Experimental Study On Soil Shrinking And Swelling Characteristics During The Alternative Drying And Wetting Processes

Soil swell-shrink is related to the change in soil volume with water content in the processes of soil drying and wetting. The swelling and shrinking of soil affect the distribution of soil pores. Thus it also possibly causes t change of soil water characteristic curve. The shrinkage is important for soil hydrological processes since the cracks caused by shrinkage provide preferential pathways for water flow and thereby affect both water and solute transport. And assessing the magnitude and geometry of cracks is important for modeling infiltration and subsequent redistribution of water within the soil. Employing the simple and reliable methods and choosing the appropriate swelling-shrinking model are very important for deep understanding of the water and solute transport and the hydrology in swelling soils. Based on the research of the swelling-shrinking characteristics of five disturbed soils with different texture, we investigated the swelling-shrinking characteristics of undisturbed soils by special devices, compared the swelling-shrinking characteristic curves of soils that with and without cracks. The main results are as follows:(1) The three straight lines model could fit the disturbed soils'shrinking and swelling characteristic curves well. The characteristic values of the curves were smaller than 1. This indicated that during the processes of soil swelling and shrinking, the change of the soil specific volume was smaller than that of the soil water content. Excepted residual zone and normal zone and structure zone, a new shrinking zone can be noticed the shrinking characteristic curve of undisturbed soils .The term,"pseudo-saturated", can be used to describe that shrinkage zone. It means that a complete saturation is never reached. While three distinct zones were generally distinguished in swelling characteristic curves.(2) Pseudo-saturated shrinking zone was observed at the higher moisture content of shrinking characteristic curves of different texture soils with different heights of the soil cores. The pseudo-saturated zone of the soil with light texture and thin-layer was more obvious. The saturation moisture content and the specific volume at saturation point were affected by the bulk density of soil sampling. The bulk density of sampling was greater, the saturation moisture content and the specific volume at saturation point were smaller. The saturation moisture content and the specific volume at saturation point of soils used were decreased with the increase of the high of soil samples. Compared to disturbed soils, the shrinking of undisturbed soils was smaller, and the relative swelling was greater because of their better structure. With the change in textures from sandy loam to silty clay, the swelling-shrinking characteristics of soils became greater. The swelling-shrinking characteristics of undisturbed soils were greater than disturbed soils.(3) Considerations in soil swelling-shrinking cracks, although the shrinking characteristic curve of Heilu soil was complex, four main shrinking zones are also referred to as: (a) residual, (b) normal, (c) structure, (d) pseudo-saturated. Its swelling characteristic curve followed the"S"shape. Different processes took place when soils drying or wetting. At the beginning of drying, only vertical shrinking occurred. If soil drying proceeded, the stress in soil increased. Horizontal and vertical shrinkage both appeared while vertical direction was predominant at the moment. When the dry soil wetted, during the first stage it undergoes three dimensional volumetric expansions, because its desiccation cracks were still opened. In the second stage, after desiccation cracks were closed, the expansion is only one dimensional, causing the rising of soil level. The geometry factor values in different phases showed that soil's swelling-shrinking in vertical and horizontal was anisotropy.(4) The stepped Laplace interpolation method proposed in this paper was practicable. The continuous shrinking characteristic curve obtained by the method was reliable and could describe the shrinking process.The disturbed soils'shrinking and swelling characteristic curves could be fitted well by the three straight lines model. While the undisturbed soils'main shrinking zones were referred four zone. And the pseudo-saturated zone of the soil with light texture and smaller height of soil cores was more obvious. The geometry factor values in different phases showed that soil's swelling-shrinking in vertical and horizontal was anisotropy. The continuous shrinking characteristic curve obtained by the stepped Laplace interpolation method was reliable and could describe the shrinking process reality. This paper provide some data and analysis for quantitative evaluation of soil swelling-shrinking, and it can be useful for a better understanding of preferential flow and contamination transport in soils.