The deterioration characteristics of the shallow surface layer of earthen heritage sites under freeze-thaw and water-salt coupling effects

A large number of earthen heritage sites are preserved in the alpine regions of northwestern China. Under the coupled action of freeze-thaw cycles and water-salt effects, the shallow surface layer of these sites undergoes repeated freezing and thawing, which induces various forms of deterioration. To investigate the deterioration characteristics of the near-surface layer of earthen heritage sites under the coupled action of freeze-thaw cycles and water-salt effects, laboratory freeze-thaw simulation tests were conducted. Based on six indicators-apparent morphology, surface temperature field, sonic wave velocity, surface hardness, microstructure, and particle size distribution of remolded samples-the deterioration characteristics of the shallow surface layer under the coupled influence of freeze-thaw cycles and salt migration were revealed. The experiments found that during freeze-thaw processes, soluble salts migrate towards the center of the samples and crystallize, thereby inducing salt weathering and forming a horizontal deterioration layer. Salt content shows a positive correlation with freeze-thaw damage, and higher Na₂SO₄ content leads to more severe damage. Freeze-thaw cycles significantly reduce sonic wave velocity and surface hardness, and disrupt the aggregated structure of soil particles, although the particle size gradation characteristics of the soil remain stable. The primary factor in the deterioration of the shallow surface layer of earthen sites is the coupled action of freeze-thaw cycles and water-salt effects, which causes salt accumulation and crystallization, damages the cementation state of soil particles, and leads to a loosened soil structure. The relevant findings can provide a reference for the prevention and mitigation of deterioration in the shallow surface layer of earthen heritage sites.