Classification, strength and water retention characteristics of lateritic regolith

Abstract

Regolith materials beneath lateritic bauxite in the Darling Range of Western Australia mostly consist of highly weathered granite and dolerite. These materials become the substrate for establishment of vegetation after bauxite mining and their physical properties affect plant growth. Disaggregation of quartz grains, volume loss, redistribution of clay and silt forming structural bridges and cementing of the fabric by rigid precipitates of iron oxides are responsible for the diverse strength and water retention characteristics of these regolith materials. A classification system has been developed for these materials and relationships of material class with water retention, strength, bulk density and field texture have been determined. Quartz rich materials have loamy sand to sandy clay textures and weak to moderately developed lenticular pedality. Clay rich materials have silty loam to silty clay textures and angular blocky pedality. Plant available water (PAW, pF 2 to 4.2) in these materials occupies 27% to 34% of total porosity compared to 24% and 21% for granitic and doleritic saprolite directly above bedrock. Plant unavailable water (pF greater than 4.2) occupies 37% to 52% (quartz rich) and 46% to 61% (clay rich) of the total porosity compared to 62% and 70% for granitic and doleritic saprolite. Both dry and wet strengths of regolith materials increase with clay content. The strength of grain supported fabrics is more responsive to changes in water content and matric potential than are matrix support fabrics which is due to the interruption of clay–water–clay bonding of the matrix by the presence of quartz grains. Materials cemented by iron oxides have high dry and wet strengths, and their strength is almost unaffected by changes in water content or matric potential. The fabric of regolith material and particularly the pore size distribution determine the location of water menisci and thus particle contact forces. These interactions greatly influence the unconfined compression strength of the materials. The strength of regolith materials is more sensitive to differences in texture and the abundance of iron oxide cement than it is to differences in bulk density.