Design of a NATM crossover at Mumbai metro line 3

The construction of underground structures in metro rail projects is always challenging as the project works normally affect a large number of adjacent surface structures with stringent settlement criteria and protection requirements. Project implementation becomes even more complex when the overburden is shallow and the area of the cross-section is comparatively large. In the case of design and build metro rail projects, the Client provides a tender stage design to the Contractor's designer; this initial design is often based on limited data and does not consider the construction phase in detail. This is a significant challenge for the Contractor's designer as the new design has to consider all relevant parties and boundary conditions with respect to the compatibility of other parts of the project. This paper describes such a challenge at Mumbai metro line 3, where a crossover structure had to be implemented during the detailed design as a change of scope. The initially proposed crossover cavern was re-arranged during the detailed design. The main emphasis of this paper is to explain the revised crossover arrangement, for optimized construction feasibility, construction staging and geotechnical stability of the excavation. After the options presented by the Contractor's designer met the original objectives, the detailed design of the primary support and modified construction sequence was carried out in accordance with the principles of NATM design.