Soil evaporative fluxes for geotechnical engineering problems

The transfer of water across the soil-atmosphere surface occurs as infiltration and evaporation. The mechanics of the infiltration into the soil surface is well understood and has been widely addressed in the literature. Alternately, the process of evaporation from the soil surface is poorly understood. A theoretical approach for evaluating the rate of evaporation from an unsaturated soil surface is developed. The theory is based on the principles of Darcy's Law and Fick's Law to describe the flow of liquid water and water vapour in the saturated-unsaturated soil below the surface. Dalton's Law and a modified form of Penman's Method for evaporation are utilized to predict evaporation from the soil surface.;Drying tests were conducted using 3 distinct soil types of sand, silt and clay. The soil surfaces were found to evaporate at the same rate as free water surfaces when saturated. The rate of evaporation begins to decline once the soil surfaces become unsaturated and total suction exceeds approximately 3,000 kPa. The rate of evaporation is proportional to total suction and continues to decline as suction increases. This principle appears independent of soil type and is universal for the three texturally distinct soils selected for testing.;The proposed theory was used to simulate the results of a 42 day evaporation test for a column of a fine, uniform, clean sand. Good agreement was generally found between the computed and measured values of evaporation rate, soil water content and soil temperature. Additional analyses were conducted using various values of the saturated coefficient of permeability and the pore-size distribution index. The computed evaporative fluxes were found to be very sensitive to the permeability of the soil. Varying the coefficient of molecular diffusion for water vapour was also found to influence the rate of evaporation.;The modified Penman expression was applied to an example evaporation problem for Saskatoon, Saskatchewan during a 10-day period in July. The evaporative fluxes were computed with the table positioned at several depths below the surface of the sand. Evaporative rates were found to vary widely between the full potential rate of 7.7 mm/day and 0.4 mm/day, depending on the position of the water table. In general, the results showed that the rate of evaporation from a soil surface depends strongly on the groundwater conditions.