Excavation of the 13.5 m diameter and 152 m deep open to sky surge shaft of Mangdechhu Hydroelectric Project through adverse geological conditions was one of the major challenges faced and successfully tackled by the Mangdechhu Hydroelectric Project Authority. The Surge Shaft was excavated through highly fractured, highly weathered to decomposed rock mass with low insitu strength. Owing to poor rock mass conditions and low values of engineering parameters such as compressive strength and shear strength, excavation of the shaft was cautiously carried out by mechanical means. Adverse geological conditions necessitated extensive grouting along with continuous steel rib support (ISMB 250 at spacing of 0.5 m c/c). Behaviour of the rock mass strata during the course of excavation was monitored by the aid of systematic geotechnical instrumentation programme. By adopting appropriate excavation methodology, adequate and optimum support system, insitu treatment of rock mass and continuous monitoring of rock mass behaviour, excavation of the 152 m deep surge shaft was successfully completed in 16 months.
