Assessment of the effect of stress anisotropy on tunnel deformation in the Kaligandaki project in the Nepal Himalaya

Abstract

Most of the analytical approaches that are available for assessing plastic deformation in tunnels that pass through weak, schistose, and foliated rock masses assume an isostatic stress state. However, in situ stresses are seldom isostatic in a tunnel passing through a varying rock overburden. The work reported here analyzed the effect of stress anisotropy on the magnitude of plastic deformation in the Kaligandaki headrace tunnel in the Nepal Himalaya, where extensive deformation monitoring plans were implemented during tunnel excavation. Recorded tunnel deformation, mapped geological information, lab-tested rock mechanical properties, and an approach reported by Hoek and Marinos (Tunn Tunn Int 32(11):45–51, 2000) were used to estimate rock mass parameters. The convergence confinement method (Carranza-Torres and Fairhurst, Tunn Undergr Space Technol 15(2):187–213, 2000) was used to assess the effective support pressure for the known tunnel deformations of 77 tunnel sections assuming an isostatic stress state. Numerical modeling was carried out to assess the effect of stress anisotropy on tunnel deformation. The analysis indicated that CCM overestimates the magnitude of tunnel deformation. This may be explained by the fact that CCM applies for a circular tunnel in the isostatic stress state, which is seldom the case. Actual measured deformations were calibrated using numerical modeling to develop equations that may be used to estimate the plastic deformation of tunnels that are subjected to stress anisotropy. However, it should be emphasized that the proposed equations are based on the data records for a single tunnel, so further validation will be needed using data records of other well-monitored tunnel projects.