Geot58-645-Experimental Pile Subjected to a Moving Slope

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Frank, R. & Pouget, P. (2008). Geotechnique 58, No. 8, 645–658 [doi: 10.1680/geot.2008.58.8.645]

645

Experimental pile subjected to long duration thrusts owing to amoving slope

R. FRANK* and P. POUGET†

An instrumented pipe pile has been installed in anunstable slope to measure long-duration P–˜y lateralreaction curves. After outlining the main features of theexperiment, results are given over a period of 16 years.These comprise the soil movements, the pile displace-ments, the bending moments and the reaction pressures.A Menard pressuremeter method and a self-boring pres-suremeter method are used to predict the P–˜y reactioncurves, which are then used in a numerical model toderive the overall pile behaviour. The results are com-pared with measured pile displacements and bendingmoments and show that the pile displacements and bend-ing moments calculated by the numerical model aremarkedly overestimated.

KEYWORDS: creep; design; field instrumentation; full-scaletests; piles; slopes

Un pieu tubulaire instrumente a ete mis en place dansune pente instable, de maniere a mesurer les courbes dereaction P–˜y de longue duree. Apres avoir decrit lesgrandes lignes de l’experimentation, on donne les resul-tats obtenus sur une periode de 16 annees. Ces resultatscomprennent les mouvements du sol, les deplacements dupieu, les moments flechissants et les pressions de reaction.Une methode de prevision a partir des resultats de l’essaiau pressiometre Menard et une methode a partir del’essai au pressiometre autoforeur sont utilisees pourprevoir les courbes de reaction P–˜y. Celles-ci sontensuite utilisees dans un modele numerique pour calculerle comportement general du pieu. Les resultats sont com-pares aux deplacements et aux moments flechissantsmesures. Cette comparaison montre que les deplacementsdu pieu et les moments flechissants calcules sont nette-ment surestimes.

INTRODUCTIONWhen a pile is submitted to a lateral load and/or a bendingmoment at its head, its stability is assured by the mobilisa-tion of the reaction pressures from the soil along its shaft.At a given depth, the soil ‘reaction’ P (in kN/m) is afunction of the pile displacement y; the P–y curve is calledthe ‘reaction curve’ of the soil.

In French practice, the P–y curve for the design of a pileis most often derived from pressuremeter tests results (eitherpre-bored Menard tests or self-boring pressuremeter tests).Full-scale tests have been performed (in particular at thePlancoet site) in soft ground, to determine the P–y curves,for short duration conditions (see Baguelin et al., 1978).

It was decided to perform this type of experiment in thecase of a pile submitted to a slow, sustained thrust. Thethrust is provided by a slope at the limit of stability (wherethe movement of the slope is linked to the level of thewater-table). The main objective of the experiment was toanalyse the lateral reaction mechanism and to determine, inparticular, the long duration (creep) P–y curves (moreprecisely, the P–˜y curves, where ˜y ¼ y � g with g the‘free’ soil movement), well after full consolidation has beenreached.

An instrumented steel pipe pile was installed in 1983downslope of an experimental embankment, constructed in1980 on the site of Salledes, in the Departement of Puy-de-Dome, Central Massif, France (Cartier & Pouget, 1988). Themeasurements, both for the pile and for the soil, have beentaken systematically over 16 years, from 1983 to 1999. More

recently, as some instruments in the pile were still workingsatisfactorily, it was possible to take some last measure-ments: in March 2004, the displacements and the bendingmoments were therefore determined down the pile, as wellas the total shear load at its head; in July 2005, only thedisplacement down the pile could be determined.

MAIN CHARACTERISTICS OF THE EXPERIMENTThe experimental site of Salledes

The analysis related to the construction problems ofembankments on unstable slopes, and their behaviour withtime, led the Laboratoires des Ponts et Chaussees (LPCs) tobuild, in 1980, an experimental site at Salledes (located30 km from Clermont-Ferrand).

The surface of the site is inclined at 7% and the ground iscomposed of colluvial marls 5 m to 8 m thick, which slideslowly at the contact with the marly, compact Oligocenesubstratum (Fig. 1). The sliding surface has been determinedfrom inclinometer readings (see Fig. 2).

In 1978, a first embankment (embankment A) was con-structed, until it reached failure at 5.40 m height. The rate ofconstruction followed usual earth moving practice. The mainobjective was to determine the geotechnical parameters ofthe ground (residual shear strength parameters), as well asthe hydrological parameters of the slope (Blondeau et al.,1983).

In 1980, a second embankment (embankment B, see Fig.2) was built in two phases (initially to a height of 3.0 m atthe crest, then to a height of 3.40 m at the crest) to analysethe evolution of the displacements with time, linked to thegeneral instability of the site (Cartier & Pouget, 1988).

Measurements with time have provided the relationshipbetween the elevation of the phreatic water surface and thedisplacement rate of the mass of colluvial marls. During theyear, the water level usually varies between 5 m and 1 mbelow ground level. Fig. 3 shows that significant displace-ments of the slope are observed when the water level

Manuscript received 14 April 2007; revised manuscript accepted 7May 2008.Discussion of this paper closes on 1 April 2009, for further detailssee p. ii* Universite Paris-Est, Ecole Nationale des Ponts et Chaussees, URNavier-CERMES, France† Laboratoire regional des ponts et chaussees, Clermont-Ferrand,France


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reaches 2 m below ground level (40 kPa for the piezometersinstalled at 6 m depth).

The steel pipe pile, 12 m long, has been installed on thissite, downslope of embankment B (see Fig. 4).

Mechanical properties of the groundThe colluvial marls are formed predominantly of carbo-

nated clays, with very thin local layers of a more sandynature (coming from volcanic pozzolana formations) andcontaining a few small granules of basalt. The main identifi-cation and shear strength parameters are rather scattered,

owing to the varying content of calcium carbonate (CaCO3)and sand. The main results are given in Table 1.

The undrained shear strengths obtained from field vanetests show variations with depth, such as the typical profileshown in Fig. 5. In particular, a relatively high value of theundrained shear strength is observed over 1.5 m near theground surface, corresponding to overconsolidation of theclay, and then the values decrease with depth to the order of50 kPa in some soundings.

Several Menard pressuremeter tests (inserting a probeinside a 60 mm diameter hole bored with an auger) and self-boring pressuremeter tests (PAF 76 type, 132 mm diameter,slenderness ratio 4, expansion rate 2 % per minute) havebeen performed. The nearest tests to the pile are locatedrespectively 1.5 m (PM1) and 4 m (PAF3) downslope. Atthese locations, the colluvial deposits are around 6.4 m thickand are underlain by the compact marls. A slight dip of thetop of the substratum is observed towards the East. Themain results of the tests are given in Figs 6 and 7. For

CD14

H 3·40 m�

Pile 10 m

10 m

Colluvial marls

Compact marls

Sliding surface

Fig. 1. Geological section at the site (at the level of embankment B)

Soundings (number)

CD14

N

10 m

0 10 m

F9N9

G2Q3

PileG3 G2

G4

G5

Instrumentation

PiezometersInclinometers

Field vane: 10CPT: 13Menard pressuremeter: 2Self-boring pressuremeter: 4Auger sample: 13Tube sample: 3

Fig. 2. Embankment B and the pile: soil investigation andinstrumentation

0

10

20

30

40

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60

0 5 10 15 20V : mm/day

Por

e pr

essu

re a

t 6 m

dep

th: k

Pa

Fig. 3. Relation between porewater pressure and rate ofdisplacement of the ground below embankment B (Pouget &Livet, 1994)

Deadman�3·40 m

Pile

39 m

7 m

Fig. 4. Location of the pile in the slope

646 FRANK AND POUGET


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Menard tests, the limit pressure pl and pressuremeter mod-ulus EM are given in Fig. 6. The results of PAF 76 tests aregiven in Fig. 7, namely po, initial pressure, p20, pressure forthe volumetric deformation of the probe ˜V/V ¼ 20 % andGo, initial tangent modulus.

The pile characteristicsThe pile (steel circular pipe) was installed 7.5 m down-

slope of the toe of the embankment B (Figs 2 and 4). Atthis location the sliding surface is at 6.15 m depth and thecompact marls lie around 7 m deep.

The main characteristics of the steel pile are the following

(a) total length: 12 m(b) embedded length: 11 m(c) internal diameter: 0.877 m(d ) external diameter: B ¼ 0.915 m (wall thickness 19 mm)(e) steel elastic limit: �e ¼ 376 MPa (measured by tension

tests on steel samples)( f ) flexural stiffness: EI ¼ 1.07 3 106 m2 kN (measured by

bending tests performed on the pile before itsinstallation; the pile was placed horizontally on twofixed supports and submitted to various vertical loads).

The objective was to impose a zero displacement conditionat the head of the pile, in order to have the maximumrelative displacement between the sliding soil and the pile,as the soil ‘flows’ around the pile. For this reason, the pilehead was held in place by a deadman anchor which islocated upslope of the embankment (Fig. 4). The link be-tween the deadman and the test pile is a pre-stressed con-crete beam (of cross-sectional area 0.4 m 3 0.4 m with apre-stressed anchor made of 12 T13s); the deadman anchoritself is a block of reinforced concrete of 20 m2 3 0.8 m.The attachment system of the pile head to the beam (Fig. 8)is composed of a yoke with four Dywidag bars. This yokesystem is located, on average, at 0.5 m above ground level(z ¼ �0.5 m). Flat jacks are placed in such a way that theycan be used to counteract any possible movement of the pilehead.

The pile was installed at the end of 1983. The hole wascreated by augering with a Benoto machine. The supportcasing to the bore was rotated back and forth as it waspenetrating the ground. A chisel was found necessary to passthrough the occasional levels of cemented marls until anembedment of 11 m in the natural ground was reached. Ametallic plate was subsequently welded to the bottom of the

Table 1. Colluvial marls: main identification and shear strength properties

ª: kN/m3 w: % Ip: % wL: % % , 2 � CaCO3: % c9: kPa j9: 8

16 to 18 30 to 55 35 to 57 55 to 85 45 to 65 18 to 55 10–20 21–28

0

1

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3

4

5

6

7

0 50 100 150

Dep

th: m

Undrained shear strength: kPa

Fig. 5. Undrained shear strength with depth (field vane sound-ing SC3)

0123456789

10

0 1 2 3

Dep

th: m

Dep

th: m

Limit pressure: MPa

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Fig. 6. Limit pressure p1 and pressuremeter modulus EM fromMenard pressuremeter tests (PM1)

0

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G0

P20

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th: m

Fig. 7. Self-boring pressuremeter results (PAF3)

TNPile

Squareplate Square

plateStiffeners

Flat jacksNut

Sleeve

Anchor

Skimcoat

Loadcells

Steel plates40 mm thick

Flat jacksupports

Nut

Yoke

Pile

4 load cells

Sleeve connection

Anchor

Dywidag bars

SpacersFlat jacks

Fig. 8. Attachment system of the pile head

EXPERIMENTAL PILE SUBJECTED TO LONG DURATION THRUSTS OWING TO A MOVING SLOPE 647


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pile to keep the inside of the pile free from water, and toprotect the instrumentation. Thus, the pile could also beinspected if needed. Reinforcement against ovalisation of thepile section was installed inside the pile at depths 0.48 m,1.52 m, 3.72 m, 5.82 m and 8.07 m (measured from groundsurface).

The instrumentationThe pile instrumentation (Fig. 9) measures movements as

well as the strains in the pile in the direction of the slope

(a) a vertical inclinometer tube G1 was fixed to the insideof the pile over its entire length and embedded 3 munder the toe of the pile in the marls of the substratum

(b) on two opposite vertical lines in the plane of thedisplacements, 28 vibrating wire strain gauges wereinstalled in pairs (11 pairs of strain gauges with aspacing of 0.75 m in the upper part of the pile, andthree pairs with a spacing of 1 m in the lower part).The strain gauge measurements allow the bendingmoments along the pile to be determined

(c) four load cells (C1 to C4) mounted on each of the

Dywidag rods making up the system for fixing the pilehead to the pre-stressed concrete beam (see Fig. 8)

(d ) furthermore, temperature gauges were used to check thedrift of the electric devices; levelling devices withmicrometric screws were also installed on the systemfixing the pile head to the pre-stressed concrete beam,to check the inclination of the pile head.

The instrumentation in the ground consisted of (Fig. 2)

(a) two inclinometer tubes (G2 and G3) installed on bothsides of the pile, at right angles to the slope and at adistance of 3 m (i.e. at three diameters) away from theaxis of the pile, to measure the ‘free’ soil movement g (z)

(b) two inclinometer tubes (G4 and G5) placed at 1.98 mand 3.36 m downslope of the pile axis, to assess thelocal influence of the pile on the movement of theslope.

THE RESULTS OF THE MEASUREMENTSNo malfunctioning of any of the measuring devices was

detected. The inclinometer tubes embedded in the groundhad to be replaced several times, however. The replacementtubes were located very close to the originals. The final plotsof ground displacements are then the accumulation of dis-placement measurements from several tubes. Measurementswere taken from 1983 to 1999. During this period, themovement of the natural ground recorded by the inclin-ometers G2 and G3 at the pile head g(0) was of the order of25 cm (Fig. 10).

The pile head was pulled back four times approximatelyto its original position because of movements linked to itsanchoring system. The deadman anchor upstream of theembankment moved slightly, although less than the grounddownslope of the embankment (see below).

The displacements of the natural groundDuring the experiment, the displacements g(z) of the

natural ground on both sides of the pile reached 27.0 cm(G2) and 24.5 cm (G3) at depth z ¼ 0.40 m. The evolutionat intervals of about two years is shown on Fig. 10 (mean ofthe inclinometer readings G2 and G3). These values arearound two times larger, over the same period of time, thanthose of the natural slope upslope of the embankment (meas-ured by inclinometer F9N9, see Fig. 2). It should also benoted that this ratio decreases slightly with time.

The shape of the ground displacements g(z) (Fig. 10)shows that the colluvium flows downslope, with some ‘pour-ing’, a phenomenon which is clearer over the first metre atthe surface.

An examination of the ‘free’ displacements at the surface(Gmean ¼ g(0) in Fig. 11) shows that movement during thefirst 8 years (around 10 cm, from 1983 to 1991) is of thesame order as that measured during the next 4 years (from1991 to 1995), which is an important increase in the rate ofdisplacement. On closer examination, the curves in Fig. 11show periods with large displacements corresponding torainy seasons with a high level of penetration of water intothe ground (for example, the springs of 1985, 1986, 1987,1992 and 1994) and periods of little displacement linked todrought seasons (for instance, years 1984, 1990, 1991, 1996and 1997).

The displacement in the ground immediately downslopeof the pile (curves for inclinometers G4 and G5 in Fig. 11)illustrate the local barrier effect played by the pile. Indeed,displacement measured by inclinometer G4 (at a distance of1.98 m downslope of the pile axis) is 20% lower than the

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Sliding surface680

Strain gauge

Inclinometer tube

Anti-ovalisation rings

Waterproof plug

Fig. 9. Pile instrumentation



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ones measured by G2 and G3. Displacement measured byG5 (located 3.36 m downslope of the pile) is of the sameorder as G2 and G3.

The displacements of the pile and of the slopeThe system linking the pile to the concrete deadman

anchor wall was designed in order to avoid insofar aspossible any movement of the pile head. The pile headmoved nevertheless and it was decided to move it back toits original position after it had moved between 2 cm and4 cm. The pile was pulled back with the help of the flatjacks incorporated into the attachment system. The evolutionwith time of the displacement of the pile head located 0.5 mabove ground level, y (z ¼ �0.5 m) (Fig. 12) shows veryclearly that the pile was pulled back four times (on 5/11/1986, 11/11/1988, 1/10/1992 and 6/5/1995). The flat jackshad reached their maximum stroke capacity, hence the pilewas never pulled back again after 1995. The sum of the fourpile head displacements was around 11 cm. In addition, afinal displacement of slightly more than 2 cm was reachedon 29 July 1999, the last day all the measurements weremade.

The measurements taken in March 2004 and July 2005,showed a further displacement at the pile head y(z ¼ �0.5 m) of respectively, 6.57 cm and 9.22 cm. Thus, in

July 2005, the last time the pile was ‘inspected’, its headhad moved by more than 11 cm since the last pulling backoperation.

It is interesting to note that during the period fromSeptember 1993 to May 1995, just before this last pullingback operation, y(�0.5 m) increased by 3 cm, while g(0)increased by roughly 5.2 cm (see Fig. 11). Thus, the pilehead displacement is 58% of the ‘free’ soil movement. Ifthis is extrapolated to the period from July 1999 to July2005, the soil can be estimated to have moved 16 cm duringthese 6 years (this is clearly a rough estimation).

0·0

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06/07/1987

31/08/1989

04/09/1991

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10/08/1995

19/08/1997

29/07/1999

Dep

th: m

Displacement: cm

Fig. 10. Displacements of the natural ground with time (mean of inclinometers G2 and G3)

0

5

10

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Dis

plac

emen

ts: c

m

1998199419901986

G5

G4

Gmean

0

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60

Por

e pr

essu

re a

t 6 m

dep

th: k

Pa

Fig. 11. Ground displacements at the ground surface and piezometer readings (whereGmean g(0) is the mean (G2 + G3)/2)

�1

0

1

2

3

4

5

Year

Dis

plac

emen

ts: c

m

1984 1986 1988 1990 1992 1994 1996 1998 1999

Fig. 12. Displacements at the level of attachment at the head ofthe pile (at 0.5 m above ground level, z 20.50 m)



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The deformations of the pile before and after each jackingoperation to pull back the pile are shown in Fig. 13. Therewas no pile movement at the toe of the pile (the zerodisplacements from the section of tube expanding beyondthe toe of the pile are not shown). The bottom graph of Fig.13, summarising all the pile deformations after jacking,shows that the pile displacement owing to the soil thrusts isgreatest in its middle part.

It should be stressed that the various pulling back opera-tions of the pile, leading to displacements of several centi-metres, created a hole at the surface in front of the pile over adepth of about 40 cm. This phenomenon has accentuated withtime, with the evolution of the displacements of the soil.

Bending moments in the pileThe strain measurements in the pile (with vibrating wire

extensometers) allow the bending moments to be determinedevery 0.75 m until 7.5 m depth (Fig. 9). They have beeninterpolated at levels 8.25 m, 9.0 m and 9.75 m from thereadings at 7.5 m, 8.5 m, 9.5 m and 10.5 m depth, to haveconstant intervals for the numerical procedure for derivingthe soil reactions (see below).

Figure 14 shows the bending moments measured in thepile before and after each jacking operation, as well as on29 July 1999, the last day all the measurements were made.An inflexion point, barely discernible, appears on the curvesaround 1 m depth; this might be owing to some degree offixation of the pile head by the yoke.

The maximum bending moment in the colluvium isobtained at 3.75 m depth, that is to say nearly in the middleof the layer. The jacking operations had a strong influenceon the maximum bending moment in the colluvium. It canbe seen that the relative variations in bending momentsowing to the operation is smaller in the compact marls thanin the sliding colluvium. This feature appears as soon as thefirst jacking operation. The bending moments vary very littlein between the last two jacking operations (for grounddisplacement larger than 16 cm). The maximum bendingmoment has reached 1500 kN m after the last two jackingoperations. This corresponds to a maximum normal stress� ¼ 128 MPa, which is well below the elastic limit of thesteel (�e ¼ 376 MPa).

In Fig. 15, a decrease with time of the maximum bendingmoment in the colluvium is observed soon after each pull-back operation. In this figure, nearly full mobilisation of the

�1·0

1·0

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�0·5 0·0 0·5 1·0 1·5 2·0 2·5 3·0 3·5 4·0: cm

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04/11/1986

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05/11/198611/11/198801/10/199206/07/199529/07/1999

(a)

(c)

(e)

(b)

(d)

Dep

th: m

Fig. 13. Pile displacements: (a) to (d)—just before and after each pulling-back operation; (e)—after all pulling-back operationsand on 29 July 1999 (depth 0 corresponds to GL; positive displacements are downslope)



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maximum bending moment is also observed at the thirdpull-back operation.

The measurements of March 2004 show a bending mo-ment at the head equal to 156.6 kN m. The maximumbending moment is equal to 520.2 kN m and it is nowlocated at 3.0 m depth (it has slightly moved up).

Reactions on the pileThe soil reactions were obtained by double differentiation

of the bending moments. They are calculated by the computer

program called ‘SLIVALIC5’ (Degny, 1985). This programapproximates the bending moment distributions M(z) byspline functions of fifth order, to solve the double differentia-tion problem. It has an inconvenience though in that constantintervals along z must be used. The reactions just before andjust after the jacking operations are presented in Fig. 16.

It is observed that the calculated reactions are always verylow over the first 2 m of the pile, indicating that very littlepressures are built up when the pile head is pulled back intoposition. In particular, there is nearly no increase in pressure(after pulling back the head of the pile) until 5 m depth. Itseems that the mean effect of the pull-back operation is toincrease the pressures in the lower part of the alluvial marls(and accordingly decrease them in the upper part of thesubstratum).

Total shear loads at the head of the pileThe total shear load is obtained by summing up the loads

measured by the four load cells installed on the Dywidagbars (cells C1 and C3 to C5, Fig. 17). This total loadincreases at each pull-back of the pile (aiming at imposingy ¼ 0), and then decreases again. It also increases with the

0

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Bending moment: kNm Bending moment: kNm

Bending moment: kNm

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Dep

th: m

Dep

th: m

Dep

th: m

(a) (b)

(c)

(e)

(d)

Fig. 14. Bending moments: (a) to (d)—just before and after each pulling-back operation; (e)—after all pulling-back operationsand on 29 July 1999 and on 30 March 2004 (partially) (positive bending moments are a bend of the pile downslope)

0

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Max

imum

ben

ding

mom

ent:

kNm

19981994199019861984 1988 1992 1996

Fitted curve

Fig. 15. Evolution with time of the maximum bending momentin the colluviums (absolute value at z 3.75 m)



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rise of the water-table, that is during sliding of the slope.Note that values are nearly identical after the last two pull-back operations. This might indicate that the soil around thepile has reached a state of limit plastic flow. The measure-ments of 29 July 1999 show a total shear load equal to545.6 kN. In March 2004, the value had relaxed down to503.6 kN.

Each load cell gives different readings, the highest loadsbeing registered by the two top cells (C3 and C5).

The P–˜y reaction curvesExperimental reaction curves are constructed from

(a) the difference in displacements (‘relative’ displacement)˜y (z) ¼ y (z) � g (z) given by the inclinometerreadings (Figs 10 and 13)

(b) the distributions of the reactions P(z) obtained bydouble differentiation of the bending moments (programSLIVALIC5), given in Fig. 16 (it is recalled that, if the‘reactions’ are P (z), the ‘reaction pressures’ are: p(z)¼ P(z)/B)

Experimental P–˜y curves have thus been established foreach level z of measurements, down to 6 m deep (Fig. 18).Below this depth, the values of ˜y ¼ y � g become too

small. To help with the presentation, these curves have allbeen drawn in the first quadrant (P . 0 and y . 0).

LPC methods for predicting P–˜y curves and comparisonwith the measurements

Reaction moduli or reaction curves, commonly used forpiles submitted to lateral loads at their head are relevant torather ‘short duration’ conditions; they must be adapted tothe case of long duration loads, in particular to thrusts owingto soil movement. The methods of the LPC for constructingP–˜y curves for long duration loadings from Menard (pre-bored) pressuremeter (PM) tests and from self-boring pres-suremeter (PAF) tests are summarized below (Bourges et al.,1980).

Menard pressuremeter (PM)The P–˜y curve is trilinear (Fig. 19). It is derived from

the pressuremeter test results obtained at the same depth,that is EM is pressuremeter modulus, pf is creep pressure andpl is limit pressure.

The initial modulus of reaction Es is the one originallyproposed by Menard

Es ¼ 18EM=f4(2:65B=Bo)ÆBo=Bþ 3Æg

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th: m

Dep

th: m

04/11/1986

05/11/1986

0

2

4

6

8

10

12

�400 �300 �200 �100 0 100 200 300 400 500

30/09/1992

01/10/1992

0

2

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6

8

10

12

�400 �300 �200 �100 0 100 200 300 400 500

05/07/1995

06/07/1995

0

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4

6

8

10

12

�400 �300 �200 �100 0 100 200 300 400 500

10/11/1988

11/11/1988

0

2

4

6

8

10

12

�400 �300 �200 �100 0 100 200 300 400 500

05/11/198611/11/198801/10/199206/07/199529/07/1999

P: kN/m P: kN/m

P: kN/m P: kN/m

P: kN/m

(a)

(c)

(e)

(b)

(d)

Fig. 16. Soil reactions: (a) to (d)—just before and after each pulling-back operation; (e)—after all pulling-back operationsand on 29 July 1999 (negative pressures are net pressures acting downslope of the pile; the base of the pile is at depth 11 m)



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where B is the diameter or width of the pile, Bo is areference diameter equal to 0.6 m and Æ is the rheologicalcoefficient. This equation derives from the long durationsettlement of a shallow foundation of width B (see Baguelinet al., 1978).

The ultimate reaction on the pile corresponds to the limitpressure of the pressuremeter, that is Pl ¼ plB. FromPf ¼ pfB to Pl ¼ plB the slope is divided by a factor of 2.

Finally, near ground surface, the following reductionfactor º is applied to the reactions (i.e. for a given displace-ment ˜y the reaction becomes ºP):

º ¼ (1 þ z=zc)=2

where z is the depth, zc is a ‘critical’ depth (zc ¼ 4B forcohesive soils and zc ¼ 2B for granular soils). Below zc,º ¼ 1.

This PM method is the one recommended by the officialCode of Practice of the Ministry in charge of public worksin France (Ministere de l’Equipement, 1993). It is fullycompatible with the requirements of Eurocode 7: Part 1. Itwill be implemented into the corresponding supplementarystandard which will be produced for applying Eurocode 7:Part 1 in France.

Self-boring pressuremeter (PAF)The method is based on the entire self-boring pressure-

meter expansion curve (p, ˜V/Vo) with a ‘dilation’ coeffi-cient equal to 2 (for long duration loads) applied along thedisplacement axis. The P–˜y curve is thus obtained bymeans of the following change of variables

P ¼ ( p� po)B

˜y ¼ B˜V=2Vo

where p (pressure), po (initial value of p) and ˜V/Vo

(corresponding to the relative expansion of the probe) areread on the PAF curve.

The ultimate pressure for the pile pu is taken equal to thepressure p20 � po obtained for ˜V/Vo ¼ 20% on the PAFexpansion curve

Pu ¼ puB ¼ (p20 � po)B

Comparison with the measured P–˜y curvesThe experimental curves could only be obtained for the

first 6 m (every 0.75 m) of depth. At most depths, thetheoretical reaction curves determined from the results ofMenard pressuremeter (PM) tests and from self-boringpressuremeter tests (PAF) are too stiff—or much too stiff—that is, the predicted reactions P are too high comparedwith the measured ones at a given relative displacement˜y. This is particularly true at the top and bottom of thecolluvial layer. Both LPC P–˜y construction methodsappear thus somewhat conservative, as they will lead to anoverestimation of the reactions for a given ‘free’ soildisplacement.

Figure 20 shows the experimental curves in the middle ofthe layer at 3 m, 3.75 m and 4.5 m depth, together with thecurves obtained from the Menard pressuremeter tests (PM)and from the self-boring pressuremeter tests (PAF) at 3 mand 4 m depth.

0

100

200

300

400

500

600

700

800

900

1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000Year

Pull back No. 1on 04/11/86



C3

C4

C1

C5


C1 C3 C4 C5� � �

C3

C4C1

C5

Load

: kN

Pile

Fig. 17. Evolution of the total shear load on the head of the pile



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Notwithstanding the scatter of the experimental curves,the comparison is altogether satisfactory, at least for this partof the pile where the reactions and displacements are signifi-cant. Nevertheless, it can be noted that the Menard curve at3 m depth is stiffer than the one at 4 m depth. The oppositeis true in the case of the self-boring pressuremeter tests.This follows directly from the results of the Menard teststhemselves, see Fig. 6. Thus, the prediction based on theMenard test (PM) is either too stiff (at 3 m), or too soft (at4 m). With the self-boring pressuremeter, the match at lowdisplacements is excellent for 3 m but it is conservative (i.e.pressures are overestimated) at 4 m depth. For both depths,

the self-boring pressuremeter slightly underpredicts the ulti-mate pressure on the pile.

CALCULATION WITH THE ‘PILATE’ PROGRAMPresentation of PILATE program

The PILATE computer program (Baguelin et al., 1976;Frank, 1981) is commonly used in France for solvingproblems of laterally loaded piles with the help of P–yreaction curves. The pile is modelled as a beam on non-linear springs, which is a generalisation of Winkler’s theoryof beams on elastic springs. The pile can be loaded at its

z 1·50 m/TN�

0

50

100

150

200

250

300

350

0 5 10 15 20 25y g– : cm y g– : cm

y g– : cm y g– : cm

y g– : cm

pB: k

N/m

pB: k

N/m

pB: k

N/m

pB: k

N/m

pB: k

N/m

pB: k

N/m

pB: k

N/m

Pull back of the pile

z 2·25 m/TN�

0

50

100

150

200

250

300

350

0 5 10 15 20 25


z 3·00 m/TN�

0

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250

300

350

0 5 10 15 20 25


z 3·75 m/TN�

0

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350

0 5 10 15 20 25


z 4·50 m/TN�

0

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0 5 10 15 20 25


z 5·25 m/TN�

0

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0 5 10 15 20 25


z 6 m/TN�

�50

0

50

100

150

200

250

300

350

0 5 10 15 20


y g– : cm

y g– : cm

Fig. 18. Experimental reaction curves between 1.5 m and 6.0 m depth



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head (by applied shear loads or bending moments) or alongits shaft, for example by lateral thrusts created by soilmovement.

The well-known equation for the equilibrium of the loadsalong a laterally loaded pile is

EId4 y(z)

dz4þ P(z) ¼ 0 (1)

where EI is the flexural stiffness of the pilez is depthy is the unknown pile displacementP is the lateral load density (or ‘reaction’) along the pile(P ¼ pB, in kN/m or MN/m, where p is the reactionpressure and B the diameter or front width of the pile).

In the case of lateral thrusts along the pile shaft, the loaddensity P(z) is assumed to be a function of the ‘relative’displacement ˜y ¼ y(z) � g(z), where y(z) is the pile–soildisplacement at equilibrium and g(z) is the ‘free’ displace-ment of the soil (in absence of the pile or far away from thepile). This approach was initiated by Marche (1973) andPoulos (1973). It has been used by the LPC for the designof foundations of bridge abutments in France, as early as1976 (Baguelin et al., 1976; Frank, 1981).

Generally speaking, the P–˜y curve can be linearisedunder the form (Fig. 21)

P(˜y) ¼ P0(˜y) þ Es(˜y)[y� g]

where P0 is an initial reaction and Es is a reaction modulus(in kPa or MPa).

In any case P(˜y) is a non-linear function; P0 and Es aresought by iterations in order to have the best linear fit to thereaction curve in the proximity of the final equilibrium

point. The pile–soil system is divided into N ‘numerical’layers, such that each layer can be represented by a uniqueP–˜y curve (with pressuremeter data the ‘thickness’ of thelayer would typically be of the order of 1 m). It is also easywith PILATE to take into account possible variations of pilestiffness EI with depth. PILATE then solves, for eachiteration j, a system of N differential equations with constantcoefficients (Euler equations with second term). For layer ithe equation is

EId4 y(z)

dz4þ E

jsi ¼ �P

j0i þ E

jsi g(z)

If g(z) is approximated by a polynomial function of orderless than or equal to 3 in layer i, the analytical solution ofthe Euler equation, at iteration j, for this layer i is simply

y ¼ e z= lo [a1 cos(z=lo) þ a2 sin(z=lo)]

þ e�z= lo [a3 cos(z=lo) þ a4 sin(z=lo)] þ gi(z)

where lo ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi4EI=Es

4p

There are four integration constants for each layer i atiteration j: a1, a2, a3 and a4. For a total of N layers (ivarying from 1 to N), PILATE must solve, for each iterationa system with 4N unknowns. The 4N equations necessaryare provided for by the 4(N�1) continuity conditions at theinterfaces between the layers (on displacements y, rotationsy9, bending moments M and shear loads T), as well as by thelimit conditions at the head and at the toe of the pile (2 atthe head and two at the toe).

Application to the Salledes pileAll the experimental stages just before (‘be’) and just

after (‘af’) pulling back the pile to the so-called y ¼ 0position have been analysed with the help of the PILATEprogram. Three sets of reaction curves have been used

(a) reaction curves derived from the Menard pressuremetertests along profile PM1, over the entire length of thepile (the calculations are noted ‘M’, that is ‘Mbe’ and‘Maf’)

(b) reaction curves derived from self-boring pressuremetertests along profile PAF3, down to depth 5.6 m; belowthis depth, as no PAF tests were performed, the reactioncurves derived from Menard pressuremeter profile PM1are used instead (the calculations are noted ‘A’, that is‘Abe’ before pulling back and ‘Aaf’ after pulling back)

(c) reaction curves derived from the measurements down to6 m depth (see Fig. 18); below 6 m, in the absence ofmeasured curves, the reaction curves derived fromMenard pressuremeter profile PM1 are used (thecalculations are noted ‘P’, that is ‘Pbe’ and ‘Paf’).

Pl

Pf

0

P

∆y

Es

Es/2

Fig. 19. Determination of the P–˜y curve from Menardpressure test results for long duration loading

0

50

100

150

200

250

300

350

0 2 4 6 8 10 12 14 16 18 20 22

3 m3·75 m4·5 mPAF 3 mPAF 4 mPM 3 mPM 4 m

PB

: mkP

a

y g– : cm

Fig. 20. Experimental reaction curves and reaction curvesderived from self-boring and Menard pressuremeter tests

P pB�

P 30

P 20

P 10

E t1s

E t2s

E t3s

P f y( )� ∆

∆y1 ∆y2 ∆y3 ∆y y z g z( ) ( )� �0

Fig. 21. Iterative process for a non-linear reaction curve usedby the PILATE program (Frank, 1981)



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Wed, 17 Mar 2010 04:49:52

The pile (embedded length D ¼ 11 m and diameterB ¼ 0.915 m) is assumed to have a linear elastic behaviour(flexural stiffness EI ¼ 1.07 3 106 kN m2 over the entirelength). For all the calculations the pile–soil system isdivided into 20 layers.

For the ‘M’ and ‘A’ calculations the P–˜y curves havebeen derived using the LPC methods described above. Forthe ‘P’ calculations, the P–˜y curves are obtained point bypoint, from the measured curves (Fig. 18). Nevertheless, thecondition P � Pmax for ˜y > (y�g)m, was imposed, (y�g)m

being the value were P reaches Pmax. In other words, afterthe (Pmax, ˜ym) point, no softening on the reaction curve istaken into account in the PILATE simulations.

Each set of calculations (‘A’, ‘M’ and ‘P’) comprises ninestages, corresponding to the dates before and after the fourpull-back operations and for the 29 July 1999, the last dayall the measurements were made.

The loading is applied through the boundary conditions atthe head (for z ¼ 0, at ground level), as well as through the‘free’ soil displacement g(z). The displacement g(z) takeninto account is the mean of the displacements measured byinclinometers G2 and G3 located at a distance of 3 m fromthe pile. For the nine selected dates, this displacement isgiven in Fig. 10. It has been approximated by a straight linein each ‘numerical’ layer. The head conditions (imposed atz ¼ 0) are the measured head displacement yo and bendingmoment Mo. The displacement yo is measured by inclin-ometer G1 which is fixed to the pile (Fig. 12 gives y at0.5 m above ground level) and the bending moment Mo isderived from the two vibrating wire strain gauges located atthe level of the natural ground (see Fig. 14). Both sets ofvalues are given in Table 2.

The toe of the pile is considered as free at all stages, thatis the conditions M (11 m) ¼ 0 and T (11 m) ¼ 0 have beenimposed.

Results of the calculations by means of PILATEFor each set of reaction curves (‘A’, ‘M’ and ‘P’), and for

each of the nine stages, PILATE yields the following resultsalong the pile

(a) displacements y(b) rotations y9(c) bending moments M(d ) shear loads T(e) distributed reactions P ¼ pB

The results obtained for displacements y and bending mo-ments M are given respectively in Fig. 22 and Fig. 23, forthe four pull-back operations, as well as for 29 July 1999.They are compared to the measured distributions of y and Mon the same dates.

The ‘P’ simulations allow a check to be made, to a certainextent, of the consistency of the whole ‘mathematical’process. All the available data that were measured along thepile are fed back into the PILATE simulations and, ideally,the calculated results should match perfectly the measuredresults. Of course, this is not the case. Nevertheless, it canbe seen that the match is overall quite satisfactory. Reasonsfor discrepancy are numerous and are of different nature.

The key point is probably the measured P–˜y curves, whichneed the assessment of the pressures on the pile (obtainedby double differentiation of the bending moments, them-selves obtained from the strain gauge readings), as well asof the displacements of the pile y and of the ‘free’ soilmovement g(z). Furthermore, no realistic curves could bederived from the measurements below 6 m depth, in thesubstratum, because the differences in the displacementsbecame very small, and beyond the ability to distinguishthem sensibly. It is interesting to note that, indeed, the matchbetween the measured results and the calculated results for‘P’ is quite acceptable in the upper part of the pile (wherethe soil is sliding), but becomes more questionable in thelower part (in the substratum).

The results from calculations with set ‘A’ and with set ‘B’are not very different from each other. Both the measureddisplacements and bending moments are clearly overesti-mated. It can be argued that the displacements are small andnever exceed 3 cm to 4 cm, along the whole pile, whateverthe stage under consideration; as to the bending moments,the calculated values lie well on the safe side. In any case,the main conclusion of this comparison is that the P–˜ycurves determined from the Menard or the self-boringpressuremeter LPC methods are altogether much too stiff.Indeed, one of the main findings of the present study is thatlower values of stiffness should be used for this type ofapplication. It is also felt, however, that more experimentaldata are required, before a precise method can be recom-mended for practical design.

CONCLUSIONSA unique contribution to pile engineering has been de-

scribed in this paper. Apparently, for the first time, readingson an instrumented pile have been taken successfully over aperiod of 16 years. A consistent picture of pile and slopebehaviour is obtained throughout this long period of time.The experimental findings will help the pile engineers tounderstand better the probable behaviour of a pile foundationsubjected to long duration thrusts. The p–y approach, usingpressuremeter data in a soil–structure interaction analysis(with the PILATE program or a similar one), proves to beapplicable for the design of such piles. Nevertheless, it isshown here that the present Menard pressuremeter and self-boring pressuremeter methods yield pile displacements andbending moments that are clearly overestimated. Thus,further work needs to be done to improve and calibrate themethods for predicting the p–y curves themselves.

NOTATIONa1, a2, a3, a4 integration constants

B pile external diameterBo ¼ 0.6 m; reference diameter in Menard settlement

formulaCaCo3ð%Þ content in calcium carbonate (%)

c9 effective cohesion interceptE

jsi value of Es in ‘numerical’ layer i at iteration j

EM pressuremeter modulus obtained with Menardpressuremeter

Table 2. Values of displacement yo and bending moment Mo imposed at the head (z 0)

Date 4/11/86(be)

5/11/86(af)

10/11/88(be)

11/11/88(af)

30/9/92(be)

1/10/92(af)

5/7/95(be)

6/7/95(af)

29/7/99(be)

yo: mm 16.2 2.4 16.5 0.9 35.2 2.0 29.8 1.8 21.7Mo: kN m 94 �209 �154 �262 �138 �313 �182 �295 �184



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Es modulus of reaction P/y or P/˜yEs(˜y) modulus of reaction as a function of ˜y

EI flexural stiffness of the pileGmean¼ g(0) mean of soil displacement at ground level measured

by inclinometers G2 and G3Go initial tangent shear modulus obtained with the self-

boring pressuremeterg soil lateral displacement

g(0) value of g at ground levelg(z) soil lateral displacement at depth zgi(z) value of g(z) in ‘numerical’ layer i

IP plasticity indexlo ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi4EI=Es

4p

elastic length (relative pile/soilstiffness)

M bending moment in the pileM(z) bending moment in the pile at depth z

P lateral soil reaction (or load density) on the pileP

jol value of Po in ‘numerical’ layer i at iteration j

Po(˜y) initial lateral soil reaction as a function of ˜yPf ¼ pfBPl ¼ plBPu ¼ puB

P(z) lateral soil reaction on the pile at depth zP(˜y) lateral soil reaction as a function of ˜y

p ¼ P/B, lateral soil reaction pressure on the pile orpressure measured with the self-boringpressuremeter

pf creep pressure obtained with Menard pressuremeterpl limit pressure obtained with Menard pressuremeter

pu ultimate lateral soil reaction pressure on the pilepo initial pressure obtained with the self-boring

pressuremeterp20 pressure at ˜V/V ¼ 20% obtained with the self-

boring pressuremeterp(z) ¼ P(z)/B, lateral soil reaction pressure on the pile at

depth z˜V/V volumetric deformation of the self-boring

pressuremeter probew water contentwL liquid limity pile lateral displacementy9 pile rotation˜y ¼ y � g, difference between displacements

y and g˜y(z) ¼ y(z) � g(z), difference between displacements y

and g at depth zy(o) value of y at ground levely(z) pile lateral displacement at depth zy9(z) pile rotation at depth z

z depth, positive downwards; z ¼ 0 at ground levelzc critical depthÆ rheological coefficient in Menard settlement

foundationª unit weight of soilº reduction factor applied to P near ground surface� normal stress in the steel of the pile�e elastic limit of the steel of the pilej9 effective angle of shearing resistance

4 and 5/11/86

0123456789

1011

�0·01 0·00 0·01 0·02 0·03Displacement : my Displacement : my

Dep

th: mz

Dep

th: mz

Dep

th: mz

Abe

Aaf

Mbe

Maf

Pbe

Paf

Measured before

Measured after

10 and 11/11/88

0123456789

1011

�0·01 0·00 0·01 0·02 0·03

Abe

Aaf

Mbe

Mal

Pbe

Paf

Measured before

Measured after

30/09 and 1/10/92

0123456789

1011

�0·01 0·00 0·01 0·02 0·03 0·04

Abe

Aaf

Mbe

Maf

Pbe

Paf

Measured before

Measured after

5 and 6/05/95

0123456789

1011

�0·01 0·00 0·01 0·02 0·03 0·04

Abe

Aaf

Mbe

Maf

Pbe

Paf

Measured before

Measured after

29/07/99

0123456789

1011

�0·01 0·00 0·01 0·02 0·03 0·04

Abe

Mbe

Pbe

Measuredbefore

Dep

th: mz

Dep

th: mz

Displacement : myDisplacement : my

Displacement : my

Fig. 22. Comparison of measured and calculated pile displacements just before and after each pulling-back operation and on 29 July1999



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REFERENCESBaguelin, F., Frank, R. & Guegan, Y. (1976). Calcul sur ordinateur

des pieux sollicites horizontalement ou subissant des pousseesparasites. (Computer calculation of horizontally loaded piles orsubmitted to lateral thrusts.) Bull. Liaison Laboratoires desPonts et Chaussees No. 84, 113–120.

Baguelin, F., Jezequel, J. F. & Shields D. H. (1978). The pressure-meter and foundation engineering. Clausthal, Germany: TransTechnical Publications.

Blondeau, F., Morin, P. & Pouget, P. (1983). Comportement d’unremblai construit jusqu’a la rupture sur un versant naturel. Siteexperimental de Salledes (Puy-de-Dome). (Behaviour of anembankment built up to failure on a natural slope. Experimentalsite of Salledes (Puy-de-Dome).) Rapport de recherche LPCN8126, Paris: Laboratoire Central des Ponts et Chaussees, p. 46.

Bourges, F., Frank, R. & Mieussens, C. (1980). Calcul des efforts etdes deplacements engendres par des poussees laterales de solsur les pieux. (Calculation of loads and displacements owing tolateral soil thrusts on piles.) Note technique du Departement desSols et Fondations, Paris: Laboratoire Central des Ponts etChaussees, p. 17.

Cartier, G. & Pouget, P. (1988). Etude du comportement d’unremblai construit sur un versant instable. Le remblai de Salledes(Puy de Dome). (Study of the behaviour of an embankment builton an instable slope. The embankment of Salledes (Puy deDome).) Rapport de recherche LPC N8153, Paris: LaboratoireCentral des Ponts et Chaussees.

Degny, E. (1985). SLIVALIC5. Programme de lissage par splinequintique. Calcul du parametre d’ajustement par une methodede validation croisee (Notice d’utilisation). (Software forsmoothening by quintic spline functions. Calculation of anadjustment parameter by a cross validation method (users’manual).) Internal report, F.A.E.R. 1.05.10.4, Paris: LaboratoireCentral des Ponts et Chaussees.

Frank, R. (1981) Design of piles subjected to lateral pressures insoft soils. Proceedings of the Colloquy of Jablonna, pp. 112–151. Gdansk: Institute of Hydroengineering.

Marche, R. (1973). Lateral pressure of clayed soils on structures.Contribution to specialty session 5. Proc. 8th Int. Conf. SoilMechs and Found. Engng, Moscow 4.3, 247–252.

Ministere de l’Equipement (1993). Regles Techniques de Conceptionet de Calcul des Fondations des Ouvrages de Genie Civil(Technical Rules for the Design of Foundations of Civil Engi-neering Structures). CCTG, Fascicule 62—Titre V; Paris: MELT.

Pouget, P. & Livet, M. (1994). Relations entre la pluviometrie, lapiezometrie et les deplacements d’un versant instable (siteexperimental de Salledes, Puy-de-Dome). (Relationships betweenpluviometry, piezometry, and the displacements of an unstableslope. (Experimental site of Salledes, Puy-de-Dome).) Etudes etrecherches des Laboratoires des Ponts et Chaussees, SerieGeotechnique, GT 57, Paris: Laboratoire Central des Ponts etChaussees.

Poulos, H. (1973). Analysis of piles undergoing lateral movement.J. Soil Mech. Found. Div., ASCE 99, No. 5, 391–406.

Dep

th: m

Dep

th: m

Moment: kNm

4 and 5/11/86

0123456789

101112

�3000�2500

�2000�1500

�1000�500

0500

10001500

20002500

Moment: kNm Moment: kNm

Dep

th: m

Dep

th: m

Dep

th: m

Measured after

Aaf

Maf

Paf

Measured before

Abe

Mbe

Pbe

10 and 11/11/88

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�3500-3000

�2500-2000

�1500-1000

�5000

5001000

15002000

2500

Measured after

Aaf

Mafl

Paf

Measured before

Abe

Mbe

Pbe

30/09 and 1/10/92

0123456789

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�3500�3000

�2500�2000

�1500�1000

�5000

5001000

15002000

25003000

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Aaf

Maf

Paf

Measured before

Abe

Mbe

Pbe

5 and 6/7/95

0123456789

101112

�3500�3000

�2500�2000

�1500�1000

�5000

5001000

15002000

25003000

Measured after

Aaf

Maf

Paf

Abe

Mbe

Pbe

Measured before

29/07/99

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101112

�3000 �2000 �1000 0 1000 2000 3000

Measured

Abe

Mbe

Pbe

Moment: kNm

Fig. 23. Comparison of measured and calculated bending moments just before and after each pulling-back operation and on 29 July1999


Documents

Geot58-645-Experimental Pile Subjected to a Moving Slope