干湿循环下气—液界面张力对黏土收缩开裂的影响

Effect of Gas-Liquid Interfacial Tension on Shrinkage Cracking of Clay as Affected by Wetting-Drying Alternation

气—液界面张力(表面张力)作为主要的外部环境因素之一,其变化势必会对土壤收缩开裂和土水特性有重要影响。开展了一系列不同温度条件下的干湿循环试验,脱湿温度设定为25℃和60℃两种,对3组不同表面张力的初始饱和试样进行干湿循环试验。在脱湿过程中,对试样的含水率变化及表面裂隙的演化过程进行定时测量拍照,利用数字图像处理技术对试样裂隙图像进行定量分析,最终得到表面收缩开裂裂隙度δ。结果表明:随着干湿循环次数的增加,试样的最终δ有所增加,但增长幅度不大;温度越高,试样初次出现裂隙的时间越快,当温度从25℃增加至60℃时,试样出现初始裂隙时对应的临界含水率从38%增至41%,试样的最终δ增长20%~40%;在同一温度环境下,土样的裂隙发育程度随着表面张力的降低而变慢,最终δ随着表面张力的降低而减小,无论何种温度环境,表面张力大小和裂隙度大小顺序一致,均为纯水试样>酒精溶液试样>肥皂水试样;含水率相同时,表面张力越大,对应试样的δ越大。

【Objective】In recent years, disastrous soil pollution incidents have been taking place quite frequently. Surface activating agents were widely used in remediation of polluted soils as it is an effective soil amendment. Addition of surfactants can accelerate remediation processes and improve soil properties, and alter the surface tension of pore water in the soil as well. It has been demonstrated in a number of studies that surface tension of pore water is substantially reduced by adding a small amount of surfactant, and altered, too, by adding some organic compounds, which are often found as residues in wastewater treated and recycled as irrigation water.The three major equations describing characteristic of the interface, i.e. Young equation, Laplace equation and Kelvin equation, are all related to surface tension.Sogas-liquid interfacial tension（also surface tension） is the main factor affecting shrinkage cracking of soil,and besides, changes in surface tension are bound to have an important impact on soil-water characteristics of soil. In addition, under actual climate conditions and natural environment, which are often very complicated, soils are generally subjected to several times of drying and wetting alternations, which may cause delay of the soil water characteristic curve, and variation of contact angle is also an important factors causing delay of the curve, while contact angle and surface tension are closely related to each other. It is,therefore, of great theoretical and practical significance to characterize soil shrinkage cracking from the view point of surface tensionas affected by wetting and drying alternation to agricultural irrigation as well as environmental protection.【Method】In order to better understand the effect of surface tension on soil shrinkage cracking, an experiment designed to have the soil samples subjected to wetting-drying alternations was carried out, with drying temperature set at 25°C and 60°C. The experiment had three groups of soil samples treated with different solutions, thus being different in surface tension when initially saturated.Changes in soil water content and evolution of surface cracks were monitored in the samples under dehumidification, and even photos were taken for quantitative analysis of fissure development degree（δon the surface of the samples using thedigital image processing technique. Then comparative analysis was done for relationships of surface tension with δ and changes in water content, and with temporal variation of δ.【Result】Results show that with the drying-wetting alternation going up in frequency, δ of the soil at the end of the experiment increases somewhat, but not large in magnitude; δ is significantly influenced by temperature： the higher the temperature, the earlier the initial fissures appear. When the drying temperature rises from 25°C and 60°C, critical water content at the onset of cracking increases from 38% to 41%, and δof the soil at the end of the experiment by 20%～40%; when the drying temperature remains the same, with declining surface tension soil fissure development slows down and δ of the soil at the end of the experiment lowered; but it is found thatδof the soilslow in surface tension may be higher than that of the soils high in surface tension at the initial stage; when water content is the same, the higher the surface tension, the more the fissure development degree. Regardless of temperature environment, in terms of surface tension and fissure development degree, the three groups of soil samples exhibits an order of samples treated with pure water samples treated with alcohol solution samples treated with soap water.【Conclusion】In a word,wetting-drying alternation does have certain influence on development of soil fissures; and temperature is a major faction directly controlling shrinkage cracking of soil; while surface tension is one restraining the processes.