Quantifying surface roughness of weathered rock - Examples from granite and limestone

It is well established within the geotechnical community that weathering affects rock in a variety of ways. Weathering not only degrades engineering properties but also changes the surface appearance. Weathering classifications are partially based on the qualitative assessment of the surface appearance of rock. This study focuses on quantifying surface roughness of weathered rock. Surface textures of both granite and limestone specimens were collected using a commercially available laboratory laser scanning system. Point clouds were analyzed using two different techniques to assess surface roughness. Granite surfaces were assessed using triangulated point clouds and surface normal vectors. Limestone surfaces were assessed using statistical methods to describe deviations from a cylindrical shape. The most highly weathered granite had the largest range of surface normal vector orientation. Less weathered granites had smaller ranges of surface normal vector orientation. Results from weathered limestones were more ambiguous. The least weathered specimens had very small deviations from a perfect cylinder whereas the most weathered specimens had the greatest deviation from a perfect cylinder. However, no clear distinction could be made between deviations from a perfect cylinder for intermediate stages of weathering for limestone specimens. Close range laser scanning was able to capture surface textures from both granite and limestone specimens, however, relationships between weathering grade and surface texture were only statistically significant for granite specimens.