Rock Profile Roughness Parameters Appropriateness for Joint Surface Geometrical Characterization

Roughness influences considerably rock joints shear strength, and, consequently, rock mass stability, especially slopes. However, the most common rock mass classification systems, as well as other analysis methods, introduce rock joint roughness by subjective personal estimation. This subjective bias can be avoided by applying roughness parameters calculated through mathematical algorithms to digitized profiles or surfaces. However, a correct rock joint roughness parametrization has not been achieved yet because of the geometrical complexity of these natural surfaces. This paper shows the behavior of the most common roughness profile parameters in different rock joints. Two physical digitization procedures, by means of stylus profilometers, were applied; one is to be applied in field and the other in laboratory; they permitted spatial sampling intervals of 0.8 and 0.0625 mm respectively. In order to select a wide range of roughness morphologies, samples from different rock types were chosen: a quartzite showing a stepped profile, a micaceous schist with a smooth but positively stepped profile, two calcareous schists similar to the micaceous but not so explicitly smooth and stepped, a dolomitic marble, smoother and more undulated than the others, and a granite, undulated and averagely rough. It was found that among the roughness parameters, the first derivative root mean square (Z2) is not appropriate for stepped profiles characterization because of its huge dependency on a single "step" in the profile, which means a big difference on Z2 value depending on just one slope among two points in the profile and consequently an important sampling bias due to the area of the joint surface where the profile is recorded. In opposition to this, the variability of the joint roughness parameters in the granite sample was much smaller, for all the roughness parameters applied.