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Unbound granular base (UGB) materials are commonly placed as part of pavement foundations. These materials are generally designed to be in an unsaturated state in order to resist the dominantly cyclic loading. Therefore, the response of UGB materials under cyclic loading needs to be studied using the framework of unsaturated soil mechanics. This requires the measurement of matric suction evolution during cyclic loading, which is a major experimental challenge to be resolved. This paper presents the development of a cyclic triaxial testing system and associated experimental techniques that meet the challenges of testing UGB materials as unsaturated soils. The testing system enables the initializing of matric suction of a specimen and facilitates direct measurement of its evolution during cyclic loading. Two types of cyclic loading could be imposed: with constant cell pressure and with cyclic cell pressure. Selected test results of both types of cyclic loading were presented to demonstrate satisfactory performance of the proposed methodology. Furthermore, a detailed analysis of these selected test results was also undertaken to show the importance of matric suction for understanding the behavior of UGB materials under cyclic loading.

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The adoption of mechanistic concepts in pavement design offers the opportunity of analysing unbound granular base (UGB) materials in the framework of soil mechanics by incorporating the recent advances in technology. In the past, the influence of fines content on the deformation behaviour of UGB is generally carried out by uni-axial cyclic loading. However, cyclic loading from moving traffic will also induce cyclic lateral stress. Recent advances in triaxial testing facility made it possible to impose cyclic loading in both axial and radial direction. This study mainly focuses on the influence of fines content on deformation behaviour of UGB behaviour under cyclic axial and radial stress. The framework of unsaturated soil mechanics is applied to analyse the test result. The test results showed that matric suction can capture the influence of fines at early stage of cyclic loading. BACKGROUNDThe pavement in the field is usually loaded by moving wheel loads experienced simultaneous pulsing of vertical and horizontal stress components accompanying with rotation of principle stress component. The commonly using constant confining stress path (CCP) triaxial test cannot simulate this type of loading condition. However, the dual axes cyclic loading offers wide spectrum of stress path by cycling both the cell pressure and vertical deviatoric stress component. Such stress path loading tests can better simulate the actual field conditions since the confining pressure in the field also cyclic in nature.This study is focussed on unbound granular base (UGB) materials consist of well graded silt sandy materials. The existing design specifications for UGB materials allows limited amount of fines content. However, the maximum allowable fines content in the UGB materials is still a debatable issue due to inconsistent research findings in the literature. Certain amount of fines content are required for UGB materials as fines assist the "binding" of particles (Hicks 1956). He suggested that fines content should be ranged 5% to 12% in the UGB materials as more than 10% fines content has adverse affect when exposed to freezing and thawing. Cote

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