A case study for geo-technical investigation for assessment of subsurface conditions for tunnel no. 2 of katra quazigund rail link project

Tunnel No. 2 between km 33.095 and km 38.450 is located on the right bank of Paikhad nallah, where the thrusted contact zone has been encountered between sandstone and the dolomitic limestone (Reasi thrust), in the rugged mountain range of the Katra-Reasi area of the sub-Himalayan and Lesser-Himalayan region. The data obtained from drilling, geophysical surveys, geological mapping and their interpretation with respect to the tunnel alignment indicate that Tunnel No. 2 will mainly pass through highly jointed and fractured dolomitic limestone. The existence of shear zones at many places may cause problem during tunnelling operations. The occurrence of water in limestone may pose problem of heavy dripping during the tunnelling operations. Problem of ground water flow may be aggravated due to existence of ‘Reasi Thrust’ in the area, which is also anticipated to be water charged. As anticipated, the progress of tunnelling work in Katra end of T-2 tunnel and Reasi end of T-5 tunnel is getting hampered because of the presence of saturated shear zone in crushed domomitic limestone and due the effect of Reasi thrust. Similar problems may be expected at the Reasi end of tunnel T-1. To overcome this problem, accurate delineation of saturated shear zone with the use of Electrical Resistivity Imaging along the alignment and also across the alignment at a few locations and detailed hydro-geological survey to identify the source recharging the sub surface flow through the saturated shear zone and the anticipated discharge are required to be carried out. The additional study shall be helpful in designing drainage/diversion of subsurface water and effective and economical grouting method, which will help in achieving faster and steady progress of tunnelling.