Structure and porosity of smectitic mudrocks as affected by experimental wetting-drying cycles and freezing-thawing cycles

The erosion rates in the Vallcebre basin (Pyrenees) are controlled predominantly by the physical weathering rate of bare regolith, i.e. mudrocks. To obtain information on the influence of thermic and moisture regimes on the weathering dynamics, laboratory experiments were carried out to investigate how freezing-thawing or wetting-drying cycles, under controlled conditions, affect structure and porosity. Three different clayey mudrocks were selected to represent materials more susceptible to erosion in the studied area. The three lithologies, identified as CA1R, CA2R, CARR, differed mainly in mineral constituents, type and quantity of the clay minerals and cation exchange capacity. An automated non-contact laser profile meter was used to analyse surface microtopography in order to monitor random roughness variations and to calculate bulk density. Porosity and pore size distribution were carried out with image analysis. Freezing-thawing cycles were the most active in modifying soil surface and bulk structure. Roughness of CA1R and CA2R was more than doubled at the end of freezing-thawing cycles. This variation was mostly associated with the whole surface elevation related to the bulk volume increase. Roughness variations in CARR were less pronounced and related to a modification of the microtopography. Micromorphological observations revealed a platy structure formation induced by repeated ice lensing. Also porosity was increased by freezing-thawing cycles, with the formation of a large number of irregular and rounded pores. Wetting-drying cycles did not affect surface roughness, and modified bulk density very little in all samples. They increased porosity by the formation of large cracks and fissures. Overall, data from wetting-drying cycles showed that changes in water content alone did not produce deep structural modifications. A mechanism of alteration of soil structure during freezing-thawing cycles is proposed.