Landslide monitoring using a road-embedded optical fiber sensor Michael Iten*, Alexander M. Puzrin, Andreas Schmid Institute for Geotechnical Engineering (IGT), Swiss Federal Institute of Technology (ETHZ), CH-8093 Zurich, Switzerland ABSTRACT A novel technique for the determination of a creeping landslide boundary is demonstrated. It is based on application of distributed optical fiber strain measurements using Brillouin Optical Time Domain Analysis (BOTDA) technology. A road crossing the St. Moritz landslide boundary was instrumented with a fiber optic cable, which turned the road, effectively, into a large scale strain gauge. The obtained monitoring data was in good agreement with visual observation and also followed the trends of the geodetical data. The presented validation of this technology allows for a conclusion that distributed fiber optic strain sensing is a promising new tool in landslide surveillance. At present, until methods and standards in this field are established and reliable, combination with traditional methods is necessary. Ongoing measurements during 2008 may strengthen the conclusions of this paper. Keywords: Creeping landslide, landslide monitoring, optical fiber sensor, strain sensor cable design, road-embedded sensor, BOTDA. 1. INTRODUCTION Creeping landslides can cause immense problems when they occur in inhabited areas. They damage the infrastructure and buildings in the sliding area and lead to high maintenance costs. Additionally, local construction laws often enforce special construction and reinforcement requirements, or limit construction within a landslide area. Therefore, in some cases it is of a crucial importance to determine the exact position of the boundary between the landslide and the stable part of the slope. Geodetic measurements can identify this border, but not necessarily with high precision. The creeping landslide, which has been monitored, is located in the heart of the renowned mountain resort town of St. Moritz, Switzerland. The building regulations of the municipality of St. Moritz specify particular requirements for construction on the landslide. These foresee, e.g., seasonal construction ban, weight compensation of removed soil due to excavation and additional reinforcement of the building. The displacements of the landslide have been monitored by geodetic measurements techniques for over 100 years and, yet, its boundary has not been clearly defined. The current legal boundary of the landslide area has been drawn by the authorities partly beyond the actual landslide area in order to be on the safe side. Disputes between landowners and the municipality about the exact landslide boundary are common. Validation of the boundary location by additional measurement techniques is, therefore, a task of a high priority. This paper is an attempt to outline a new landslide monitoring technique by means of continuous strain measurements in an optical fiber embedded into a road. This method is based on a simple idea, that if a landslide boundary is intersected by a road, this road can be seen as a large scale strain gage, provided it is properly instrumented. The necessary technology of measuring continuous longitudinal strain in an optical fiber is nowadays commercially available. It is based on Brillouin Optical Time Domain Analysis (BOTDA) and strain data along a fiber can be obtained using a measurement unit such as the DiTeSt TM (Omnisens, 2007 [1] ). In 2006, a 89 meter long stretch of a road crossing the boundary was instrumented by embedding a hand-coated optical fiber cable into a trench cut in asphalt along the road. The strain in the fiber was first measured following the integration and again 7 months later. A first estimate of the boundary was then performed. The estimated position of the boundary correlated well with the damage pattern in the surrounding buildings. Furthermore, the strain measurements suggested that the sliding occurs not perpendicular, as believed before, but at about 45 degrees angle to the road, which was later also confirmed by the field observations. The measured strain values correlated well with the observed landslide displacements. *michael.iten@igt.baug.ethz.ch ; phone ++41 44 633 6785; fax ++41 44 633 1429; www.igt.ethz.ch Smart Sensor Phenomena, Technology, Networks, and Systems 2008, edited by Wolfgang Ecke, Kara J. Peters, Norbert G. Meyendorf, Proc. of SPIE Vol. 6933, 693315, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.774515 Proc. of SPIE Vol. 6933 693315-1 2008 SPIE Digital Library -- Subscriber Archive Copy