Multiple Thixotropisms of Liquid Limit–Consistency Clays Unraveled by Multiscale Experimentation

Multiscale experiments were conducted to investigate the thixotropic behavior of a naturally occurring illite-rich marine clay with a moderate salinity and of a manufactured, highly purified kaolin clay at respective liquid limits. The temporal evolution in undrained shear strength su and small-strain shear modulus Gmax was assessed using fall cone and bender element testing, and one-dimensional X-ray diffraction and electron microscopy were employed to probe microfabric changes. Results showed that both the su and Gmax of the two very soft clays exhibited a time-dependent increase by as much as 330% and 386% over ∼64 days, respectively, which can be divided into two stages with drastically different rates: a much higher rate during the initial 2–7 days, and a lower rate over extended time. Such a transition in the hardening rate manifested dissimilar thixotropic mechanisms dominating the two stages; whereas the initial stage is controlled by fabric aging chiefly dictating su, including particle rearrangements (e.g., reorientation, aggregation, and flocculation) and homogenization of defects and disturbance, the later stage is controlled by contact aging dominating the long-term increase in Gmax. A temporal increase in both the su and Gmax fit the three-unit Burgers model well, further confirming that multiple mechanisms contribute to different thixotropic stages with dissimilar rates.