Deriving roughness characteristics of rock mass discontinuities from terrestrial laser scan data

In this study, 3D terrestrial laser scan data have been used to quantify the surface roughness of rock mass discontinuities, to determine the roughness scale effect and to determine the influence of the range of precision. The Roughness-length method was applied to calculate fractal parameters (fractal dimension and roughness amplitude). The study showed that it is possible to calculate the fractal parameters from laser scan data of discontinuity surfaces. It has been found that the fractal dimension and roughness amplitude are very high for raw laser scan data, due to range measurement error of the used scanner. The discontinuity surface has been reconstructed from the raw point data using a scattered data interpolation technique. The range measurement error can be removed using this data interpolation technique. The reconstructed surface visually resembles the actual discontinuity surface. The fractal parameters of the reconstructed surface have subsequently been calculated. By subtracting the fractal parameters of the raw data from the fractal parameters of the reconstructed surface, the range error of the laser scan data can be estimated. The fractal parameters have also been calculated using different sample sizes. The fractal parameters show an increase with increasing sample size. This means that the roughness complexity of the discontinuity surface is also increasing with increasing sample size. From this observation it can be concluded that the roughness of discontinuity surfaces has a significant scale effect. The maximum resolution of the laser scan data used in this study is 5 mm (1 point per 5 mm). The laser scan data with 5 mm resolution can only acquire roughness information from roughness features larger than 5 mm. Consequently, small-scale roughness (roughness features smaller than 5 mm) can not be obtained from the data. Therefore the resolution of the laser scan data (point density) needs to be increased to quantify discontinuity surface roughness accurately as a smaller scale