Measuring Orientations of Individual Concealed Sub-Vertical Discontinuities in Sandstone Rock Cuts Integrating Ground Penetrating Radar and Terrestrial LIDAR

Vertical or sub-vertical discontinuities striking parallel to rock cuts are dangerous because toppling and spontaneous raveling failures can initiate from these surfaces, creating hazards below. At the same time, the surfaces of these discontinuities are often concealed because they do not "daylight" into the rock, and any trace of the discontinuity that might be seen at the top of the rock cut is obscured by overburden. These hidden discontinuities can often be detected by ground penetrating radar (GPR). Our new method uses GPR in conjunction with terrestrial LIDAR (light detection and ranging) to accurately measure the orientation of these hidden discontinuities. The method presented in this article establishes three control points on the surface of the rock cut. At each control point the global coordinates are remotely measured using LIDAR. GPR soundings at each control point are used to measure "the perpendicular horizontal distance" (depth) from each control point on the rock cut face to any discontinuities hidden behind the rock cut face. The true perpendicular distance is added to the GPR coordinates at each control point to form three new control points on the surface of each of the hidden discontinuities. Using the three-point method, the orientation of the hidden discontinuity is calculated.