Development of a continuous 3D-monitoring system for unstable slopes using TDR

In recent years Time Domain Reflectometry (TDR) has been introduced as a new system for landslide monitoring. Until now no standards have been defined, how to install a TDR system or how to process the received data, especially when multiple TDR measuring points are connected to produce a 3D model of the deformation zone. A TDR system basically consists of two components: a coaxial cable and a TDR measuring device ("cable tester"). The cable is installed into a borehole and coupled to the surrounding rock mass with grout. The cable tester sends an electric pulse through the coaxial cable and receives and analyses its reflection. Changes in the geometry of the coaxial cable due to movements in the surrounding rock mass produce characteristic TDR signatures, which allow an approximate determination of type and amount of movement. The exact location of the deformation can be acquired by measuring the time span between the initiation of the electric pulse and the detection of its reflection, since the propagation velocity of the electric pulse within the coaxial cable is constant. TDR measurements are influenced by a great variety of parameters. Beside the deformation itself, the used cable and grout types are surely the most significant factors which have to be taken into account. In ongoing research attempts are made to define a couple of different TDR measuring-system configurations, where each is designated for a specific geological environment. These configurations are then calibrated in laboratory shear tests and finally tested in the field in real landslides, if possible by comparing them with inclinometer measurements. By combining several of these calibrated TDR measurements positioned in a pattern or along a profile within a landslide, better knowledge of the position, width and type of the deformation zone can be achieved at comparably low cost. Since the TDR data can be acquired continuously as well as remotely, the effect of outside influences (e.g. rainfall) on the landslide can be determined nearly in real time.