Analysis of Stress Induced Deformation and Evaluation of Performance of Tunnel Boring Machine in Extraordinary Geological Conditions Encountered During Excavation of Longest Head Race Tunnel in Himalaya: A Case Study

The paper reviews major geological problems encountered during construction of the longest Head Race Tunnel (HRT) of length 31.5 km in the hydropower projects of India through Tunnel Boring Machine (TBM). Major tunneling hindrances encountered in the TBM face include rock bursting, shear zone with groundwater ingress, high cutter consumption due to abrasive nature of rockmass and frequent detachment of rockmass above cutter head of TBM causing overbreak. The remedial measures adopted to successfully overcome these challenges are discussed. The paper also back analyzes magnitude of in-situ stresses in rock bursting zones using numerical modelling to determine role of depth of overburden and field stress ratio in causing rock bursting. The result indicates that yielding zones increase with depth of overburden and decrease in field stress ratios. TBM performance was evaluated using QTBM (Barton, 2000) parameters and the observed penetration rate (PR) during boring in quartzite, granite gneiss and chlorite schist. An empirical relationship developed between QTBM and PR indicates exponential increase of penetration rate with QTBM under stress induced conditions.