sensors Article New Distributed Fibre Optic 3DSensor with Thermal Self-Compensation System: Design, Research and Field Proof Application Inside Geotechnical Structure Lukasz Bednarski 1 , Rafal Sie ´ nko 2 , Marcin Grygierek 3 and Tomasz Howiacki 2,4, *   Citation: Bednarski, L.; Sie ´ nko, R.; Grygierek, M.; Howiacki, T. New Distributed Fibre Optic 3DSensor with Thermal Self-Compensation System: Design, Research and Field Proof Application Inside Geotechnical Structure. Sensors 2021, 21, 5089. https://doi.org/10.3390/ s21155089 Academic Editors: Paulo Antunes, Jaroslaw Rybak, Marian Drusa and Andrea Segalini Received: 31 May 2021 Accepted: 26 July 2021 Published: 27 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Mechanics and Vibroacoustics, Faculty of Mechanical Engineering and Robotics, AGH University of Science and Technology in Krakow, Mickiewicza 30, 30-059 Krakow, Poland; lukaszb@agh.edu.pl 2 Reinforced Concrete Structures Division, Faculty of Civil Engineering, Cracow University of Technology, Warszawska 24, 31-155 Krakow, Poland; rafal.sienko@pk.edu.pl 3 Faculty of Civil Engineering, Silesian University of Technology, Akademicka 5, 44-100 Gliwice, Poland; marcin.grygierek@polsl.pl 4 SHM System Sp. z o.o. Sp. komandytowa., Libertów, ul. Jana Pawla II 82A, 30-444 Krakow, Poland * Correspondence: th@shmsystem.pl or howiacki.tomasz@gmail.com; Tel.: +48-505-870-669 Abstract: Thanks to the dynamic development of advanced building technologies as well as the growing awareness, experience and responsibilities of engineers, structural health monitoring sys- tems (SHM) are increasingly applied in civil engineering and geotechnical applications. This is also facilitated by the construction law and standard requirements, e.g., the observation method for geotechnical structures described in the Eurocode 7. Still, the most common approach is to apply spot sensors in selected points of the structure to validate theoretical models, numerical simulations and support technical assessments by involving statistic and approximation methods. The main limitation of spot sensing is the inability to detect localized damages such as cracks, fractures, sink- holes or shear planes. Thus, such analysis is subject to considerable uncertainty, especially within geotechnical structures, characterized by random mechanical parameters that change with location, but also over time. Another approach is based on distributed fibre optic sensors (DFOS), which are finding a growing acceptance in laboratory and field projects, overcoming limitations of conventional measurements. The design and applications of new DFOS dedicated for 3D displacement sensing are described hereafter in the article. The novelty of the presented solution lies in several features, including design, application, production technology and materials. This article is focused on the operational rules governing DFOS and proving their effectiveness in laboratory and geotechnical field applications. Keywords: distributed fibre optic sensing DFOS; composite; 3DSensor; displacements; settlements; in situ measurements; thermal compensation; embankment; geotechnics; laboratory 1. Introduction 1.1. General Background Modern civil engineering is not only about creating unique building objects, but above all about using new materials, construction solutions and improving existing technologies. The optimization process should provide both financial savings as well as an appropri- ate and acceptable level of safety, expressed by reasonably low failure probability and risk [1]. Optimal decision making [2] is a challenging task, not only while managing the construction site at the initial stage on investment, but also through the entire life cycle of the structure [3]. This simple insight is reflected in recommendations included in the basic European standard for structural design EN-1990 [4], where control procedures relevant to the particular project must be specified for design, production, execution and further operation of the structure. Sensors 2021, 21, 5089. https://doi.org/10.3390/s21155089 https://www.mdpi.com/journal/sensors