On the potential of Ground Penetrating Radar to help rock fall hazard assessment: A case study of a limestone slab, Gorges de la Bourne (French Alps)

The stability of a potentially unstable rock mass is highly sensitive to the continuity of the joints cutting it. The rock fall hazard assessment, generally based on surface geological observations and on a simple geomechanical model, suffers from the lack of information on the 3D geometry and the properties of the joints. This case study investigates the potential of Ground Penetrating Radar (GPR) measurements regarding hazard assessment in hard rock. GPR was used to detect and characterize joints that could cause the failure of an overhanging rock mass located above a road in the "Gorges de la Bourne" (France). GPR data were acquired using high frequency shielded antennae (500 and 800 MHz) to assess the extension of these joints and to detect other possible fractures. Combined with a velocity profile deduced from a Common Mid-Point (CMP) analysis, these measurements provided detailed images of the fracture network, which were locally consistent with turn rate velocity measurements performed within two horizontal boreholes. The rock bridge percentage in the critical joints was estimated from GPR data to be too low to ensure the stability of the overhanging rock mass. Consequently, mining of the rock mass was decided on. Afterwards, a comparison between GPR images and the real joints observed after mining also confirmed the GPR interpretation for the location and the extension of the joints. (c) 2007 Elsevier B.V. All rights reserved.