Sub-surface real time monitoring and early warning system (DMS) of the Åknes rockslide
|Location||International Geological Congress,oslo 2008|
|Author||Lovisolo, Mario۱; Foglino, Luigi۱; Blikra, Lars Harald۲|
|Holding Date||04 October 2008|
The presentation focus on the subsurface real time monitoring technique and early warning system (DMS) installed at the Aknes rockslide. The DMS’s continuous monitoring is part of the Åknes/Tafjord Early Warning Centre, which is going to handle the risk related to the possible rockslide generated tsunami in the municipalities around Storfjorden in Norway.
The intrumentation includes 3 DMS IUT automatic monitoring columns emplaced in deep boreholes of the Åknes rockslide.
The multi-parametric DMS instrumentation consists at 2 borehole locations (upper and middle) of 48 digital IT modules (bx tiltmeter + temp) and 1+1 IUT modules (bx tiltmeter + temp + piezometer) and 98 IT and 1+1 IUT at lower site. The present instrumentation allow us to have the control of both the displacements of rock along multi-sliding zones, as well as the fluctuations of the water table and the temperature in the same borehole.
Based on the core analysis and geophysical logging in the preliminary stage, the monitoring columns were installed in autumn 2006 at 32-82 m bgl (upper) and 16.5-66.5 mbgl (middle). A repositioning was done in autumn 2007 at 82-133 mbgl (upper) and 31.5-81.5 mbgl (middle) respectively. The third will be positioned at 20-120 mbgl at the lower borehole. The results so far show that the DMS instrumentation has detected several sliding planes from about 30 m to more than 100 m depth. It also has demonstrated the close relationship between sliding and water-level fluctuations The data thus contributes significantly to the understanding of the complex geological model (e.g. multi-sliding zones), and will be fundamental for input to numerical modeling.
Precision analysis (Dunnicliff criterion) A statistical analysis of the inclinometric data sets (upper and middle site in place columns) has been performed specifically for a 3 months continuous interval with 1 record pr hour (11.11.07-10.02.08). Measurement uncertainty is well within the requirements for In Place Inclinometers (Dunnicliff & al, 2000). The top positioning of the upper and middle sites strings were defined with a precision ±0.85mm along both the roll and the pitch axis for the test period (4 times better than the Dunnicliff requirements of ± 3.3mm/50m string). The high precision of the data demonstrates the systems high capacity of detecting micro movements, which is critical related to early warning issues and public safety.
Early warning management
The data from the DMS are sent through RS485 to the control unit, which compares them with threshold values continuously. In case of displacements or water table fluctuations larger than threshold values, the control unit sends a warning SMS/direct call to the reference mobile phones. Warning levels are counted from 1 to 4, in a order of danger.
A new modular software (DMS EW) was designed specifically for the Åknes/Tafjord Early Warning Centre to improve the real-time monitoring and data analysis in the decision-making support stage.