Structure development in surficial heavy clay soils: A synthesis of mechanisms

This paper presents a synthesis of mechanisms related to structure development of surficial heavy clay soils. These clay soils develop specific structural features due to wet/dry cycles and desiccation cracking they undergo during soil "ripening". There is substantial field and laboratory evidence to indicate that clay soils generally develop stable structures with stable material properties when they ripen under repeated wet/dry cycles of climatic change. This development occurs as a result of the re-arrangement of the soil particles to minimise the potential or free energy. Available evidence indicates that under field climatic conditions, swelling/shrinkage of clay soils occur predominantly due to water loss from interparticle and interaggregation pores. Vertisols or heavy clay surficial soils can develop special geomorphological features such as gilgai. Mechanisms of gilgai formation are also analysed, and their origin is related to the initial pattern of soil desiccation cracking. The process of shrinkage cracking and associated volume change in soils is explained on the basis of unsaturated soil mechanics theory. Crack patterns are divided into orthogonal and non-orthogonal patterns, and the conditions that lead to the development of these crack patterns are highlighted. Finally, a conceptual approach for modelling of the desiccation cracking process is presented.