TIME-DEPENDENT BEHAVIOUR OF EXCAVATION SUPPORT SYSTEM IN SOFT CLAY

Abstract

An elastic-plastic model taking consolidation of the soil mass into consideration has been developed to analyse the time-dependent behaviour of excavation support system in soft clay due to dissipation of excess negative pore pressure. The model selected to describe soil behaviour in this study is an elastic-plastic model using Mohr-Coulomb yield criterion with associated flow rule expressed in terms of effective stresses. The use of effective stress-strain parameters associated with dissipation of excess pore pressure represents a significant improvement over existing analyses based on undrained parameters. The model can simulate actual excavation sequence as well as uneven excavation, and predict the magnitude and rate of change of deformation of the supporting system and strut or anchor loads. The analytical results predicted for two instrumented excavations show good agreement with field measurements of lateral deformation, base heave and strut/anchor loads. The rate of increase in maximum lateral deformation for strutted excavation in soft marine clay in this study is 1.5-3.0 mm/day during excavation and about 0.2-0. 7 mm/day in between consecutive stages of excavation. Heave development with time at the final stage of excavation is predicted to be about 4 to 6 mm/day which is consistent with field measurements. Studies using the proposed consolidation analysis showed that judicious preloading reduces bending moment in the retaining wall as well as lateral deformation.