Prevention Of Basal Erosion Caused By Salt Deterioration For Earthen Sites Based On SH Hydrophobic Cushion Layer

Earthen sites in the arid region of Northwest China have important social,historical,scientific,cultural,and artistic values,but they have been damaged seriously due to the destructive effects of natural environment and human factors.Basal erosion is one of the most threatening patterns for the destruction of earthen sites,and soluble salt,sand-driving wind,and precipitation are important reasons for the development of the basal erosion.At present,ramming earth or adobe masonry combined with anchoring and grouting has become a common method to repair the basal erosion.However,after the basal erosion is repaired,it sometimes reoccurs within a short period of time.Therefore,it is still necessary to explore the method to effectively inhibit the redevelopment of the basal erosion.This study presented the cushion layer with the excellent physical and mechanical properties,and the damp-proof functions to prevent the development of basal erosion.The excellent physical and mechanical properties not only resist the damaging effects of sand-driving wind,precipitation,and freeze-thaw cycle,but also avoid the damage caused by uneven settlement and poor air permeability.The damp-proof functions can prevent the damage of soluble salt to the basal part of earthen wall.For the excellent physical and mechanical properties,a lot of studies have been carried out.However,for the damp-proof functions,few studies have been conducted on earthen sites in Northwest China.Therefore,after the hydrophobic additives are identified,this study mainly discusses the feasibility of the hydrophobic cushion layer to prevent the basal erosion caused by salt deterioration in Northwest China.First,the physical and mechanical properties of the additive composite soils were compared,and the best additive was selected.Then,the influencing factors related to the hydrophobicity were analyzed,and the changes in the hydrophobicity of optimum additive composite soils after dry-wet and freeze-thaw cycles were discussed.Next,the effects of the hydrophobicity and the optimum additive on the soil-water characteristic curve(SWCC)were studied.Finally,the approximate one-dimensional soil column,three-dimensional model and numerical model tests were used to verify the feasibility of the SH hydrophobic cushion layer to prevent the basal erosion of earthen sites in Northwest China caused by salt deterioration.The main contents and results of this study are as follows:(1)This study compared the physical and mechanical properties of the SH(a kind of polyvinyl alcohol),sticky rice,cooked tung oil,and tung oil composite soils such as hydrophobicity,unconfined compressive strength,water vapor permeability,and color difference.The results showed that the physical and mechanical properties of the SH composite soil were better than those of the sticky rice,cooked tung oil,and tung oil composite soils.Therefore,the SH solution was recommended as the additive of the hydrophobic cushion layer.(2)This study discussed the effects of SH content,temperature,type and content of soluble salt,type and content of clay mineral,etc.on the hydrophobicity of the SH composite soil.Fourier transform infrared spectroscopy(FTIR)combined with some theoretical models was used to analyze the influencing factors,and the reasons for the soil hydrophobicity induced by SH and the influence of the above factors on the hydrophobicity were discussed.The results of the above influencing factors were comprehensively compared,and the applicable scope of the SH hydrophobic cushion layer was determined.The SH hydrophobic cushion layer should not be used in the environment with the temperature of 60 ℃ or more,and the soil with the salt content ≥1% and the montmorillonite content ≥ 15%.(3)This study discussed the effects of dry-wet and freeze-thaw cycles on the hydrophobicity of the SH composite soils.The results showed that dry-wet and freezethaw cycles decreased the soil hydrophobicity,and the samples with 1% SH content had the best resistance to the effects of dry-wet and freeze-thaw cycles.Combined with the results of influencing factors and the theoretical models,the experimental phenomena can be explained reasonably.Freeze-thaw and dry-wet cycles can change the distribution of hydroxyl groups and carbon chains.The changes in the hydrophobicity were mainly due to the destruction of hydrogen bonds and the curling(or stretching)of carbon chains.For the samples thawed or dried at room temperature,the SH composite soil can still exhibit the hydrophobic properties after several dry-wet and freeze-thaw cycles,which indicated that the SH hydrophobic cushion layer was still effective after the change of environment.(4)This study proposed a new method which isolated the effect of soil hydrophobicity caused by organic additives from other factors.The influence of soil hydrophobicity as a single factor on the SWCC was evaluated by this method.When matric suction was 100-400 k Pa,soil hydrophobicity significantly accelerated drainage.When matric suction was 2.7-298.7 MPa,the inhibitory effect of the hydrophobicity on water vapor adsorption was weak.The addition of the SH not only changed the soil hydrophobicity,but also changed the specific surface area and pore size distribution of the soil.Therefore,the addition of SH increased the water retention capacity of the soil when the suction was in the range of 100-400 k Pa,but it decreased the water vapor adsorption capacity of the soil with the suction in the range of 2.7-298.7 MPa.The SH composite soil as a hydrophobic cushion layer decreased the amount of water vapor adsorption,and thus did not enhance salt damage.(5)This study designed two temperatures to treat the soil columns with the cushion layer,and the effects of the SH content,the position of cushion layer,the thickness of cushion layer and the type of salt solution on the capillary test were discussed.The results showed that cushion layer after heat treatment(100 °C)cannot prevent the water-salt capillary rise.However,for the samples dried at 20°C,1.0% SH cushion layer can obviously prevent the capillary rise.The prevention of water-salt capillary rise by SH cushion layer depended on the hydrophobicity rather than the pore size.The modified Green-Ampt model was proposed to describe the phenomenon of capillary rise in the layered soil.The modified Green-Ampt model combined with the capillary theory can explain the experimental phenomenon reasonably,and this study enriched the existing theories.(6)This study designed three-dimensional model and numerical simulation tests to analyze the earthen walls on site.The results of the three-dimensional model tests showed that soluble salt can cause the development of the basal erosion,and the SH hydrophobic cushion layer can change the water-salt migration path,thereby preventing the occurrence of basal erosion.The soluble salt was mainly concentrated on the ground on both sides of the wall,and the SH hydrophobic cushion layer should extend to the direction of the earthen wall.Numerical simulation results showed that only when the hydrophobic cushion layer was fully laid at the bottom of the earthen wall,the development of the basal erosion can be effectively inhibited.This study presented a technical concept to change the water-salt migration path by SH hydrophobic cushion layer,and thus prevented the development of the basal erosion.The results confirmed that the SH hydrophobic cushion layer was effective from multiple scales.This study provided the theoretical basis and engineering guidance for the effective prevention of basal erosion,and provided innovative explorations for the application of the consolidation materials for earthen sites.