Experimental study on the thixotropic mechanism of deep-sea clay from the perspective of microstructure and bound water

Soil thixotropy plays an important role in many engineering practices, which recently has been drawn an increasing attention by many researchers. In this paper, an experimental investigation on the thixotropy of deepwater marine clays in the South China Sea was carried out. The thixotropy-induced changes of undrained shear strength were measured by fall cone method, and the microscale evolution of soil structure, particularly the change of various types of water, was observed and quantified via employing scanning electron microscopy and thermogravimetric analysis, respectively. The test results show that the thixotropy strength ratios of all deep-sea clays increase quickly at the early stage (before about 10 days), then followed by a gradually lower increasing rate with prolonged time. The microstructural evolution of clay during thixotropic hardening is characterized by the transformation from a dispersed structure with parallel-arranged particles to a well flocculated one. The different types of water and their changes of mass fraction with thixotropic time could be well classified and quantitatively measured by thermogravimetric analysis, in which both the weakly and strongly bound water contents decrease, with the corresponding increase in the free water fraction. The thixotropic mechanism was then revealed from the perspective of the microstructural evolution and the adjustment of the thickness of bound water. The recovery of strength is majorly attributed to the forming and strengthening of inter-particle bonds or contacts and the adjustment of bound water layer. This research could help to provide a deeper insight into the inner reason of soil thixotropy.