Monitoring of Pre-stressing Anchors in Geotechnical Applications using Contact Free Sensors

Shahid Islam

In the proceedings of: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: SOIL AND TERRAIN CHARACTERIZATION - VII Instrumentation & Monitoring

ABSTRACT: Our infrastructure is expanding and the existing infrastructure is aging.Health monitoringandevaluation of structureshave become more critical than ever.A large number of retaining walls, bridge abutments, dams and other geotechnicalstructures aredesigned withpre-stressedanchors.There is agrowingtrend inusingre-stressable anchors. Theengineer needs to know the force in the anchor to decide ifanchor is performing as intended and ifre-stressing isneeded. Among available instrumentsstrain gauges, load cells or fiber optics are pre-installed.Mounting of strain gauges and fiber optics are challenging. Load cells when used require larger block-out to stress andlarger cap to protect the anchor. Most do not provide reliable data after few years in service.Intense research and testingwere performed to develop a sensor based on elasto-magnetic characteristic of steel,which will not requireanyattachmentto the steel element andwillnotbe subjected to compression loading.Thegoal wasto designsuch asensorwhich wouldhaveasimilarservicelife to that of the surrounding structure.Its application in force monitoringin retainingwall, force transfer in bond zone and tie-backforcemonitoring in a tunnel is presented.R•SUM•Notreinfrastructure est en expansion et l'existante vieillit. Surveillance de sant† et †valuation des structures sontdevenus plus critiques que jamais. Nombre de murs de sout‡nement, cul†es, barrages et autres ouvragesg†otechniques sont con…us avec ancrages pr†contraints. Il y a tendance croissante — l'utilisation d– ancragesretensionnables. L'ing†nieur a besoin de connaƒtre la force dans l'ancrage pour evaluer son fonctionnement et besoin deretensionnement si n†cessaire. Extensom‡tres, cellules de charge oufibre optique sont certains instrumentsdisponibles. Le montage d–extensom‡tres et fibre optique peut etre complexe. Les cellules de charge n†cessitent nichesplus grandes pour mise en tension et plus grand capuchon de protection. La plupart ne fournissent pas de donn†esfiables apr‡s quelques ann†es de service. Recherche et essais intensifs ont †t† effectu†s pour d†velopper un capteurbas† sur la caract†ristique †lasto-magn†tique de l'acier, qui ne n†cessite pas de fixation sur l'†l†ment en acier et n–estpas soumis — une charge de compression . Le but †tait de concevoir un capteur qui aurait une dur†e de vie similaire —celle de la structure environnante. Son application pour surveillance de la force dans un mur de sout‡nement, transfertde force dans zone d–ancrage et tirants dans un tunnel est pr†sent†e.1INTRODUCTIONOur infrastructure is expanding and the existinginfrastructure is aging. Some structures are expensive tomaintain and some occasionally fail.There is a largedemand for implementing health monitoring of structures.Many tie-backs,retaining walls, dams and othergeotechnicalstructures are post-tensioned and many ofthem need repair using a post-tensioned system.Onmanyoccasionsduringconstructionandservicelifeofthestructure,itisrequiredtoknowtheforceinthegroundanchorin the bonded and un-bondedlength.There is a trend in design specificationswhere the engineer calls for re-stressable anchor.The engineer needs to know the force in theanchor to decide if re-stressing is required. If notinstrumented there is no way to check how theseanchors are performing and also the condition afterany major event takes place. Among availableinstrumentsstrain gauges, load cells or fiber optics arepre-installed. Mounting of strain gauges and fiber opticsare challenging. Load cells when used require largerblock-out to stress and larger cap to protect the end ofanchor. Most do not provide reliable data after few yearsin service.The elasto-magnetic (EM) technology is a novelapproach to monitor forces in steel elements(Wang,et.al).DYNAForcesensor is built based on EMtechnology (See Figure1). Itis composed of a primarycoil and a secondary coil (sensing coil). By passingcurrent through the primary coil, ferromagnetic material ismagnetized. The sensing coil picks up inducedelectromotiveforce that is proportional to change rate ofapplied magnetic flux (Faraday's law)and relativepermeability. As permeability of core changes, outputvoltage changes. The output voltage is calibrated tomeasure the force.Areadout unitisdesigned tomagnetically energize the steel through the sensor andmeasure the response of the steel to the process. Thereadout unit then converts the response intoadirectforcereading. The sensor has an embedded thermistorwhich allows for temperature compensation.Due to the

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