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 pre dominantly by the physical weathering rate of bare regolith, i.e. mudr ocks. To obtain information on the influence of thermic and moisture r egimes on the weathering dynamics, laboratory experiments were carried out to investigate how freezing-thawing or wetting-drying cycles, und er controlled conditions, affect structure and porosity. Three differe nt clayey mudrocks were selected to represent materials more susceptib le to erosion in the studied area. The three lithologies, identified a s CA1R, CA2R, CARR, differed mainly in mineral constituents, type and quantity of the clay minerals and cation exchange capacity. An automat ed non-contact laser profile meter was used to analyse surface microto pography in order to monitor random roughness variations and to calcul ate 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 vari ation was mostly associated with the whole surface elevation related t o the bulk volume increase. Roughness variations in CARR were less pro nounced and related to a modification of the microtopography. Micromor phological observations revealed a platy structure formation induced b y repeated ice lensing. Also porosity was increased by freezing-thawin g cycles, with the formation of a large number of irregular and rounde d pores. Wetting-drying cycles did not affect surface roughness, and m odified bulk density very little in all samples. They increased porosi ty by the formation of large cracks and fissures. Overall, data from w etting-drying cycles showed that changes in water content alone did no t produce deep structural modifications. A mechanism of alteration of soil structure during freezing-thawing cycles is proposed.