Abstract The process of physical ripening of a soil developed on recently exposed marine clay has been studied in detail by examining the swelling of soil clods subjected to a number of drying and wetting cycles. During the wetting cycle, solutions with three different CaCl 2 concentrations were used to study the effect of salts on the swelling. The swelling behaviour was represented by a linear swelling characteristic showing that the concentration of solutions clearly influences the swelling. Based on the swelling characteristics, quantitative aspects of physical ripening were investigated in terms of the irreversibility of extracted soil water. A linear relationship was noted between the moisture ratio at the threshold of irreversible extraction and the initial moisture ratio. Based on the relationship, a physical ripening index (PRI) is proposed, which can be used together with the initial moisture ratio to indicate stages in the ripening process for these soil materials. A greater proportion of water could be reversibly extracted from soils which had undergone increasing degrees of ripening.