Protecting earthen sites by soil hydrophobicity under freeze–thaw and dry–wet cycles

Abstract Earthen sites have important social, historical, artistic, and scientific value. Basal erosion, which is closely related to soluble salt, is one of the most important reasons for the destruction of earthen sites. Soil hydrophobicity may change the path of water–salt migration. Thus, it can protect earthen sites by inhibiting basal erosion at the bottom of earthen walls. However, the changes in the hydrophobicity after freeze–thaw and dry–wet cycles are still unclear. Therefore, the primary aim of this study is to investigate the effects of these cycles on the hydrophobicity of PVA composite soil with a high alcoholysis degree. The results indicate that soil hydrophobicity decreased with increasing number of freeze–thaw and dry–wet cycles. At a dry temperature of 80 °C, the soil lost its hydrophobicity. The surface hydrophobicity of immersed samples dried at room temperature decreased or was lost; however, the hydrophobicity inside samples still remained after five dry–wet cycles. Furthermore, an indicator based on the water inlet velocity was presented to judge the interior wettability. The changes in the wettability of the PVA composite soil were mainly controlled by changes in hydrogen bonds, the coiling and stretching of alkyl chains, and the orientation of free hydroxyl groups.