Research On BOTDA Based Sensing Technology And Hydropower Structure Monitoring

The distributed optical fiber strain monitoring technology based on Brillouin Optical Time Domain Analysis/Reflectometer(BOTDA/R)has been applied in civil engineering for more than a decade,and a wealth of data and experience have been reported.With the increasing of application cases,problems about spatial resolution of instrument and applicability of sensing fiber become outstanding.Especially in hydropower engineering field where the traditional monitoring means have been developed for a long history,special study on distributed optical fiber strain monitoring technology need to be carried out to prove the feasibility of this new technology.The preloading spiral case has the feature of largescale,wide strain change range,short test period,reference with calculatedvalue and electrical sensor data.The filed experiment for spiral case was carried out to prove the applicability of distributed optical fiber strain monitoring technology for concrete structure and steel structure in hydropower engineering.The theoretical and experimental research results and several new knowledge/innovations are summarized as the following:1)Spatial resolution of BOTDA and BOTDR were studied.The spatial resolution has a notable influence on measured data series,which is verified by lab tests and on-site experiments.Two calibration methods,including 10%-90%Step-Function method and Hot-Spot method,as well as their feasibility were discussed.A new calibration method named as Dissimilar-Fiber-Splicing method,which means two types of fibers with distinguishable initial center frequency were alternately spliced in series with different length section,was recommended and tested.2)The sensing cable classification such as specification,methods of testing and manufacture elements were proposed.Four new styles of strain sensing cable were invented or improved according to the characteristics of hydropower engineering applications.A series of research and experiments including uniform strength beam calibration test,calibration frame tensile test,steel I-beam bending test,steel pile driving tests,concrete beam bending test,concrete bamboo tank compression test,and office building reinforcing process test,were carried out to verify the intact ratio anddeformation/strain detection capabilityof sensing cables.3)The application proposal of BOTDA based distributed optical fiber strain monitoring for several typical hydropower structures were designed,including surface strain and temperature distribution monitoring for high arch dam,concrete faceplate strain and crack monitoring for high rockfill dam,concrete lining strain monitoring along long diversion tunnel,strain distribution monitoring for large surge chamber,and internal deformation monitoring for surrounding rock of underground cavern.4)BOTDA was used in an on-site monitoring project for large-scaled sprical case during water-filling test and preloading process.Special sensing cables were attached onto the surface of steel spiral case and into the surrounding concrete to measure the hoop strain distribution of cross sections.With the internal pressure of 2.6MPa and 4.1 MPa,the measured hoop strain were about 330?? and 590??,which were very close to calculated value.Stress adjustment phenomenon of steel liner at 1MPa pressure and residual strain about-20?? after pressure relief were observed by BOTDA.The shrinkage strain of surrounding concrete about 70?100?? was also measured,according to which the gapwidth of 0.51?0.75mm between steel case and concrete could be estimated.The data of optical fiber has the features of reliability and larger quantity,which is meaningful for the verification and the optimizing of preloading spiral case design.