Generation of an early warning system for landslide and slope instability by optical fiber technology

Arslan, Arzu
The purpose of this study is to develop an early warning system for all kinds of mass movements regardless of the failure mechanism and lithology type. For this purpose, an optical fiber system was preferred due to its superiority in field conditions and continuous data measurement capability. Two different optical fiber systems, namely, the Optical Time Domain Reflectometer (OTDR) and the Brillouin Optical Time Domain Analyzer (BOTDA) were experimented with alternative fiber cables and their competency was investigated for a real case landslide located in a hazard prone region in Bahçecik Settlement Area in Kocaeli Province. Before the field experiment, the systems were tested first in laboratory scale. For the laboratory studies, a landslide simulation model having an inclination mechanism designed to represent a slope was used. After these experiments, their applicability in the field or for a real case was conducted. Experiments revealed that the OTDR allows sensitive measurement in laboratory scale but is not suitable for field application due to its energy loss based measurement nature. Therefore, the BOTDA system capable of measuring strain without power loss but detecting frequency shift was preferred for the field studies. Once the measurements from the optical fiber system were gathered, it was necessary to compare these results with the displacements that occurred on the studied mass. The displacements that take place in a small scale laboratory landslide simulator are rather obvious; however this is not the case for field application. Therefore, slope stability analyses were conducted in order to compare the strain results collected from fiber cables with displacements on the moved mass. In order to accomplish this objective, a back analysis study was implemented to reach the mobilized shear strength parameters of the moved mass by utilizing three profiles. Then, the landslide was modeled based on deformation analysis via the finite element method to compute the displacements of the critical region within circular failure. The main purpose of the finite element analysis was to determine the most critical part of the failure region and to examine the sensitivity of the study by locating the optical fiber system at this region. Thus, the reliability of field results can be better understood. In conclusion, finite element modelling results showed that the displacement values calculated by modelling were in good agreement with those obtained through field monitoring with BOTDA.