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Dfi2005 pecker

  1. 1. DESIGN AND CONSTRUCTION OF THE RION ANTIRION BRIDGE FOUNDATIONSAlain Pecker, Géodynamique & Structure, Bagneux, FranceThe choice of a design concept for a bridge foundation is guided by various factors; severalof these factors are indeed of technical origin, like the environmental conditions in a broadsense, but others non technical factors may also have a profound impact on the final designconcept. The foundations solutions adopted for the Rion Antirion bridge are described andan attempt is made to pinpoint the major factors that have guided the final choices. The RionAntirion bridge is exemplar in that respect: the foundation concept combines the simplicity ofcapacity design, the conceptual facility of construction and enhances the foundation safety.The design of these foundations was a very challenging task which required full cooperationand close interaction with all the parties involved: concessionaire, contractor, designers anddesign checker.INTRODUCTIONThe Rion-Antirion bridge project is a BOT contractgranted by the Greek Government to a consortiumled by the French company Vinci Construction(formerly Dumez-GTM). It is located in Greece, nearPatras, will constitute a fixed link between thePeloponese and the Continent across the westernend of the gulf of Corinth and is intended to replacean existing ferry system (Fig. 1). The solutionadopted for this bridge is a multiple spans cablestayed bridge with four main piers; the three centralspans are 560 m long each and are extended by twoadjacent spans (one on each side) 286 m long. Thetotal length of the bridge, with the approach viaducts,is approximately 2.9 kilometers (Fig. 2). The call fortender was launched in 1992, the contract, awardedto the consortium in 1996, took effect in December1997. Construction started in 1998 and has beencompleted in summer 2004 and opened to traffic onthe 11thof August, five months ahead of schedule.The bridge has to be designed for severeenvironmental conditions (e.g., Teyssandier et al.2000; Teyssandier 2002): weak alluvium deposits,high water depth, highly seismic area, possibleoccurrence of large tectonic displacements. Veryearly in the design process it turned out that designwas mainly controlled by the seismic demandimposed to the foundations.Figure 1: Location of the bridgeIn order to alleviate potential damage to the structuredue to the above adverse conditions and to carry thelarge earthquake forces brought to the foundation(shear force of the order of 500 MN and overturningmoment of the order of 18 000 MNm for a verticalbuoyant pier weight of 750 MN), an innovativefoundation concept was finally adopted whichconsists of a gravity caisson (90 m in diameter at thesea bed level) resting on top of the reinforced naturalground (Combault et al. 2000). The groundreinforcement is composed of steel tubular pipes, 2 min diameter, 20 mm thick, 25 to 30 m long driven at agrid of 7 m x 7 m below and outside the foundation,
  2. 2. covering a circular area of approximately 8 000 m2.The total number of inclusions under each foundationis of the order of 150 to 200. In addition, the safety ofthe foundation is greatly enhanced by interposing agravel bed layer, 2.8 m thick, on top of the inclusionsjust below the foundation raft with no structuralconnection between the raft and the inclusionsheads. This concept (inclusions plus gravel layer), inaddition to minimizing the hazards related todifferential settlements, enforces a capacity designphilosophy in the seismic foundation design, (Pecker1998).In the following we will focus on the foundations ofthe main bridge, because the foundations of theapproach viaducts (pile foundations) do not deservemuch comments.Figure 2: Bridge elevationGEOLOGICAL AND GEOTECHNICALENVIRONMENTThe site has been subjected to extensive offshoresoil investigations performed either from floatingbarges or from a ship, controlled with a dynamicpositioning system; these investigations includedcored boreholes, static Cone Penetration Tests withpore pressure measurements (CPTU), StandardPenetration tests (SPT), vane tests and dilatometertests, seismic cone tests and sampling of intact soilsamples for laboratory testing. All the boringsreached depths ranging from 60 m to 100 m belowthe sea bed. Under each of the main bridge pierthree continuous boreholes, three CPTU, two seismiccones, one SPT/dilatometer boring have been drilled.Approximately 300 samples have been retrieved andsubjected to advanced laboratory testing. Based onthe results of these investigations representative soilprofiles have been defined at the locations of themain bridge piers and ranges of soil mechanicalcharacteristics have been derived.The water depth in the middle of the strait reaches 65m. The soil profile consists of weak alluvial stratadeposited in alternate layers, with individual thicknessof a few meters, of silty sands, sandy clays andmedium plasticity clays. In the top hundred metersinvestigated by the soil survey, the clay, or silty clay,layers predominate (Fig. 3). No bedrock wasencountered during the investigations and based ongeological studies and geophysical surveys its depthis believed to be greater than 500 m.65m65 mCLAYSILTSAND-GRAVELFigure 3: Soil profileThe mechanical characteristics of the offshore layersare rather poor with undrained shear strengths of thecohesive strata increasing slowly with depth fromapproximately 30-50 kN/m2at the sea bed level to80-100 kN/m2at 50 m depth (Fig. 4). A majordifference occurs in the cohesionless strata: some ofthese layers are prone to significant pore pressurebuildup, even possibly to liquefaction, under thedesign earthquake. Accordingly, the undrainedstrengths of the cohesionless layers are taken equaleither to their cyclic undrained shear strengths or totheir residual strengths.Based on the results of the laboratory onedimensional compressibility tests and of correlationswith CPT results or with the undrained shearstrengths, a slight overconsolidation, of the order of150 kN/m2, of the upper strata was evidenced.The shear wave velocities are also small, increasingfrom 100-150 m/s at the ground surface to 350-400 m/s at 100 m depth (Fig. 5).It is worth noting that for design, as shown in Fig.4and Fig.5, two sets of soil characteristics have beenused instead of a single set based on characteristicvalues; this decision is guided by the fundamentalnecessity to maintain compatibility in the wholeseismic design process: the seismic demand iscalculated assuming one set of properties(successively lower bound and upper bound) and thefoundation capacity is checked with the associatedstrengths (respectively lower bound and upper
  3. 3. bound); the capacity is never checked with lowproperties when the forces are calculated with highproperties and vice versa. In addition, given the largefoundation dimensions, special attention has beenpaid to the spatial variability of the soil propertiesacross anyone foundation; this variability may havean impact on the differential settlement and tilt of apylon. Fig. 6 gives an example of the variability of thecone point resistance across a foundation, variabilitywhich is obviously reflected by variability in the shearstrength and compressibility of the soil strata.0204060801000 100 200 300 400 500Undrained shear strength (kN/m2)Depthbelowgroundsurface(m)Figure 4: Undrained shear strengthENVIRONMENTAL CONDITIONSThe environmental conditions are defined by threemajor possible events likely to occur during thebridge life time: ship impact on a main bridge pier,occurrence of a major earthquake in the vicinity of thebridge, long term tectonic movements.Ship impactThe hazard represented by this impact correspondsto 160 000 Mg tanker hitting one pier at a speed of 16knots (8.2 m/s). This impact induces an horizontalshear force of 480 MN acting at 70 m above thefoundation level; at the foundation level thecorresponding forces are: shear force of 480 MN andoverturning moment of 34 000 MNm.Earthquake eventThe bridge is located in one of the most seismic areain Europe. In the past 35 years three earthquakesexceeding 6.5 on the Richter scale have occurred inthe Gulf of Corinth. The 1995 Aigion earthquake tookplace less than 30 km east of the site. Fig. 7presents the epicenters of the major earthquakes feltin the Gulf of Corinth along with the major tectonicfaults. The contract fixed for the design motion areturn period of 2 000 years. A comprehensiveseismic hazard analysis has defined the governingevent as a 7.0 surface wave magnitude earthquakeoriginating on the Psathopyrgos fault (shown with anarrow in Fig. 7) only 8.5 km east of the site (circle inFig. 7).0204060801000 100 200 300 400 500 600Shear wave velocity (m/s)Depthbelowgroundsurface(m)Figure 5: Shear wave velocityDepthbelowseabedDistance-60-80-12040-60 600 206555452515535-100-40 -20Figure 6: Spatial variability of cone pointresistance across Pier M3In recognition of the influence of the soilcharacteristics on the ground surface motion, thedesign response spectrum at the sea bed elevation is
  4. 4. defined from specific site response analyses basedon the actual soil characteristics and on the designrock motion defined by the seismic hazard analysis.The 5% damped response spectrum is shown in Fig.8: the peak ground acceleration is equal to 0.5g, theplateau at 1.2 g is extending from 0.2 s to 1.1 s andat 2 s the spectral acceleration is still high, equal to0.62 g.Figure 7: Locations of major earthquakesTectonic movementsThe seismic threat arises from the prehistoric drift inthe earths crust that shifted the Peloponese awayfrom mainland Greece. The peninsula continues tomove away from the mainland by a few millimeterseach year. As a result the bridge must accommodatea 2 m differential tectonic displacement in anydirection and between any two piers. 1.0 2.0 3.0 4.0 5.Period0(s)Pseudoacceleration(g)Figure 8: Design ground surface responsespectrumMAIN BRIDGE FOUNDATIONSVery soon during the design stage it appeared thatthe foundations design will be a major issue. On theone hand, the unfavorable geotechnical conditionswith no competent layer at shallow depth, the largewater depth, typical of depths currently encounteredin offshore engineering and the high seismicenvironment represent a combination of challengingtasks. It was also realized that the earthquakedemand will govern the concept and dimensioning ofthe foundations. On the other hand, the time allowedfor design turned out to be a key factor: thanks to thecontractor who decided to anticipate the difficulties,the design studies started one year ahead of theofficial effective date. Advantage was taken of thistime lapse to fully investigate alternative foundationsolutions, to develop and to validate the innovativeconcept that was finally implemented. The amount oftime spent initially for the development of the designconcept was worthwhile and resulted in a substantialsaving for the foundation. In addition, the closecooperation that existed from the beginning within thedesign team between structural and geotechnicalengineers, between the design team and theconstruction team on one hand, and between thedesign team and the design checker on the otherhand, was a key to the success.Investigated foundation solutionsAfter a careful examination of all the environmentalfactors listed above, no solution seems to dominate.Several solutions were investigated: piled foundation,caisson foundation, surface foundation. Piles werequickly abandoned for two reasons: the difficulty torealize the structural connection between the slaband the piles in a deep water depth, and the ratherpoor behavior of floating piles in seismic areas asobserved in Mexico city during the 1985 Michoacanearthquake. Caissons foundations were hazardousdue to the presence of a gravel layer at the groundsurface (Fig. 3), which may induce some difficultiesduring penetration of the caisson. Surface foundationwas clearly impossible in view of the poor foundationbearing capacity and of the high anticipatedsettlements. However, it was quickly realized thatsurface foundation was the only viable alternativefrom a construction point of view: constructiontechniques used for offshore gravity base structuresare well proven and could easily be implemented forthe Rion-Antirion bridge.The problem posed by the poor soil conditions stillremains to be solved. Soil improvement is required
  5. 5. in order to ensure an adequate bearing capacity andto limit the settlements to acceptable values for thesuperstructure. Several techniques werecontemplated from soil dredging and backfilling (soilsubstitution), to in situ treatment with stone columns,grouted stone columns, lime columns. The need fora significant high shear resistance of the improvedsoil and for a good quality control of the achievedtreatment led to the use of driven steel pipes, atechnique derived from offshore engineering, toreinforce the soil beneath the foundations. Toprevent any confusion with piles foundations, whichbehave differently than steel pipes, those are namedinclusions.Adopted foundation concept: description andfunctioningIn order to alleviate potential damage to the structuredue to the adverse environmental conditions and tocarry the large earthquake forces brought to thefoundation (shear force of the order of 500 MN andoverturning moment of the order of 18 000 MNm for avertical buoyant pier weight of 750 MN), theinnovative foundation design concept finally adopted(Fig. 9) consists of a gravity caisson (90 m indiameter at the sea bed level) resting on top of thereinforced natural ground (Teyssandier 2002,Teyssandier 2003). The ground reinforcement (Fig.10) is composed of steel tubular pipes, 2 m indiameter, 20 mm thick, 25 to 30 m long driven at agrid of 7 m x 7 m below and outside the foundationcovering a circular area of approximately 8 000 m2The total number of inclusions under each foundationis therefore of the order of 150 to 200. In addition, asshown below, the safety of the foundation is greatlyenhanced by interposing a gravel bed layer, 2.8 mthick, on top of the inclusions just below thefoundation raft with no structural connection betweenthe raft and the inclusions heads (Fig. 10). Thereinforcement scheme is implemented under three ofthe four piers: M1, M2 and M3. Under pier M4, whichrests on a 20 m thick gravel layer, soil reinforcementis not required and the caisson rests directly on a 1 mthick ballast layer without inclusions.The concept (inclusions plus gravel layer) enforces acapacity design philosophy in the foundation design(Pecker 1998). The gravel layer is equivalent to the"plastic hinge" where inelastic deformation andenergy dissipation take place and the "overstrength"is provided by the ground reinforcement whichprevents the development of deep seated failuremechanisms involving rotational failure modes of thefoundation. These rotational failure modes would bevery detrimental to the high rise pylon (230 m). If thedesign seismic forces were exceeded the "failure"mode would be pure sliding at the gravel-foundationinterface; this "failure mechanism" can beaccommodated by the bridge, which is designed formuch larger tectonic displacements than thepermanent seismically induced ones. The concept isalso somehow similar to a base isolation system witha limitation of the forces transmitted to thesuperstructure whenever sliding occurs.65m90m230m65m90m230mFigure 9: View of one pylon of the Rion-AntirionbridgeFigure 10: Foundation reinforcementJUSTIFICATION OF THE FOUNDATIONSThe foundations have to be designed with respect tostatic loads (dead loads, live loads and tectonicdifferential movements) and with respect to dynamicloads (ship impact and earthquake). With regards tothe static loads, the main issue is the settlementsevaluation: absolute settlement and differentialsettlements inducing tilt, to which the 230 m highpylon is very sensitive. With regards to theearthquake loads, several problems have to besolved: evaluation of the seismic demand, i.e. the
  6. 6. forces transmitted to the foundation during anearthquake, and check of the foundation capacity, i.e.calculation of the ultimate bearing capacity andearthquake induced permanent displacements.Static loadsThe immediate settlement of the foundation is not amajor concern since approximately 80% of the totalpermanent load (750 MN) is brought by the pierbelow the deck level. More important are theconsolidation settlements of the clay strata whichinduces differed settlements. The settlements arecomputed with the classical one dimensionalconsolidation theory but the stress distribution in thesoil is computed from a three dimensional finiteelement analysis in which the inclusions aremodeled. It is found that the total load is sharedbetween the inclusions for approximately 35 to 45 %and the soil for the remaining 55 to 65 %. Theaverage total settlement varies from 17 cm to 28 cmfor Piers M1 to M3. For Pier M4 the computedsettlement is much smaller, of the order of 2cm. Thefigures given above are the maximum settlementspredicted before construction; they were computedassuming the most conservative soil characteristics,neglecting for instance the slight overconsolidation ofthe upper strata. Consolidation settlements wereestimated to last for 6 to 8 months after completion ofthe pier.The differential settlements are evaluated on thebasis of the spatial variability of the compressibilitycharacteristics assessed from the variability of thepoint cone resistances (Fig. 6). The computedfoundation tilts range from 7. 10-4to 1.8 10-3radians,smaller than those that would have been computedwithout inclusions: the inclusions homogenize the soilproperties in the top 25 m and transfer part of theloads to the deeper, stiffer, strata.Although the previous figures were acceptable, it wasdecided, in order to increase the safety of thestructure, to preload the pier during construction.This preloading was achieved by filling the centralpart of the cone pier (Fig. 9) with water duringconstruction to a total weight slightly larger than thefinal pier weight. Deballasting was carried out asconstruction proceeded, maintaining the total weightapproximately at its final value. The beneficial effectof this preloading is readily apparent in Fig. 11, whichshows the recorded settlement and applied loadversus time during construction. The final settlementamounts to 8 cm as compared to 22 cm originallyanticipated, and occurred more rapidly. This isclearly a beneficial effect of the overconsolidation, asback calculations have shown. More important thefoundation tilts are almost negligible, less than 3 10-4.020040060080010000 200 400 600 800Fréquence (Hz)TotalLoad(MN)-12.0-10.0-8.0-6.0-4.0- time (days)Settlement(cm)Figure 11: Settlement versus time – Pier M3Seismic loadsThe evaluation of the seismic behavior of thefoundation requires that the seismic demand and theseismic capacity be calculated. However, as for anyseismic design, it is important to first define therequired performance of the structure. For the RionAntirion bridge this performance criterion was clearlystated in the technical specifications: "The foundationperformance under seismic loading is checked on thebasis of induced displacements and rotations beingacceptable for ensuring reusability of the bridge afterthe seismic event". In other words, it is accepted thatthe foundation experiences permanent displacementsafter a seismic event, provided the induceddisplacements remain limited and do not impedefuture use of the bridge. Full advantage of thisallowance was taken in the definition of the designconcept: sliding of the pier on the gravel layer ispossible (and tolerated) but rotational mode offailures are prevented (and forbidden).Seismic capacityBecause of the innovative concept and therefore ofits lack of precedence in seismic areas, itsjustification calls for the development of new designtools and extensive validation. A very efficient threestages process was implemented to this end:• Development of design tools based on a limitanalysis theory to estimate the ultimate capacity of
  7. 7. the foundation system and to define the inclusionslayout: length and spacing. As any limit analysismethod, this tool cannot give any indication on theinduced displacements and rotations.• Verification of the final layout with a non lineartwo, or three, dimensional finite element analysis.These analyses provide non only a check of theultimate capacity but also the non linear stressstrain behavior of the foundation that will beincluded in the structural model for the seismiccalculations of the bridge.• Experimental verification of the design toolsdeveloped in step 1 with centrifuge model tests.As shown below all three approaches give resultswhich are within ±15% of each other, which increasesthe confidence in the analyses performed for design.The design tools are based on the Yield Designtheory (Salençon 1983), further extended toreinforced earth media, (de Buhan and Salençon1990). The kinematic approach of the Yield Designtheory applied to mechanisms of the type shown inFig. 12, (Pecker and Salençon 1999), permits thedetermination, for a wrench of forces (N verticalforce, V horizontal force and M overturning moment)applied to the foundation, of the sets of allowableloads; this set defines in the loading spaceparameters a bounding surface with an equation:( , , ) 0N V MΦ = (1)Any set of loads located within the bounding surfacecan be safely supported by the foundation, whereasany set located outside corresponds to an unsafesituation, at least for permanently imposed loads.ΩωBλΒεα µδFFigure 12: Kinematic mechanismFig. 13 presents a cross section of the boundingsurface by a plane N=constant, corresponding to thevertical weight of one pier. Two domains arerepresented in the figure: the smallest onecorrespond to the soil without the inclusions, thelargest one to the soil reinforced with the inclusions.The increase in capacity due to the inclusions isobvious and allows the foundation to supportsignificantly larger loads (V and M). Furthermore, thevertical ascending branch on the bounding surface,on the right of the figure, corresponds to sliding at thesoil-foundation interface in the gravel bed layer.When moving on the bounding surface from the point(M=0, V=560 MN), sliding at the interface is thegoverning failure mechanism until the overturningmoment reaches a value of approximately 20 000MN; for larger values of the overturning moment,rotational mechanisms tend to be the governingmechanisms and the maximum allowable horizontalforce decreases. The height of the vertical segment,corresponding to a sliding mechanism, is controlledby the inclusions layout and can therefore beadjusted as necessary. The design philosophy isbased on that feature: for a structure like the pylonwith a response governed by the fundamental mode,there is proportionality between M and V, thecoefficient of proportionality being equal to the heightof the center of gravity above the foundation. WhenV increases the point representative of the loadsmoves in the plane of Fig. 13 along a straight linepassing through the origin, assuming that the verticalforce is constant; it eventually reaches the boundingsurface defining foundation failure.050001000015000200002500030000350000 100 200 300 400 500 600 700Horizontal force at foundation level (MN)Overturningmoment(MN-m)Figure 13: Cross section of the boundingsurface: Doted line without inclusions; solid linewith inclusions
  8. 8. The inclusions layout is then determined in a waysuch that this point be located on the verticalascending branch of the bounding surface.The previous analysis provides the ultimate capacityof the foundation but no information on thedisplacements developed at that stage. Thesedisplacements are calculated from a non linear finiteelement analysis. Most of the calculations areperformed with a 2D model, but some additionalchecks are made with a 3D model. For thosecalculations, the parameters entering the non linearelastoplastic soil constitutive model are determinedfrom the laboratory tests carried out on theundisturbed soil samples; interface elements withlimiting shear resistance and zero tensile capacity areintroduced between, on one hand, each inclusion andthe soil and, on the other hand, the raft and the soil.These models are loaded to failure under increasingmonotonic loads such that M/V=constant (Fig. 14).Results are compared to those obtained from theYield Design theory in Fig. 13; a very goodagreement is achieved with differences of the orderof ±12% for all the calculations made for the project.However, it is worth noting that the finite elementanalyses, because of the large computer timedemand, could not have been used to make thepreliminary design; the Yield Design theory is, in thatrespect, a more efficient tool: to establish the fullcross section of the bounding surface represented inFig. 10 requires only 10 to 15 minutes on a PC; a nonlinear finite element analysis, i.e. a single point of thecurve, requires more than 4 hours of computing in 2Dand 15 hours in 3D on a workstation with 4 parallelprocessors.This totally innovative concept, at least in seismicareas, clearly calls for extensive theoretical analysesand experimental validation. As sophisticated asthey can be, the theoretical and numerical tools donot have the capacity for modeling all the details ofthe behavior of this complex scheme during anearthquake. Centrifuge model tests were thereforeundertaken with a three-fold objective:• To validate the theoretical predictions of theultimate bearing capacity of the foundation undermonotonically increasing shear force andoverturning moment,• To identify the failure mechanism of thefoundation under these combined loads,• To assess the behavior of the foundation undervarious cyclic load paths.Four tests have been performed in the 200 g-tongeotechnical centrifuge at the LCPC Nantes center,(Pecker and Garnier 1999). It is designed to carry a2000 kg payload to 100 g accelerations, the bucketsurface is at a radius of 5.5 m and the platform has aworking space of 1.4 m by 1.1 m. All tests have beencarried out at 100 g on models at a scale of 1/100.The dimensions of the corresponding prototype areas follows:• radius of the circular footing: Bf = 30 m,• inclusions length and diameter: L = 8.5 m andB=0.67 m,• Wall thickness t = 6.7 mm (steel), Stiffness EI =158 MN.m2,• Thickness of the ballast layer: 1.2 m.01002003004005006007000.00 0.05 0.10 0.15 0.20 0.25 0.30Displacement at foundation level (m)Horizontalforce(MN)M = 50 VM = 30 V0500010000150002000025000300000.0E+00 1.0E-03 2.0E-03 3.0E-03 4.0E-03 5.0E-03Rotation at foundation level (rd)Overturningmoment(MN-m)M = 50 VM = 30 VFigure 14: Finite element analyses: force-displacement curvesThe soil material has been sampled at the location ofpier N17 of the Antirion approach viaduct and sent tothe laboratory where it was reconsolidated prior tothe tests to reproduce the in situ shear strengthprofile. The loads applied to the foundation consist ofa constant vertical force and of a cyclic shear forceand overturning moment; at the end of the tests thespecimens are loaded to failure under monotonicallyincreasing loads with a constant ratio M/V. The mainfindings of the test are summarized in Fig. 15 whichcompares the theoretical predictions of the failureloads (class A prediction according to the terminology
  9. 9. introduced by Whitman) to the measured failureloads. Disregarding the preliminary tests that werecarried out with another equipment (CESTAcentrifuge in Bordeaux), all four tests yield valueswithin ±15% of the predictions obtained with the YieldDesign theory.The centrifuge tests not only provide the ultimateloads but also valuable information on factors thateither could not be easily apprehended by theanalysis or need experimental verification. Ofprimary interest is the fact that, even under severalcycles of loading at amplitudes representing 75% ofthe failure load, the foundation system does notdegrade; no tendency for increased displacementwith the number of cycles is noted. The equivalentdamping ratios calculated from the hysteresis loopsrecorded during the tests are significant with valuesas high as 20%; it is interesting to note that thesevalues have been confirmed by numerical analysesand can be attributed, to a large extent, to thepresence of the inclusions (Dobry et al. 2003).Finally, a more quantitative information is given bythe failure mechanisms observed in the centrifuge,which compare favorably either with the mechanismsassumed a priori in the Yield Design theory (Fig. 12),or with those computed from the non linear finiteelement analyses (Fig. 16).The approach explained above takes care of thejustification of the inclusions. Another importantissue is the behavior of the gravel bed layer; afundamental requirement for this layer is to exhibit awell defined and controlled shear resistance, whichdefines the ultimate capacity for the sliding modes offailure. This requirement can only be achieved if nopore pressure buildup occurs in the layer during theearthquake; the development of any excess porepressure means that the shear resistance isgoverned by the soil undrained cyclic shear strength,a parameter highly variable and difficult to assesswith accuracy. The grain size distribution of thegravel bed layer (10-80 mm) is therefore chosen toensure a fully drained behavior (Pecker and al 2001).Furthermore, the friction coefficient between the slaband the gravel has been measured on site withfriction tests using a concrete block pushed on top ofthe gravel bed layer; a rather stable value (0.53 to0.57) has been measured for that coefficient.Seismic demandFor the seismic analyses it is mandatory to take intoaccount soil structure interaction, which is obviouslysignificant given the soft soil conditions and large piermass. In the state of practice, the action of theunderlying soil is represented with the so-calledimpedance functions, the simplified version of themconsisting, for each degree of freedom, of constantsprings and dashpots. In the calculation of thesesprings and dashpots the soil modulus is calibratedon the free field strains, neglecting any further nonlinearity developed in the vicinity of the foundation(Pecker and Pender 2000). Such an approach maybe inadequate because it implicitly assumes that theforces generated on the foundation are independentof its yielding. Therefore there is some inconsistencyin checking the foundation capacity for those forces.Recent studies, (e.g., Paoluci 1997, Pedretti 1998,Cremer et al. 2001, Cremer et al. 2002), throw somelight on this assumption and clearly show that closeto the ultimate capacity it is no longer valid. This istypically the situation faced for the foundation of theRion-Antirion bridge. Therefore it was decided toimplement a more realistic approach which closelyreflects the physics of the interaction. Sincenumerous parametric studies are necessary for thedesign, a dynamic non linear soil finite element modelis not the proper tool.0204060801001200 50 100Measured horizontal failure load (MN)Computedhorizontalfailureload(MN)Figure 15: Computed versus measured failureloads in the centrifuge tests:Preliminary tests (diamonds); final tests(triangles)In the same spirit as the impedance functions, theaction of the soil (and inclusions) below thefoundation slab is represented by what is called a
  10. 10. macro-element (Cremer and al 2001, Cremer and al2002). The concept of macro-element is based on apartitioning of the soil foundation into (Fig. 17):• A near field in which all the non linearities linked tothe interaction between the soil, the inclusionsand the slab are lumped; these non linearities aregeometrical ones (sliding, uplift of the foundation)or material ones (soil yielding). The energydissipation is of hysteretic nature.• A far field in which the non linearities aregoverned by the propagation of the seismicwaves; the energy dissipation is essentially of aviscous type.Figure 16: Failure mechanisms: centrifuge test(upper left), FE analysis (lower right)In its more complete form the macro-element modelcouples all the degrees of freedom (verticaldisplacement, horizontal displacement and rotation)(Cremer et al. 2001). For the design studies of thebridge a simplified version of it, in which the degreesof freedom are uncoupled, is used. Conceptually thiselement can be represented by the rheological modelshown at the bottom of Fig.17; it consists of anassemblage of springs and Coulomb sliders for thenear field and of a spring and a dashpot for the farfield. One such model is connected in each directionat the base of the structural model; the parametersdefining the rheological model are calibrated on themonotonic force-displacement and moment-rotationcurves computed from the non linear static finiteelement analyses (Fig. 14). For unloading-reloadinga Masing type behavior is assumed; the validity ofthis assumption is backed up by the results of thecentrifuge tests (Pecker and Garnier 1999) and ofcyclic finite element analyses (Dobry et al. 2003).The adequacy of the macro-element to correctlyaccount for the complex non linear soil structureinteraction is checked by comparison with fewdynamic non linear finite element analyses includinga spatial modeling of the foundation soil andinclusions.Fig. 18 compares the overturning moment at thefoundation elevation calculated with both models; avery good agreement is achieved, not only in terms ofamplitudes but also in terms of phases.This model has been subsequently used by thestructural design team for all the seismic analyses ofthe bridge. It allows for the calculations of the cyclicand permanent earthquake displacements; Fig.19shows the displacement of the foundation center(bottom) and the variation of the forces in the (N-V)plane (top); sliding in the gravel bed layer occurs atthose time steps when tanV N φ= .Aλ4λ3λ2λ1C0FoundationNear FieldFar FieldK0K1 K2 K3NEAR FIELDMFAR FIELDθK0 C0Figure 17: Macro element for soil structureinteraction-16000-800008000160000 5 10 15 20 25 30 35 40 45 50Time (s)Overturningmoment(MN-m)Simplified modelFinite elementFigure 18: Time history of foundation overturningmoment
  11. 11. CONSTRUCTION METHODSThe construction methods for the foundations,described in details by Teyssandier (2002 and 2003),are those commonly used for the construction ofoffshore concrete gravity base structures:• construction of the foundation footings in a drydock up to a height of 15 m in order to providesufficient buoyancy;• towing and mooring of these footings at a wetdock site;• construction of the conical part of the foundationsat the wet dock site;• towing and immersion of the foundations at itsfinal position.However some features of this project make theconstruction process of its foundations quiteexceptional.Verticalforce(MN)12008004000 2000 60040000VH = V tgφHorizontal force (MN)tanV N φ=Uy(m)-- 0.15-- 0.10-- 0.0500.0500.1000.15-0.35 -0.25 -0.15 -0.05UxyUx (m)Figure 19: Time history of foundationdisplacementTop – variation of forces in the (N-V) planeBottom - displacement of foundation centerThe dry dock has been established near the site. Itwas 200 m long, 100 m wide, 14m deep, and couldaccommodate the simultaneous construction of twofoundations. It had an unusual closure system: thefirst foundation was built behind the protection of adyke, but once towed out, the second foundation, theconstruction of which had already started, wasfloated to the front place and used as a dock gate.Dredging the seabed, driving of inclusions, placingand levelling the gravel layer on the top, with a waterdepth reaching 65 m, was a major marine operationwhich necessitated special equipment andprocedures. In fact, a tension-leg barge has beencustom-made, based on the well known concept oftension-leg platforms but used for the first time formovable equipment.This concept was based on active vertical anchorageto dead weights lying on the seabed (Fig. 20). Thetension in these vertical anchor lines was adjusted inorder to give the required stability to the barge withrespect to sea movements and loads handled by thecrane fixed on its deck. By increasing the tension inthe anchor lines, the buoyancy of the barge allowedthe anchor weights to be lifted from the seabed, thenthe barge, including its weights, could be floatedaway to a new position.Figure 20: Tension-leg bargeAs already stated, once completed the foundationsare towed then sunk at their final position.Compartments created in the footings by the radialbeams can be used to control tilt by differential
  12. 12. ballasting. Then the foundations are filled with waterto accelerate settlements. This pre-loading wasmaintained during pier shaft and pier headconstruction, thus allowing a correction for potentialdifferential settlements before erecting pylons.The deck of the main bridge is erected using thebalance cantilever technique, with prefabricated deckelements 12 m long comprising also their concreteslab (another unusual feature).ACKNOWLEDGMENTSThe development of this original foundation conceptwas made possible thanks to the full cooperationbetween all the parties involved in the project: theConcessionaire (Gefyra SA), the Contractor (GefyraKinopraxia), the Design JV and the Design Checker(Buckland & Taylor Ltd). The author is indebted toMr. Teyssandier from Gefyra SA, Mr De Maublancand Morand from Gefyra Kinopraxia, Mr Tourtoisfrom the Design JV and Dr. Taylor for their long termcollaboration and confidence. He also wishes tospecially thank Dr Peck, Dr. Dobry and Dr Gohl,consultants to the Checker, for their invaluablecontribution through their constant willingness to helphim clarify and develop the concept.More information on the project can be found on theweb site www.gefyra.gr.REFERENCESCombault, J., Morand, P., Pecker, A. (2000)."Structural response of the Rion Antirion Bridge."Proc. of the 12th World Conf. on Earthq. Eng.,Auckland, Australia.Cremer, C., Pecker, A., Davenne L. (2001). "Cyclicmacro-element for soil structure interaction - Materialand geometrical non linearities." Num. Methods inGeomech., 25, 1257-1284.Cremer, C., Pecker, A., Davenne L. (2002)."Modelling of non linear dynamic behaviour of ashallow strip foundation with macro-element." J. ofEarthq. Eng., 6(2), 175-212.de Buhan, P., Salençon, J. (1990). "Yield strength ofreinforced soils as anisotropic media." in Yielding,Damage and Failure of Anisotropic Solids; EGF5,Mechanical Engineering Publications, London.Dobry, T., Abdoun, T., ORourke, T.D., Goh, S.H.(2003). "Single piles in lateral spreads: Field bendingmoment evaluation". J. Geotech. and Geoenv. Eng.,ASCE, 129(110), 879-889.Dobry, R., Pecker, A., Mavroeidis, G., Zeghal, M.,Gohl, B., Yang, D. (2003). "Damping/Global EnergyBalance in FE Model of Bridge Foundation LateralResponse." J. of Soil Dynamics and Earthq. Eng.,23(6), 483-495.Paolucci, R. (1997). "Simplified evaluation ofearthquake induced permanent displacement ofshallow foundations." J. of Earthq. Eng., 1(3), 563-579.Pecker, A. (1998). "Capacity Design Principles ForShallow Foundations In Seismic Areas." Proc. 11thEuropean. Conf.. on Earthq. Eng., A.A. BalkemaPublishing.Pecker, A., Salençon, J. (1999). "GroundReinforcement In Seismic Area." Proc. of the XIPanamerican Conf. on Soil Mech. and Geotech.Eng., Iguasu, 799-808.Pecker, A., Garnier, J. (1999). "Use of CentrifugeTests for the Validation of Innovative Concepts inFoundation Engineering." Proc. 2ndInt. Conf. onEarthq. Geotech. Eng., Lisbon, 433-439.Pecker, A., Pender, M.J. (2000). "Earthquakeresistant design of foundations : new construction."GeoEng2000 , 1, 313-332.Pecker, A., Prevost, J.H., Dormieux, L. (2001)."Analysis of pore pressure generation and dissipationin cohesionless materials during seismic loading." J.of Earthq. Eng., 5(4), 441-464.Pedretti, S. (1998). "Nonlinear seismic soil-foundationinteraction: analysis and modeling method." PhDThesis, Politecnico di Milano, Milan.Salençon, J. (1983). "Introduction to the yield designtheory." European Journal of Mechanics A/Solids9(5), 477-500.Teyssandier, J.P., Combault, J., Pecker, A. (2000)."Rion Antirion: le pont qui défie les séismes." LaRecherche, 334, 42-46.Teyssandier, J.P. (2002). "Corinthian Crossing." CivilEngineering, 72(10), 43 -49.
  13. 13. Teyssandier, J.P. (2003). "The Rion-Antirion bridge,Design and construction." ACI InternationalConference on Seismic Bridge Design and Retrofit.La Jolla, California.