Abstract
Abstract. With micro-strain resolution and the capability to sample at rates of 100 Hz and higher, fiber optic (FO) strain sensors offer exciting new possibilities for in-situ landslide monitoring. Here we describe a new FO monitoring system based on long-gauge fiber Bragg grating sensors installed at the Randa Rockslide Laboratory in southern Switzerland. The new FO monitoring system can detect sub-micrometer scale deformations in both triggered-dynamic and continuous measurements. Two types of sensors have been installed: (1) fully embedded borehole sensors and (2) surface extensometers. Dynamic measurements are triggered by sensor deformation and recorded at 100 Hz, while continuous data are logged every 5 min. Deformation time series for all sensors show displacements consistent with previous monitoring. Accelerated shortening following installation of the borehole sensors is likely related to long-term shrinkage of the grout. A number of transient signals have been observed, which in some cases were large enough to trigger rapid sampling. The combination of short- and long-term observation offers new insight into the deformation process. Accelerated surface crack opening in spring is shown to have a diurnal trend, which we attribute to the effect of snowmelt seeping into the crack void space and freezing at night to generate pressure on the crack walls. Controlled-source tests investigated the sensor response to dynamic inputs, which compared an independent measure of ground motion against the strain measured across a surface crack. Low frequency signals were comparable but the FO record suffered from aliasing, where undersampling of higher frequency signals generated spectral peaks not related to ground motion.
Highlights
In-situ monitoring of landslide displacements can be a critical means of evaluating the internal mechanisms and deformation characteristics of slope instabilities for hazard and risk assessment
In the case of coseismic slope deformations, traditional instruments are rarely capable of detecting small dynamic displacements, which are necessary data in conjunction with local ground motion to better understand the seismic behavior of landslides (Harp and Jibson, 1995)
Fiber optic (FO) borehole sensors were emplaced to span known active fractures showing normal mode offset
Summary
In-situ monitoring of landslide displacements can be a critical means of evaluating the internal mechanisms and deformation characteristics of slope instabilities for hazard and risk assessment. Traditional in-situ sensors (e.g. mechanical or electrical extensometers) have long-established success for monitoring local movements, but are limited in the scope of their application by generally low resolution and sampling rates. In the case of coseismic slope deformations, traditional instruments are rarely capable of detecting small dynamic displacements, which are necessary data in conjunction with local ground motion to better understand the seismic behavior of landslides (Harp and Jibson, 1995). Fiber optic (FO) strain sensors are a promising new technology for advancing the state of the art in in-situ landslide monitoring. There are only a few cases where FO strain sensors have been used for monitoring slope deformations in landslide investigations (notably Brunner et al, 2007; Woschitz and Brunner, 2008). Moore et al.: Rockslide deformation monitoring with fiber optic strain sensors. Disadvantages of the system, highlight relevant first results and their implications, and compare FO time series data to that from traditional instruments already installed on site
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