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

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Abstract:

Kolkata East West Metro is a predominantly underground corridor traversing the most congested part of the city. The twin tunnels pass below an existing North-South Metro underground Station at a depth of 8.593 m from base slab. It was discovered that there was a temporary steel decking for maintaining vehicular traffic during construction of the underground station and parts of some huge redundant steel joist piles were still remaining under the base-slab of existing station which would obstruct the alignment of twin tunnels of new Metro line being bored with EPB tunnel boring machines. Those machines were not capable to cut such steel joists and it was obligatory to remove those joist piles before tunneling. As the steel joist are existing below base slab of an operational metro station, there was no scope for physical extraction of those piles from surface. Proper methodology was devised for removal of those piles by construction of pilot tunnel by New Austrian Tunneling Method [NATM] which was a unique challenge in soft alluvial geology of Kolkata. The pilot tunnel was subsequently suitably filled up for boring of main tunnels. Due to sensitive location of the work, proper measures for minimizing ground settlement was adopted to avert any distress to the structure of the existing metro station as well as metro tracks. Elaborate instrumentation schemes for monitoring ground movement parameters were undertaken during NATM pilot tunnel as well as subsequent work of main tunnels. The geotechnical parameters and design issues associated with this soft ground NATM pilot tunnel and subsequent passage of main tunnels and measures adopted to restrict ground movement are discussed in this case study. The instrumentation schemes for ground movement are also elaborated and the field results are compared with predicted values.

BISWANATH; Mr. BISWANATH DEWANJEE

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