The headrace tunnel (HRT) of ongoing Nathpa Jhakri Project in Himachal Pradesh, India is 27.4 km long with a diameter of 10.15 m. It is the longest power tunnel in the world and is being constructed in Higher Himalaya in a complex geological setting. The paper presents rock mass classifications based on rock structure rating (RSR), rock mass rating (RMR), rock mass quality (Q) and rock mass index (RMi) systems besides using rock condition rating (RCR) and rock mass number (N). As many as 685 tunnel sections covering more than 22 km length of the tunnel have been considered. An attempt has been made to develop correlations between RSR, RMR, Q, RMi, RCR and N for Himalaya. New SRF-values have been proposed for moderately jointed rocks experiencing high stresses and a new correlation has also been developed for this purpose. The paper concludes by developing new correlations for:(i) estimation of support pressure in blocky ground using RMi support recommendations and (ii) prediction of ground conditions based on joint roughness and alteration.