Getting TBMs up and digging!

With the rising demand for underground infrastructure primarily due to space constraints, the role of TBMs or Tunnel Boring Machines are assuming greater relevance to excavate tunnels, across various types of soil strata. Since India does not presently manufacture TBMs, all those in use are imported. Of late, L&T has been employing several TBMs across project sites having launched a few recently at the metro rail projects in Mumbai, Bengaluru, and Ahmedabad, in several cases without much help from the OEMs.

At the Mumbai Coastal Road Project (MCRP), Sandeep Singh, Project Director of Package 4 has an 80 M long TBM with a 12.19 M diameter, weighing 2,300 T with an installed capacity of 7,280 kW to assemble. “It is the largest in the country,” he says seriously, “specially procured to excavate in deep overburden through a compound stratum of basalt, breccia and shale for which it is equipped with a mixed cutter head with eight spokes and eight panels to bore in the complicated strata for a long distance.” Assembling such a complex piece of equipment is Sandeep’s biggest challenge.

Certainly, the process of assembling, launching, and putting a TBM to work calls for knowledge, precision, and high safety standards, recognizing which L&T’s Tunnelling Excellence Academy is training and preparing tunnelling experts.

Safely building it at site

Once safely unloaded at site, the team must put their minds and muscle to building the TBM. The sequence is crucial, and the shaft determines the order of reassembly. Some L&T projects have vertical shafts that only allow the shield to be lowered followed by the cutter head by cranes, carefully chosen considering their lifting capacity and capability to lower all the TBM components above the shaft. “The Mumbai Metro Line 3 used a 1000 T crane and they had to divert traffic for nearly 3 months in an extremely people and vehicle dense location to complete the task,” remarks Michael with a roll of his eyes.

Through all this, Safety remains the key consideration. Understanding the risk of every activity is crucial for the team and then to plan and work together on every aspect from design, P&M, EHS, execution. Safety is top priority for the Tunnel Manager too because any load failures can be catastrophic. “Every day brings new activities and fresh challenges,” chips in G Divakar, EHS Compliance Head, Heavy Civil IC, who has been in the thick of TBM action at several Heavy Civil sites.

The challenge for Project Manager, Vivek Maruti Pai at the other Bangalore Metro package RT 02 project was to assemble and operate Slurry TBMs to suit Bangalore’s geological conditions, an entirely new proposition. To add to their troubles, the excavation of the shaft was becoming extremely difficult due to certain geological constraints. “We were forced to use an umbilical system that is more complicated both in assembly and operation, but we succeeded.” Vivek is both exultant and relieved.

All in it together

Although there are experts, everyone involved in the TBM assembly must take specific tunnel inductions as different tunnels pose different hazards and safety risks. An EHS Tunnel personnel guide lays down the EHS systems to ensure a safe assembly. All likely hazards and risk must be prior identified and controlled, captured in risk assessment and method statements that are submitted to the client for approval before starting the process.

Lowering after the assembly

Once the TBM shield is built, the gantries are lowered. Each gantry has a specific function and has been designed specifically with safety in mind. As the build continues, every part of the TBM must be tested, under a Permit. “Here, the left hand must know what the right is doing,” smiles Michael. “Accidents occur during the assembling and commissioning stage as the machines become very confined so wearing PPE is a must especially protection for the eyes during welding and handling of pressurised hoses,” he stresses. Fire protection systems and personnel must remain on red alert till the build is complete. Work at height needs to be considered too when assembling the shield and the gantries in the shaft area. “The shield is round and if you miss your step walking on the top, you can land several meters below so fall protection is important,” points out Divakar. Gantries require handrails as walkways over 1 m as anyone can fall, because the existing tunnel lining is used for protection of falls during tunnelling. Therefore, only authorised persons are allowed into the shaft to ensure safety.