You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Brillouin Distributed Fiber Sensors: An Overview and Applications 
Chapter Title:  Geotechnical Structures Monitoring. 
1. Abstract of Cited Paper (BIB001):  We report on the development of a complete system for spatially resolved detection of critical soil displacement in ::: river embankments. The system uses Brillouin frequency domain analysis (BOFDA) for distributed measurement ::: of strain in silica optical fibers. Our development consists of the measurement unit, an adequate coating for the ::: optical fibers and a technique to integrate the coated optical fibers into geotextiles as they are commonly used in ::: dike construction. We present several laboratory and field tests that prove the capability of the system to detect ::: areas of soil displacement as small as 2 meters. These are the first tests of truly distributed strain measurements ::: on optical fibers embedded into geosynthetics. 
2. Abstract of Cited Paper (BIB002):  For the understanding of the bearing behavior of a loaded ground anchor, the measuring and monitoring of the stress ::: distribution in the anchor tendon is essential. This paper proposes a novel monitoring ground anchor using embedded ::: optical fibers for the continuous strain assessment along the anchor tendon. In a first step, optical sensors have been ::: integrated into short tendons using different methods and laboratory strain testing was performed on these instrumented ::: tendons. The evaluation of the laboratory testing enabled the design and development of an 8m long monitoring ground ::: anchor for field application. In 2009, this anchor has been placed into a wall supporting an excavation pit and ::: subsequently, anchor pullout test was carried out. The anchor was loaded stepwise up to 470kN, almost reaching its ::: ultimate bearing capacity. Optical measurements were taken successfully at each load step. Comparison of the optical ::: data with data acquired using conventional methods indicated good consistency of the results. To a geotechnical ::: engineer, this proposed monitoring anchor provides a powerful tool for the measuring of the pullout load, the anchor ::: head displacement and the load distribution in the anchor tendon. 
3. Abstract of Cited Paper (BIB003):  Cross-borders smuggling tunnels enable unmonitored movement of people, drugs and weapons and pose a very serious threat to homeland security. Recent advances in strain measurements using optical fibers allow the development of smart underground security fences that could detect the excavation of smuggling tunnels. This paper presents the first stages in the development of such a fence using Brillouin optical time domain reflectometry (BOTDR). Two fiber optic layouts are considered and evaluated in a feasibility study that includes evaluation of false detection and sensitivity: (1) horizontally laid fiber buried at a shallow depth, and (2) fibers embedded in vertical mini-piles. In the simulation study, two different ground displacement models are used in order to evaluate the robustness of the system against imperfect modeling. In both cases, soil–fiber and soil-structure interactions are considered. Measurement errors, and surface disturbances (obtained from a field test) are also included in the calibration and validation stages of the system. The proposed detection system is based on wavelet decomposition of the BOTDR signal, followed by a neural network that is trained to recognize the tunnel signature in the wavelet coefficients. The results indicate that the proposed system is capable of detecting even small tunnel (0.5 m diameter) as deep as 20 m (under the horizontal fiber) or as far as 10 m aside from the mini-pile (vertical fiber), if the volume loss is greater than 0.5%. 
4. Abstract of Cited Paper (BIB004):  In this paper, an overview of optical sensor development, testing and evaluation for several geotechnical monitoring ::: applications is presented. Additionally, sensor integration and data interpretation are addressed as key influences to the ::: overall success of the monitoring project. They should be taken into consideration already in the design stage. ::: Particular focus is given on strain sensor development to minimize the slippage of the fiber inside the protection. For the ::: first time, slippage progression monitoring by high spatially resolved Brillouin measurements is presented as a new tool ::: for sensor testing and evaluation for geotechnical projects. ::: The main findings of the study are that in a geotechnical monitoring project, special care has to be taken by choosing the ::: sensor slippage properties, longitudinal stiffness and robustness, as well as in the design of the sensor system itself ::: (fixation, gauge length and bond strength). With appropriate alignment of these factors, reasonable monitoring data can ::: be obtained, as shown in the applications proposed in this manuscript.

Literature Review:
To understand the bearing behavior of a loaded ground anchor, the measuring and monitoring of the stress distribution in the anchor tendon is essential. To provide this information to the geotechnical engineers, novel monitoring ground anchors using embedded optical fibers along the anchor tendon were developed using a BOTDA technique BIB003 BIB005 . In a first step, optical sensors have been integrated into short tendons using different methods and laboratory strain testing was performed on these instrumented tendons (Figure 14) . The evaluation of the laboratory testing enabled the design and development of an 8 m long monitoring ground anchor for field application. In 2009, this anchor has been placed into a wall supporting an excavation pit and, subsequently, anchor pullout test was carried out. The anchor was loaded stepwise up to 470 kN, almost reaching its ultimate bearing capacity. Optical measurements were taken successfully at each load step. Authors report comparison of the optical data with data acquired using conventional methods, and they claim good consistency of the results. Geotextiles are generally used to perform functions of separation, reinforcement, filtration, and/or drainage, and they have been used to solve civil engineering problems over more than three decades. Since fabrication processes of geotextiles and optical fiber have substantially evolved, nowadays it is possible to combine both materials to produce a nonconventional optical sensor. Thus, several geotextiles with embedded optical fibers have been proposed and developed for optical sensing (Figure 15(a) ). By using Brillouin interrogation techniques smart structures capable of monitoring and/or measuring the strain and/or temperature distribution in 2-dimentions can be developed. Employing geotextiles, distributed measurements of critical mechanical deformations (or soil displacements) on dikes with several kilometers length can be carried out. Additionally, they are commonly used in dikes for reinforcement of the dike body and erosion prevention, among other possibilities. A BOFDA monitoring system has been optimized to fit the demands on dike monitoring: detection of mechanical deformation (strain) with a spatial resolution of 5 m over a distance range of up to 10 km. The functionality of the monitoring system and the fiber-sensors-equipped geotextiles has been proven in several installations and field tests in dikes and dams such as the gravity dam in Solina, Poland in August 2006. A thin soil layer of several 10 cm put onto the geomats after installation has been proven to be a sufficient protection of the textile-integrated glass fiber cables against heavy machinery and construction work. It must also be noticed that on the laboratory dike (15 m long) at Hannover University, Germany, applications like the before tested were carried out. Sensor-based geotextiles were installed on top of the dike and were covered with a thin soil layer (Figure 15(b) ). To simulate a mechanical deformation/soil displacement, a lifting bag was embedded into the soil and was inflated by air pressure. This induced a break of the inner slope of the dike and a soil displacement (Figure 15(c) ). The soil displacements were clearly detected and localized by the BOFDA system BIB001 BIB002 BIB002   . Cross-border smuggling tunnels enable unmonitored movement of people or forbidden stuff (drugs, weapons, etc.), and they may suppose a threat to homeland security. To contribute to overcome this risk, a method for detecting the excavation of small (diameter <1 m) tunnels in clayey soils using Brillouin optical time domain reflectometry (BOTDR) and a neural network was proposed BIB004 . The reported architecture includes two fiber optic layouts. One is a horizontal fiber buried at a shallow depth below ground surface, and the other one has a fiber embedded in vertical minipiles to detect very deep tunnels. In both configurations, strains would develop in the fiber due to the soil displacements induced by the tunnel excavation. It was demonstrated by the authors that the proposed system was capable of detecting even small tunnel, 0.5 m diameter, as deep as 20 m under the horizontal fiber or as far as 10 m aside from the minipile, if the volume loss is greater than 0.5%.