Loess and fragipans: Development of polygonal-crack-network structures in fragipan horizons in loess ground

One of the defining features of the fragipan horizon is the presence of a blocky polygonal network structure. In loess soils, this network structure can be explained by contraction forces (due to drying) operating after hydrocollapse due to loading and wetting, as in the Bryant hypothesis for fragipan formation. Three stages are identified in the formation of a fragipan horizon in loess ground. There is a deposition phase in which the aeolian deposition of loess material produces certain ground properties. A collapse stage allows the soil structure to deform under the influence of loading and wetting. This collapsed material develops internal tensile forces as drying contraction proceeds and these cause the development of a characteristic crack network. The crack network can be modelled using a very simple Monte Carlo approach and the two dimensional structure produced gives a good representation of fragipan cracking. The collapse-contraction process for fragipan formation offers explanations for the strength and hardness of fragipans, the constant depth to fragipan horizons, the slaking in water (predominance of short range contact bonds) and the mineralogical similarities throughout the system. The fragipan horizon impedes drainage, and this becomes increasingly important as land use becomes more widespread.