Estimation of deformation behaviour of deep excavation in under-consolidated soft ground

The under-consolidated (UC) state affects the deformation behaviour of deep excavation in soft soil and poses potential risk to the safety of adjacent facilities. However, deformation models for deep excavation in UC ground have not been well investigated. This paper presents a series of numerical analyses on the deformation characteristics of deep excavations in UC and normally consolidated (NC) ground, each retained by diaphragm walls with rigid struts and a bottom improvement layer. UC cases without excavation activities (simplified as consolidated cases) are included for comparison. The modelling results showed that, with the lateral constraint of the inner rigid support system, the UC ground resulted in only a slight increase in lateral wall deformation but a significant increase in ground settlement as compared with NC ground. The UC ground with lower initial average consolidation ratio, thicker surface fill, higher permeability and longer construction period produced greater wall deformation and ground settlement during excavation. An empirical method is proposed to estimate the settlement envelope for deep excavation in UC ground as the superposition of two parts: settlement induced by excavation activities and settlement induced by residual consolidation with consideration of average consolidation ratios before and after excavation.