Rockslide deformation monitoring with fiber optic strain sensors

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.

[1]  Alan G. Green,et al.  Microseismic investigation of an unstable mountain slope in the Swiss Alps , 2007 .

[2]  E. Eberhardt,et al.  Internal structure and deformation of an unstable crystalline rock mass above Randa (Switzerland): Part II — Three-dimensional deformation patterns , 2008 .

[3]  Edward G. Nawy,et al.  Concrete Construction Engineering Handbook , 2008 .

[4]  F. Brunner,et al.  MONITORING OF DEEP-SEATED MASS MOVEMENTS , 2007 .

[5]  Sandra Lloret Barrera.  A deformation measurement system for dynamic structural monitoring , 2001 .

[6]  Cornelia Schmidt-Hattenberger,et al.  Dynamic strain detection using a fiber Bragg grating sensor array for geotechnical applications , 2003, SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring.

[7]  Steven W. Smith,et al.  The Scientist and Engineer's Guide to Digital Signal Processing , 1997 .

[8]  E. Harp,et al.  Seismic instrumentation of landslides: Building a better model of dynamic landslide behavior , 1995 .

[9]  C. Tsang,et al.  Estimation of fracture flow parameters through numerical analysis of hydromechanical pressure pulses , 2008 .

[10]  C. Schindler,et al.  Die Ereignisse vom 18. April und 9. Mai 1991 bei Randa (VS): ein atypischer Bergsturz in Raten , 1993 .

[11]  Valentin Gischig,et al.  Identification of active release planes using ground-based differential InSAR at the Randa rock slope instability, Switzerland , 2009 .

[12]  E. Eberhardt,et al.  Internal structure and deformation of an unstable crystalline rock mass above Randa (Switzerland): Part I — Internal structure from integrated geological and geophysical investigations , 2008 .

[13]  Cornelia Schmidt-Hattenberger,et al.  Temperature-controlled fiber Bragg grating dynamic strain detection system , 2005 .

[14]  Cornelia Schmidt-Hattenberger,et al.  Bragg grating seismic monitoring system , 1999, Optics East.

[15]  Cornelia Schmidt-Hattenberger,et al.  Fiber Bragg grating strain measurements in comparison with additional techniques for rock mechanical testing , 2003 .

[16]  Daniele Inaudi,et al.  Geostructural monitoring with long-gage interferometric sensors , 2000, Smart Structures.

[17]  F. Brunner,et al.  Monitoring a deep-seated mass movement using a large strain rosette , 2008 .

[18]  E. Eberhardta,et al.  Numerical analysis of initiation and progressive failure in natural rock slopes—the 1991 Randa rockslide , 2003 .

[19]  M. Jaboyedoff,et al.  Kinematics of the 1991 Randa rockslides (Valais, Switzerland) , 2003 .

[20]  Pietro Ferraro,et al.  On the possible use of optical fiber Bragg gratings as strain sensors for geodynamical monitoring , 2002 .

[21]  M. Jobmann,et al.  The response of Opalinus clay due to heating: A combined analysis of in situ measurements, laboratory investigations and numerical calculations , 2007 .

[22]  Philipp M. Nellen,et al.  Optical fiber Bragg gratings for tunnel surveillance , 2000, Smart Structures.

[23]  Cornelia Schmidt-Hattenberger,et al.  Strain measurements by fiber Bragg grating sensors for in situ pile loading tests , 2003, SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring.

[24]  A. Green,et al.  Characterizing an unstable mountain slope using shallow 2D and 3D seismic tomography , 2006 .

[25]  Y. Guglielmi,et al.  Use of in situ fiber optic sensors to characterize highly heterogeneous elastic displacement fields in fractured rocks , 2006 .

[26]  Doug Stead,et al.  Numerical analysis of initiation and progressive failure in natural rock slopes—the 1991 Randa rockslide , 2004 .