Probabilistic stability analysis of qanat tunnels in c-φ soil considering soil spatial variability

Some newly built high-speed railways inevitably pass through the qanat region, which may potentially cause the failure of the qanat tunnels upon embankment and traffic loadings. Thus, it is of great importance to evaluate the stability of the qanat tunnels under surcharge for the design of railways. In the context of the Tehran–Isfahan railway construction in Iran, the soil properties obtained from a series of site investigation and laboratory tests show an important spatial variability. To better consider the spatial variability of soil strength parameters, a series of random adaptive finite element limit analysis (RAFELA) is performed in this study to investigate the stability of the qanat tunnels in the cohesive-frictional soils. The emphasis is placed on the effects of the coefficient of variation and vertical spatial correlation length of the strength parameters (i.e. soil cohesion c and effective fiction angle φ) and the qanat tunnel depth (H/D). Results show that considering the variabilities of both c and φ is more unfavourable to the qanat stability compared to the cases with only one strength parameter (c or φ) variability being considered. Detailed stability evaluation charts are obtained to assess the probability of failure of the qanat tunnels under different conditions.