Comparison of predicted and measured performance of a large cavern in the Himalayas

Abstract

Detailed investigative and performance monitoring studies have been carried out at the site of an underground powerhouse cavern in the Himalayan Region of India. The updated empirical (Q-system) and numerical (UDEC-BB) approaches, applied for predicting the behaviour of the rock mass prior to the construction of the underground cavern (20 × 49 × 216 m), have been compared with the instrumentation data from multi point borehole extensometers (MPBX). Upon completion of the first numerical excavation step (20 m span arch), a relatively high stress-strength ratio and a maximum deformation of approx. 18 mm was predicted in the roof of the cavern. MPBX readings in the arch have indicated maximum deformations in the range 19–24 mm with the 20m span fully excavated. The results of numerically excavating the cavern to its full height (49 m), have indicated maximum deformations in the range 43–45 mm in the walls of the cavern. Upon completion of the ongoing benching operations, the measured performance from the walls of the cavern will be available for comparison with the existing numerical results. Permanent rock support in the cavern consists of systematic bolting of alternating lenghts and mesh reinforced shotcrete S(mr). However, rock support design recommendations based on the Norwegian Method of Tunnelling (NMT), which employs wet process fibre reinforced shotcrete S(fr) instead of S(mr), have been numerically tested and verified.