Insights from a combination of surface and deep measurements to set a long-term monitoring system of a complex, slow-moving landslide in Lower Austria (Austria)

Landslides are one of the most important and frequent geological hazards worldwide. Among the many different types and processes, slow and very slow mass movements are often underestimated, even if they can impact local infrastructures and permanently affect agricultural practices and land use planning. Slow-moving landslides are common in clay-rich layers and areas that are typically characterized by mechanically weak materials. In the field of slow-moving landslide monitoring, understanding the factors driving the slope instability is the key to assessing the landslide hazard and to supporting local authorities in hazard management.

In this study, the first results of an ongoing monitoring setup for a complex, slow-moving earth-slide system in Lower Austria are presented. The Brandstatt landslide is located in a complex geological transition zone between the Flysch and Klippen zones, which is known to be prone to shallow and deep landslides because its susceptibility to sliding processes has been investigated in recent years. Considering the predisposing conditions (geological and climatic settings), the unstable slope can be considered as a representative site of complex landslide processes in this region.

Landslide movements monitoring includes a combination of surface and subsurface methods to investigate the spatio-temporal evolution of factors that prepare, trigger, and control landslide dynamics. Geological characterization of the subsurface was obtained through a dynamic penetration test (DPH) campaign and percussion drilling. In addition, the subsurface displacements and potential shear planes were evaluated using repeated inclinometric measurements. A meteorological station is also installed on-site, as well as piezometers and time-domain reflectometry (TDR) sensors in selected locations on the slope. These instruments provide high temporal resolution data, which are automatically transmitted to a server for the real-time monitoring of hydrometeorological conditions. However, the monitoring strategy to detect surficial changes is currently limited to the application of Terrestrial Laser Scanning (TLS) because an Unmanned Aerial Vehicle (UAV)-based Structure from Motion (SfM) is not possible for vegetation cover issues.

The current results suggest the following: i) the connection between soil properties, soil moisture, and changes in groundwater level in the evolution of the slope instability, ii) potential shear surfaces within the shallow layers of the unstable slope, and iii) the importance of combining hydrological and geotechnical monitoring to set up an integrated network for landslide interpretation. Accordingly, obtaining information from a multi-parameter monitoring system is fundamental to identifying the relationship between the triggering and kinematic mechanisms of a complex, slow-moving landslide. However, the nonlinear behavior of slow movements restricts the temporal capability to properly understand the processes of complex mass movements. Consequently, landslide dynamics need to be further understood to establish a long-term monitoring system.