TBM Makes Unique “Hidden” Breakthrough at Chennai Central

On this past Wednesday (Jan 27), a Robbins mixed ground EPB Tunnel Boring Machine operated by the Afcons-Transtonnelstroy JV made a breakthrough at Chennai’s Central station! The TBM was commissioned from the May Day Park shaft in early February 2015 and made a breakthrough for the Airport-Washermenpet line’s down-line tunnel after traveling 1025 meters and building 732 rings along the way. On its 11 month journey the TBM crossed beneath several buildings, the Cooum river and the Southern Railways’ tracks.

Now for the unique part – the TBM made a “hidden” breakthrough since the TBM receiving shaft on the western end of the Central station was not fully excavated. In the other words, the TBM broke-through the shaft’s diaphragm wall and is now inside the shaft, but the soil on top of it has not yet been excavated. As such, the TBM is not visible to anyone and literally hidden beneath piles of soil.

Alignment of Chennai Metro’s May Day Park shaft to Central Station section – view Chennai Metro Map

Chennai’s Central metro station site across from the Victoria Public Hall on Jan 30 – Photo Copyright: Biju M

In mid-January, the TBM crew stopped stopped tunneling after building 724 rings (1013m) , just 11m short of the shaft and was told to wait for the shaft to be completed by the end of February. However, a change of plan was made (perhaps under pressure from the CMRL) and the team went on to make a breakthrough without the shaft being ready. Excavation will take another month after which the TBM’s cutterhead & shields will be retrieved. In the meanwhile, the TBM’s backup gantry will be disassembled and taken out from its rear-end and retrieved from May Day Park shaft.

To backtrack for a minute, another unique TBM breakthrough, the first of its kind in the country, was made on this section’s parallel up-line tunnel at an adjacent shaft at Central station on July 10, 2014. In that case, the shaft was filled with bentonite and water up to concourse slab prior to the breakthrough making the event also hidden and not visible to anyone.

Up-line breakthrough at Chennai’s Central Station – Photo Copyright: Chennai Metro Rail

With this breakthrough out of the way, only 4 more breakthroughs are left on the UAA01 section between Egmore/May Day Park to Washermenpet. All 4 will take place at Central station – 2 from the Egmore station side for which 1 TBM is yet to be commissioned (targeted for Feb 10) and 2 from the High Court station side which are currently encountering hard rock and need daily intervention to progress. For a more detailed report with progress made on all tunnel sections, please see my January 2016 Tunneling Update.

For more updates on Chennai’s metro, check out the Chennai section of The Metro Rail Guy!

– TMRG