Anchor Force Measurement with Load Cells: The most common way of determining the anchor force in geotechnical applications is by means of force sensors or load cells. Load cells exploit various measuring principles such as piezo technology and/or methods based on fluid pressure. Installation takes place on site where the load cell is set between the anchor plate and anchor nut or the wedge carrier. The anchor force is measured using a Bourdon manometer or by electric means with a digital manometer. Depending on the manufacturer and measuring principle, measuring accuracy is roughly ±1% if installed properly. However, practical findings have shown that given eccentric installation of the load cell or pronounced temperature fluctuations, the results can be significantly affected. In addition, in case of high anchor forces (e.g., slope stabilization or dam retrofittings), large, heavy load cells have to be used. This can make installation more difficult, particularly given remote terrain or if hoisting machines are not available. Furthermore, large-sized anchor head structures represent additional obstacles for site traffic, as they can be damaged and, therefore, no longer available for moni- toring purposes. Therefore, load cells are only suitable to a limited degree given constricted space conditions. Innovative Contactless System DSI developed a monitoring system for measuring the tensile load of steel tendons with Dyna Force Elastomagnetic sensors. The sensor measures without actual contact and, therefore, is a nondestructive method. Force is measured based on the elastomagnetic properties of ferromagnetic materials, where the magnetic permeability (m) of steel within a magnetic field changes as a function of the mechanical normal stress state in the steel. The magnetic properties of a steel tendon depend on the material composition and the normal stress state of the steel tendon. By measuring the relative change in the magnetic permea- bility (Dm), the normal stress within the steel tendon can be derived. The sensor is equipped with two coils: an exciter coil and an induction coil. During measurement, electric voltage is produced within the exciter coil and a magnetic field is produced, which induces electric voltage within the induction coil. The induced voltages are ultimately processed and stored by a readout unit. The quotient B/H (magnetic flow density/magnetic field strength) provides the magnetic permeability. Given that the hysteresis curve of a prestressed strand and of a nonpretensioned strand, the change in the magnetic flow density depends on the mechanical normal stress state, the magnetic field strength or the magnetic permeability of the strand. Providing the interrelationships are known, it is possible to derive the mechanical force in the steel tendon from the induced electric voltage within it. Prior to utilizing sensors onsite, the material relationship between relative permeability and mechanical load must be calibrated. Load cell with Bourdon manometer System Components: The measuring system consists of elastomagnetic sensors, a readout unit, a multiplexer and extension cables. By means of a modem and controller, it is possible to remotely transfer the measured data. The sensors are connected to the readout unit where the force in the steel tendon can be measured and read. If several sensors have to be read at the same time, a multiplexer can be installed. To allow for cable installation, a groove is cut into the anchor head structure and a cable fed through it, after which the groove is filled with a sealing agent. Readout Possibilities: Where measure- ments are recorded manually, a sensor is connected directly to the readout unit, and the sensors are connected singly to the readout unit and evaluated one at a time. When simultaneous and automatic reading of all sensors is performed, a multiplexer is connected to the readout unit or a connected PC. When continuous recording of the sensors is performed, the measured results can be read from any location by maintaining an operable connection of the sensors to multiplexer, readout unit and integrated network modem. The control unit can regulate the power supply and, therefore, the reading interval. Applications: The elastomagnetic sensors Schematic of sensor and qualitative presentation of hysteresis curves of a ferromagnetic material 78 • DEEP FOUNDATIONS • JULY/AUG 2017 are hollow formed cylinders, which are assembled over the strands or bars at the
