Cyclic behaviour of an unsaturated silt at various suctions and temperatures

Subgrade soils are constantly subjected to daily variations of temperature and suction in the field. Their deformation under cyclic traffic loads can greatly affect the performance of a pavement. Yet, the behaviour of unsaturated subgrade soils under cyclic loads at various temperatures and suctions is not fully understood. In this study, a new suction- and temperature-controlled cyclic triaxial apparatus was developed. Using temperature-compensated Hall effect transducers, the accumulated plastic axial and volumetric strains and resilient modulus of an unsaturated silt under cyclic loads were measured at suctions ranging from 0 to 60 kPa and temperatures ranging from 20 to 60°C. Experimental results show that yield stress increases with increasing suction (suction-induced hardening), but decreases with increasing temperature (thermal-induced softening). Consequently, axial plastic strain accumulated during cyclic loading–unloading is larger at lower suctions and higher temperatures. At a given temperature, measured resilient modulus significantly increases when suction increases. On the other hand, the influence of temperature on resilient modulus depends on suction level. At zero suction, resilient modulus is slightly affected by temperature. At suction larger than zero, an increase in temperature induces an obvious decrease in resilient modulus. It is evident that thermal effects on resilient modulus are more significant at unsaturated than saturated states.