Study on the Structural Evolution and Mechanical Behavior of Soils in Archaeological Sites Under Wet–Dry Cycling

Archeological sites in humid regions are particularly susceptible to mechanical degradation induced by rainfall-driven wet–dry (W-D) cycles after excavation. In this study, representative archeological soils from the Suzhou region were investigated to quantify strength attenuation and pore structure evolution under cyclic moisture disturbance. Laboratory W-D cycling tests were conducted on samples prepared using static compaction and layered compaction methods, with cycle numbers up to nine and cycle amplitudes of 1–4 days. Unconfined compressive strength (UCS), direct shear strength, scanning electron microscopy, and mercury intrusion porosimetry were used for multiscale characterization. Results show that UCS decreases by approximately 40–50% after six to nine W-D cycles, accompanied by a porosity increase of 4.0–5.5% for statically compacted samples and 6.5–8.0% for layered-compacted samples. Layered-compacted specimens exhibit an average strength reduction of about 20% within the first three cycles, significantly higher than that of statically compacted soils. Microstructural observations reveal a progressive transformation from micropore-dominated structures (<10 μm, initially 70–80%) to interconnected meso- and macropores (>50 μm, up to 30–40%), leading to increased permeability (from ~10 −8 to 10 −6 cm/s). A semi-empirical model incorporating cycle number and amplitude successfully captures the non-linear evolution of porosity and strength degradation. These findings provide quantitative criteria for assessing excavation stability and long-term deterioration risks of archeological sites in humid environments.