Composite Microscopic Pore Structure Model for Saline Lean Clay during Freezing and Thawing under Confining Pressure Based on the NMR Fractal Theory

This study investigates the evolution of pore structure and unfrozen water content in saline lean clay under varying confining pressures during freeze–thaw cycles, which is critical for artificial ground freezing (AGF) in deep underground construction. A custom-designed nuclear magnetic resonance (NMR) test system was developed to enable simultaneous control of temperature and confining pressure. A composite pore model was established, combining a sphere–cylinder structure for macropores and a capillary model for meso- and micropores, with fractal theory used to characterize pore complexity. Results showed that the unfrozen water content increased by 1.8% when confining pressure rose from 0 to 800 kPa at −15°C. The volume fraction of capillary pores increased from 26% to 50% as temperature dropped, while sphere–cylinder pore volume fraction declined. The transition around 400 kPa marked a shift from overconsolidated to normally consolidated behavior, leading to abrupt changes in pore size distribution and fractal dimension (macropore fractal dimension reduced from 2.5 to 2.2), as well as a nonlinear reduction in macropore volume fraction with increasing confining pressure (e.g., from 0.5 to 0.26). A correction coefficient β was introduced to improve the accuracy of cumulative pore size distribution predictions, achieving high model fit. These findings provide a quantitative basis for predicting pore behavior in saline clays under freeze–thaw conditions and offer practical guidance for AGF applications in coastal tunneling projects, where AGF is widely used for connection passages and launching or receiving shafts, inevitably intersecting saline soil layers.