Development of a fiber-optic system for monitoring geotechnical structures

The paper presents the concept of a point amplitude sensor for the registration of displacement of geotextile, a synthetic fabric that is used to reinforce geotechnical structures such as a dam. The implementation of a system for continuous monitoring of the structural condition of a building based on the concept of a “smart” geotextile has the potential to significantly enhance the safety of the structure. Such a system could provide early warning of the necessity for unscheduled repairs, the occurrence of an emergency situation, and the need for the immediate cessation of building operations, evacuation of personnel or population. The capabilities of existing technical solutions for displacement sensors have been evaluated. It is not feasible to apply existing monitoring systems utilizing fiber Bragg Grating Sensors (FBG) in the context of geotextile. This is due to the greater pliability of the soil which exhibits minimal elastic deformation. In addition, FBG sensors are much more expensive in production compared to telecommunication optical fiber. The single-mode fiber which constitutes the sensing element, forms one or more loops that are placed between movable stops that are attached to the sensor body and to the movable activator. At the point of macro bending of the reinforcing fiber, the phenomenon of total internal reflection is disrupted, which in turn gives rise to amplitude modulation of the radiation. The macro bending is proportional to the displacement of the activator attached to the geotextile. This paper presents the design, dimensions and mathematical relationships of the sensing element as well as the dimensions and characteristics of the design elements for signal processing. The sensor model is constructed from ABS plastic and fiber Corning SMF-28. An experimental setup was constructed to test the proposed concept which involved controlling the displacement of the activator, the input and output of radiation. The dependences of the output power on the fiber bending diameter, ranging from 25 to 11 mm, and the displacement, up to 14 mm, at a radiation wavelength of 1550 nm, were determined. It was demonstrated that the obtained dependences were monotonic and exhibited quasi-linear plots. The kinks at the small diameter of the fiber bend are caused by two factors: the intensive radiation output from the core to the cladding and scattering within it; and at the large diameter, they are due to small bending losses. The conducted studies have demonstrated that the sensor is capable of reliably detecting displacements up to 0.5 mm. The results exhibited good repeatability. The proposed sensor demonstrated inferior accuracy compared to FBG sensors. Conversely, at comparable accuracy of ground displacement registration, the proposed sensor was observed to be an order of magnitude more cost-effective than FBG sensors.

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