Estimation of the appropriate sampling interval for rock joints roughness using laser scanning

The rock joint roughness greatly affects the mechanical behaviors and permeability characteristics of rock mass. To accurately and effectively estimate the roughness of rock joints, an appropriate sampling interval is required to be specified during the point cloud collection using laser scanning systems and the roughness coefficient calculation. Firstly, seventeen point clouds of rock joints with sampling size ranging from 60×60 to 2000×2000 mm was collected using a terrestrial laser scanner in the field to investigate the effect of sampling size on the selection of the appropriate sampling interval. Then, a portable laser scanner was employed to acquire the dense point clouds of thirteen small-scale rock joint specimens, which have various roughness levels, to assess the influence of roughness level on the appropriate sampling interval. Furthermore, fitting analyses were performed to establish relationships between the appropriate sampling interval vs. sampling size and the appropriate sampling interval vs. roughness level, respectively. The results show that the curves of sampling interval vs. roughness coefficient followed a power distribution, and the inflection point with the maximum curvature could be regarded as the appropriate sampling interval; the appropriate sampling interval increased with the sampling size, while a negative correlation between the appropriate sampling interval and roughness level was observed. To facilitate the application of findings into practical engineering, a simplified workflow was provided to determine the appropriate sampling interval: 0.015×sampling size and 0.03×sampling size were recommended to be considered as appropriate sampling intervals for the rock joints with high-level and low-level roughness, respectively.