Effect of spatial variability on undrained behaviour of tunnel with improved surroundings

Cement treated ground is extensively used to limit ground disturbance during tunneling in soft soils in Singapore. The cement treated columns are installed in an overlapping fashion to form a circular surrounding which resists the deformation during excavation. In practical design, the cement treated ground is usually assumed to be homogenous and elastic-perfectly plastic although it is highly spatially variable and exhibits significant strain softening and hardening behaviours. In this study, random finite element method (RFEM) is used to account for the effect of spatial variability of unconfined compressive strength of cement treated soil. The complex constitutive behaviour is simulated using the Cohesive Cam Clay (C3) model which is developed to describe the complex hardening and softening constitutive behaviour. The critical state is defined as the moment when volume loss suddenly increases and the critical stress release ratio is marked. A parametric study was conducted to find the relationship between quality control parameters and equivalent strengths. A rational guideline on design strength is provided based on the probabilistic distribution of equivalent strength