Effect of compaction state on the soil water characteristic curves of sand-natural expansive clay mixtures

Proper understanding and interpretation of theperformance of sand-clay liners requires the consideration of its unsaturated behaviour. A cornerstone in the understanding of the unsaturated hydraulic efficiency is the soil water characteristic curve (SWCC). This study discusses the results of an experimental programme performed to investigate the influence of clay content and compaction state on the SWCCs of a proposed sand-natural clay liner. The proposed liner is a mixture of sand and natural expansive clay obtained from the city of Al-Qatif, Saudi Arabia. Experimental results revealed that the SWCC of sand-natural clay mixture is mainly dependent on the sand-clay composition and compaction state. The influence of clay content on the SWCC of sand-natural clay mixtures was marked by an increase in water retention capacity with increase in clay content ranging between 0% and 60%. This increase in water retention capacity was attributed to the formation of micropores due to the addition of clay. In addition, compacted specimens simulating optimum, dry-of-optimum and wet-of-optimum conditions showed two forms of SWCCs: unimodal and bimodal SWCC. The unimodal SWCC was observed to be dominant for sand-clay mixtures with low clay content (less than 5%) and for specimens compacted at wet-of-optimum conditions. The bimodal SWCC was observed for optimum and dry-of-optimum water content specimens. These forms of SWCCs were used to interpret mixture structure and pore size distribution arising from different clay content and compaction conditions.