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-Paper

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Kolkata’s East–West Metro tunnels had to pass beneath the operational Esplanade station on the North–South line. Investigations revealed redundant steel joist piles left from temporary road decking during the station’s construction. The piles fouled the planned tunnel alignment, and the earth-pressure-balance TBMs could not cut through them. Removing them from the surface was impossible because they lay beneath the station’s base slab.<br /><br />The project team rejected hyperbaric cutting because of risks to workers and instead built a New Austrian Tunneling Method (NATM) pilot tunnel with branch tunnels to access and remove the piles. This was a challenging operation in Kolkata’s soft alluvial ground, directly below an active station. The pilot tunnel used short excavation rounds and fibre-reinforced shotcrete, with steel ribs, forepoling and face anchors where required. Numerical analyses informed the support design and predicted ground movement.<br /><br />Workers exposed the joists from the tunnel, cut them into sections and extracted them using hydraulic jacks. Afterward, the pilot tunnel was filled with grout and contact-grouted to eliminate voids.<br /><br />Because settlement could threaten the station and tracks, monitoring included settlement markers, extensometers, piezometers, inclinometers and optical targets, with close monitoring of track levels. Maximum surface settlement reached 7 mm, compared with a design estimate of 6.2 mm; settlement beneath the station base slab was 8.5 mm, below the predicted 10.33 mm. Differential settlement of the North–South Metro tracks was limited to 1 mm.<br /><br />The work caused no distress or disruption to metro operations. The case demonstrates that carefully designed excavation, execution and monitoring can address unforeseen obstructions in sensitive urban infrastructure. It also highlights the value of preserving records of temporary works for future projects.

Keywords

Kolkata Metro

East-West Metro

Esplanade station

tunnel obstruction

steel joist piles

NATM pilot tunnel

TBM alignment

soft alluvial ground

ground settlement monitoring

urban tunneling