M. Bustamante, L. Gianeselli,

Appraisal of the foundations and underpinning

of a Napoleonian masonry bridge

M. Bustamante, <*> L. Gianeselli, <'> J.-L. Ledoux <*>

(v Laboratoire Central des Fonts et Chaussees, Paris, France

EMail: [email protected] [email protected]

\2)Laboratoire Regional des Fonts et Chaussees, Bordeaux, France

EMail: jledoux@cete33. equipement.gouv.fr

Abstract

During a survey of the oldest bridge in Bordeaux, severe damages have beenobserved on the piers adjacent to the left bank abutment. They have beenattributed to the combined effect of differential settlement and scouring. Afterconsidering various strengthening schemes, underpinning by micropiling wasadopted. The residual capability of the timber piling was also taken into account.The suitability of the micropile solution and its feasability have been checkedout in situ. A comprehensive monitoring of the underpinned bridge has beencarried out. The reported monitoring observations span over a period of 5 years,from June 1993 to December 1998.

1 Introduction

The Pont de Pierre was built in Bordeaux by order of Napoleon, to expedite theGaronne river crossing of his army, marching to Spain. Commenced in 1810, thebridge was only completed in 1821 (Fig.l) after the collapse of the First Empire,and a delay due to difficulties encountered during construction [1].

The most noticeable features are a deck length of 487 m, supported by 17 archesof 23 m span. The fabric is a brick and stone masonry resting on a 1.5 m thicklime concrete footing, built on a timber platform supported by approximately250 piles (Fig.2). The piles are driven piles of pine wood, about 300 mm indiameter, and of 8 to 10 m length below the footing. They were driven to

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780 Structural Studies, Repairs and Maintenance of Historical Buildings

'refusal' with a conventionnal 375 kg drop hammer commonly used in thesedays.

Figure 1: Opening of the Pont-de-Pierre after a print from 1820.

UPSTREAM

0.00 N.G.F

lime concrete footing

3BHF^B_

timber pile

Figure 2: Typical cross section through pier.


Structural Studies, Repairs and Maintenance of Historical Buildings 781

The piers were protected by a conventional rockflll. Successive refilling and thepresence of 16 piers have substantially reduced the river flow, with acorresponding increase values of the river and tidal current speed. Nowadays,this speed can reach value as high as 5 m/sec. River currents and tidal streamshave eroded the river bed so severely that the scour has become critical in theimmediate vicinity of the piers. Figure 3 shows the foundation condition as onJanuary 1995.

-^ — «s Garonne River

<x

If

llUilllllllllll

? ^

\

\̂

9 +3.75 H.W

yl.81 L.W4#"

L scouring extentX C January 1995

Fig 3 : Section on pier n°7 before remedial work. Note the toe timber pile levelwith respect to the scour hole level.

Figure 4: Typical crack on pier n°2. Note horizontal coring in progress

Just after construction, the left bank abutment and piers n°l to 9 of the Pont-de-Pierre started to settle. Since its completion, i.e for a period of approximately175 years, pier n°3, which is the most affected, settled down up to 595 mm. The



other piers, n°l, 2, 4, 5, 6, 7, 8, 9, have settled 375, 483, 312, 181, 200, 150,100, 120 mm, respectively. The down- or up-stream tilting have not beenrecorded because considered of minor importance. In 1982, due to theimportance of the bridge for Bordeaux traffic and its historic value, the DirectionD6partementale des Fonts et Chaussdes initiated a comprehensive survey of themovements and structural disorders. A ten year survey period, from 1982 to1992, revealed that the disorders were continuing (Fig.4). The most affected pierswere again the piers adjacent to the left bank. While the settlement was notexceeding an average of 1.5 mm/per year for the majority of the foundations,piers n°2 and n°3 continued to move down at a rate of 20 to 40 mm/per year,with a clear accelerating trend as shown in Fig.5.

1983

-70

Figure 5 : Time-settlement curves for different piers since 1983.

2 Remedial programme

Before adopting a strengthening scheme it was decided to evaluate :a) the foundation residual capability,b) the adequacy of various underpinning techniques,c) the risk of collapse of bridge piers due to scouring.

Keeping in mind the understandable inaccuracy of such analysis and using anassumed value of the pile skin friction as the main input parameter, it wascalculated that the most threatened piers, i.e. n°l, 2, 3, 4, indicated a factor ofsafety against failure Fs = 1.1 to 1.3. Consequently, for ensuring to the pierweighing about 60 MN a factor Fs > 1.5, it was decided to transfer about 40% of



this load to a complementary foundation system. All this analysis was based onthe assumption that the existing timber piling was still sound and fit to bring anappreciable contribution to the foundation.

Various remedial techniques have been considered :a) soil improvement by adequate grouting ;b) micropiling or conventional piling installed around the pier ;c) micropiling installed through the pier.

L.bank 1 9 10 11 12 13 14 15 16R.bank

-20 J

mud + softclay

dense to very densesand + gravel

calcareous mail bedrock

Figure 6: Subsoil stratigraphy along Pont de Pierre, with piling toe level.

The first scheme, based on conventional grouting or jet-grouting, was consideredof a questionable efficiency in soils like recent mud and soft clay. Furthermore,the integrity control after such treatment was felt rather difficult. The secondsolution based on micropiling or conventional large diameter piles installedaround the pier had also to be rejected, because it involved the removal of therockfill, thus endangering an already grossly over-loaded foundation. In addition,both techniques appeared non cost efficient.

Micropiling installed through the pier, a well-proven underpinning technique,was finally regarded as the most suitable scheme and presented the advantage ofa reduced disturbance on the existing structure. The number of micropiles had tobe limited to avoid weakening the pier masonry and damaging the timber pilesduring drilling. It was decided to install 16 vertical micropiles with a workingload equal to 1.5 MN, and a bond length Lg = 9 m into the calcareous marls. Thestrengthening programme was divided into two distinct phases:



1°/ consolidation of the most threatened piers n°2 and n°3, to be carried out assoon as possible, just after subsoil and structure investigation is completed,i.e. at the end of 1992;

2°/ remedial action on piers n°l and n°4, to be carried out following theprevious phase in 1993.

3 Soil and masonry investigations

The soil investigation consisted of boreholes and in situ tests (PM and CPT). Thegeneral stratigraphy along the bridge is shown in Fig.6. The typical soil profileconsists of 5 to 12 m of a recent mud and soft clay overlying 1 to 10 m ofinterbedded sands and gravels. The bedrock was found 20 m below the left bank,and 27 m below the central portion of the bridge. The bedrock consists ofcalcareous marl, weathered and definitively marly in the upper part, becominggradually stiff to very hard limestone. The mud and soft clay layer is thickerbeneath the left bank, with a marked tendency to wedge out in the direction of theright bank. The ground water level is controled by the Garonne river regimeunder the direct influence of the tide, from the nearby ocean.

The masonry investigation consisted of boreholes drilled through pier andfooting. It revealed a material in good condition with compressive resistance R*values as follows:

2.7 <Rc< 77 MPa, for stone only,1.2 <Rc< 21 MPa, for stone + mortar joint.

The pier masonry permeability k was ranging between 2 and 7x10"* m/s. Anadditional investigation, indicated that the pile toe for piers n°l to n°5 werelocated in soft clay. This fact, quite consistent with the settlement patternobserved along the bridge, was recognised as one of the main reason of themovements. Partial timber decay and voids observed beneath the timber platformand possibly caused by scouring, could also be blamed for the aggravation of thedisorders.

4 Checking the suitability of the micropile scheme

Because of the high working loads to be supported by the micropiles, theremedial works were preceded by a test programme aimed to:a) make a full scale study of the behaviour of the selected micropile technique

under 3 MN, i.e. twice the proposed design load ;b) evaluate the feasibility problems and the performance of the post-grouting ;c) measure the resisting forces developed along the micropile bond length.

The test has been performed on a micropile equipped with a recoverableextensometer [2]. Neither the ultimate load Qu nor the critical creep load Qccould be reached. By extrapolating the extensometric results it was assessed thatQc was at least equal to 3 MN, with Qu > 4.2 MN. Figures 7 and 8 show that:



a) a noticeable elastic shortening of the micropile, up to 25 mm under 3 MN ;b) the load is mainly resisted by skin friction ;c) a high unit friction develops in calcareous marl, qs > 100 kPa.

The test proved that the selected micropile performed very well and couldsupport a working load greater than 2.4 MN. For the required load of 1.5 MN,the total settlement was 11.4 mm, with an elastic shortening of 11 mm.

load QQ (MN)0 0.5 1. 1.5 2. 2.5 3. 3.5

-10

-20 _

-30-

-40

KN\_

I design load. 1.5JMNxn>

\

Ho— — o pile he»•••••* point

SQ

\

X

ad IJ

0 g 2: 3. MN 'Q 2 42MN

\\

X\

load QO (MN)0.5 1. 1.5

Figure 7 : Head and point settlementsversus load versus load

Figure 8 : Load distribution alongthe shaft

5 Pier underpinning works

A period of 7 months was necessary to complete the remedial work on pier n°2and n°3 [3] [4], A total of 16 micropiles of the post-grouted type were installedthrough piers (Fig.9). Technically similar to the test micropile, all of them had abond length L$ = 9 m in calcareous marl. The construction was carried out fromthe deck, from Friday afternoon to Monday morning, to minimize trafficdisturbance. The bonding of the micropile to the pier masonry, was done after allthe 16 micropiles have been post-grouted.

pier extent ,' ' o o o o o o o o o o !

' ' o o o o o o o o o o o <• o o o o o o o o o ^l O O O O O O O O O O ^

o o o o o o o o oi o o o o o o o o o o o o o o o o o o o o o

i O O O^3 O O^p O O A,^ o o o o o o o o o o o o o o o o o o o o o o o. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

I 2. I 2.45 I 2.95 I 2.7 I

ooo existing piles ® ® underpinning micropiles

Figure 9: Micropiling pattern with respect to existing timber piling



Pier settlement and tilting were monitored during the construction period. Theglobal settlement measured for pier n°2 and n°3 was 15 and 20 mm respectively.

1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997

Figure 10. Time-settlement curves showing the consequence of remedial action.

Time-settlement curves for piers n°2 and n°3 over an observation period of 13years are given in Fig. 10. The underpinning was considered successful!, since inthe 2.5 years from June 1993 to December 1995 following completion, levellingmeasurements indicated a settlement of between 4.9 to 5.9 mm, and a furthersettlement of between 0.8 to 0.6 mm, from December 1995 to December 1997.

2/3/96 31/8/96 1/3/97 30/8/97 28/2/98

Figure 11 : Seasonal pier movement



-o.i

|-0.2

-0.3

piern

5 10 15 20time (h)

Figure 12 : Tidal pier movement

25

In order to stop the crack development but also to provide a better load spreadfrom the pier down to the micropiling, pier bottom part has been equiped with ahooping system consisting of 3 steel tendons, stressed up to 0.4 MN each.

Later on, from December 1993 to June 1994, similar strengthening works wereundertaken on pier n°l and n°4. They appeared to be as successful as theprevious ones carried out on pier n°2 and n°3 (see Table 1).

Table 1. Measured settlements for typical periods.

remedialwork phase

1st

2nd

piern°2314

ASi(mm)14.6619.305.0011.20

ASz(mm)4.945.901.253.80

AS](mm)0.860.620.361.52

In Table 1, settlement ASi, AS], AS] correspond to the following duration ;ASi - 7 months underpinning sequence, for piers n°2 , n°3, n°l, n°4 ;AS] - 30 months period, from the remedial work completion to December 1995

for pier n°2 & n°3, and a period of 18 months for pier n°l & n°4ASg - 2 years period, from December 1995 to December 1997 for all piers.

It was also clearly observed that the pier exhibited cyclic settlement, seasonal(Fig. 11) and tidal (Fig. 12). A final third phase of more complex works, aimed tostop scouring around the bridge was carried out in 1996 and 1997.



These works involved:a) a slope reprofiling of the up- and down-stream river bed around the piers,b) the placement of a protecting mattress consisting mainly of layers of

armoured stone blocks.

6 Concluding remarks

1°/ The opportunity to access to the existing 19th century archive plans of thePont-de-Pierre was decisive for understanding the disorder mechanismaffecting the bridge and had an incidence on the selected remedial scheme.

2°/ The adopted underpinning technique seems to be successful!, and levellingsurvey indicated small settlements over the five years preriod followingcompletion. However, additional settlement due to the micropile installationwas significant and did represent about 50% of the total movement for themost affected piers, i.e. n°3 & n°4.

3°/ Data obtained from the preliminary full scale load test carried out oninstrumented micropiles, were of a great help to check the validity of theadopted strengthening solution.

References

[1] Bustamante, M., Gianeselli, L. & Ledoux J-L., Underpinning of theBordeaux Pont-de-Pierre: A first Empire masonry bridge, Geotechnical Eng.for the preservation of monuments and historic sites, ed. C. Viggiani,Balkema, Rotterdam, pp.551-559,1997.

[2] Bustamante M. & Doix B., A new model of LPC removable extensometers,Proc. of the 4-th Int. Conf on Piling and Deep Foundations, Stresa, Italy,April 7-12, 1991.

[3] Ledoux, J-L. & Massoutier, A. Travaux sous hautes surveillance.Renforcement des fondations de 2 piles du Pont de Pierre, Bull de Liaisondes LPC, n° 210, juil.-aout, 1997.

[4] Ledoux J-L., Renovation du Pont de Pierre de Bordeaux - Projet derenforcement des fondations des piles 2 et 3. Revue generale des Routes,numero special dejuin, pp.92-94, 1997.


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