ABSTRACT
Creation of underground space in a given topography and geology comes with its own challenges; both foreseen and unforeseen. There is no ‘one-size-fits-all’. In the current case of ‘Hutatma Chowk’ metro station which is part of Mumbai Metro line 3, a fully underground public transport system, executing New Austrian Tunnelling Method (NATM) works at a shallow crown level of around 16.0 m within the heritage precinct was the challenge. The station size is around 233 m x 31.0 m with an average rail level of 23.25 m situated on a busy narrow street of Mumbai. Due to high dense habitation, the station was planned to be constructed with a combination of Cut & Cover and NATM with further integration by means of 8 adits. Adding to this complexity, the station was situated in a heritage precinct with over 100-year-old functional buildings wherein the NATM works were executed with innovative micro blasting technique as the geology is predominantly presented with rocky strata ranging from Grade I to Grade III. In view of shallow overburden for NATM section, additional steel supports were also installed in the arches of heritage buildings. More than 10,000 controlled micro blasts with low charge and long delays were taken in the heritage precinct by limiting the vibration intensity to 2.54 mm/sec (Peak Particle Velocity) with further excavation carried out using drum cutter. A 1.5 m deep ‘free face’ around the overt portion was created prior to micro blasts in every cycle to limit the intensity of vibration due to blasting. This article presents the detailed construction sequence and methodology of the innovative approach adopted with the combination of ‘micro blasting with free face’ and ‘excavation with drum cutter’ which led towards safe and successful completion of ‘Hutatma Chowk’ metro station in heritage precinct.
