Stability analysis of tunnel composite liner using convergence-confinement method and support capacity diagram – A case study of the Notog railway tunnel in Indonesia

This paper presents the findings of a case study of the stability of a tunnel composite liner using the convergence-confinement method (CCM) and a support capacity diagram by means of 2-D numerical analysis. The height of the loose zone and the induced loads exerted on the composite liner from the numerical analysis are compared to the empirical calculation proposed by Terzaghi’s rock load theory. The case study is taken from a recently completed double-track railway tunnel located in Notog district, Central Java, Indonesia. The numerical result shows the loose zone height is 15 m or roughly 1.5B, where B = tunnel width, giving rock load of 32 ton/m² and composite liner FoS of 2.4. This is within the estimates obtained from Terzaghi’s rock load theory, which amounts to 12-21 m, giving the rock load of 25-46 ton/m² and corresponding composite liner FoS of 3.1 to 1.7. It is also observed that each supporting element in the composite liner can sustain the induced loads due to excavation, with FoS well beyond 2.0. Compared to the supporting capacity of each supporting element, the loads carried by the steel-sets are significantly lower than those carried by the shotcrete.