Open Access

Berlin Main Station is the largest multi-level station in Europe. Its daily passenger number amounts to over 300.000. Structures built for such a large number of people require a high-level safety standard. The station was built on the inner city site of the historic Lehrter Bahnhof. The conditions for building and start of operation were challenging by several reasons. The typical sandy ground with a high level of groundwater makes the permanent static stability of such a complex structure difficult. Several completed, ongoing, and planned construction activities in the immediate vicinity of the station influence the ground settlement of the whole area. On basis of the structural design an impact prediction was calculated, which expected certain vertical displacements particularly between the single columns of the outer concrete bridges of the building. These columns support the glass roof construction, which only allows a defined limit of displacement. In order to avoid damage, a concept for monitoring and adjusting potentially occurring displacements was developed for installation at the outer bridges of the station.

Rocks and stone are common construction materials in geomechanics. Great difficulties arise from the inhomogeneous nature of the mechanicalhydraulic materials behavior and their materials properties. However, well-tried mathematically based engineering concepts are only valid if reliable figures of the materials properties are known. One of the important measurement tools is the triaxial fest cell. Essentially used for conducting crack and creep tests, they apply mechanical pressure to the front sides of a cylindrical rock sample. Additionally, the rock sample is exposed to a triaxial pressure by creating a hydraulic pressure inside the test cell to simulate the original environmental conditions which can be found at the site where the rock samples were extracted. Deformation measuring sensors inside the test cell are exposed to those extreme conditions as well; sensor designs have to withstand pressures up to 2000 bar. In this work, a novel fiber based deformation sensor is presented in detail.

Fiber-optic strain sensors are increasingly used in very different technical fields. Sensors are provided with specifications defined by the manufacturer or ascertained by the interested user. If deformation sensors are to be used to evaluate the long-term behavior of safety-relevant structures or to monitor critical structure components, their performance and signal stability must be of high quality to enable reliable data recording. The measurement system must therefore be validated according to established technical rules and standards before its application and after. In some cases, not all details of the complex characteristic and performance of applied fiber-optic sensors are sufficiently understood, or can be validated because of a lack of knowledge and methods to check the sensors' behavior. This contribution focusses therefore on the importance of serious validation in avoiding a decrease or even deterioration of the sensors' function. Methods for validation of applied sensors are discussed and should reveal weaknesses in validation of embedded or integrated fiber-optic deformation and/or strain sensors. An outlook to some research work that has to be carried out to ensure a well-accepted practical use of fiber-optic sensors is given.