Study of the stability of the 90 year old Millswood Underpass protected by revetment walls in unsaturated Keswick Clay

William S. Kaggwa, Evan W Jones, Yih H Koh, Bradley J Tiver and Marcus A H Wong

In recent times, there has been much debate regarding the use of unsaturated soil mechanics for the design of earth retaining structures in Adelaide’s semi-arid conditions. The soil-water characteristic curve (SWCC) is the main design tool within this field as a means to incorporate the additional shear strength associated with total soil suction. However, the majority of research to date only incorporates matric suction. This paper is focused on adopting the more commonly measured total suction, and to more effectively capture the behaviour of Keswick Clay as osmotic suction is the dominant component of total suction. Using the Millswood Underpass as a case study, weather data over a 120 year period was used to examine the variations of the stability of slopes in unsaturated Keswick Clay. The reduction in stability due to precipitation-induced flooding, and water leakage due to buried services were also considered. This paper demonstrates the significance of total soil suction to the shear strength of Keswick Clay that is well-known to design consultants but rarely used in slope stability problems.