Effects of freeze-thaw conditions and salt content on porosity and pore size distribution of chloride silty clay

Artificial ground freezing (AGF) technology is frequently utilized to ensure tunnel stability during the construction of undersea tunnels. However, freeze-thaw cycles cause substantial alterations in the soil’s porosity and pore size distribution, which can result in deformation and even collapse of the soil. Therefore, it is imperative to thoroughly explore the impact of freeze-thaw and salt content on the porosity and pore size distribution of soil. For this reason, two tests, namely X-ray computed tomography technology (CT) test and Nuclear magnetic resonance spectroscopy technology (NMR) test were performed on the silty clay at different freeze-thaw conditions (freeze-thaw FT cycle: 0, 1; freezing temperature: -10 ℃, -20 ℃, -30 ℃) and salt contents (0%, 1%, 2%, 3%, 4%) in this study. The test results revealed that after freeze-thaw, the total porosity increased, with higher freezing temperatures resulting in greater total porosity. As the salt content increased, the total porosity initially increased and then decreased. The porosity in both the XZ and YZ planes followed a normal distribution, while multiple peaks appeared in the XY plane with the smallest coefficient of variation. The porosity uniformity in the vertical direction of the specimen was good, and the spatial anisotropy was significant. Freeze-thaw led to a reduction in the number of micropores, while simultaneously causing an increase in pore size. Moreover, the conversion rate of micropores to macropores decreased as the freezing temperature lowered. The total porosity measured by CT test was much smaller than that of NMR test. The pore size distribution curves derived from NMR and CT tests were found to be complementary. The method for constructing full-size pore size distribution curves containing different scales was proposed. The results provide important perspectives on the microstructure characteristics of marine soft clay under the application of AGF technology.