Monitoring of an Unstable Slope in Kocaeli (Türkiye) by an Optical Fiber Based System
Authors: Arslan Kelam A. , Koçkar M. K. , Akgün H. 
Published: September 2023

ABSTRACT
This paper assesses the stages and the field application results of a slope monitoring system developed for continuous long-term monitoring applications using an optical fiber system to decrease the risks associated with slope instabilities. During this study, a system composed of optical fiber cables and a device referred to as the Brillouin Optical Time Domain Analyzer (BOTDA) have been used. The system is a distributed system that measures the strain along an optical fiber cable when slope instability displaces the cable and causes differentiation of the initial location of the cable. This system has been implemented on a risky unstable slope in the Bahçecik region of Kocaeli, one of the active landslide-prone regions in the Marmara region, Türkiye. The strain data collected through this real case field application has enabled the investigation of the effects of hydrological and hydrogeological conditions along with seismic activity on slope movement for a period of 12 months. For this purpose, in addition to the optical fiber system, a piezometer and an acceleration seismometer were deployed in the field. The evaluation of the data revealed that the change in strain was directly proportional to precipitation and groundwater head. Moreover, the seismic activity also influenced the deformation as a secondary triggering effect. In addition, the slope movement characteristics were validated by surface topography monitoring measurements. The results showed that utilization of the optical fibers is reasonable and the system is well-suited for developing monitoring systems for deformation-related engineering applications such as slope instabilities.

INTRODUCTION
Mass movements are one of the most frequent and destructive natural hazards that occur all around the world and they can be triggered by many different factors such as geological properties, geomorphologic and topographic parameters, precipitation characteristics, groundwater level changes, daily temperature changes, snow melt, and seismic effect. Awareness about mass movements and studies regarding monitoring them has become more popular day by day. Correspondingly, the importance given to the concept of risk assessment has been increasing, and, early warning systems have started to gain more attention (Li et al. 2012; Pecoraro et al. 2019). Today, there are different techniques to monitor mass movements and they have their advantages and disadvantages. Inclinometers, tiltmeters, extensometers, GPS (Global Positioning System), ground-based LIDAR (Light Detection and Ranging), satellite images, and air photography are examples of these techniques (Yin et al. 2010; Pei et al. 2011; Zhang et al. 2018). Although all of these methods are used to monitor and detect deformation, optical fiber systems are advantageous in terms of their easy and fast data transfer capability, smaller dimensions, light weight, sensitivity to strain and temperature change, wide band range, resistance to environmental and electromagnetic effects, and real-time monitoring properties (Gupta 2012; Wu et al. 2019). Due to their advantages, the usage of optical fiber systems has been increasing with the help of the vast development in fiber technology (Zhang et al. 2014).