Equivalent Strength for Tunnels in Cement-Admixed Soil Columns with Spatial Variability and Positioning Error

Abstract

Cement-admixed soil columns are often used to strengthen the surrounding soft soil for enhancing tunnel stability. Such improved soil surrounds possess high spatial variability and heterogeneity in the strength and stiffness properties. In this paper, three-dimensional random finite-element analysis is conducted to compute the equivalent strength and stiffness for tunnels constructed in overlapping cement-admixed soil columns surrounded by soft clays. The heterogeneity and spatial variability of the improved soil surround are studied by varying coefficient of variation, considering anisotropic scale of fluctuation in mean strength and stiffness, as well as varying positioning error arising due to the off-verticality in cement-treated columns. Finally, strength reduction factors are computed for obtaining the equivalent strength of tunnels in such spatially variable improved soils. The proposed range of strength reduction factors can be used in numerical analysis by engineers in practice to compute the appropriate values of equivalent strength and stiffness of homogeneous improved soil surrounds around tunnels. The influence of varying permeability of improved soil surround and time of excavation of tunnel opening on the equivalent strength is also studied and was not found to be significant.