Several feet below the ground and with hard rocky surface in front, it is actually a blind drive for the tunnel boring machines that are at work for Namma Metro in Bangalore. With very limited scope for deviation from the planned path, the 95-metre long TBMs that weigh about 280 tonnes each, are inching forward in the right alignment using a survey instrument.

Theodolite, a survey instrument with self-rotating telescope, is used to gauge the path of the TBMs. The instrument is fixed on the rear of a TBM and is read from the control room. Though the instrument is about 100 m behind the TBM head, it gives almost an accurate reading about the alignment.

“It is something like reading the front portion of your body from the rear,” R. Balasubramanian, Senior Resident Engineer of BMRCL’s General Consultants, told The Hindu.

The control room is positioned behind the TBM, along with which several other back-end operating machines, including grouting system are lined up, he added.

BMRCL engineers have fixed a maximum permissible deviation of 50 mm for themselves from the fixed alignment in this extremely difficult work area. So far, deviations up to 35 mm were observed even as the entry of TBMs from Majestic to Central College underground station went through with near precision, said Subrahmanya Gudge, Deputy Chief Engineer (Underground) of BMRCL, who is in-charge of the East-West underground section.

He said the entire TBM operations are regulated from the control room even as entry towards TBM cutter head is highly restricted. It has been quite a challenge for the engineers in Bangalore that has rocky and mixed strata (rocky and soil) to run the TBM.