Mechanics of unsaturated soils and applications

Professor Daichao Sheng

July 13, 2017EH Davis Lecture

This biennial lecture commemorates the work in geomechanics by Professor Edward Hughesdon Davis. Professor Davis was one of the pioneers of geomechanics in Australia. He perceived that progress and understanding of the theoretical basis of geomechanics would only come if consistent, sound, but simple models of soil behaviour were used. The two topics that were the central focus of his research were the application of the theory of elasticity to foundation deformation and the theory of plasticity to stability. He also made a major contribution to the theory of consolidation of clay soils. Ted Davis was always extremely conscious of the link between theory and practice, the relationship between the idealised and the real material, and the engineering significance of his work.

Unsaturated soils in a narrow definition refer to soils that are partially saturated with liquid water, or in a broader definition to porous materials that contain more than one fluid in their pores. They naturally occur in arid and semi-arid climates like Australia’s and are also common in constructed grounds like compacted fills. These soils can inflict significant damages to structures they support when subjected to environmental actions like wetting and drying. Principles in classical soil mechanics based on Karl Terzaghi (1883-1963) do not directly apply to unsaturated soils. Understanding their behaviour is therefore crucially important for designing building foundations, dams, highways, earth slopes, tunnels and waste containment facilities. The lecture will outline the fundamental principles that possibly govern the volume change, shear strength, water retention and general hydro-mechanical behaviour of unsaturated soils. It will discuss the most recent advances in these aspects. The lecture will finish with some examples of practical applications of unsaturated soil mechanics, including the deformation and stability analysis of a high-speed railway embankment and an airport pavement on deep fill foundation.