Sammendrag
A serious cave-in failure in a headrace tunnel in Buon Kuop, Vietnam is analysed using convergence confinement (CC-method) and 2D numerical modelling (Phase 2). Results from convergence confinement method show that the method is an excellent tool for parameter study to obtain the reasonable rock mass properties and the appropriate failure criterion. The results from the analyses indicate that the properties obtained by observing and classifying the material in the weakness zone may replace expensive in-situ tests during the pre-investigation. The results also indicate that non-linear failure criteria, such as the Hoek-Brown failure criterion, are most suitable for analysing the problem. Subsequently, the obtained rock mass properties and the recommended failure criterion are used for 2D numerical analyses, in which the cave-in is well indicated. Excavation and support measures are also modelled. Even though, the methods produce results that match relatively good with what happens in reality, there are some limitations concerning groundwater and the support properties that one needs to be aware of. These limitations are discussed.