Case Study for Removal of Redundant Steel Joist Piles under Base Slab of an Operational Metro Station by NATM Pilot Tunnel for Clearing the Alignment for East West Metro Line Tunnels in Kolkata, India-Video

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This case study describes removing undocumented steel joists beneath Kolkata’s operational Esplanade metro station, where they obstructed the planned east–west line tunnels. Because the alignment could not be changed and closing the station was impractical, the team selected a NATM pilot tunnel instead of risky hyperbaric intervention.<br /><br />The tunnel was designed for alluvial ground using PLAXIS 2D and Phase2 analyses, with staged excavation, fiber-reinforced shotcrete, lattice girders, forepoling, and ground bolts. GFRP supports were used where conventional steel could interfere with the future TBM route. Extensive monitoring tracked settlement in the station, tracks, roads, and nearby buildings.<br /><br />Construction uncovered 21 joists—more than the nine initially identified—and required 84 metres of pilot tunnel. All joists were removed successfully, and the TBM tunnels later passed through without issue. Measured settlement was 8.5 mm, below the predicted 10.33 mm, with no station distress. The project demonstrates that carefully designed, executed, and monitored NATM can work effectively in urban soft ground.

Keywords

NATM pilot tunnel

Esplanade metro station

undocumented steel joists

alluvial ground excavation

TBM tunneling