Influence of matric suction on resilient modulus and CBR of compacted Ballina clay

Road infrastructure is often built above the groundwater table. The materials used are usually compacted and thus generally remain under unsaturated conditions throughout their service life. This paper presents experimental results that highlight the influence of matric suction on the resilient modulus (MR) and California bearing ratio (CBR) of a compacted subgrade Ballina clay (typically found in NSW, Australia). The soil specimens were prepared in a range of water contents and dry unit weights, tested using a series of repeated load triaxial tests and CBR tests, and the associated matric suction was measured using the filter paper method. The tests were complemented by the study of the macrostructure of the compacted specimens using X-ray computed tomography (CT). Test results show that there are intimate relationships between the soil suctions and resilient modulus as well as CBR on the compacted clay at different moisture contents and that both parameters can be defined empirically through matric suction. A linear trend was established between matric suction and MR with a high coefficient of correlation of 0.99. CT scan results reveal that increasing soil moisture increased the inter-pores volume and the aggregations became more compressible and present probable matrix-dominated macrostructure during compaction. While soils compacted at the dry side of optimum moisture content yield distinct aggregations owing to the flocculation and aggregation of soil structure.