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盐分及干湿循环对壤土蒸发特性与裂隙发育影响的机理探究

Mechanism explorations of salt and drying-wetting cycle affecting evaporation and crack development of loam soil

  • 摘要: 为揭示盐渍土在干湿循环作用下的蒸发特性和裂隙发育特性,进而探究其内在影响机理,该研究设置5种盐度(0、0.3%、0.6%、1%、2%)的Na2SO4、CaCl2、NaCl型盐土,开展恒温蒸发试验,同时对Na2SO4型盐土进行干湿循环试验,结合数字图像处理技术对蒸发过程中土壤干缩裂缝网络几何形态特征进行定量分析,并进一步系统揭示水分蒸发与收缩开裂互作以及盐分(包括盐分类型和含盐量)和干湿循环对水分蒸发和收缩开裂的作用机制。结果表明:1)不同处理蒸发过程具有相似性,即蒸发过程均包含一个线性阶段和一个非线性阶段。2)盐分抑制了土壤水分蒸发,随着土壤含盐量的增加,抑制作用也不断增强;盐分类型和干湿循环能够改变土壤减速段的蒸发速率。3)盐分类型对裂缝发展影响显著。对于Na2SO4型盐土,土壤盐分可以抑制表面裂缝的形成和发展,盐渍土的裂隙面积密度、裂隙总长度和平均宽度分别降低了4.5%~9.4%、0.01%~7.9%和10.5%~21.3%;而对于CaCl2和NaCl型盐土,随土壤含盐量的增加,裂隙面积密度分别增加了2.8%~5.5%和3.5%~8.3%,裂隙总长度分别增加了17.7%~35.0%和11.9%~36.9%,而裂隙平均宽度分别降低15.5%~22.1%和8.8%~21.5%。4)干湿循环对裂隙指标的影响表现为低盐时抑制、高盐时促进,并且随着干湿循环次数的提高,这种影响也不断增强。机理分析表明,不同类型盐土的溶质势和晶体形态是影响土壤蒸发特性和裂隙发育的重要因素;钠质土壤相较钙质土壤具有更大的扩散双层,降低了土壤的抗拉强度;Na2SO4通过促进微团聚体间胶结和堵塞土壤孔隙等途径抑制地表开裂;干湿循环通过膨胀引起的裂纹愈合促进表面开裂。

     

    Abstract: Soil shrinkage and swelling have been characterized by the variation in the soil volume in response to water content in agricultural production. The soils can then generate some cracks in arid and semi-arid areas. Desiccation cracks can also destroy the internal structure of the soil to form the preferential circulation channels for rainwater and irrigation water, leading to nutrient loss and groundwater pollution. Meanwhile, the cracks typically occur in shrink-swell soils with high contents of clay minerals. Furthermore, the porous medium can often turn into a variable-solid-skeleton soil, particularly with the typically randomly distributed cracks. As such, it is difficult to predict the fluid transport in the shrink-swell soils. Specially for the saline soils, salts can accumulate around cracks, due to the water loss at cracks. The partial salt accumulation can change the hydraulic properties, and then destabilize soil particles, leading to destroy the soil structure. Besides, the salt flow can be easily into the deep soil or groundwater through preferential channels that are formed by cracks. Therefore, there are more serious potential threats in the cracks under cyclical and regional changes in environmental conditions. This study aims to reveal the evaporation and crack development of saline soil under dry-wet cycles. The soil evaporation experiments were conducted on the Na2SO4, CaCl2, and NaCl-type saline soils with five salt contents (0, 0.3%, 0.6%, 1%, and 2%) at constant temperature. Dry-wet cycle experiments were on the Na2SO4-type soils. Digital image processing was combined to quantitatively analyze the geometrical characteristics of soil drying and shrinkage crack network during evaporation. A systematic investigation was then implemented on the interactions between water evaporation and shrinkage cracking, in order to explore the mechanisms of soil salt (including salt type and content) and dry-wet cycle. The results showed that: 1) There was similar evaporation in the different treatments. The evaporation also included a linear and a nonlinear stage. 2) Soil salt inhibited the water evaporation. The inhibition effect also increased with the increase in soil salt content; The type of salt and the dry-wet cycles were used to change the evaporation rate of the soil deceleration section. 3) The salt type shared a significant influence on the crack development. Soil salt also inhibited the formation and development of surface cracks in the Na2SO4-type saline soil. The crack area density, total length of cracks, and average width of cracks of saline soils decreased by 4.5%-9.4%, 0.01%-7.9%, and 10.5%-21.3%, respectively, compared with the non-saline soil. Furthermore, the area density of cracks increased by 2.8%-5.5% and 3.5%-8.3%, respectively, and the total length of cracks increased by 17.7%-35.0% and 11.9%-36.9%, respectively, while the average width of cracks decreased by 15.5%-22.1% and 8.8%-21.5%, respectively, with the increase of soil salt content in the CaCl2-type saline and NaCl-type saline soils. 4) The dry-wet cycles also inhibited the crack indexes at the low salinity but promoted them at the high salinity. This effect increased with the increase in the number of dry-wet cycles. The mechanism analysis showed that the solute potential and crystal morphology were the important influencing factors on the soil evaporation and crack development in the different types of saline soils; Sodium soil shared a larger diffusion bilayer than calcium soil, which reduced the tensile strength of soil. The Na2SO4 promoted the cementation between micro aggregates and plugging soil pores, leading to the suppression of surface cracking. Therefore, the dry-wet cycle can promote surface cracking through swelling-induced crack healing, in the case of hydrophilic clay minerals in contact with water.

     

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