Comparisons of weathered lateritic, granitic and volcanic soils : compressibility and shear strength

Abstract

Lateritic soil is rich in oxides of iron and aluminum (sesquioxide). The effects of these oxides on its mechanical behaviour are not well-understood. In this study, the compression and shear behaviour of a lateritic (LAT) soil was tested and compared to those of two other weathered granitic and volcanic soils (CDG and CDV) for the first time within the critical state framework. The mineralogy, chemical compositions and microstructure of these three soils were also measured by x-ray diffraction (XRD), x-ray fluorescence (XRF) and scanning electron microscopy (SEM) respectively. It is found that only LAT contains goethite, while other minerals are similar in all the soils. The content of iron and aluminum oxides in LAT is about 40% higher than that in CDV and CDG. LAT is found to be less compressible than CDV and CDG by 18% and 36% respectively, even though the former has a higher clay content than the latter two soils. The lowest compressibility of LAT is mainly attributed to its high content of sesquioxide, which induces the formation of more aggregates. The LAT has more intra-aggregate pores and hence less inter-aggregate pores than CDV and CDG. A smaller volume of inter-aggregate pores of LAT results in a lower compressibility. It is found that the critical state friction angles of LAT, CDV and CDG are 42° 33° and 38° respectively. The highest friction angle of LAT is likely attributed to the content of large particles and presence of goethite. Due to the high sesquioxide contents of LAT, many clay fines also form large aggregates leading to a more granular microstructure. Consequently, the number of particle contacts is increased and therefore enhancing particle interlocking. Goethite mineral found in LAT could enhance inter-aggregate interlocking through its contiguous singly coordinated OH groups.