Seite - 684 - in Book of Full Papers - Symposium Hydro Engineering

monitor long lengths through a single cable, thus simplifying installation and increasing data density. A different installation technique is chosen for the 2 sections: in the penstock, where precise and accurate monitoring under water is required, the sensing cable (flat profile) is directly glued on the internal surface. The steel penstock is sand-blasted to offer a smooth and clean installation surface where 510 m, linear length, of sensing cable is glued along 4 different lines, Fig. 10. Fig. 10 Strain sensing cable installation inside the penstock On the other hand, for the access tunnel a mixed installation technique was selected: sensing cable was directly glued on concrete for most of its length, but fixed with stainless steel bracket where wide cracks were already visible and developing; this decision was taken in order to preserve sensor from breaking in case the crack keep developing, Fig. 11. This installation technique allows a precise and accurate monitoring over the whole length of this tunnel of approximately 70 m. Fig. 11 Strain sensing cable installation on penstock access tunnel After 3 years of monitoring, the collected results are in line and good agreement with the mathematical predictions and other geo-matic measurements provided by additional monitoring systems installed at site. A typical example of Strain distribution measured in the penstock access tunnel clearly shows the location of open developing cracks, peaks can be seen and easily localized along the sensing cable length, Fig. 12. 684