Back on December 8, tunnel boring machine (TBM) S-639 operated by Afcons Infrastructure broke through the northern diaphragm wall of Mahakaran Station on Kolkata Metro’s 16.34 km Line-2, roughly six months after it crossed beneath the Hooghly River and entered into Kolkata’s city limits.
Like TBM S-640 (Prerna) which arrived here on November 26, the Herrenknecht manufactured machine nicknamed Rachna wasn’t visible to workers as soil excavation work for the station’s box hadn’t commenced. Throughout the rest of December, Afcons continued building temporary rings within the station box towards Esplanade and broke through Mahakaran Station’s southern diaphragm wall in early January. The temporary rings built within the station box will be dismantled during excavation this year. Similar ‘hidden’ TBM breakthroughs, not visible to workers, were recently performed by Afcons at Chennai’s Sir Theagaraya College Station (By S-711 and 219-364) as well as locally by TBM S-639 at Howrah Station.
In response to my LinkedIn post announcing S-640’s breakthrough in November, Virinder Kaul – the General Manager of Afcons Infra left behind this comment about the diaphragm wall’s construction technique which some of you might find interesting –
TMRG this breakthrough was achieved in one hour. Instead of traditional GFRP bars and concrete we have been using only styrofoam in the soft eye zone. This has been done first time in India and very rarely world wide. This idea was proposed by our previous Project Manager Mr P P Boni a tunneling veteran. With this innovation the breakthrough time is reduced to 1-2 hours, TBMS don’t have to cut concrete and subsequent intervention and replacement of cutting tools is reduced to almost zero.
Image of Mahakaran Station’s site shared on Twitter –
As mentioned earlier, the roughly 2 km journey from Howrah Station to Mahakaran wasn’t an easy one, as the influence zone of the TBM consisted of structurally weak buildings on Brabourne Road (which required temporary evacuation of residents to hotels) and a cluster of heritage monuments & religious structures which required special 24/7 monitoring of soil settlement and grouting injections to strengthen their weak foundations. On this past Sunday, excavation work came to a halt for Sunday Mass at St. Andrew’s Church, which is visible in the image above.
As of early February 2018, both S-639 and S-640 have only about 200m each left to reach Esplanade Station – their final destination. Both will again make ‘hidden’ breakthroughs there and remain in captivity until the station’s structure is ready and soil is excavated from its base slab – a process which will take another 6-9 months.
For more updates, check out the Kolkata section of The Metro Rail Guy!