another six months, Mr. Kharola added. The corridor will be ready for train operation by March-April next.