Use of unsaturated small-strain soil stiffness to the design of wall deflection and ground movement adjacent to deep excavation

Small-strain soil stiffness is an important parameter for the design of wall deflection and ground movements around deep excavations in urban cities. However, the use of unsaturated small-strain soil stiffness in the design of excavation is rarely reported, although the ground condition often becomes unsaturated due to de-watering. The objective of this technical communication is to report and illustrate the effects of suction-dependent small-strain soil stiffness on the design of wall deflection and ground movements due to a 15-m deep excavation in unsaturated soils in Tianjin, China. A small-strain stiffness model for unsaturated soils incorporated into the Hardening Soil-Small Strain (HSS) model was adopted. Two analyses, with and without considering suction-dependent small-strain soil stiffness, were carried out to provide design and construction guidelines to control the progress of excavation. By comparing the measured data with the two different analyses, it is clear that the analysis without considering unsaturated small-strain soil stiffness significantly overestimated the deflection of pile wall by 85%, ground surface settlement by 55% and basement heave by 40%. On the contrary, by considering unsaturated soil stiffness, more accurate predictions were obtained to save construction time and to reduce construction costs.