Ingenieurbau

Génie civil

Civil engineering

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IntroductionLa réfection et le remplacement dela piste de l’aéroport de Genèveau droit du tunnel de Ferney ontpu être effectués grâce à la miseen œuvre de techniques inéditeset originales exploitant au mieuxles matériaux à disposition et latechnologie du béton et de laprécontrainte.La dalle du tunnel a été rempla-cée par un système de poussagecadencé intervenant durant leshoraires restreints de fermeturenocturne de la piste, soit entre0h00 et 05h00 du matin. Duranttoute la durée du chantier, lesavions ont décollés et atterris surune dalle posée sur des patinsglissants soutenue par un étayagecomplexe et sécurisé par des vé-rins latéraux de guidage. A l’issue de l’opération, le bilanest très positif : l’ouvrage a étéréalisé selon les budgets annon-cés, et surtout, sans aucune per-turbation du trafic aérien et del’exploitation de l’aéroport. Aucunvol n’a été ni retardé, ni détour-né. De plus, toutes les dispositionsavaient été prises pour permettrel’accueil éventuel de vols d’urgen-ce tels que les vols sanitaires,

même durant les opérations noc-turnes de travaux.Une opération complexe où laqualité de la préparation initialedu dossier et du projet est pri-mordiale, tout comme la collabo-ration et la synergie entre les dif-férents intervenants, qu’ils soientexploitants de l’aéroport, exploi-tants du domaine routier, ingé-nieurs ou entreprises.

Problème spécifique dutunnel de FerneyLa construction du tunnel de Fer-ney a été imposée par la prolon-gation de la piste en 1962. Il s’agitd’une tranchée couverte de plusde 400 m de longueur croisant lapiste en son milieu. La toiture dutunnel forme directement la struc-ture de la piste d’envol. Le tunnel a été dimensionné poursupporter les charges d’un avionde 200 t avec des réactions maxi-males par bogie de 125 t. Dans lesannées 70, à l’arrivée des Jumbo-jet, d’un poids maximal de 400 t,une expertise utilisant les nou-veaux moyens informatiques del’époque a permis de démontrerque les réserves de capacité aucentre de la piste étaient suffisan-tes pour permettre l’accueil de cetype de gros porteurs sur l’aéro-port de Genève. En 2005, plusieurs paramètres ontincité l’aéroport de Genève àengager des travaux de grandeimportance permettant la réhabi-litation de la piste au droit dutunnel :1. La dégradation de l’état de

surface de la piste est en voied’accélération et pourrait poserrapidement un problème desécurité pour les utilisateurs.

2. Le développement de l’aviationcivile incite les constructeurs aproduire des avions de taille su-périeure (600 t). La toiture dutunnel dans son état actuel est

IntroductionThe renovation and replacementof the Geneva airport runwayabove the Ferney tunnel wereachieved by means of unique andunprecedented techniques, usingin the most optimal method, avail-able materials as well as concreteand prestressing technologies.The tunnel slab was replacedusing an incremental launchingmethod, occurring only during thelimited hours of runway closureat night, that is between 12:00 pmand 5:00 am. During the wholeconstruction period, aircraft tookoff and landed on a slab laid onsliding bearings, supported by acomplex shoring system, and se-cured by lateral guiding jacks.The project was completed verysuccessfully: estimated costs weremet, and above all air traffic wasnever disturbed, nor were airportoperations. No flight was delayedor rerouted. Besides, specific mea-sures had been taken to ensureemergency flights landings, such asair-ambulance flights, even duringconstruction hours at night. Such a complex operation relieson a thorough preparation of theproject, as well as a very good

Renforcement de la piste de l’aéroport de Genève au-dessus du tunnel de FerneyUpgrade of Geneva airport runway – the Ferney Tunnel

Jean-François Klein

Fig. 1

Situation du tunnel et de la piste de l’aéroport.Situation of the tunnel and the runway.

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coordination of the differentpeople involved in the organizati-on: airport operating services,road operating services, engi-neers and contractors.

Specific problem of theFerney tunnelThe tunnel was constructed usingthe cut-and-cover method, inter-secting the Geneva airport run-way with a very shallow cover.In that part, the runway (i.e. thetunnel cover slab) was designedto carry 200 ton aircraft, with amaximum load of 125 tons perbogey. In the 1970s, when JumboJets with a weight of up to 400tons came on the scene, an inves-tigation of the tunnel carried outwith new computer models avail-able at the time showed that thestructure was strong enough towithstand this type of aircraft. In 2005, several factors led Gene-va airport to initiate major worksto renovate the runway aroundthe tunnel:1. The surface of the runway was

in worsening condition, a safetyissue became a growing con-cern.

2. Aircraft industry developmentsled to larger planes with aweight of 600 tons, starting2007. The covering slab of thetunnel was unable to with-stand such loads.

Two types of works were thenenvisioned:1. Reinforcement of the part of

the tunnel under the runway

incapable de supporter cettecharge.

A cela se greffe le problème del’entretien du tunnel qui n’a pasété effectué depuis de très nom-breuses années. L’état de Genève,propriétaire de l’ouvrage souhai-te profiter de ces travaux de gran-de envergure pour assurer la pé-rennité de l’ouvrage par un grosentretien ainsi que de procéder àla mise à jour des installations tech-niques et de sécurité. Les travauxengagés sont ainsi de deux na-tures :1. Renforcement et entretien de

la piste au droit du tunnel deFerney.

2. Assainissement et gros entre-tien de l’ensemble du tunnel,mise à jour des installationsélectromécaniques, création deniches SOS et adaptation desportails d’entrée.

Augmentation de la capa-cité portanteL’augmentation de la capacitéportante permettant de suppor-ter les avions de 600 t (3 x la capa-cité portante du projet d’origine)a pu être obtenue par modificati-on de la structure et utilisation dematériaux performants. Ainsi :– La dalle portante est rempla-

cée par une nouvelle dalle de60 cm d’épaisseur en béton àhaute performance, fortementprécontrainte dans les deux di-rections

– Des encastrements puissantsavec précontrainte verticale

and renovation of the runwayitself

2. Renovation of the whole tun-nel, upgrading of electro-me-chanical equipment, creationof emergency wall niches, andadaptation of the entrances.

Increase of the loadingcapacityThe increase of the loading capa-city for 600 ton aircraft (3 timesthe original one) was achieved bymodifying the structure and usingbetter materials:– The new 60 cm-thick slab is

highly pre-stressed in twodirections

– Strong fixed ends of the slabsare ensured by vertical pre-stressing in new abutmentwalls behind the bearing wallsof the tunnel. This increasessignificantly the global stiff-ness.

– The runway slabs are cast withhigh strength concrete

Description of the methodsusedIn order to achieve the replace-ment while meeting the scheduleconstraints, a launching methodwas used, replacing progressivelythe former slab. The work on therunway surface was then limitedto what is usually done to replacethe runway slab in other parts onthe ground. The sequence of constructionwork can be sum up as follows(A–H):

Fig. 2

Section existante et section future renforcée.Old and new cross section.

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dans les murs sont créés sousforme de culées latérales à l’ar-rière des murs porteurs per-mettant une forte augmentati-on de la rigidité globale

Description de la méthodeemployéeLa méthode développée pourmener à bien ces travaux dans lerespect des contraintes (géomé-triques et d’exploitation) est unesolution de poussage cadencéremplaçant progressivement ladalle actuelle. Les interventions desurface nécessaires pour mener àbien ces travaux sont ainsi trèslimitées. Les opérations principa-les peuvent être schématiséescomme suit (A–H):A. Forage de pieux au travers dela piste actuelle de part et d’au-tres du tunnel.Ces pieux serviront essentielle-ment aux phase de constructionsafin de permettre la réalisationdes culées du tunnel en taupe,sous la circulation des avions, enarrière des murs du tunnel enplace et à l’abri d’une paroi berli-

A. Drilling of piles through thecurrent runway slab on each sideof the tunnel.These piles are mostly used duringthe construction phase to build theabutments underground underthe aircraft traffic. The abutmentsare built between the walls of thecurrent tunnel and a Berlin-typewall (US: soldier pile wall) in bet-ween the steel piles. The drillingprocedure uses special equipmentlike epoxy sealed 40 cm-thick con-crete cover to allow immediatetraffic on top of the just drilledand concreted pile.B. Replacement of the runwayslabs on each side of the tunnel(those had not been changeduntil now), and laying of a tem-porary link element between theside slabs and the tunnel slab.The former side slabs (containingno reinforcement steel bars) arelifted up by a high capacity travel-ling crane from the airport, usinghundreds of dowels sealed in theconcrete. 6 m-wide by 14 m-longelements are thus removed in oneoperation.

noise formée par les pieux métal-liques. L’exécution de ces pieux anécessité la mise au point de dis-positifs spéciaux tels que l’emploide couvercles préfabriqués en bé-ton de 40 cm scellés à l’époxy rapi-de après le bétonnage du pieupermettant le passage immédiatdes avions.B. Remplacement des dalles debords sur le terrain (dalles encoreinchangées à ce jour), pose d’unélément provisoire de raccordentre les dalles de bord et la toi-ture actuelle du tunnel.Les anciennes dalles de bord nonarmées sont soulevés à l’aide desportiques de grande capacité del’aéroport, par le biais de centai-nes de goujons scellés dans lebéton. Des éléments de 6 m delarge par env. 14 m de long d’unpoids de 120 à 160 t sont ainsiremplacés en une opération. Les nouvelles dalles de bords sontposées sur un réseau de boudinsgonflables à l’eau permettant derégler leur niveau et assurant leurpositionnement pendant le tempsde durcissement du coulis d’injec-tion à prise rapide remplissant l’in-tervalle entre le remblai et la dallede bord. L’extrémité de la dalle debord présente un renfort avec uncorbeau et des réservations ve-nant s’enficher sur les têtes depieux. Un élément préfabriqué à géo-métrie variable posé entre le cor-beau de la dalle de bord et le cor-beau du tunnel existant sert dedalle de transition. Il permettral’excavation entre le mur et laparoi berlinoise formée par les

Fig. 3

Coupes en travers des dalles de bord.Description of the lateral elements.

Fig. 4

Construction des nouvelles culées en arrière des murs actuels –étayage temporaire de la dalle actuelle avec appuis glissants.Construction of new abutment walls next to the Berlin-typewall – temporary shoring under the runway slab.

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The new side slabs are then laidon water-inflated tubes, allowingthem to adjust their height andposition before the space bet-ween the ground and the slab isfilled with rapid-setting grout.The end of the side slab next tothe tunnel has a reinforced corbelwith holes enclosing the steelpiles’ top ends.C. Construction of the new abut-ments underneath, while keepingthe clearance free for traffic inthe tunnel. This work is carriedout next to a Berlin-type wall inbetween the steel piles.

pieux tout en assurant le passagedes avions. Cet élément permetégalement de compenser les ef-fets de la courbure du tunnel etde faire un ripage rectiligne mal-gré la courbure.C. Construction en taupe de nou-velles culées sous la piste actuelleet à l’intérieur du gabarit du tun-nel. Exécution de ces travaux àl’abri du bouclier formé par uneparoi berlinoise réalisée entre lespieux forés au travers de la piste.D. Pose d’un étayage provisoiresupportant l’ancienne dalle detoiture du tunnel. Cet étayage est

D. Demolition of the tunnel walls,replaced by temporary shoring,bearing the current slab of thetunnel. This steel shoring, replacing thetunnel walls is designed to pro-gressively transfer the aircraftloads, while disconnecting theslab to be pushed out. E. Creation of two large openingsin the tunnel slab on each side ofthe runway.On each side of the runway, out-side the aircraft traffic, two ope-nings are created. They are 8 and10 m long, and are as wide as the

Fig. 5

Elévation de l’étayage, situation des trémies de pose et d’évacuation.Cut and removal of an element to create openings on each side of the runway.

Fig. 6

Principe général de la méthode de ripage.General principle of the launching method.

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au droit des anciens murs et assu-re progressivement la reprise desefforts. La totalité de la dalle estainsi désolidarisée de ses murs etposée sur des voies de ripage. E. Ouverture de deux trémiesdans la dalle du tunnel de part etd’autre de la piste.De part et d’autre de la piste, endehors des zones de circulationdes avions, deux ouvertures de 8et 10 m de longueur sur toute lalargeur du tunnel sont créées. Cesouvertures serviront à poser lesnouveaux éléments préfabriquésde dalle d’un coté et de d’enleverles anciens de l’autre coté après laphase de poussage. En phase opé-rationnelle de l’aéroport, ces ou-vertures sont protégées par descouvercles métalliques coulissantsur des rails capables de supporterles charges des avions en cas desortie de piste. Ces «tiroirs» sontainsi ouverts et fermés en foncti-on des heures d’exploitation de lapiste d’envol. F. Pose d’éléments préfabriquésde dalle précontrainte, étanchéi-té, bétonnage de la dalle d’usureet évacuation de l’ancienne dalle.Dès l’atterrissage du dernier avionvers 23h45, le couvercle de pro-tection de la trémie de pose estouvert. Le portique de levage dé-pose un élément préfabriqué dedalle de 36 m de longueur par 3 m

tunnel. They are used to lay newprecast slab sections on one side,and to remove old sections on theother side after each launchingstep. During the day when theairport is running, these openingsare covered with steel plates slid-ing on rails, which are capable oftaking the load of an aircraft (incase a plane accidentally exits therunway). These sliding covers areopened and closed according tothe air traffic hours on the run-way.F. Laying of new precast prestress-ed slab sections, proofing layer,casting of runway top concrete,removal of the old section of thecurrent slab.As soon as the last aircarft landsaround 11:45 pm, the openingcover is slid out. The travellingcrane brings a new 36 m-wide by3 m-long slab section and puts itin the opening. This section isfixed to the previous one byepoxy sealing and post-tensionedusing thirty 50 mm diameter barsconnected to the previous secti-on. After two of these elementsare laid in the opening, giving 6 mof slab, the proofing layer isinstalled and the 30 cm-thick topconcrete of the runway is cast.On the other side of the runway,the 6 m-long old slab sectionpushed out is removed from the

de largeur dans la trémie. Celui-ciest connecté par collage époxyaux dalles précédentes et mis enprécontrainte par une trentainede barres diamètre 50 mm con-nectées à celles de l’élément pré-cédent. Après la pose de deux élé-ments de dalles formant 6 m denouvelle couverture du tunnel,l’étanchéité est complétée et ladalle d’usure de 30 cm d’épaisseurest bétonnée. Du coté de la trémie d’évacuati-on, le morceau de 6 m de dalleprécédemment poussé en dehorsde la piste est découpé et évacué.La piste est rendue au traficaérien à 05h30.G. Ripage progressif de l’anciennedalle et des nouveaux élémentspar étapes de 6 m de longueur.Tous les 6 m, l’ensemble de la dalleest poussée au travers de la piste,déplaçant ainsi la partie résiduel-le de l’ancienne dalle vers la trémiede démolition, et introduisant lesnouvelles dalles placées dans latrémie de pose dans le chemine-ment de la piste. La surface deroulement est ainsi progressive-ment remplacée sans interventionmajeure dans le chemin d’évoluti-on des avions. A la fin de chaquephase de ripage, des vérins deblocage latéraux intégrés dansl’épaisseur de la dalle viennentbloquer tout le système, afin que

Fig. 7

Pose d’un nouvel élément dans la trémie – collage époxy et post-tension des éléments à joints conjugués.Laying a new element in the opening – epoxy sealing and post-tensioning connection against the previous one.

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other opening. The runway isgiven back to air traffic at 5:30am.G. Progressive launching of thestructure in six metre steps.Every six metres, the structure ispushed across the runway, eachtime pushing a portion of the oldslab out of the runway towardsthe demolition opening, and push-ing in a portion of the new slab.Thus, the runway is replaced andreinforced gradually, without dis-turbing air traffic. At the end ofevery pushing step, lateral jacksare used to block the whole struc-ture, so that aircraft can land andtake off without creating any dis-placement of the slab laid on slid-ing bearings. With this system,even if there is a failure in thepushing system, the structure canbe blocked and secured at anymoment and the runway used byaircraft. In the case of the arrivalof an emergency or air-ambulanceaircraft, the runway can be readyin just a few minutes.H. End of the pushing sequence,connection to the abutments.Once the whole new structure ispushed in place, the back walls ofthe abutments are connected to

les avions puissent évoluer sur letunnel sans risque de faire bougerla dalle posée sur ses appuis deglissement. De cette manière,même en cas de blocage du ripa-ge, le système peut être à toutmoment sécurisé et la piste ex-ploitée. En cas d’arrivée impromp-tue d’un avion sanitaire ou d’ur-gence, la piste peut ainsi être opé-rationnelle en quelques minutes. H. Fin du ripage, solidarisation desculées et de la dalle du tunnel. A la fin du poussage, les mursarrières des culées sont connectésà la dalle par des barres de pré-contrainte verticales, formantainsi des encastrement puissantspermettant l’augmentation decapacité portante de la dalle. Cesmurs remplacent progressivementles étayages qui sont démontésau fur et à mesure des transfertsde charge sur les murs. Au niveau de la dalle, de la pré-contrainte longitudinale complé-mentaire et continue complèteles barres couplées. De même,dans la dalle d’usure, des câblesde précontrainte enfilés à la finpermettent de lutter partielle-ment contre les effets de la fissu-ration due aux effets différés.

the slab using vertical post-ten-sioning bars, creating strong fixedends to the slab to increase theloading-bearing capacity. Newwalls gradually replace the tem-porary shoring, and this is thendismantled bit by bit once the loadis transferred to the walls.Additional longitudinal prestress-ing in the slab comes on top ofthe connected bars. Similarly, pre-stressing cables are slid into thetop concrete of the runway toprevent cracking due to long-term effects.

Auteur/Author

Jean-François Klein Dr ès sc. techn., ing. civil dipl. EPFL Administrateur de T ingénierie saCH-1211 GenèveEmail: gva@t-ingenierie.com

Fig. 8

Vue de la piste après 3 étapes de ripage.View of the runway after 3 pushing steps.

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IntroduzioneNell’ambito degli interventi di po-tenziamento delle infrastrutturedestinate al turismo da diporto fi-nanziate dalla Regione Lombardia,rivestono un ruolo importante lerealizzazioni dei due pontili gal-leggianti di Porto Valtravaglia eMaccagno situati entrambi sullasponda est del Lago Maggiore. Ciascuna delle due strutture è for-mata da moduli prefabbricati incalcestruzzo armato assemblati inacqua per mezzo di un sistema diprecompressione esterna. Graziealla particolare forma ad arco ealla notevole mole del manufattoassemblato, le oscillazioni prodot-te dal moto ondoso agente sullastruttura risultano contenute atutto vantaggio di un notevolecomfort per gli utilizzatori delmolo.

Azioni esterne preponde-rantiL’azione che genera le sollecita-zioni più importanti all’interno diun corpo galleggiante è senz’altroil vento. Il suo effetto si esplicaattraverso un’azione diretta rap-presentata dalla pressione cineti-ca applicata alla struttura stessa eai natanti ad essa ormeggiati edun’azione indiretta rappresentatadal moto ondoso indotto dalvento.I parametri che maggiormente de-terminano l’intensità di tali azionie condizionano quindi il dimen-sionamento del sistema, sonol’estensione dello specchio d’ac-qua (fetch) e il regime dei ventidella zona che si estende attornoalla struttura. Dal fetch e dalladurata dei venti nel tempo dipen-dono in particolare le lunghezzed’onda dei moti ondosi che si pos-sono sviluppare in un determina-to bacino. Dalle intensità dei

venti dipendono in generale lealtezze delle onde che possonoimpegnare la struttura.

Principali tipologie di moligalleggianti in calcestruzzoarmatoLe tipologie di moli galleggiantiin calcestruzzo armato attual-mente impiegate per creare unadarsena possono essere classifica-te in base al sistema di ormeggioutilizzato e in base alla rigidezzapropria posseduta dalla struttura. Con riferimento alle modalità diormeggio, si possono distinguereun primo tipo di soluzioni che uti-lizzano sistemi di catene ancorateal fondo per mezzo di corpi mortio pali infissi e un secondo tipo disoluzioni che utilizzano sistemi dibielle rigide (o bracci oscillanti)ancorate al fondo per mezzo dipali.Nel caso dei pontili in oggetto si èoptato per la soluzione rigida delsecondo tipo accoppiata ad unsistema di ormeggio formato daben 34 catene opportunamentetesate ed orientate in modo dagarantire spostamenti relativi frapasserella di accesso e corpo gal-leggiante entro le tolleranze diprogetto.

AnalisiLa struttura qui in esame ha il ca-rattere di prototipo. Per la deter-minazione delle sollecitazioni in-terne indotte dal moto ondosoassociato a diverse direzioni di

IntroductionWithin the scope of the works forthe development of infrastructu-re for the tourist industry financ-ed by the Region of Lombardy,the creation of two floating land-ing stages at Porto Valtravagliaand Maccagno play an importantrole. Both are situated on the eastbank of Lake Maggiore.Each of the two structures com-prises prefabricated reinforcedconcrete modules assembled inthe water by means of an exter-nal prestressing system. Thanks tothe particular arched shape andconsiderable inertia of the as-sembled product, the oscillationgenerated by the movement ofthe waves acting on the structureproves to be contained for thegreat convenience of those usingthe jetty.

Main external actionThe force which generates moststress inside a floating body is thewind. Its effect is developedthrough direct action representedby the kinetic pressure applied tothe structure in question and tothe craft moored to it as well asindirect action due to the move-ment of the waves, which thewind creates in the fluid accom-modating the body.The main parameters which de-termine the intensity of such ac-tion and therefore condition thedimensioning of the system, arethe extent of open water (lengthof the free area available to thewind or fetch) and the windspeed (kinetic pressure) in thearea that extends around thestructure. In particular, the length of thewaves in the wave movement,which may be generated in a par-ticular basin, depend on the fetch

Pontili galleggianti in calcestruzzo precompresso sul Lago MaggioreFloating landing stages in prestressed concrete on Lake Maggiore

Antonio Paronesso, François Prongué

Aknowledge La realizzazione delle opere illustratenel presente articolo è stata resa pos-sibile grazie al contributo fornitodallo Studio Ambrosetti-Colombo,Varese, Italia, da M. Jartoux, daFreyssinet SA Suisse e da AR&PAEngineering, Lugano Svizzera.

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and duration of the wind. Ingeneral, the height of the wavesthat could affect the structuredepends on the intensity (kineticpressure) of the wind.

Main types of floating jet-ties made of reinforcedconcreteThe types of floating jetties madeof reinforced concrete currentlyused to create a dock can be clas-sified according to the mooringsystem used and the inherent rigi-dity of the structure. With regard to mooring methods,there is one type of solution,which uses systems with chainsanchored to the bottom by meansof anchor blocks or piles, and an-other type of solution, which usessystems with rigid connectingrods (or swivel arms) anchored tothe bottom via piles.

propagazione d’onde, sono stateeffettuate analisi idrodinamichein frequenza del solo corpo rigidogalleggiante e analisi statiche edinamiche non lineari nel tempodel sistema completo di catene.

Geometria del manufatto etecnica costruttivaIl molo è formato da 20 moduliprefabbricati in calcestruzzo ar-mato di forma trapezoidale dilunghezza media 5,6 m, sezionetrasversale di larghezza 4 m ealtezza 3 m. I due moduli di testa-ta si differenziano per organizza-zione e dimensioni della strutturainterna dagli altri 18 della zonacentrale in quanto destinati adaccogliere le teste di ancoraggiodei cavi di precompressione. Lospessore della soletta di fondo deimoduli è pari a 22 cm mentre quel-lo della soletta superiore è delle

In the case of the landing stagesin question, the rigid (i.e. second)type of solution was chosen cou-pled with a mooring system form-ed of 34 chains tightened and ori-entated so as to guarantee therelevant displacement betweenthe access bridge and the floatingbody within the project tolerancerange.

AnalysisThe structure under considerationis like a prototype. In order to de-termine the internal stress induc-ed by wave action coming fromdifferent directions, some hydro-dynamic analyses were carriedout in the frequency domain forthe floating rigid body alone aswell as non-linear static analysesand non-linear dynamic analysesperformed in time domain for thesystem with chains.

Fig. 1

Messa in acqua dei moduli per mezzo di un autogru.Placing in the water of the modules by means of mobile crane.

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pareti verticali è di circa 18,5 cm.La stazza totale del pontile è paria circa 1200 t. Una volta assem-blata, la struttura assume laforma di un arco con lunghezzauguale a circa 120 m e concavitàrivolta verso riva. I moduli sono prefabbricati in sta-bilimento a Varese grazie all’im-piego di un cassero metallico. Peril loro confezionamento è utiliz-zato calcestruzzo di classe C40/50e acciaio B450C. La gabbia dellearmature lente del singolo ele-mento è preassemblata a pièd’opera e quindi inserita nel cas-sero prima del getto del calce-struzzo. Segue il montaggio degliinserti necessari per il passaggiodelle barre di preassemblaggiodei moduli e quelli per la forma-zione delle selle di appoggio edeviazione (diabolos) dei cavi diprecompressione. Si procede quin-di al getto simultaneo della solet-ta di fondo e delle quattro paretiverticali del modulo, seguito adistanza di 24 a 36 ore da quellodi completamento della solettasuperiore. Gli elementi prefabbricati sono inseguito trasportati su strada sino

Product’s geometry anddesign techniqueThe jetty consists of 20 prefabri-cated modules made of reinforc-ed concrete, trapezoidal in shapewith an average length of 5.6 m,a cross section of 4 m width andheight of 3 m. The two pierheadmodules differ in their arrange-ment and dimensions of internalstructure from the other 18 in thecentral zone in that they are forhousing the anchorage heads ofthe prestressed cables. The thick-ness of the bottom slab of themodules is 22 cm while that ofthe upper slab and vertical wallsis about 18.5 cm. The total regis-tered tonnage of the landingstage is about 1,200 tons. Onceassembled the structure took onthe shape of an arch with an ave-rage length of about 120 m andconcavity in the direction of thebank. The modules were manufacturedin the plant at Varese thanks tothe use of a steel formwork. ClassC40/50 concrete and B450C steelwere used. The frame for the con-crete steel reinforcement of thesingle block was preassembled on

alla darsena di varo di Sant’Annasituata presso Sesto Calende. Quiavviene la messa in acqua (fig. 1)dei moduli e un primo assemblag-gio a gruppi di tre. Questa opera-zione è effettuata utilizzando 8barre Ø 40, fpk = 1030 N/mm2, dis-poste su ciascuna interfaccia dicollegamento. In questa fase lebarre vengono tesate ad un valo-re non superiore a 100 kN.Successivamente viene realizzatoil giunto di continuità fra modulimediante iniezione di maltacementizia debolmente espansivapraticata nell’intercapedine crea-ta dall’accostamento dei moduli edall’interposizione fra di essi di unnastro di neoprene posto lungo ilperimetro delle facce da accop-piare. Avvenuta la maturazionedella malta di iniezione dei giun-ti, le barre sono tesate una secon-da volta ad un valore limite ugua-le a circa 750 kN.Ciascun gruppo formato da tremoduli è quindi trasportato viaacqua dalla darsena di varo allazona temporanea di ormeggioposta a circa 300 metri dalla riva.In questo luogo, al riparo da motiondosi importanti (fig. 2), sono

Fig. 2

Ultime operazioni di accopiamento dei gruppi formati da tre moduli.Last connecting operations of the units formed of three modules.

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site and therefore inserted in thesteel formwork before casting theconcrete. Then followed the as-sembly of the inserts necessary toaccommodate the preassembledmodule bars and those for theformation of the saddle and de-viation supports (diabolos) for theprestressed cables. Next, simul-taneous casting of the bottomslab and four vertical walls of themodule took place, followedafter 24 to 36 hours by that forcompletion of the upper slab. The prefabricated items werethen transported by road as far asthe Sant’Anna dock slipway situ-ated near Sesto Calende. Here themodules were placed in the water(Fig. 1) and assembled in groupsof three. This operation was car-ried out using eight prestressingbars arranged on each connectinginterface with Ø 40 and fpk =1,030 N/mm2. In this phase thebars were tightened to a value ofnot more than 100 kN.Subsequently, a continuity jointwas created between modules bymeans of an injection of slightlyexpansive mortar, made in thecavity created by the line-up of

effettuate le ultime operazioni diaccoppiamento dei gruppi prece-dentemente assemblati. Una voltacompletato il collegamento ditutti e 20 i moduli si procede allamessa in opera del sistema di pre-compressione costituito da 8 cavipostesi ciascuno formato da 12trefoli di 0,6’, fpk = 1860 N/mm2, ealla successiva messa in tensionedei cavi per una forza totale diprecompressione misurata al mar-tinetto pari a circa 18 750 kN.Il manufatto finito completo deiflaps di stabilizzazione idrodina-mica è quindi trasportato via lagoal luogo d’impianto definitivo(fig. 3) dove sono in attesa le ca-tene di ormeggio preventivamen-te messe in opera.

Sistema e tecnica di messain precompressioneLo stato di sollecitazione dellastruttura in oggetto è caratteriz-zato da variazioni cicliche cheimpongono l’utilizzo di una pre-compressione centrata con ten-sioni uniformi su ciascuna sezionetrasversale del pontile. La tecnicadi precompressione esterna quiadottata (con cavi che corrono

the modules and the insertionbetween them of a neopreneband, placed along the perimeterof the surfaces to be joined.When the mortar injected intothe joints had hardened, the barswere tightened a second time toa limit value of about 750 kN.Each group formed of three mo-dules was then carried by waterfrom the dock slipway to the tem-porary mooring zone located atabout 300 metres from the bank.Here, sheltered from significantwave movement (Fig. 2), the lastconnecting operations of theunits previously assembled werecarried out. Once the connectionof all 20 modules had been com-pleted, setting up of the prestres-sed system took place, comprising8 cables, each one formed of 12strands of 0.6’, fpk = 1,860 N/mm2

and subsequent tightening of thecables for a total prestressedforce measured with a jack ofabout 18,750 kN.The finished product, completewith hydrodynamic stabilizingflaps, was then carried by lake tothe final place of installation (Fig.3) where it awaited the mooring

Fig. 3

Il manufatto al luogo d’impianto definitivo.The product at the place of final installation.

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chains which had been set up pre-viously.

Prestressing system andtechniqueThe state of stress of the structu-re in question is characterized bycyclical variations, which imposethe use of prestressing centredwith uniform tension on eachcross section of the landing stage.The external prestressing techni-que adopted here (with cablesthat run in the internal free spaceof the modules), presents variousadvantages compared to othersystems currently in use. Theexternal alignment of the cablesallows containment of the thick-ness of the slabs and walls of themodule, which must satisfy, in thiscase, only static requirements andthose concerning guaranteedminimum concrete cover to rein-forcement.The use of greased and PE sheath-ed strands located inside HDPEtubes, reduces the coefficient offriction to the minimum betweenthese items and facilitates thestressing of cables 120 m longfrom one end. This makes it possi-ble to carry out the stressing ope-rations outside the head modules

nello spazio libero interno deimoduli) presenta diversi vantaggirispetto ad altri sistemi corrente-mente utilizzati. L’andamento es-terno dei cavi permette di conte-nere gli spessori delle solette edelle pareti del modulo che devo-no soddisfare, in questo caso, asole esigenze statiche e di copri-ferro minimo garantito. L’impiego di trefoli inguainati elubrificati posti all’interno di tubiguaina PEHD, riduce al minimo ilcoefficiente di attrito fra tali ele-menti e consente la messa in ten-sione di cavi di lunghezza 120 m apartire da una sola estremità. Ciòrende possibile effettuare le ope-razioni di tesatura all’esterno deimoduli di testata grazie all’ado-zione di cavi ad andamento incro-ciato. La tecnica utilizzata per il confe-zionamento dei cavi prevede chel’inserimento nel tubo guaina diciascun trefolo inguainato e lubri-ficato avvenga in maniera indi-pendente dagli altri grazie all’uti-lizzo di un sistema di trascina-mento a spola di filo che garantis-ce un perfetto allineamento deitrefoli che compongono lo stessocavo. La successiva iniezione deltubo guaina con malta cementizia

Fig. 4

Impiego di due martinetti monotrefolo.Use of two single strand jacks.

AcknowledgmentCompletion of the works described inthis article was made possible thanksto the contribution provided byStudio Ambrosetti-Colombo, Varese,Italy, M. Jartoux, Freyssinet SASwitzerland and AR&PA Engineering,Lugano Switzerland.

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thanks to the use of cables incrossed alignment.The technique used for makingthe cables provides for insertionin the HDPE tube of each strandto take place independently ofthe others thanks to the use of anetting needle drag system,which ensures perfect alignmentof the strands comprising thecable. The subsequent injectionof the cladding tube with slightexpansion mortar guarantees thecorrect distance between strands,which can be tightened in thisway, independently of each otherensuring greater actual flexibilityand better tightening control.Protection of each individualstrand is ensured by a triple safetysystem formed by the externalHDPE tube, the sheathing of thestrand itself and the grease thatsurrounds it.Another advantage of the chosensystem is the possibility of makingprovision for additional lengthsof strand required for assembly,thanks to which it will always bepossible to adjust each individualitem for the purpose of control-ling tightening over time andpossibly readjusting its value fordealing with loss due to creep in

debolmente espansiva garantisceil corretto distanziamento dei tre-foli che possono così essere tesatiin modo indipendente l’unodall’altro garantendo maggiorflessibilità esecutiva e un controllomigliore della tensione. La protezione del singolo trefoloè assicurata da un triplice sistemadi sicurezza formato dalla guainaesterna del tubo, da quella ester-na del trefolo stesso e dal grassoche lo avvolge. Altro vantaggio del sistema sceltoè la possibilità di prevedere dellesovralunghezze di montaggio deitrefoli, grazie alle quali sarà sem-pre possibile agire sul singolo ele-mento al fine di controllarne latensione nel tempo ed eventual-mente riaggiustarne il valore perfar fronte alle perdite per fluagedel calcestruzzo e per rilassamen-to dell’acciaio. Gli interventi di tesatura sonostati portati a termine mediantel’impiego di due martinetti mono-trefolo (fig. 4) e di un martinettomultitrefolo (fig. 5).

the concrete and steel relaxation. The tightening work was perfec-ted and completed by using twosingle strand jacks (Fig. 4) andone multi strand jack (Fig. 5).

Autori/Authors

Antonio ParonessoDott. ing. dipl. EPFLAR&PA Engineering SaglCH-6963 Lugano-Pregassonaantonio.paronesso@arpa-enginee-ring.ch

François PronguéIng. dipl. EPFLFreyssinet SACH-1510 Moudonprongue@bluewin.ch

Fig. 5

Impiego di un martinetto multitrefolo.Use of a multi strand jack.