A critical state framework for unsaturated soil.

A critical state framework for unsaturated compacted clay is proposed. Five state variables are included: mean net stress, deviator stress, suction, specific volume and water content. The proposed framework includes three equations to define isotropic normal compression states, three equations to define critical states and two equations to define a state boundary.
Support for the proposed critical state framework is provided by results from a series of controlled suction triaxial stress path tests on samples of unsaturated compacted kaolin. Unsaturated 50 mm diameter samples were produced by static compaction in 9 layers, prior to consolidation to virgin states at selected values of mean net stress and suction. Shear tests were conducted with a variety of different stress paths and drainage conditions, in a double-walled triaxial cell with independent computer control of cell pressure, pore air pressure and pore water pressure.
The experimental results indicated that all test paths originated on a unique isotropic normal compression hyperline and terminated on a unique critical state hyper-line. To a first approximation all stress paths traversed a unique state boundary that could be defined by the expansion of a yield surface with elliptical cross-sections at constant values of suction.
Possible applications of a critical state model for unsaturated soil include: providing a qualitative framework to aid understanding of unsaturated soil behaviour; guiding the choice of strength parameters to be used in conventional stability calculations; and providing a formalized mathematical stress-strain model for incorporation in a finite element program.