Application of Observational Method in the Successful Construction of Underground Structures, Sardar Sarovar (Narmada) Project, Gujarat, India

The Sardar Sarovar (Narmada) underground powerhouse is located in Deccan Basalt flows in the lower Narmada valley in Gujarat state. These flows are intruded by dolerite dykes and sill. Basalt flows and dolerite rocks are considered good tunneling media for locating underground structures. Therefore, initial designing of supports was done considering good rock mass conditions. However, during construction numerous geotechnical problems were encountered necessitating review of support system. Rock falls and collapses were observed in tunnel sections passing through dolerite dykes and sills. Cracks were observed in the walls of the powerhouse cavern. During progressive excavations back analysis was done to know the causes of distress in rock mass and structures The ‘Observational Method’ adopted during construction resulted in the safe execution of structures by timely modification and installation of adequate support system