A constant cell volume triaxial testing technique for evaluating critical state parameters of unsaturated soils

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Abstract
SUMMARY: This paper describes the development of a constant cell volume triaxial compression testing machine. This apparatus features a method for applying the deviator load to a soil specimen contained in a conventional triaxial testing machine without altering the volume of the fluid jacket. The technique utilizes the stiffness or compliance of the closed hydraulic system to monitor the volume changes taking place in the soil sample during shear failure. This information is obtained from the pressure changes taking place in the closed fluid system.The behaviour of a prototype apparatus of this type in testing a normally consolidated soil sample is analysed in some detail. These results, and others not discussed in the paper, demonstrate that the method has considerable scope in the rapid acquisition of quantitative data on the strength and volume change behaviour of unsaturated soils. A mathematical model of the system is developed and used to evaluate its performance and exhibits good agreement with experimental observations. The model also highlights how the provision of a variable compliance facility extends the range of state paths that can be achieved by the system.