Delhi Metro’s precision tunnelling: A major engineering breakthrough beneath an operating corridor

The Delhi Metro Rail Corporation (DMRC) has completed one of its most demanding underground works to date as part of the Phase 4 expansion. Engineers successfully advanced a Tunnel Boring Machine (TBM) beneath the operational Red Line between Pulbangash and Sadar Bazar without suspending train movement. The viaduct above stands on open foundations and incorporates a balanced cantilever span. This configuration made the structure highly sensitive to any subsurface movement. Even minor ground deformation had the potential to affect pier stability and the running line, which carries approximately 700,000 passengers daily. The task therefore required a highly controlled geotechnical response supported by continuous monitoring.

Before tunnelling commenced, DMRC implemented an extensive Tube-a-Manchette (TAM) grouting program to stabilise the soil surrounding the Red Line pier. Engineers drilled 180 TAM boreholes and injected high-strength cementitious grout to fill voids, enhance shear strength and reduce the risk of differential settlement. Alongside ground improvement, a comprehensive instrumentation network was installed to monitor in real time parameters such as settlement, tilt and lateral movement. Devices included deep inclinometers, building settlement points, surface settlement markers, pier tilt meters, load cells and an Automatic Total Station. Specialised personnel supervised the readings around the clock. All recorded values remained within acceptable limits throughout the down-line tunnelling sequence.

With the down-line tunnel completed safely, work on the up-line tunnel is progressing under the same monitoring protocols. DMRC notes that maintaining continuous service on the Red Line while advancing the TBM below required strict adherence to geotechnical control measures. The successful execution of this operation demonstrates the capability of controlled tunnelling beneath sensitive infrastructure in dense urban environments. It also provides a benchmark for similar future works where underground expansion intersects with operational transport systems.