Comparative Study of Laboratory Based Methodologies for Determination of the Resilient Performance of Clay Subgrades

Abstract

For the development of performance-based design for pavement foundations appropriate laboratory assessment of the underlying subgrade is required. To achieve this, assessment of the resilient behaviour (stiffness) and resistance to permanent deformation of the subgrade must be made, under realistic loading conditions. This paper compares data from variably confined repeated load tests (utilising the recently developed “Springbox” apparatus), simplified repeated load triaxial tests (using standard triaxial apparatus) and repeated load triaxial tests. Tests were undertaken on a series of remoulded clay subgrade materials for a range of water contents prepared between the plastic limit and a predicted long-term equilibrium value. Correlations of the resilient deformation (stiffness) measured in the various tests are presented, together with comments on their applicability for pavement design. Having accommodated the differences in the confining stress, acceptable comparisons
between the data from the various devices are achieved. However, the quality of the resilient data from the simplified procedures was affected by the resolution of instrumentation, producing better correlations at higher applied stresses where the stiffness approaches a constant value termed the “stiffness asymptote”. A correlation between stiffness asymptote and sample plasticity is also presented. To conclude, the suitability of the various test devices is discussed and outstanding design/sample preparation issues considered. Significantly this includes the need to predict and manufacture samples representative of long term (equilibrium) subgrade water contents.