Stability evaluation of draft tube tunnels of Palamuru Ranga Reddy lift irrigation scheme lift-Ⅲ, Telangana state, India

Abstract

Geological and geotechnical evaluation for the 7–9 m diameter and 100 m length, 10 numbers of parallel draft tube tunnels were carried out. The draft tubes are constructed to connect the underground pump house and surge pool for lifting the water. Due to adverse geological conditions and the complex geometry of the structure, a sequential excavation method and controlled blasting were adopted, which minimized the risk of instability during the excavation of the draft tubes. To classify the rock masses and recommendation of support, detailed engineering geological investigations were done. The rocks encountered during the excavation of these tunnels are granites/granitic gneiss which are traversed by mafic dykes of Dharwar Craton. Prominent three to four joint sets were recorded during geological mapping and rock mass shows W-I to W-II weathering grade. For the determination of principal stresses, an in-situ hydrofracturing test was conducted in the borehole. Q-values were determined based on the rock joints and their characteristics, 3D mapping of geological units and stress measurement conducted inside the borehole. The Q-system provides a quantitative assessment of rock mass stability in jointed rock for underground tunnel design and construction. Good stability is indicated by high Q-values, while poor stability is shown by low values. Based on the “Q” system, the entire lengths of the tunnels were characterised as poor to fair rock mass categories. Wedge analysis was conducted along the tunnel alignments using RocScience Unwedge software and accordingly, stability was checked. Engineering geological investigations identified potential geotechnical issues, and corresponding engineering solutions were recommended. A support system was designed based on the Q-system and site-specific geological conditions.