Influence of Fines Content on Unbound Granular Base Materials (UGB) under Cyclic Axial and Radial Stress

Abstract

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