Effect of specimen preparation techniques on dynamic properties of unsaturated fine-grained soil at high suctions

The seismic response of soil depends on proper evaluation and use of soil dynamic properties, including shear modulus and damping ratio at various strain levels. Despite extensive studies on the shear modulus and damping ratio of saturated soils, research on the dynamic properties of unsaturated fine-grained soils - especially at high suction - is limited. This study aims to investigate the dynamic properties of loess at a variety of initial states resulting from different specimen preparation techniques (reconstituted, recompacted, and intact) and their evolutions due to suction-induced desiccation. Results of resonant column tests show that at initial states, the specimen preparation technique has a negligible effect on the normalized modulus (G/Gmax) and damping degradation pattern, while the influence becomes significant at a high suction (40 MPa). This is attributed to the microstructural evolution of specimens with different initial states that were subjected to suction-induced desiccation. More specifically, the elastic shear strain threshold decreases (reduction of elastic range) while shear modulus increases as suction reaches 40 MPa. Furthermore, the rate of increase in the damping ratio as well as degradation of the shear modulus for specimens at high suction is faster than their initial states. Based on the scanning electron microscopy observations, these findings may be attributed to the aggregation of larger silt-clay assemblies induced by suction increase. Consequently, soil with larger aggregates behaves more like granular sandy soil than saturated silt or clay.