Laser elevation measurements of a smectite-rich mudrock following freeze-thawing and wet-drying cycles

Abstract

A laboratory experiment was conducted to study the effect of thermic and water content changes on the disintegration of a smectite-rich mudrock near Vallcebre, in the Eastern Pyrenees range. Undisturbed, unaltered fresh mudrock samples were subjected to freeze-thawing and wet-drying cycles and surface roughness changes were measured with an automated non-contact laser profile meter. Each freezing event during a freeze-thawing cycle produced a vertical expansion of the sample due to ice-crystal growth and swelling. Total volume changes were recorded as elevation differences between consecutive microrelief measurements. After each thawing cycle, the samples only partially recovered, indicating the presence of hysteresis effects due to physical weathering. Photographs of vertically oriented thin sections impregnated with fluorescent resin, before and after thermic treatments, showed significant modifications of the internal structure. Conversely, wet-drying cycles showed less disruption of the sample structure, although there were some indications of compaction after repeated drying. The laser profiling technique therefore enabled measurements to be made of volumetric and microtopographic changes induced by freeze-thawing and wet-drying cycles in the samples studied. The re-arrangement of the bulk micro and macrostructure is not reflected in the roughness index (σ R) of the soil surface.