Impact of coating and weathering on the properties of chalk fracture surfaces

The surface topography of a natural fracture intersecting a chalk formation was mapped using a high-resolution laser-scanning device, and its roughnes s was evaluated using five different criteria. Two coated and two uncoated slices of the natural fracture surface were encased in flow cells and expos ed to shea (24 and 8 hours) interactions with percolating synthetic rainwat er and industrial wastewater, followed by long drying periods (weeks), Thes e flow experiments simulated the intermittent infiltration of surface runof f and industrial effluents typical of the study area and resulted in erosio n of the fracture surfaces and modification of their roughness, The tempora l variations in surface roughness were evaluated using three commonly used criteria (standard deviation from mean heights, differences between maximum and minimum heights, and the root-mean-square roughness) and two newly dev eloped measures, namely, the slopes on the surfaces and a calculated roughn ess-related ratio (RRR). The mean slope maps and RRR calculations were foun d to be good criteria for evaluating surface topography and its uniformity across the fracture surface. The coated surfaces were significantly rougher , had greater relief, more heterogeneous topography, and a larger surface a rea than the uncoated surfaces. The results suggest that coating and fast w eathering (caused by intermittent flow events) significantly change roughne ss, surface area, relief, and the heterogeneity of a fracture surface: Thes e, in turn, may affect the spatial distribution of flow paths across that f racture surface, Consequently, when exploring the properties of fracture su rfaces and fracture flow, the impact of surface skin and variations of the surface properties over short timescales should be taken into account.