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 $s_u$ and small-strain shear modulus $G_{\max }$ 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 $s_u$ and $G_{\max }$ of the two very soft clays exhibited a time-dependent increase by as much as 330% and 386% over $\sim 64\mathrm{days}$, 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 $s_u$, 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 $G_{\max }$. A temporal increase in both the $s_u$ and $G_{\max }$ fit the three-unit Burgers model well, further confirming that multiple mechanisms contribute to different thixotropic stages with dissimilar rates.