Segregated ice and liquefaction effects on compaction of fragipans

The brittleness of fragipans is generally ascribed to the close-packing fabric arrangement acquired at the initial step of pedogenesis thanks to physical processes. However, there is an on-going debate over the agent causing soil densification. In this work, we tested the plausibility that ice segregation or liquefaction could have been the cause of the compaction of four fragipans. Two of them are located in nonseismic areas that have experienced periglacial conditions; one is from a strongly seismic area not affected by periglacial conditions, while the fourth site underwent moderate seismic activity and slight periglacial conditions. After disaggregation in the laboratory, soil specimens were submitted to freeze-thaw cycles and vibrations at different amplitude and duration, either dry or water-saturated. Analyses of aggregate stability, bulk density, porosity, and pore-size distribution were made on natural and treated specimens. Results indicated that the compactness arose mainly from the close-packing arrangement of particles. The freeze-thaw cycles were able to reproduce only some of the features in the water-saturated specimens, independent of whether they came from periglacial or seismic areas, while those from seismic areas successfully acquired the original arrangement after vibrations-induced liquefaction. This different behavior could be partly explained by the fact that consolidation after liquefaction occurs only if a material with proper particle-size distribution and mineralogical assemblage is saturated by solutions able to disperse phyllosilicates and promote their face-to-face arrangement. Our findings support the hypothesis that liquefaction of soil material due to earthquakes could indeed provide a dense parent material in which the fragipan may develop through pedogenesis.