Volume change behaviour of a saturated lateritic clay under thermal cycles

Abstract

Many lateritic soils contain goethite and hematite, which have a low thermal expansion coefficient and also enhance the formation of soil aggregates. The influence of these minerals on the thermo-mechanical soil behaviour, however, has not been well understood. In this study, cyclic thermal strain of a small number of specimens of saturated lateritic clay was investigated over a temperature range of 5 to 70 °C, using a thermal oedometer equipped with an invar ring. Both compacted and reconstituted specimens, which were expected to have different degrees of aggregation, were tested. The soil was characterised in terms of its mineralogy and microstructure determined respectively using x-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that a semi-empirical equation over-estimated the thermal expansion coefficient of normally consolidated (NC) lateritic clay by about 3 times. The discrepancy is mainly because the goethite and hematite in the lateritic clay enhance the formation of aggregates and stiffen the specimen. Even the reconstituted specimen contains many aggregates, as clearly revealed by the result SEM tests. Goethite and hematite have much lower thermal expansion coefficient (TEC) than many clay minerals. On the other hand, it was found that both NC compacted and reconstituted specimens showed an accumulation of irreversible contraction under cyclic heating and cooling, but at a decreasing rate, where for a given number of thermal cycles, the measured thermal strain of NC reconstituted specimens was about 30% lower than that of the NC compacted specimens. This is mainly because the density of NC reconstituted specimen is much higher than the NC compacted specimen, even though they had experienced the same effective stress.