Engineering geological characteristics of the underground surge pool cavern: a case study, India

For better rock mass characterization, 3D engineering geological mapping and in-situ testing was carried for the heading portion of surge pool of Palamuru Ranga Reddy lift irrigation scheme lift-II. The direction of the longest axis of the surge pool cavern was finalized based on the in-situ hydrofracturing testing inside the borehole and aligning the appurtenant structures of pump house and surge pool cavern on the ground. Rock types mapped were grey and pink granites belongs to the Peninsular Gneissic Complex of Archaean age. The assessment of Tunnel Quality Index ‘Q’ for the exposed granitic rock mass was done based on the information available of the rock joints and their nature, 3D geological mapping and in-situ stress measurement. The rock mass quality (Q) is related with the ultimate support pressure requirement. Excavation Support Ratio (ESR) as given in the ESR updated classification standard of NMT Q-system is applied to 1.0 for this cavern. On the basis of NMT Q-system chart and site geological characteristics, support system is recommended and its efficacy is assessed.