Engineering geology plays a key role in planning, design and construction of underground cavern storage facilities. The geological assessment of the site is made not only during investigation but also corroborated and updated during construction. In case of cavern storage project, relatively smaller dimensioned water curtain tunnels, which lie above the main storage cavern, are always excavated in advance before excavation of the main large storage caverns. This water curtain tunnel acts as a pilot tunnel and helps assessment of the likely geological setting in advance. As part of the water curtain system, large numbers of boreholes are drilled from the water curtain tunnel and are hydraulically charged to maintain a saturated rock mass surrounding the storage caverns, revealing additional information about the geological and hydro-geological setting above the cavern. Thus a 3D geological (structural) model of the site is updated through an active design process with focus on geological hot spots intersecting the cavern. During construction of caverns, a careful approach is adopted along these hot spot segments. These adverse rock mass conditions are probed, assessed and supported in accordance with special design supports. The geological model is further updated during excavation of cavern heading and becomes useful for excavation of benching, for verification of the rock support in heading and in further optimisation of rock support during bench excavations. The paper presents two case studies of underground storage cavern sites, where in the active design process has been adopted for geological assessment of both the sites.
