Application of Suction Monitoring for Cyclic Triaxial Testing of Compacted Soils

The performance of compacted soils that are used for the construction of railway embankments are impacted by weather-driven suction and water content fluctuations. Hence, the accurate measurement of the water retention properties of soil during cyclic loading is essential to evaluate the soil cyclic behavior under repeated train-induced loads. This paper presents a suction monitoring setup for soil cyclic triaxial testing and its application to evaluate soil water retention and volumetric response. The setup uses a high-capacity tensiometer to measure suction and on-sample displacement transducers to measure volume changes mounted at the midheight of the soil sample. Compacted soil samples were subjected to cyclic loads under different testing conditions: (1) testing a saturated sample under a free-to-drain condition showed the accumulation and dissipation of excess pore-water pressure during cyclic loading, (2) testing unsaturated samples under constant water content conditions demonstrated that the accumulated volumetric strain was smaller than the saturated sample and suction decreased during cyclic loading due to an increase in the degree of saturation, and (3) applying successive packets of cyclic loads and wetting to unsaturated samples showed a progressive increase in the volumetric strain and degree of saturation, leading to loss of suction. The accumulation of the volumetric strain measured for the tested soil was dependent on the magnitude of the confining stress, cyclic deviator stress, and suction. The soil water retention response during cyclic loading was governed predominantly by the suction level and the distance between the current soil water retention state and the main water retention curves. The accurate measurements of the soil water retention properties allowed the evaluation of the evolution of the stress path during cyclic loading in terms of Bishop's stress and the dynamic nature of water retention properties of the soil. The void ratio of the tested soil measured in the resilient state was found to be dependent on the applied stress levels and degree of saturation, and changes in the void ratio also affected the water retention behavior under cyclic loading.