Effect of Cyclic Suction History (Hydraulic Hysteresis) on Resilient Modulus of Unsaturated Fine-Grained Soil

This study explored the effect of hydraulic hysteresis on the resilient modulus (M_r) of subgrade soils. For that purpose, suction-controlled M_r tests were performed on compacted specimens along the primary drying and wetting and secondary drying and wetting paths. First, M_r tests were performed continuously on the same sample at various suction values along all of the paths (drying, wetting, etc.). A relationship between M_r and matric suction was obtained and identified as the resilient modulus characteristic curve (MRCC). MRCC results indicated that M_r increased with suction along the drying curve. Results along the primary wetting curve indicated a higher M_r than along primary drying and secondary drying curves. The second series of tests, referred to as virgin tests, were performed at suction values without subjecting the sample to previous M_r testing. Results indicated that M_r values were lower compared with results from the other type of test (i.e., with continuous M_r testing), and that M_r results were influenced by the continuous stress loading. However, M_r values along the wetting curves were still higher than for the drying curves for these virgin tests and therefore indicated that results were also influenced by hydraulic hysteresis (i.e., cyclic suction stress loading). New models to predict the MRCC results during drying and wetting were proposed on the basis of the soil–water characteristic curve hysteresis. The models favorably predicted the drying and then the wetting results with the use of the soil–water characteristic curve at all stress levels.