Hydro-geophysical observations for an Advanced Understanding of Clayey Landslides

As a consequence of climate change and, thus, in hydrological cycles, the risk of landslides is growing worldwide. Therefore, there is an urgent demand of a short-time landslide prediction tool towards risk mitigation. Here, we propose a multidisciplinary approach which consists in the development of hydrogeological and geophysical measurement and interpretation techniques for the real-time monitoring of processes in landslides. To achieve this, our approach combines the implementation of emerging technology combined with advanced numerical modelling. The advantage of the approach here proposed is that our monitoring system permits the collection of hydrogeophysical data with high spatio-temporal resolution, but over long time continuous monitoring, as required to understand the deformation and triggering factors in landslides. We propose the application of the Induced Polarization (IP), an extension of the electrical resistivity method, to delineate the occurrence of clay minerals, which are characterized by a characteristic IP response. For a better interpretation of the IP imaging results and to understand the landslide triggering mechanism before the onset of a displacement, additional parameters have to be monitored. Typical approaches consider GPS receivers and total station benchmarks at the surface, or inclinometers at depths, which provide only punctual 1D information, but have limitations at high displacement rates. To solve interpretation ambiguities, but also to account for spatial changes, in our proposal we consider horizontally and vertically (borehole) distributed displacement/strain measurements. In addition to this, new approaches will be applied, namely temperature and strain monitoring at high frequency with Fiber-Optic (FO) cables both at the surface and in boreholes and sensing of surface deformation with Ultra-High Resolution (UHR, 20 cm) optical images. The combined application of these methods for landslide monitoring is very rare. Coupled multi-physical modelling simulations to gain the understanding of underlying processes will support the joint interpretation of available monitoring datasets. Our proposal benefits from the European landslide-monitoring network established in 2009 by the Geological Survey of Austria (GSA) in collaboration with national and international partner such as the French Observatory on Landslides (OMIV), established by the National Centre of Scientific Research (CNRS). In this regard, the project account with significant amount of information, collected over the last years to calibrate and evaluate predictions resulting from models developed in our investigations. The co-application in this proposal by the GSA and CNRS provides continuity to this international and multidisciplinary approach, but also permits to involve other research institutions, such as the Vienna University of Technology (TUW) to contribute in relevant investigations and the knowledge transfer.

The goal of the Hydroslide project was to gain a better understanding of the hydrological conditions and their changes over time in clay-rich landslides, in order to make a better estimate of the mechanisms of the respective slope movement activity. In order to implement this project, three landslides (one in Austria, two in France) were equipped with monitoring systems. Additionally geophysical, geotechnical, geological and petrophysical investigations were carried out. In particular, the two locations in France (La Valette, Ldeve) have been already under scientific observation for years, so some data was already available. The location in Austria (Wolfsegg am Hausruck) was selected in cooperation with the torrent and avalanche control (WLV) and the ZT Moser/ Jaritz, since there was a large demand in knowledge about the current slope activity. The expensive drilling was financed by the WLV with funds from the Disaster Fund. Additionally data acquired by the WLV was provided for scientific use. The Austrian part of the project was largely devoted to the development of a new geoelectric monitoring system that was extended to induced polarization (IP). In addition to the electrical resistivity, this method allows to measure the chargeability of the substrate. In the case of clay-rich soil, with this parameter conclusions can be drawn about the clay mineralogy and the pore space distribution. In monitoring operation, the temporal changes in water saturation and pore space distribution are of interest. At the end of the project the new IP instrument (Geomon4D-IP) was ready for standard field; extensive tests are still necessary for monitoring operation. Due to this situation, the predecessor measuring system (Geomon4D), which only records the electrical resistivity, was used. To get information about the chargeability, commercial measuring devices were used for repeated IP measurements. In addition, a short-term IP monitoring could be implemented at another location in Austria (Gresten (Lower Austria)). In this way, it was possible to produce a data record that includes the change of chargeability in time. This data set has shown that the relationship between the IP effect and other influences is extremely complex, but can provide interesting information. In the course of the extensive IP (repeat) measurements, improvements in data acquisition and subsequent data evaluation were developed. In addition, methods that enable an optimal merging of diverse (monitoring) data were improved for a detailed interpretation (geological, hydrological). In total, geological and hydrological characterizations could be created for all three locations (different level of detail). The relationship of the hydrological conditions with different phases of slope activity could only be partially understood. Here the enormous complexity of the mechanisms seems to prevent a better understanding or the available data does not seem to be sufficient.