ice 2006 foundation design of a large arch bridge

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FoundationdesignforalargearchbridgeonalluvialsoilsARTICLEinGEOTECHNICALENGINEERINGJANUARY2006ImpactFactor:1.06DOI:10.1680/geng.2006.159.1.19

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Geotechnical Engineering 159 Issue GE1 Foundation design for a large arch bridge on alluvial soils Monnet et al. 19

Proceeding of the Institution of Civil Engineers Geotechnical Engineering 159 January 2006 Issue GEI Pages 19-28 Paper 13533 Received 10/10/2003 Accepted 08/08/2005 Keywords bridges / design methods & aids / site investigation

Jacques Monnet D.ominique Allagnat Jean Teston Pierre Billet Franois Baguelin Associate Professor, LIRIGM Manager, Scetauroute Building Manager, AREA Associate Professor, LIRIGM Technical Manager, Universit Joseph Fourier, 38180, Seyssins, France Bron, France Universit Joseph Fourier, FondaConcept, la Plaine St Grenoble, France Grenoble, France Denis, france

Foundation design for a large arch bridge on alluvial soils

J. Monnet, D. Allagnat, J. Teston, P. Billet and F. Baguelin The Crozet bridge is located on the Grenoble-Col du Fau motorway, on the Grenoble-Sisteron route, fifteen kilometres south of Grenoble, France. It crosses a 350 m wide valley and the RN75 national road. The adaptation of the bridge into the landscape has involved an arch design with three bays (direction Grenoble-Sisteron) and one bay (direction Sisteron-Grenoble). The supports of the structure were difficult to build because of the huge horizontal force and the low displacement tolerance. The low stiffness and strength characteristics foreseen led to a geotechnical investigation by cyclic pressuremeter tests with a friction angle and cohesion interpretation. The foundation calculations were carried out by a Finite Element calculation using CESAR-LCPC program to determine the support rigidity. The complete computation of the bridge was done with the calculated support rigidity which showed that displacements of the arches were lower than the tolerance. The bridge is located in low seismic area of France and the design takes into account the maximum foreseeable magnitude, and analysis of the soil liquefaction risk. Monitoring carried out for completion of the bridge in 1999 and along the surveying shows displacements lower than tolerance values. Since 1999, the bridge has withstood huge service weights without any difficulty.

NOTATION a radius of the borehole amax static horizontal acceleration equivalent to a seism aN nominal acceleration c cohesion Cr correction coefficient from Seed function of K0EM pressuremeter modulus from Mnard test Ee elastic modulus F safety coefficient g gravity acceleration G shear modulus of the soil h depth of the sample khx , khy , ks , kv , k1 , k2 : stiffness of the soil for displacements and rotations K0 coefficient of earth pressure at rest n number of cycles Pl limit Pressure of Mnard test Rd reduce coefficient from Seed function of depth ua displacement at the borehole wall V, H, M vertical, horizontal forces and bending moment applied to the foundation z depth of the pressuremeter test unit weight of the soil

sat unit saturated specific weight Poissons ratio d effective deviatoric stress v0 effective initial vertical stress 3 lateral stress on triaxial test eq equivalent shearing stress l cyclic shearing stress [l]n cyclic equivalent shearing stress at n cycles drained friction angle interparticle angle of friction dilatancy angle

1. INTRODUCTION The Crozet bridge is located on the A51 motorway Grenoble-Col du Fau on the Grenoble-Sisteron route fifteen kilometres to the south of Grenoble. It spans a distance of 350 m across a small valley where the national road RN75 lies. An embankment was initially foreseen, but a bridge has now been built with two separate decks. The first deck is on a single arch (Eastern way Sisteron-Grenoble) and the second, on three arches (Western way Grenoble-Sisteron). The arch design was chosen from an architectural point of view, so that the bridge could be integrated into the natural site of the Crozet valley. The design of the foundations on simple supports is conventional (drilled piles), but the design of the foundations of the arches is rather complex due to the low stiffness and strength characteristics of the soil. This paper presents the original geotechnical approach for the soil investigation, the design of the foundation and the design of the bridge, which takes into account the displacements of the structure and the soil, and the arrangements used to build the foundation. Foundation displacements were measured to monitor the behaviour of the bridge in this seismic area.

2. DESCRIPTION OF THE BRIDGE The Crozet bridge (Fig. 1) runs from northeast to southwest and is made up of two decks of 313 m for the eastern deck and 335 m for the western deck (Fig. 2). The arch design is chosen from an architectural point of view, so that the bridge is integrated into the natural site of the small Crozet valley. Each deck supports three lanes. This paper focuses on the eastern deck (Fig. 3), which is made up of spans 13 to 20 m long (locally 28 m for RN75


Fig. 1: General view of the bridge

Fig. 2: Plan view of the bridge

crossing), and an arch of a 120 m radius and 143.5 m long which is supported on two end columns and six central columns of a 1 m diameter. For the western deck, land surveying and architectural control led to a three-arch design, two of which are 101.5 m in long, and the third one is 87 m long supported on 6 and 5 small columns. The total height of the bridge is 30 m above the Crozet stream 3. GEOTECHNICAL ENVIRONMENT 3.1 Geological and geotechnical environment The geology of the Crozet valley is quite simple. It is a fluvio-glacial deposit (Fig. 3), which is modelled by a thalweg with gentle slopes. At the bottom, a small stream flows. The site is classified as a low seismic area. Various investigations reveal recent deposits of a few meter thickness above quaternary alluvial and glacial deposits, the thickness of which is greater than 10 m. The bedrock comprises black marl and was not reached by the boreholes at 40 m depth. Recent deposits are made up of gravely clay and modern alluvial deposits found at depth ranging from 7 to 10 m. The former alluvial and glacial deposits can be described in four different families of soils, which are function of the depth: (a) Family F1: clayey gravel with sandy levels of around

100 mm. The thickness of this loose layer ranges from 8 to 15 m. This thickness is of 2 m in the thalweg due to channelling of the recent alluvial deposits. This stratum is part of the Wrm Formation.

(b) Family F2: Clayey and loamy sandy moraine grey, very stiff, with scarce sandy and gravely levels. This formation is channelled by the family above, and different thicknesses are found.

(c) Family F3: Grey yellow sandy moraine with some clay; it is very stiff and its thickness varies from 15 to 8 m

Fig. 3: Geological situation and elevation view of the bridge


from north to south, and it disappears under the south side of the thalweg.

(d) Family F4: Clayey loamy grey moraine, very stiff, over-consolidated, with a thickness greater than 25 m, very homogeneous.

The last three levels are part of Riss formation. 3.2 Results of investigation and tests Investigations were conducted with a lot of in situ and laboratory tests, sixteen classical pressuremeter boreholes, three boreholes with undisturbed samples for laboratory tests, two boreholes were drilled for fifty-nine high-pressure pressuremeter tests with three cyclical loadings undertaken in the four old formations (F1-F4), several triaxial tests were performed to measure the shearing characteristics of the soils. The statistical analysis allows the determination of the mean values shown on Table 1. 3.2.1 Triaxial Drained Consolidated Tests. Tests were carried out in the LIRIGM laboratory of the Joseph Fourier University in Grenoble on remoulded samples, matched to the in-situ effective lateral pressure with a pore pressure of 100kPa. There was lateral drainage during the consolidation phase. Dimensions of the samples were 70 mm in diameter and 150 mm in height without any lubrification on each end. The test was drained with a speed of 0.01 mm/mn. The final consolidation wasreached in 90 minutes and the test took 22 h, which is 15 times the consolidation duration. The measurement of the volume variation is done from the measurement of the inner volume of the sample. The results of triaxial tests are shown in Table 2 and physical characteristics of samples are shown in Table 3. The interparticle angle of friction is the minimum value of the friction angle as shown by Rowe1. It measures the friction between two different particles of soil and it is found by the Frydman et al.2 theory.

3.2.2 Cyclical pressuremeter tests. The pressuremeter was designed by Menard3,4 to find the pressuremeter

modulus and limit pressures of the soil. The pressuremeter tests are used here to find the elastic modulus and the friction angle of the soil. The tests were carried out with a slotted tube of 63 mm external diameter 1090 mm long and slots was 915 mm long. The thickness of the tube is 2 mm. The interpretation of the tests was carried out by the Gaiatech5 patent, which takes into account the corrections to volume and pressure based in the French standard6, but also takes into account the influence of the shape of the probe under pressure as shown by Fawaz et al.7, the non-uniformity of the pressure distribution along the probe, which was found by Basudhar and Kumar8, and the difference between the external and internal radii of the probe. The process leads to the determination of the mechanical characteristics in four different steps: (a) The determination of the angle of interparticle friction

measured on the consolidated drained triaxial test, as reported by Monnet and Gielly9.

(b) The determination of the elastic shearing modulus along the unloading-reloading cycle of the pressuremeter test. The cycle is carried out in the so-called linear range of the soil behaviour of the pressuremeter test.

(c) The determination of the angle of internal friction of the soil by measurement of the slope of the linear relation between logarithms of the pressure versus radial strain of the pressuremeter results as shown by Hughes et al.10 and Monnet and Khlif11. The undrained cohesion is determined by the slope of the linear relation between the pressure and the logarithm of the radial strain, as shown by Gibson and Anderson12 and Monnet and Chemaa13.

(d) Control of the mechanical characteristic of elasticity (shear modulus) and strength (cohesion or friction angle). The Gaiapress14 program is used to check the correct comparison between experimental and theoretical pressuremeter curves (Fig. 4) and experimental and theoretical limit pressures. The shear modulus value is controlled by the correspondence between experimental and theoretical cycles. Values of friction angle and cohesion are controlled by the correspondence between

Family of soil Description EM

(MPa) Pl(MPa)

Made ground 3 0.3 Colluvium 5 0.5

Recent deposits

Modern alluvial deposits 6 0.7 F1 Clayey gravel 30 2 F2 Clayey and loamy sandy moraine 60 5.4 F3 Grey yellow sandy moraine 100 6.5

Quaternary alluvial and glacial deposits

F4 Clayey loamy grey moraine 60 6.2 Table 1: Results of the Menard pressuremeter tests Family of soil

Description Depth (m)

Number of tests

(degree)

c'(kPa)

' (degree)

F2 Loamy sandy moraine (Riss) 10 3 33.8 7 41.5 F2 Clayey sandy moraine stiff (Riss) 15.7 3 21.5 80 31 F3 Grey yellow sandy moraine (Riss) 29 3 29.5 0 37.5 F4 Clayey loamy grey moraine (Riss) 31 3 31.9 4 38.5

Table 2: Results of Drained Consolidated triaxial tests on intact samples.


Family of soil

Description Depth (m)

Methyl blue test (g/100g)

Per cent Passing 75 sieve

Classification ASTM D32-82

Classification USCS

F2 Loamy sandy moraine 12.5 0.85 90.5 A6 CL F2 Clayey sandy moraine 20.8 1.19 94.3 A6 CL F3 Grey yellow sandy moraine 25 0.23 30. A2-4 SM F3 Grey yellow sandy moraine 32.5 0.23 33.5 A2-4 SM F4 Clayey loamy grey moraine 32 0.30 55.2 A6 CL Table 3: Results of physical classification of soils

Fig. 4: Control of mechanical characteristics by comparison between experimental and theoretical curves: Test 23 m deep. experimental and theoretical pressuremeter curves above the creep pressure. The theoretical curve for the granular soil (1) assumes an elasto-plastic behaviour, with effective stress, a non-standard dilatancy and a three dimensional equilibrium as shown by Monnet and Khlif11 :

1 ( ) ( ) ( zLnpLnCn

au

Ln a ...1. 1 =

+ )

( ) ( )

++ 10 .21...1 C

GnzKLn

where

2

Nn+=11 and ( )( ) ( )( )

+

++

+=

pzn

GzKNnp

znaun

Ca

..1

.2...1..1.. 0

1

where

3 'sin1'sin1

+=N

4 +=

sin1sin1n

and

5 = - The theoretical curve for the cohesive soil (6) assumes an elasto-plastic behaviour, with total stress, no volume variation when plasticity occurs and a three dimensional equilibrium as shown by Monnet and Chemaa13 :

6 ( )

++=

+G

cGzKLn

cz

cp

Gc

au

Ln uuu

ua

.2.2..1.

.2 0

Results of pressuremeter interpretation are shown on Table 4. The interpretation is made with the assumption of no friction for a cohesive soil and no cohesion for a friction soil. As the pressuremeter test mainly shears the soil in the horizontal plane between the radial and the circumferential stress, it can be considered as equivalent to a single Mohr circle. For soil with cohesion and friction, the value of the equivalent friction angle, measured by this method, is overestimated by the imposed condition of no cohesion. On the other hand, the value of the equivalent cohesion is also overestimated by the imposed condition of no friction. The mean value of the ratio between elastic modulus and pressuremeter modulus Ee/EM is 3.07 in the fluvial-glacial deposit with a large variation from 1.37 to 4.77. The ratio is 3.31 in the grey clay and 2.06 in the grey sand. The elastic modulus is high and greater than 80 MPa. The fluvio glacial deposits and grey sandy moraine (F1 and F2) have a huge

Soil family Description Elastic modulus Ee :

MPa Ratio Ee / EM

cu : kPa : degree F1 Clayey gravel 80 to 280 3.07 +/- 1.70 900 to 1200 0 F2 Clayey sandy moraine very stiff 100 to 220 3.31 +/- 0.88 600 to 1000 0 F3 Grey yellow sandy moraine, stiff 180 to 350 2.06 +/- 0.67 0 40 to 45 Table 4: Results of cyclical pressuremeter tests.

undrained cohesion without friction angle. For family F3, the interpretation is made with an assumption of no cohesion, and all the shearing strength of the soil is calculated with a friction. For the grey sand, this leads to a friction angle of 40 to 45, which is in the range of expected values. 3.2.3 Cyclical triaxial tests. These tests were conducted in the LIRIGM laboratory of the Joseph Fourier University in Grenoble. Samples from the F2 family were selected to study liquefaction under seismic excitation. The Association Franaise de Gnie Parasismique (AFPS) calculation was followed to define test procedure. Samples 70 mm in diameter and 140 mm high were used. The cell is connected to a cyclical hydraulic press of 20kN maximum load. This press is connected to two hydraulic systems, one for the axial force, the other one for the lateral pressure of the triaxial cell. For a constant lateral pressure, the number of cycles, axial displacement, axial force and pore pressure are recorded. The bridge is located in low seismic area (low magnitude IB classification of the French territory by French authorities) and is classified category C (public construction on motorway, with a high level of risk due to frequent utilization and huge economic importance). This leads to a nominal acceleration aN equal to 2 m.s-2 ( Table 5) The current sample is tested under the equivalent seismic shearing eq imposed by AFPS 9215:

7 kPaR

ga

h dsateq 36...32 max =

=

8 [ ] kPaCn

d

n

drn

=

= '

3

'

'.

'

1 .3.78.2.

where Cr equal to 0.6; Rd is the reduction coefficient defined by Seed and Idriss16 and Seed17 equal to 0.7; amax equal to 0.9. aN; sat equal to 21kN/m3; h the depth of the foundation equal to 16 m; v0 equal to 261kPa; d is equal to (1 - 3); [l]n is the shearing resistance of the sample when liquefaction appears after n cycles. A safety coefficient F is defined as the ratio between [l]n the laboratory stress which leads to liquefaction phenomena, and eq the calculated equivalent stress for n cycles so that

Classes Areas B C D

IA 1.0 1.5 2.0 IB 1.5 2.0 2.5 II 2.5 3.0 3.5 III 3.5 4.0 4.5

Table 5 : Values of aN (m/s2) for theFrench classification

9 Fd .459.0'

3

'

=

The experimental conditions are: 10 3 = 100kPa, d = 45.9.F kPa, 0.5 Hz frequency.

We have carried out a test with d equal to 58.4 kPa. The AFPS 9215 rules that the liquefaction shear stress must be lower than the of the limit shear stress of the soil. It appears for 2.5% of strain after 5 cycles in IB area, 10 cycles in II area, 25 cycles in III area. For a liquefaction phenomenon that occurs on the fifth cycle, the safety coefficient is 1.27. By applying a deformation criterion (2.5% of strain) it can be seen on Fig. 5 that liquefaction does not appear in the first five cycles but at the 17th cycle. Thus, the risk of liquefaction is very low. 4. FOUNDATION DESIGN

The general design of the bridge was carried out so as to integrate the anticipated foundation stiffness into the structural analysis according to the organization of calculations indicated in Fig. 6.

4.1 General design of arch foundations Preliminary studies for the design of the principal supports of the arch led to massive and rigid foundations made of a unit of orthogonal bars organised in a box with three webs. The continuity of the reinforcements had to be ensured. This solution was regarded as the basis of Contractor tenders for the project. As the deformation of the structure had to be limited, it was necessary to use a very rigid foundation. As there is no major hydraulic constraint, the Contractor suggested elliptic box foundations. For the arch of the eastern deck, the boxes are 13 m long, 10 m wide for a maximum depth of 16 m. These hollow boxes are filled with soil for stability, and rest on a 2 m thick reinforced concrete raft foundation (Fig. 7). The well foundations excavated by 1 m steps are followed immediately by a reinforced concrete lateral support and an elliptic truncated cone formwork. After setting up a reinforcement cage , a second form is placed in order to

Fig. 5: Results of triaxial liquefaction tests: displacements against number of cycles


Fig. 6: Hierarchical organisation used for the bridge design

Fig. 7: Mesh used for finite element calculations and the mesh used for the foundation well make the 2 m thick final walls. After filling with granular soil, a concrete roof slab, 1 m thick, is poured to close the foundation box. A cement grouting system with pipes uniformly distributed perpendicular to the lateral supporting walls allows the improvement of the contact between the surrounding soil and the foundation. For the supports that bears the force of a single arch, the horizontal component is very large (close to 47.2 MN), and equal to the vertical component (close to 47.9 MN) for support P13/P14 of eastern deck) with a stabilizing moment of 122 MN.m for the permanent service load. The elliptical shape of the foundation improves the rigidity of the foundation as the long axis of the ellipse is in the direction of the horizontal component. The aim is to keep horizontal displacement under a 2 cm threshold. 4.2 Organization of the calculation The method of the bridge structural calculation requires knowledge of the stiffness matrix of the soil mass around the

foundations, which are introduced in the general seismic calculation of the bridge. The model suggested by the Contractor for the foundation of the P5/P6 supports of the eastern and western arch decks is an elastic node-beam where the elliptical well foundation is modelled (Fig. 8) by a vertical beam (along z) and two orthogonal horizontal beams (along x and y). Along the vertical beam, there are continuous horizontal springs khx and khy so that the horizontal reactions are mobilized along the body of the well foundation. At the ends of the horizontal beams, there are vertical springs of rigidity ks that represent the side friction mobilized along the side of the well foundation. A vertical spring of rigidity kv is placed at the lower end of the vertical beam to represent the ground vertical stiffness under the foundation, reduced by the four springs stiffness ks. The model has also local differential springs of stiffness k1 and

Fig. 8: The node beam model used to represent the well foundation in 3D FEM calculus



k2 to represent the difference of vertical pressure from the anchorage along the foundation that are linked to the rotation of the horizontal beams. The 3D model that is used for the foundation support is calibrated based on finite element calculation (Fig. 7 and 8) with the LCPC CESAR program. The calibration of the node-beam model is carried out as follows.

(a) k1 and k2 are calculated from the model of axial rigidity as defined in Fascicule 6218 as a deep foundation and from the inertia of the foundation base (elliptic surface)

(b) khx and khy are calculated from the results of pressuremeter tests EM(z) as proposed in Fascicule 6119

(c) ks is found from the calculated settlement of the foundation well, by finite element program CESAR-3D

(d) kv is found from the calculated result of the rigidity at the top kv0, by the finite element calculation, reduced by the ks rigidity:

11 kv = kv0 4. ks

(e) The length of beam l is based on the results of CESAR-

3D calculation so that a similar rotation is obtained. The sensitivity of this type of structure (arch bridge) to the foundation stiffness was investigated by parametric calculations with characteristics raised by 30% (stiff ground) or undervalued by 30% (soft ground).

4.3 Results 4.3.1 The CESAR 3D model The model of the southern arch foundation was 45 m deep, 110 m wide and 110 m long leading to more than 6500 elements and 7000 nodes (Lac20). An elasto-plastic calculation with non-standard Mohr-Coulomb criteria was carried out to check the validity of the elastic solution. The results showed that the foundation behaves in elastic conditions, which validates the previous elastic analysis. The linear elastic calibration of the node-beam model was carried out without contact elements. However the model includes an intermediate layer between the external side of the foundation and the soil in order to take into account decompression of the near field soil due to excavation of the well foundation. As the injected volumes were small, it was decided to give the same characteristics of the intermediate layer as the soil. The values of the geotechnical parameters are shown in table 6 and the loading are shown in table 7. The values of the elastic modulus were chosen in the range of the results given by cyclic pressuremeter analysis. The shearing characteristics came from the pressuremeter analysis. The boundary conditions are no vertical displacement at the bottom of the mesh and no horizontal displacements at the vertical edges of the mesh. Other displacements are free. The 3D finite element simulation allows the determination of the elastic settlement of the foundation and its rotation. The settlement obtained for this support is 7 mm in the center of the foundation, with a rotation of 6.10-4rad (settlements at the two ends of the box are 2 mm and 10 mm). The ground under the raft is under compression (max

= 450kPa) except at the end of the long axis where tension of 50kPa is found. This tension was removed by elasto-plastic computing and the interface elements were suppressed in tension area. Complementary parameter calculations show that the separation is sensitive (extent and value of the tensile stress) to an increase in the stiffness of the soil under the raft. The numerical results are shown in table 7. 4.3.2 The node-beam model The explicit structural node-beam model along (xyz) was applied to all the supports by taking into account the results of the 3D model, especially for the settlement of the box and the magnitude of the rotation. This calibration allows the lateral compression of the box to be taken into account. For the south support (P13) of the eastern deck, the results are shown in Table 7 for the permanent loads. 5. CONSTRUCTION AND MONITORING

5.1 Final controls The final controls were focused on three different objects, stratification of the soil, methods used for the construction, displacements of the foundations under the loading forces. 5.1.1 Stratification of soil during excavation. During construction of the foundation box (Fig. 9), the excavation of the foundation well allowed confirmation of the position of soil layers, and the geological and geotechnical models, which were used in the design. Many boreholes were made and the knowledge gained during excavation did not show any significant difference from the geotechnical model of the preliminary study. This control also confirmed the stiffness of the Riss formations, which were used for the direct support of the foundations. 5.1.2 Effects of the method of construction. The control used during the construction checked that the building process does not unload the soils. Following special methods of construction ensured this limitation: (a) The excavation and construction of the retaining wall (of

the well foundation) are made in a very short delay. Thus, the construction management led to a depth of excavation of 1 m per stage, with a delay of 18h to build the retaining wall. This wall was made of concrete sleeves of 0.3 m thickness.

(b) The excavation is made by mechanical power (high power shovel)

(c) Decompression of the soil is restrained by injections after concrete work of the foundation well. These injections were made into the interface between soil and foundation under low pressure. with one injection point per 1 m2. The construction work did not exhibit special difficulties. The injected volumes were small, lower than 1 liter/m2 of interface with a pressure close to 1 MPa.

5.1.3 Displacements of foundations under loading forces. During construction special measurements were

made, with special techniques such as the measurement of forces on the jacks supporting the arches by the knowledge Family Soil description E : MPa c : kPa :

degree :

degree Modern alluvial deposits 30 0.3 0 30 0 F1 Clayey gravel 162 0.3 0 30 0 F2 Clayey and loamy sandy moraine 220 0.3 0 40 10 F3 Grey yellow sandy moraine 270 0.3 0 40 10 F4 Clayey loamy grey moraine 270 0.3 0 38 8 Table 6: Values of the geotechnical parameters used in the FEM calculation

Applied forces

MN Settlement

mm Horizontal displacement

mm Rotation 1000.rad

Soft Hypothesis Stiff Hypothesis Measurements

41.54 6.7 3.8 2.4

3.7 2

0.5

0.4 0.2 0


45.11 8 4.3

3.8 2

0.5 0.2


60.09 9.5 5.2 3.8

4.8 2.6 1

0.5 0.3 0


74.52 11.4 6.2

5.4 2.9

0.6 0.3


96.8 16.4 9

5.2

6 3.2 1.5

1 0.5 0


93.85 15.9 8.6

5.9 3.2

0.9 0.4

Table 7: Displacements found by finite element analysis and measurements for the south support (P13) of the eastern deck

Fig. 9: General view of the foundation well of the hydraulic pressures, the measurement of relative displacements between the foundations and the basis of the arches by four gages, the measurement of foundation displacements and rotations by topographic surveying of targets with a motorised theodolite and by clinometers fixed on top of the foundation, the measurement of the displacements of the arch by topographic targets. During the construction of the deck, topographic measurements were carried out. New periodical controls are

planned to observe the long-term behaviour of the foundations and to ensure a creep smaller than tolerance. These measurements (Table 7) allowed the experimental loading curve to be drawn and to compare it with the theoretical ones (Fig. 10), which are drawn for two different stiffness of the soil. The arch was built on a support, which allow rotation so that bending moments are not transferred to the foundation. Furthermore, the foundation rotation has no influence on the stability of the arch. As expected, the measured rotations are so small that the clinometers could not measure anything. On Fig. 10. one can see that the soil is stiffer then the stiff hypothesis. Long-term measurements will be carried out to measure creep of the soil under permanent loads.

5.2 Performance Monitoring and Surveying The Crozet bridge is located in south east France, and is maintained by company AREA (socit des Autoroutes RhnE Alpes), whose goals are to keep each structure functional and safe. Bridges usually longer than 100 m, allow deep and wide breaches to be crossed. They are essential to the continuity of the motorway traffic but also to the stability of economic exchanges for the area crossed. They are supervised and monitored as a main priority. Moreover, maintenance and repair of these bridges are very


sensitive because of their length and because each working site causes great constraint to traffic over a very long time.

Figure 10: Achievement control for the total displacements on the support P13 The management of AREA construction works is based on periodic detailed inspections as defined in ITSEOA21. Inspections are carried out every 6 years. The damages are visually observed and the corresponding reports give a state index based on IQOA22 classification given by the French Road Authority so that the maintenance work can be carried out. The Crozet bridge was built in 1998 and was inspected for the first time in 1999 before it was used. The inspection report described a construction in good condition (index IQOA: 1). The next detailed inspection is planned for 2005. Because of its particular structure, decks supported by prestressed concrete arches founded on a soil with a medium bearing capacity, it is necessary to check the movements of the arches precisely and especially the separation between the arch foundations. Calculation showed that the bridge stability can be lost when the arches move apart from one another. For the eastern arch of 140 m, a maximum displacement of 5cm is allowed. For the western deck which is supported by three arches, the displacement of the central supports are not taken into account because it is supposed to be compensated for by the efforts of the two end arches, so that the maximum displacements of one support are 1.5 cm maximum allowed for the arch in the Grenoble direction, 87 m long., 2.5 cm maximum allowed for the arch in the Sisteron direction, 102 m long. The relative displacements of one support to the other one are twice these values. When these displacements are reached, special execution processes with recovery of the hydraulic jacks of the arches will be carried out. The process is designed so that it is possible to tighten the prestressed cables of the foundation boxes. Topographical reference targets were fixed on the construction so that displacements can be measured:


(a) Reference targets of medallion type fixed on each foundation boxes, 0.8 m above ground level,

(b) Reference targets of target type fixed at the top of the columns and on arches,

(c) Reference targets of rivet type fixed on the pavements of the bridge.

Finally XYZ measurements were carried out on 28th May 1999. A new series of measurements was carried out on 17th August 2002. No significant displacement was found within the precision of the measurement, which are 1 mm along vertical and 2 mm along horizontal.

5. CONCLUSION The Crozet bridge is an arch viaduct founded on soil, which is not a stiff formation for this structure with strict displacement limitations. The calculation which was used for the design, took into account of both characteristics of structural deformation and subgrade deformation with an iterative process. The stiffness of the soil was a function of the displacements but the displacements modified the forces on the foundation. A total analysis both in static and dynamic mode was considered and the seismic risk was taken into account. To achieve this aim, geotechnical modelling was carried out with the help of cyclical pressuremeter tests with slotted tube and laboratory cyclical triaxial tests for liquefaction analysis. Performance tests were performed at each stage of construction, and when the bridge was used for the first time. This showed that displacements were lower then expected. The design and the grouting of the space between the well foundation and the soil allowed the achievement of a foundation with high horizontal force but low horizontal displacement. The bridge has been used since 1999 without any trouble. This design method is now available for other arch bridges founded on soft ground.

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4. MENARD L, Mesures des proprits physiques des sols, Annales des Pon s e Chausses, Paris, 1957, 14, N3, 357-377.

t t

r

5. GAIATECH, Procd d'essai de forage, Brevet Franais, 1989, N 89 09674, Lyon.

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