Seri nala fault zone
587 m excavation under Seri Nala took the tunnel more than 4 years from the time it was encountered on 13 December 2011 to 03 January 2016. The rest of the 8.5 km tunnel took the same time. It is the largest shear zone crossed by any highway tunnel in the world.
The fault zone is situated between Chainage 1+887 to Chainage 2+474 where tunnel supporting system was required to be redesigned due to the shear zone with heavy ingress of water up to 140 litres/sec. The anticipated fault zone was between Ch 2200 – 2800 m, with contact plane of Quartizitic Schist and Migmatites. But during the actual excavation, the rock types encountered are Quartzitic Schist and Phyllitic Quartzite.
Several additional methods of tunnel supports as well as targeted consolidation grouting of the surrounding environment near the tunnel.
The other remedial measures adopted were: dewatering pipes installed single and multistage grouting using rockbolts or pipes of 76 mm, sequential excavation with top heading division into small panels and immediate support using fibre shotcrete, wiremesh and rockbolts, single/double pipe roofing, PU grouting, pilot tunnel and temporary invert of top heading.
Based on the unfolding ground condition and deformation trend, the design convergence fixed for Class 4 rock at 80 mm had to be changed to 150 mm and finally to 200 mm and for Class 5 rock, increased to even 250 mm to avoid re-profiling.