Effects of Temperature and Suction on Plastic Deformation of Unsaturated Silt under Cyclic Loads

Soils are natural and environmentally friendly materials that are widely used for constructing earth structures such as pavement. In these structures, soils are often unsaturated and subjected to cyclic loads and daily variations in suction and temperature. Up to now, suction and thermal effects on plastic-strain accumulation under cyclic loads have not been fully understood. In this study, cyclic triaxial tests were carried out on unsaturated compacted silt using a temperature and suction-controlled triaxial system. Suctions from 0 to 60 kilopascals (kPa) and temperatures from 20 to 60°C were considered. It is found that there is a threshold cyclic deviator stress below which the soil specimen reaches an essentially reversible response (plastic shakedown) within 100 cycles. The threshold value increases with an increase in suction, but it seems to be insensitive to temperature changes. Conversely, at a given cyclic deviator stress, measured soil-plastic strain at zero suction is about four times of that at suction of 60 kPa. Similarly, soil plastic strain induced by a given cyclic deviator stress almost doubles when soil temperature increases from 20 to 60°C. The observed thermal effects imply that conventional design methods in geotechnical and pavement engineering may underestimate soil-plastic strain when the ground temperature is significantly higher than the design temperature. Furthermore, the observed suction and thermal effects on strain accumulations are mainly because yield stress of unsaturated soil increases with increasing suction (suction hardening), but decreases with increasing temperature (thermal softening). With a higher-yield stress, smaller plastic strain would be induced by cyclic loads.