Guidelines for TBM tunnelling in squeezing conditions – a case study

This paper describes a study undertaken to derive hazard scenarios for the excavation of the Kishanganga hydroelectric project headrace tunnel, Kashmir, India. The tunnel is to be driven in poor rock mass conditions (meta-sandstones, meta-siltstones and shales) with expected squeezing behaviour under average to high overburden. The excavation is to be carried out using a double shield universal tunnel boring machine (TBM), with concurrent installation of a segmental lining (SL). The highly variable rock mass conditions and the limited data available, due to difficulty of access to the site, required a detailed preliminary study of the most relevant critical factors such as tunnel overburden, in situ state of stress and rock mass properties. Increasing levels of detail were adopted once the significant hazard scenarios for TBM jamming or overloading of the SL in short- and long-term conditions were outlined. The scenarios were first derived by simplified design methods with the rock mass represented as an elasto-plastic continuum. Subsequently, and following the results of laboratory tests on representative samples, numerical modelling studies were performed with the rock mass being modelled as an elasto-visco-plastic continuum.