The structural clay of the Zhanjiang Formation exhibits significant thixotropy, and there are considerable differences in the ultimate bearing capacity of pulled-out piles under different resting times. Using the structural clay from the Zhanjiang Formation as the foundation, direct shear tests on the soil surrounding nine groups of model single piles of different sizes were conducted at various resting times, along with static pullout tests on the pile foundations. The results indicate that the cohesion and internal friction angle of the surrounding soil increase following a logarithmic function with increasing resting time; specifically, the growth rate is rapid in the early resting period and gradually slows down in the later period. A quantitative relationship describing the variation of cohesion and internal friction angle over time was established. The load-displacement curves for single piles at different resting times exhibit a distinct steep drop. The uplift single pile exhibits significant time-dependency, with the ultimate uplift bearing capacity increasing more rapidly in the early stages and gradually stabilizing in the later stages. Under different resting times, for each load level, the maximum side friction resistance of the pile gradually shifts to the middle and lower parts of the pile body, while the ultimate side friction resistance is evenly distributed along the lower part of the pile body, with the side friction resistance of the pile bearing the uplift load. Based on the quantitative relationship of the cohesion and internal friction angle of the surrounding soil around the pile varying with time, a predictive formula for the axial pullout ultimate bearing capacity of a single pile in the Zhanjiang Group structured clay foundation has been established. Using existing pile foundation projects, model experiments were designed to verify the validity of the formula; however, there is a lack of field-scale validation. The research findings can provide a reference for predicting the axial pullout ultimate bearing capacity of single piles in practical engineering applications.

The thixotropy of lime-modified loess is a key engineering problem in large-scale mountain levelling and urban construction on the Loess Plateau in China. We analyse the thixotropic factors and establish thixotropic models of modified loess at macroscales and microscales to interpret the evolution of the thixotropic mechanism of lime-modified loess. A custom-made volume-preserving thixotropic instrument is used to eliminate the influence of consolidation deformation on thixotropy and simulate soil consolidation in the field. Consolidated undrained triaxial tests, nuclear magnetic resonance analysis, and electron microscopy are used to investigate the thixotropy of soils with different thixotropic periods (0 days, 7 days, 14 days, 21 days, 42 days, 84 days, 126 days, and 168 days). The results show that the failure strength increases and the growth rate decreases with the thixotropic period length. The failure strength increases rapidly in the early thixotropic stage; the inflexion point occurs at 21 days, and stabilisation is observed at about 42 days. The internal friction angle and cohesive force increase over time, the cohesive force increased more obviously, which was 2.94 times of the initial thixotropic period, the increase in internal friction angle is within 4 degrees. The pore distribution is more uniform at the microscopic level, and large and small pores are transformed into medium pores over time. As the thixotropic period increases, the amount of cementitious material generated in the modified loess and the cementation degree increase, and the number of surface pits and large pores on the particles substantially decreases, resulting in numerous flower-shaped and grid structures. The thixotropic mechanism of modified loess consists of pore homogenisation, gravitational repulsion between particles, and cementation caused by the lime reaction.