Performance evaluation of an FBG-based sensing beam for measuring the dynamic lateral displacements of soil in a shaking table test

D.S. Xu, J.H. Yin, H.F. Pei

In the proceedings of: GeoManitoba 2012: 65th Canadian Geotechnical Conference

Session: W1D - Exploration and Instrumentation

ABSTRACT: Internal displacement is one of the most important parameters to reflect the safety condition of geotechnical structures. The application of traditional sensors has encountered many practical limitations, such as large size, non-compatibility in stiffness and low accuracy for measuring small lateral displacements. To overcome these limitations, Fiber Bragg Grating (FBG) sensors have been used to fabricate a FBG-based sensing beam. In this paper, the design and working mechanism of the FBG-based sensing beam are described along with the installation procedures used for the shaking table test. The results of the sensing beam and laser displacement sensors are compared. It is found that the FBG-based sensing beam is capable of measuring dynamic lateral displacements with good accuracy in the soil mass.

RÉSUMÉ: Le déplacement interne est l'un des paramètres les plus importants afin de refléter l'état de sécurité des ouvrages géotechniques. L'application des capteurs traditionnels a rencontré de nombreuses limitations pratiques, telles que la taille grande, non-compatibilité de la rigidité et la précision pour la mesure de faible petits déplacements latéraux. Pour surmonter ces limitations, fibres à réseaux de Bragg (FBG) ont été utilisés pour fabriquer un faisceau FBG basée sur la détection. Dans cet article, la conception et le mécanisme de travail de la poutre FBG basée sur la détection sont décrites ainsi que les procédures d'installation utilisés pour l'épreuve table à secousses. Les résultats de la poutre de détection et de capteurs de déplacement laser sont comparés. Il se trouve que le faisceau FBG basée sur la détection est capable de mesurer des déplacements latéraux dynamiques avec une bonne précision dans la masse du sol. INTRODUCTION Deformation measurement plays a vital role to evaluate the safety of geotechnical structures. In the past few decades, numerous types of transducers such as laser displacement sensors (LDS), linear variable differential transformer (LVDT), dial gauges, global positioning systems (GPS) have been developed for displacement measurement. However, their performances are not satisfactory for some cases due to limited accuracy or only being suitable for surface measurements. The interior small displacement measurement has been a main factor for evaluating the performance of geotechnical structures. Recently, there has been a comprehensive development in the field of fiber optic sensing technology, which can be effectively utilized to overcome the earlier mentioned limitations. In comparison with conventional transducers, FBG sensors have apparent advantages such as small size, light weight, high precision, resistance to electromagnetic interference, and excellent sensing and transmission capacities. The fiber optic sensors can be connected in series to form a quasi-distribution or multiplexed array in parallels (Yin, 2008; Zhu, 2010). The use of FBG sensing technology for geotechnical monitoring has been advocated by many researchers (Todd et. al. 1999; Majumder et. al., 2008). Recently, FBG sensors have been successfully employed in many geotechnical applications in Europe, North America and Asia (Nellen et. al. 2000; Liu et. al. 2002; Lee et. al. 2004). The FBG sensor has been widely proved to be an effective tool to determine many geotechnical properties (Sato et. al. 2000; Yang et al. 2007). The purpose of this study is to develop a sensing beam based on the fiber Bragg grating (FBG) sensors to measure the internal dynamic lateral displacements of sand mass in a shaking table test. A flexible FBG based sensing beam was designed and fabricated. To verify the performance of the FBG sensing beam, a shaking table test was conducted with installation of the FBG sensing beam in saturated sand. At the same time, other transducers, such as the LDS, accelerators and pore water pressure transducers were also installed for comparison. The details of the test set-up and results are presented and discussed in this paper. PRINCIPLES OF FIBER OPTIC SENSOR The FBG sensors used in this study were manufactured with phase mask method in The Hong Kong Polytechnic University. After a spatial pattern of ultraviolet light passes through the phase mask and is exposed on the bare fiber, the core refractive index of the fiber will be permanently

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