Combining structural data with monitoring data in open pit mines to interpret the failure mechanism and calibrate radar alarms

In a moderately jointed rock mass, slope failure is generally defined by the presence and orientation of discontinuities that act as planes of weakness within the rock mass, thus controlling the size and the direction of ground movement. For this reason, when dealing with structurally-controlled slope movements, a method to support the interpretation of monitoring data could be to combine displacement records with structural data, especially when the monitoring system relies on radar. Since movement measurements provided by radar are line-of-sight (LOS), thus providing only magnitudes but not directions of slope displacement, they cannot be used for understanding the 3D failure kinematics and the corresponding behaviour. This poses a challenge in the interpretation of the deformation data provided by the radar and in the set-up of alarm thresholds that commonly assume that monitoring data provides true (total vector) movement, which could be significantly different from that measured by radar. In order to reconcile the directional component of radar measurements with actual movement, the authors investigated the use of kinematic analyses to interpret true movement from LOS data. An evolution of the kinematic analysis process was introduced with the concept of the ‘kinematic hazard index’ for each instability mechanism. Based on such a concept, it is possible to obtain maps of the relative probability of each of the different failure modes (plane failure, wedge, direct toppling, flexural toppling, and rockfall). The kinematic maps can become an essential input for the interpretation of radar data, allowing the slope engineer to associate measured ground movements with a possible failure mechanism. Moreover, from the kinematic maps, it is possible to derive the most likely direction of movement of each kinematic block, thus enabling the estimation of the sensitivity to movement measured by radar to an estimation of true movement for each sector of the pit. As a consequence, kinematic maps can also be used to refine the radar alarm thresholds by correcting the LOS measurements with respect to their sensitivity to the expected direction of movement. Within the framework of a research project funded by the LOPII, a methodology to implement the above described approach aimed at calibrating the radar alarms and at supporting the interpretation of radar alarms was developed and tested in a few selected mine sites owned by LOPII sponsors, where structurally controlled slope movements at different spatial scales represent a common issue and where slope monitoring radar units and robotic total stations were already deployed. The proposed and tested approach represents an operational methodology to support the understanding of the relationship between observed deformations and structural stability controls. In addition, the output of the analysis is aimed at improving the effectiveness of the slope monitoring program thanks to the input provided to a better selection of alarm thresholds.