Stability of Tunnels in Cement-Admixed Soft Soils with Spatial Variability

Abstract

At present, there is no well-accepted method for the design of tunnels built in cement-admixed soils. Moreover, spatial variability and heterogeneity in strength have been reported for such treated soils. This paper presents a two-dimensional plane-strain study of tunnel stability in cement-admixed soft soil surrounds considering spatial variability. The influence of spatial variability was studied by varying the coefficient of variation and scale of fluctuation in the mean strength and stiffness of improved soil surround using random finite-element analysis. The critical tunnel support pressures from the random analyses were compared with results of deterministic analyses to compute appropriate design strengths and strength reduction factors for varying spatial variability in treated soils. By using appropriate values of design strength or the strength reduction factor, equivalent strength and stiffness can be prescribed for homogeneous improved soil surrounds in deterministic finite-element analyses. The worst scale of fluctuation was also investigated and found to be approximately two-thirds of the thickness of improved soil surround.