The hydraulic behaviour of sand and silt soils around the residual-state condition

Abstract

Geotechnical and geo-environmental engineering problems may require the computation of near-ground-surface water balances. Evaluation of the unsaturated coefficient of permeability function is often necessary in order to undertake numerical simulations associated with the water balance evaluations. Evaporation at ground surface has the potential to reduce the water content of the soil to values less than the residual water content. However, it appears that the accuracy of commonly used methods for the determination of the permeability function around residual-state conditions is unknown. There may be lack of accuracy due to an oversimplification of the physics of water movement around the residual-state condition. Evaluation of the coefficient of permeability function around the residual-state condition requires reliable experimental data in the low water-content range. In this study, the concept of residual-state condition is reviewed, and a definition of the conditions suitable for geotechnical engineering practice is suggested. A transition zone for the soil-water content/soil-suction profile is defined for steady-state flow systems. A possible link between the limits of the transition zone and the residual-state condition is proposed. A method is developed for predicting the unsaturated coefficient of permeability, based on a new definition of the residual-state condition. The method is based on the theory of vapour-phase flow and on the soil-water characteristic curve. A series of evaporation tests were conducted in an environmentally controlled room on two different types of soil samples: sand and clayey silt. The unsaturated coefficient of permeability functions for the selected soils were established. The steady-state evaporation method used in this study proved to measure the unsaturated coefficient of permeability function in the low-water content range. The results obtained from the predictive method proposed in this thesis are compared to the experimental data and to the data predicted by the previously proposed methods. Predictions computed when using the new method appear to be more accurate than those from previously proposed methods. It was not possible to draw firm conclusions from the tests performed regarding the relationship between the residual-state condition and the upper and lower limits of the transition zone of the water-content/soil-suction profile.