ABSTRACT
Underground and tunneling works have been recently implemented in infrastructures across Thailand. The Phayao tunnel, which is one of the challenging sections, is partly excavated in sheared flysch rock masses. This leads to major tunnel deformations and instability encountered during the construction stage. This paper presents the characteristics of the geological and geotechnical conditions in the flysch rock masses of the Phayao tunnel. The unexpected changes in various conditions along the tunnel result in significant variations in the quality of the rock mass and its evaluations during the design and construction stages. The selected monitoring sections of convergence measurements of the primary support and numerical analysis results are presented and discussed. The observations and experiences gained from the construction of this tunnel provide a basis for references and useful information for the design and construction of similar engineering projects for new tunnels in heterogenous flysch formations in this region.
