Pre-injection versus post-injection grouting – a review of a case from the Himalaya

Safety and stability are important issues in underground excavations. Making tunnels water tight plays an important role in this respect. In general, most of the rocks are significant barrier against groundwater movement and have very low porosity, with the exception of certain young sedimentary and volcanic rocks. However, existence of discontinuities in the rock mass increases the permeability. The Himalaya rock mass are in general deeply weathered, faulted and fractured. This leads to an increase in the rock mass permeability in the region. If the purpose of tunnel excavation is to transport (convey) water, two issues must be addressed during tunnel excavation. The first one is to secure tunnel against instability. The second is to make tunnel water tight so that risk of water loss through unlined tunnels is kept to a minimum during operational life of the project. This paper discusses the use of pre-injection and post-injection grouting in controlling water leakage from a shotcrete lined/unlined headrace tunnel used to convey water for hydropower generation. Pre-injection data are evaluated and strength and weaknesses of the pre-injection and post-injection grouting are discussed. The effectiveness of grouting in controlling construction costs and completion time are also briefly highlighted.