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Earthen sites have important historical, cultural, artistic, social, and scientific values. However, they have been seriously destroyed by weathering. The methods of in situ replacement and sacrificial layer were previously presented to protect the surface of the earthen sites. This work presents the feasibility of the polyvinyl alcohol (PVA)-treated soil in a mud state as the anti-weathering material for these two methods. The desiccation cracking behaviour of soil was the main concern, and the surface peeling, hardness, and colour difference were used as the auxiliary evaluation criteria. The results showed that the PVA solution with a high degree of alcoholysis effectively inhibited soil cracking, but cannot eliminate all cracks even if the content reached 1%. The cracks of samples with the PVA mainly developed when the water content exceeded water content corresponding to air-entry value. Filamentous (or meshy) connections and cementation between particles were the main reason to inhibit the development of the cracks. The obvious advantages of the PVA in the decohesion index, hardness, colour difference, and so on cannot be also ignored. Based on these evaluation indicators, a PVA content of 0.8% may be the best choice for anti-weathering of earthen sites. [ABSTRACT FROM AUTHOR]

Earthen sites with important historical, cultural, artistic, social, and scientific values have been seriously damaged. Basal erosion, which is caused by soluble salts, water runoff, and sand-driving wind, is an extremely threatening erosion pattern for earthen sites. A cushion layer which has a damp-proofing function and good physical and mechanical properties can effectively decrease basal erosion. The primary aim of this study is to investigate material selection for the hydrophobic cushion layer for earthen sites in northwest China. The results indicate that cooked tung oil and sticky rice liquid, traditionally used in China, cannot meet the fundamental requirements of sufficient hydrophobicity for the cushion layer. Although polyvinyl alcohol (PVA) solution and tung oil exhibit similar properties in terms of soil hydrophobicity, water vapour permeability, water resistance, surface hardness, and colour difference for samples with 0.0-1.0% content, the PVA solution is significantly better than tung oil in unconfined compressive strength, deformation modulus, wave velocity, and viscosity. Therefore, the PVA solution should be preferentially selected as a hydrophobic cushion layer material according to the results of this work. When the mixing method is adopted, 0.6-1.0% PVA content is the best choice. [ABSTRACT FROM AUTHOR]

The Subashi Buddhist Temple Ruins, which contain a lot of coarse-grained soils, have been severely damaged by surface weathering. Modified polyvinyl alcohol (SH) which can improve the mechanical properties of fine-grained earthen ruins has never been reported in the protection of coarse-grained earthen ruins, and thus, it is of great practical significance to investigate the impregnation and consolidation effects of SH solution on coarse-grained soils. First, the characteristic changes obtained by SH impregnation with respect to consolidation radius, impregnation rate, impregnation time, and cumulative impregnation were analysed for soil specimens with different dry densities and grain gradations. Then, disintegration, drying–wetting cycle, and freeze–thaw cycle experiments were carried out, and the results of these experiments were analysed. Finally, the impregnation and consolidation effects of the SH solution were evaluated together. The feasibility of protecting the Subashi Buddhist Temple Ruins with SH by impregnation was studied, and it is suggested that the methods of surface spraying and borehole impregnation for anti-weathering should be combined to protect these ruins. [ABSTRACT FROM AUTHOR]

Soil wettability is an important property that affects the behaviour of fine-grained soils. Previous studies have shown that hydrophobicity induced by organic or silane additives may affect the soil-water retention curve (SWRC). However, current findings regarding the effect of hydrophobicity on the SWRC are controversial. Previously, organic or silane additives were assumed to change only the soil wettability. Nevertheless, this study shows that polyvinyl alcohol (PVA) can change the hydrophobicity, pore size distribution (PSD), and specific surface area (SSA) of soil, thus affecting the SWRC. PVA-treated soil is hydrophobic when cured at 20 ℃, but hydrophilic when cured at 100 ℃. No thermal degradation is indicated in the PVA at 100 ℃ based on thermogravimetric analysis and a comparison of mass loss, and the sole difference between the PVA-treated soils cured at 20 and 100 ℃ is their wettability. This study managed to make the hydrophobicity caused by PVA to become an independent variable separated from the PSD and SSA by controlling the ambient temperature. The wetting and drying curves are measured under isopiestic humidity control in the high-suction domain (2.7–298.7 MPa), whereas pressure plates are used for the drying curves in the low-suction domain (0–800 kPa). The results obtained indicate that soil hydrophobicity significantly accelerates drainage when the matric suction is between 100 and 400 kPa, and an increase in PVA content increases the water retention capacity of soil. When the matric suction exceeds 2.7 MPa, the effect of soil hydrophobicity on the SWRC of the soil weakens. At this time, the SWRC is dominated by the van der Waals force, and the SSA contributes significantly to the process. Additionally, this study enables some controversial findings from previous studies to be explained, which is of great significance in soil and agriculture sciences. • A new method to isolate effect of hydrophobicity from other factors is proposed. • Soil hydrophobicity enhances water drainage in the low-suction domain. • Soil hydrophobicity has a weak effect in the high-suction domain. • The reason for the controversial conclusions in past studies is discussed. [ABSTRACT FROM AUTHOR]

Water-salt migration is an important reason for the development of basal erosion which seriously threatens the conservation of earthen sites. Studies in cutting-off or redirection of the water-salt migration path become vital. Polyvinyl alcohol (PVA) composite soil with extreme hydrophobicity shows the potential as the cushion layer with the damp-proof function, but the feasibility of preventing basal erosion based on the PVA cushion layer still lacks verification. In this study, for the embedded and non-embedded PVA cushion layers, water-salt distribution and morphology of earthen wall after repeated injection of salt solution were compared. The results showed that PVA cushion layer changed the water-salt migration path and promoted the accumulation of soluble salt on the ground outside the wall rather than the bottom of cushion layer, which effectively prevented basal erosion. A numerical model based on HYDRUS-2D confirmed that a partial laying of hydrophobic cushion layer under the wall cannot effectively prevent the water-salt migration, and a full laying of cushion layer should be adopted to prevent basal erosion. The cushion layer was recommended to extend towards the direction of the earthen wall instead of towards the sides, which also resisted the destructive effects of sand-driving wind and runoff erosion. • Basal erosion caused by water-salt migration is directly confirmed. • Water-salt distribution for models with/without PVA cushion layer is explored. • PVA cushion layer can inhibit basal erosion. • Reasonable laying modes of the hydrophobic cushion layer is presented. [ABSTRACT FROM AUTHOR]

• PVA-treated soil is confirmed to prevent water-salt capillary rise. • Effectiveness of 1.0% PVA-treated soil is best. • Interface effects are proposed to explain experimental phenomena. • Capillary rise of layered soil is described by modified Green-Ampt model. In the loess region of northwest China, the accumulation of soluble salt in the soil due to the capillary rise of water often damages earthen buildings. Thus, studies on the cutting off of the capillary-rise path are vital. As a hydrophobicity inducer, polyvinyl alcohol (PVA) solution with water as the solvent has the potential to function as a barrier. This study mainly aims to evaluate the feasibility of using PVA-treated soil as a waterproof layer to prevent water-salt capillary rise through soil column experiments, and the effects of the waterproof layer thickness, waterproof layer position, PVA content, ambient temperature, and type of salt solution on the capillary action. The results indicate that a 1.0% PVA-treated soil with extreme water repellency (i.e. dried at 20 °C) can perfectly prevent the water-salt from migrating to the top of the sample, regardless of the waterproof layer position, waterproof layer thickness, and type of soluble salt. For a 0.5% PVA-treated soil dried at 20 °C, the lower waterproof layer prevents the water-salt capillary rise, while the middle and upper layers exhibit less effectiveness. However, the PVA-treated soil that loses its hydrophobicity after a treatment at 100 °C cannot prevent the water-salt capillary rise, confirming that the prevention of capillary rise is due to the soil hydrophobicity. The waterproof layer thickness and type of soluble salt are less significant than the waterproof layer position, PVA content, and ambient temperature. This study proposes a modified Green-Ampt model to describe the capillary rise, and the hydraulic conductivity and pore size are obtained using the modified model. These parameters and the interface effect are used to explain the experimental results. Finally, the application of PVA-treated soil as a waterproof layer to inhibit water-salt capillary rise is illustrated. [ABSTRACT FROM AUTHOR]

• Hydrophobicity of PVA treated soil decreases after dry–wet and freeze–thaw cycles. • a = 20 is used as the ideal critical value to judge internal wettability. • Wettability of treated soil is mainly controlled by alkyl chain, and hydroxyl group. 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. [ABSTRACT FROM AUTHOR]