Resolution of Critical Geological Issue by Seismic Tomography in a Northern Himalayan Hydro Tunnel

Geotechnical exploration in hydro power investigation plays a very vital role. Conventional exploration techniques like drilling/drifting are expensive and time- consuming. New age geophysical techniques of the investigations are fast, economical and capable of providing finer details and continuous coverage of the investigation area. Parbati-II Hydro Power Project, located in the Northern Himalaya, is presently under construction. Out of 31.5 km long HRT, 8.5 km is being bored through TBM. At chainage 4118 m huge water ingress with silt was encountered and tunnel construction was stalled for a considerable period. As per geological assessment, a fault zone was apprehended ahead of TBM face. An innovative approach of seismic tomographic scanning was utilized to define the accurate geometry and location of the apprehended fault zone. Four inclined drill holes in upward and downward directions were made through a side tunnel covering the area of interest. Strategic planning and implementation of seismic tomographic scanning of about 4200 m3 volume led to accurate assessment of the apprehended fault zone geometry and rock mass conditions. The most important finding of the survey was identification of good rock mass quality at invert level and this helped in the successful advancement of TBM, which was stranded for several years. In the present paper, details of field measurements, data processing and results are discussed.