Effects of temperature and suction on secant shear modulus of unsaturated soil

Understanding of the secant shear modulus (G) of soil at small strains (lower than 1%) is essential to predict the performance of many earth structures in working conditions. Furthermore, soils at shallow depth are often unsaturated and subjected to variations of suction and temperature. Yet the effects of suction and temperature on G of unsaturated soils at small strains are often neglected and not fully understood. In this study, the secant shear modulus of an unsaturated silt was measured at various suctions and temperatures using a suction- and temperature-controlled triaxial apparatus with local strain measurement. The experimental results confirm that the shear modulus of unsaturated soil at small strains considerably increases with increasing suction under isothermal conditions, due to increases in the average skeleton stress and the stabilisation effects on the soil skeleton induced by meniscus water. On the other hand, at constant suction, G decreases as temperature increases. This is likely due to a reduction of the air–water surface tension with an increase in soil temperature, leading to a reduced influence of suction on G values.