Negotiating through geological hazards of deccan trap for NATM tunnel

NATM tunneling is an art rather than a precise set of guidelines to be adhered to while executing the tunnel. Deccan trap geology is considered a safe rock mass to mine as a NATM tunnel. However, it presents many surprises that are not identified even through a systematic investigation. The shear zones, flow joints, and contact joints imposed certain constraints that were inherent in selecting the tunnel alignment. The twin tunnels for six lane traffic were planned for a length of 1.762 km to establish proper connectivity. Risk analysis was carried out in the initial stages to understand their impact. The FEM Analysis was adopted while analyzing these problematic areas and actual behaviour manifested through instrumentation data is presented. Requisite redressal measures were planned accordingly in a broad sense. The paper presents some of the complexities of the geological setup and the measures evolved while negotiating these zones.