Compressibility and shear behaviour of saturated and unsaturated lateritic clay rich in sesquioxide

Abstract

Lateritic soil covers about 15% of the area in the world and is predominant in tropical areas. It is extensively used as construction materials in many geotechnical projects. Compared to other soils, lateritic soil is richer in oxides of iron and aluminum (sesquioxide). The sesquioxide is well recognized to enhance the formation of soil aggregates. Lateritic soil is less studied, and effects of the sesquioxide- induced aggregates on the microstructure, compressibility and shear behaviour of saturated and unsaturated lateritic soil have not been well understood. Most of the existing experimental results of lateritic soil are not interpreted within the critical state framework. Moreover, the influence of thermal cycles on behaviour of lateritic soil have not been previously investigated. The objective of this study is to understand the mechanical behavior of a lateritic clay from Nigeria at saturated and unsaturated states within the critical state framework. Series of saturated tests were first conducted to study the compressibility and shear strength of compacted, intact and reconstituted specimens. Thereafter, suction effects on volumetric and shear behaviour of the compacted specimen were investigated using suction-controlled triaxial apparatus. Finally, the volume change under thermal cycles was studied in a temperature-controlled oedometer. Moreover, the soil microstructure was investigated using mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) techniques. At saturated state, the compacted lateritic (LT) specimen is found to be less compressible than a completely decomposed volcanic (CDV) and granitic (CDG) soils by 18% and 36% respectively, even though LT has a higher clay content than the latter two soils. The highest critical state friction angle is observed in LT, estimated to be 42°. The low compressibility and high friction angle of LT is due to its high sesquioxide contents. Many clay particles in LT form large aggregates, and hence particle/aggregate interlocking is enhanced. Furthermore, the compressibility of compacted (CL) and reconstituted LT (RL) specimens is found to be 90% larger than intact LT (IL). The difference is mainly because the aggregates in IL were 90% larger compared to CL and RL specimens. The compressibility of LT is over-estimated by about 3 times using a widely-used empirical relationship between plasticity index (PI) and compression index. As suction increased from 0 to 150 kPa, the compressibility of compacted LT increased by about 100%, mainly because the sizes of inter-aggregate pores increased due to shrinkage of aggregate. The critical state friction angle appeared to be independent of suction. The contribution of suction to shear strength is found to decrease with increasing net stress, mainly because the LT undergoes high desaturation rate as stress increases. As a result, the inter-aggregate pores have low degree of saturation and the contribution of water meniscus to shear strength is low. The normally consolidated LT specimens showed accumulation of irreversible contraction under cyclic heating and cooling. The thermal strain of LT is over-estimated by about 3 times using a popular semi-empirical equation, which was proposed based on experimental results of non-lateritic soils. The discrepancy is because different from other soils, LT contains goethite and hematite with low thermal expansion coefficient.

Date of Award	2019
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology