Differential frost heave and sorted patterned ground: field measurements and a laboratory experiment

Small-scale sorted stripes and circles dominate over the alpine regions in the Upper Engadin, Swiss Alps. Most of these features are spaced at less than 30 cm intervals by coarse margins shallower than 5 cm. Monitoring of sorted stripes indicates that shallow diurnal freezing often induces differences in amount and timing of frost heave by needle ice growth between fine and coarse stripes, while deeper diurnal freezing and seasonal freezing rarely differentiate heave amounts of the two materials. A laboratory simulation is designed to explore the contribution of differential frost heave to the sorting process at the monitoring site. The device consists of a fine part (loam) and a coarse part (a granule layer mounted on loam). Freeze–thaw tests with a granule mantle of 0.5, 2.5 and 5 cm thick demonstrate that the heave amount on the coarse part is greatly reduced by the 5-cm-thick mantle, and that the coarse part always heaves later but subsides earlier than the fine. As a result of this time lag in frost heaving, lateral soil movement towards the coarse part is generated within the surficial soil. Differential heave associated with diurnal freeze–thaw cycles is capable of sorting of the top 5 cm of soil.