The Delhi Metro Rail Corporation (DMRC) has achieved a unique engineering feat in its Phase 4 expansion work by successfully constructing an underground tunnel at Pulbangash below the already operational Red Line of the Delhi Metro network. This tunnel has been constructed as part of the Janakpuri West – RK Ashram Marg extension of the Magenta Line.
This tunnelling work between the Pulbangash and Sadar Bazar stations was especially challenging, as trains on the busy Red Line above were not halted for even a single day, even as the Tunnel Boring Machine (TBM) carefully constructed the tunnel below the elevated viaduct on which trains were operational. Moreover, the Red Line viaduct here stands on open foundations with a balanced cantilever span. Tunnelling beneath such a structure required extraordinary engineering precautions to ensure its safety.
A number of measures were taken by DMRC’s engineers to achieve this feat. Some of them are mentioned below:
• Extensive Ground Strengthening Around the Existing Pier: A detailed strategy was adopted to strengthen the area around the pier of the Red Line. A grouting programme called TAM (Tube-a-Manchette) was implemented. As per this technology, 180 TAM boreholes were arranged around the pier. After that, high-strength cement grouting was done to stabilise the soil, fill the voids and in the process, increase the load-bearing capacity. In addition, it was also ensured that all possibilities of settlement of soil during tunnelling were eliminated.
• World Class Instrumentation and Real Time Monitoring: Elaborate instrumentation was deployed during the tunnelling work to monitor ground movement, pier behaviour, and building safety on a real-time basis. Some of these equipment were Surface Settlement Markers, Deep Inclinometers, Building Settlement Points, Pier Tilt Meters and Load Cells, Automatic Total Station (ATC).
Specialised personnel were deployed round the clock to monitor the readings of these advanced equipment. All parameters remained well within permissible limits during the entire operation.
The construction of the downline tunnel on this section has now been completed. Work on the up-line tunnel is in progress with the same level of precautions and monitoring. This was an extremely critical construction work since the Red Line, on average, carries about seven lakh passengers every day. Any disruption to the movement of trains on this line would have caused tremendous inconvenience to the people.
