DMRC completes tunnel under red line

The Delhi Metro Rail Corporation (DMRC) has completed a technically challenging tunnelling task in its phase four expansion by constructing a tunnel beneath the operational Red Line at Pulbangash without disrupting train services.  The tunnel is part of the Janakpuri West-” RK Ashram Marg extension of the Magenta Line and connects the Pulbangash and Sadar Bazar stretch.

According to the DMRC, trains on the busy Red Line continued to run throughout the operation, even as the tunnel boring machine (TBM) worked just below the elevated viaduct. The Red Line structure at the site sits on open foundations with a balanced cantilever span, making the task particularly sensitive, it stated, adding that engineers said tunnelling below such a structure required extraordinary safety measures.

“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,” it said.

The DMRC also deployed an extensive set of instrumentation to monitor ground movement, pier behaviour, and building safety on a real-time basis. Some of this equipment were Surface Settlement Markers, Deep Inclinometers, Building Settlement Points, Pier Tilt Meters and Load Cells, Automatic Total Station (ATC).

The DMRC said 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,” it added.