Digital rock mass discontinuity survey

Abstract

Rock mass discontinuity survey is conducted to examine rock mass stability. The use of remote sensing techniques including photogrammetry and laser scanning could generate point cloud data for computing the discontinuity data of rock slope. The objective of this project was to study to what extent a digital rock mass discontinuity survey using the remote sensing techniques and existing data analysis approach are effective in automatically identifying rock discontinuities under different undesirable site condition or accessibility. This study focused on the performance on discontinuity plane orientation which is the primary parameter required in kinematic analysis of rock slope stability. Two registered rock slopes Feature No. 11NW-D/C80 at Shun Yung Street and No. 11NW-A/C134 at Wing Cho Street were chosen in this project. The major focus was to compare how the presence of disturbances like wire mesh, vegetation, and distance from slope would affect the point cloud quality. Conventional mapping was conducted to manually collect the rock discontinuity data as the reference dataset. Point cloud data of the rock slopes was obtained from three methods, the photogrammetric method of Structure from Motion, laser scanning using mobile laser scanners, and existing point cloud data obtained from GEO (2021) and the consulting company of the slope works of C134. The processed data was input into Discontinuity Set Extractor (DSE) to identify the discontinuities in point cloud. The results showed no clear evidence on whether close-range photogrammetry or close-range mobile laser scanning was more accurate in terms of the identified discontinuity plane orientations and discontinuity sets. For data collected closely to the slope, the presence of wire mesh did not cause great variations in the discontinuity identification results. Point cloud collected distantly with vegetation and wire mesh on slopes were unsatisfactory as the noises were difficult to be removed during data processing.