New Delhi: In a remarkable engineering achievement, the Delhi Metro Rail Corporation (DMRC) has successfully completed the construction of an underground tunnel beneath the operational elevated Red Line at Pulbangash, without halting train services even for a single day. The breakthrough was accomplished as part of the Janakpuri West–RK Ashram Marg extension of the Magenta Line under Phase 4.
The tunnel lies between the upcoming Pulbangash and Sadar Bazar stations, in a zone considered one of the most technically challenging sections of the ongoing project. The operational Red Line runs above on an elevated viaduct supported by open foundations and a balanced cantilever span, a configuration that made subterranean tunnelling exceptionally sensitive. As DMRC highlighted, tunnelling beneath such a structure required “extraordinary engineering precautions to ensure its safety.”
A key focus was stabilising the soil around the existing pier of the Red Line. Engineers carried out extensive ground strengthening using the Tube-a-Manchette (TAM) grouting technique. DMRC explained that “180 TAM boreholes were arranged around the pier,” after which high-strength cement grouting was injected to fill voids, improve soil strength and eliminate any chance of settlement during tunnelling. The process significantly enhanced the load-bearing capacity around the pier, allowing the Tunnel Boring Machine (TBM) to pass safely beneath.
In addition, a sophisticated real-time monitoring system was deployed to track every possible movement in the ground, buildings and the viaduct structure.
DMRC used advanced instruments—including settlement markers, inclinometers, tilt meters, load cells, and ATS—monitored round the clock to ensure safety. With the down-line tunnel complete, up-line work continues, maintaining uninterrupted service for seven lakh daily Red
Line passengers.