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Polyvinyl alcohol (PVA) as an additive has been widely concerned in soil improvement. In the past, the consolidation effect of the PVA in various soils showed obvious differences. However, the reasons for the difference remain unclear. Clay mineral and soluble salt are important components of the soil. Therefore, this study mainly discussed the effects of clay mineral and chloride salt on the unconfined compressive strength (UCS) of PVA-treated soil. The results indicated that CaCl2 and MgCl2 significantly increased the UCS of the PVA-treated soil, whereas NaCl decreased the UCS. Illite and kaolinite had little effect on the UCS, while the UCS first decreased and then increased with the increase in the montmorillonite content. When the MgCl2 or CaCl2 content approached 3%, the soil samples cracked after curing at room temperature for 28 days due to the formation of crystalline water, which sometimes decreased the UCS of the samples. According to the results of multiple linear regression, for the effect of the UCS of the PVA-treated soil, MgCl2 > NaCl > CaCl2 > montmorillonite > kaolinite > illite. The effects of clay mineral and chloride salt on the UCS of the PVA-treated soil mainly depended on the changes in the soil structure, hydrogen bonding, and cross-linking reactions according to the results of the microscopic tests. The PVA as a consolidation material is not suitable for the seaside, and it should be applied to the soil with no more than 2% MgCl2 or CaCl2. [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]

Earthen sites with historical, scientific, social, cultural, and artistic values can be destroyed by weathering. Chemical consolidation is an effective method to protect earthen sites, and organic–inorganic composites are often considered ideal. The primary aim of this study was to investigate the effect of polyvinyl alcohol (PVA) with a high degree of alcoholysis on the expansion and cracking behaviour of quicklime-treated soil. The expansion and surface cracking behaviour of the composite soil were investigated with a Crack Image Analysis System. Changes inside the soil samples were observed and analysed by X-ray computed tomography and ImageJ. The results indicated that the expansion of quicklime-treated soil decreased with increasing PVA content, and the cracking behaviour on the surface was different from the inside of the samples. Surface cracks were significantly weakened when PVA was added to soil with a quicklime content ≤ 5%. As the quicklime content increased to 10% or more, the PVA transformed long and wide surface cracks into short and narrow cracks. The development of internal cracks was not as evident as those developing at the surface. Soil samples with 1.0% PVA had no obvious internal cracks at 0–25% lime content. Samples with 0.5% PVA content had internal cracks only when the quicklime content was ≥ 20%. A mechanism for the cracking phenomenon was proposed considering the results obtained from X-ray diffractometry. Soil particles were cemented together through physicochemical reactions with PVA, and these soil particles shared the expansion resulting from the hydration reaction of quicklime instead of through local expansion. [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]

Earthen sites (i.e., ancient earthen buildings) with important historical, cultural, artistic, social, and scientific value have been seriously damaged. Soil treatment with chemical additives can decrease the degradation rate of earthen sites. In the past, polyvinyl alcohol (PVA) and quicklime as chemical additives were studied separately for the protection of earthen sites. In arid and semi-arid northwest China, the early strength of the lime-treated soil may be insufficient due to an inappropriate reaction environment, and the PVA as the additive can quickly increase the soil strength. However, calcium carbonate has been proven to not be decomposed for hundreds of years, while there is still a lack of experimental evidence to show that the PVA cannot be decomposed in decades. The PVA and lime as the additives show the potential to make up for the defects each other. Therefore, the physical and mechanical properties of the PVA and lime composite soil are analysed, and the microscopic properties of the composite soil are discussed in this study. The results show that the addition of the PVA decreases water vapour permeability and pore size, which causes a delay in the carbonization reaction of lime, but it acts as a cementing agent to improve soil strength before the lime appropriately reacts. The addition of quicklime can decrease the hydrophobicity of the PVA-treated soil. The mixing of these two materials is conducive to the colour control of the sample. For the composite soil applied to earthen sites in northwest China, the optimal ratio is 10% lime and 1.0% PVA. [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]