Field investigation of deformation characteristics and stress mobilisation of a soil slope

Stress mobilisation and deformation of a slope are important for engineers to carry out reliable design of retaining systems. However, most case histories reported mainly on the response of pore-water pressure (PWP), whereas knowledge about the stress-deformation characteristics of slope is limited. In this study, a saprolitic soil slope was instrumented to monitor not only the responses of PWP but also horizontal stress and horizontal displacement. To assist in the interpretation of field data, a series of laboratory tests was conducted to characterise the volume change behaviour of soil taken from the site, under the effects of both net stress and suction. During a rainstorm event when positive PWP built up, a remarkably large displacement of 20 mm was recorded between 5.5 and 6 m depths, and the top 5 m of the slope exhibited translational down-slope movement. This caused an increase in effective horizontal stress by 350%, which reached a peak value close to 40% of an effective passive stress. During the subsequent dry season when suction was recovered, an up-slope rebound of 10 mm was recorded. Comparison of field and laboratory data reveals that the rebound was attributed to suction-induced soil shrinkage. This rebound led to a decrease in the effective horizontal stress previously built up during the storm event.