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Re-visitation of Actual Evaporation Theories
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- Author / Creator
- Tran, Dat T.Q.
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Evaporation from a deposit of thickened or paste tailings or cover systems is increasingly becoming a big challenge for geotechnical engineers. Accurate calculation of the actual evaporation from a saturated-unsaturated surface requires accurate specification of vapour pressure or relative humidity at ground surface. Flux of water at the soil boundary is also a boundary condition of moisture for the analysis of the problem of the flow of water at the ground surface. The evaporation of water from a water surface known as potential evaporation is quite well understood. However, the evaporation of water from a saturated-unsaturated surface known as actual evaporation needs to be re-evaluated. Several methods of estimating evaporation from unsaturated soil surfaces can be found in the literature. According to these methods, the actual rate of evaporation has been calculated on basis of the total suction or relative humidity predicted at the soil surfaces. Total suction not only depends on the character of the soil matrix but also on salt concentration in the pore-water. Currently, the accuracy of these methods can be questioned since they overestimate the actual rate of actual evaporation. To overcome this deficiency, either a variety of adjustment factors of total suction or modified relative humidity at the soil surface have been used to compute evaporative flux from the soil surface. In this thesis’s work, the fundamental physics of water transfer from a soil surface are re-considered. The mechanism of mass and heat transfer and the derivation of the equation of evaporation are also re-visited. At the end, a theoretical model (i.e., new soil-atmosphere flux equation) is developed for prediction of evaporation rate from a soil surface using the concept of “surface resistance” to vapour water diffusion from the soil surface to atmosphere. Soil suction and the corresponding water content at which the actual rate of evaporation begins to depart from potential rate of evaporation during drying process are re-assessed using a series of laboratory data (i.e., thin soil section drying tests and soil column drying tests) collected from the research literature. It is observed that soil suction at which the actual rate of evaporation begins to reduce from potential one for soil columns may be different from thin soil sections. For example, the value of suction at evaporation-rate reduction point appears to be approximately 3,000 kPa for the thin soil sections regardless of the soil texture. However, it is observed that this suction appears to be in between the air-entry value and residual soil suction for the soil columns. As a result, a formula to determine soil suction at evaporation-rate reduction point is derived for soil columns. A new set of equations related to the coefficient of surface moisture availability, vapour pressure at soil surface and soil surface resistance is then proposed. The effect of pore-water salinity on the evaporation rate from salinized soils was also considered. A function of osmotic suction which depends on initial salt content and volumetric water content at soil surface is derived for thin soil layers during drying process and verified using data of osmotic suction measured in the laboratory testing program. Drying tests on thin soil layers as well as thick soil layers were conducted using the non-saline and salinized soils (i.e., the selected sand and silt). The obtained results were utilized to verify the proposed equations. Good agreement was generally found between the computed and measured rate of evaporation. In addition, these equations were also verified using the evaporative data collected from the research literature. The findings throughout this thesis will help solve the challenge of predicting evaporation from non-saline and salinized soil surfaces with which the geotechnical engineers are facing in many practical problems.
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- Graduation date
- Fall 2013
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.