Moisture Diffusion in Shallow Clay Masses

Seasonal cycles of moisture and suction variation in shallow clay masses create repeating episodes of soil shrinkage and swelling that can adversely affect a wide variety of structures including pavements, shallow foundations, piers, and slopes. Design of such structures requires a means of adequately characterizing the depth of this moisture active zone and the magnitude of suction variations within the zone. This paper describes an analytical framework for characterizing suction variations in the moisture active zone and for estimating the soil mass moisture diffusion coefficient, one of the critical parameters governing the rate of moisture penetration in the soil. The paper presents extensions to an existing analysis for sinusoidal variations in surface suction to general nonsinusoidal conditions. It also presents a review of moisture diffusion coefficient data obtained from field measurements and shows that these data exceed laboratory measurements on intact soil specimens by up to two orders of magnitude. A conceptual model of moisture diffusion in a fractured soil mass provides an explanation of the differences between the field and laboratory values of the moisture diffusion coefficient.