On the variability of fracture surfaces in unsaturated chalk

The topographical variations in a fractured chalk surface were studied by laser scanning in laboratory experiments after cycles of immersion in tap water and air drying. The surfaces were found to erode by up to 0.295 and 0.352 mm following wetting for 10 min and 14 hours, respectively. Topographical changes were related to the nonuniform release of particles from the surface under no-shear flow conditions. The total amount of particles released decreased exponentially with time during a 96-hour experiment, from 11 mg/L following a wetting period of 10 min (at the beginning of the experiment) to 1 mg/L following a wetting period of 48 hours (at the end of the experiment). These preliminary results suggest that under conditions of variable water content (highly significant in arid and semiarid regions), the aperture, roughness, and flow channels of fractures in soft rocks are transient properties. In the field site, fracture apertures are >1 order of magnitude smaller than expected from the laboratory observations. This discrepancy is related among other possible reasons, to partial wetting and drying cycles and to the development of flow channels within the filling materials rather than along the fracture surfaces.