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  Challenges for NATM design of Grant Road Station in Mumbai Metro Line-3

  Grant Road underground station is being constructed at the core of densely inhabited part of Mumbai city with old and dilapidated buildings at surface. Because of space constraints, conventional cut & cover station is not possible. Therefore, for upline platform New Austrian Tunnelling Method (NATM) is proposed below buildings and for downline platform narrow cut-and-cover box is proposed below available road width. The geology of this area consists of 8-10m of alluvial soil and completely weathered to slightly weathered breccia rock underneath, having Uniaxial Compressive Strength (UCS) below 10 MPa. Excavation in soil is proposed using secant pile as water table is near to ground. Due to constricted space, there is only 4m rock column available between NATM and station box outer wall. This narrow rock column is to carry the overburden on tunnel and surcharge load from buildings at surface. Two-Dimensional Finite Element Method (2D FEM) analysis indicates that simultaneous excavation for NATM and cut & cover results in high stress concentration in rock-column, lower Factor of Safety (FOS) and higher ground settlement. Different construction options for tunnel and station are studied. Finally, it is found that higher FOS and lower ground settlement is achieved when NATM excavation starts after completion of cut-and-cover box till mezzanine level which provide confinement of rock-column zone.

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