Contactless laser positioning system for tracking three-dimensional kinematic behaviour of rock slope fractures and active faults tested on a large rockslide in the Western Alps

Discontinuities within large rock slope failures are able to provide critical insights into the prevailing stress and strain regimes governing the behaviour of these unstable rock masses. To capture the true displacement vectors-essential for accurate failure mode characterisation and computation of the near-surface stress state-information about their kinematic behaviour must be obtained in three dimensions. Yet only a few extensometers have been designed for this purpose. This technical note presents an innovative contactless laser positioning system that employs a triangulation approach to generate three-dimensional records of fault and fracture behaviour in near real time. The positioning system comprises a sensing unit-a pair of laser modules, a digital camera, and a customised target-and a control unit together with a processing unit hosted on a remote server. Images of the target are acquired at regular intervals and transmitted to the server, where a semiautomated image processing protocol transforms the beam patterns into accurate three-dimensional measurements. Its accuracy is characterised by a maximal standard deviation in a single-axis measurement of 0.04 mm, and its precision is estimated to be better than 0.02 mm. Two of these devices have been tested on a large rock slope failure located on the eastern flanks of the Aiguilles Rouges in south-eastern France. Although the site posed a range of technical and logistical challenges, it is clear that the laser positioning system is capable of detecting complex kinematic fracture behaviour, opening the door for its integration into more comprehensive landslide monitoring programs.