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Proceedings of the XVI ECSMGEGeotechnical Engineering for Infrastructure and DevelopmentISBN 978-0-7277-6067-8

© The authors and ICE Publishing: All rights reserved, 2015doi:10.1680/ecsmge.60678

by Misra and Chen (2004). The model seems to be suitable to interpret the mechanical field behaviour of the studied micropiles. In perspective, for the evalua-tion of both the bearing capacity and displacements of reinforced resin micropiles, more results from ex-perimental tests in soils of different typology are needed.

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Settl

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t mm

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Figure 3. Measured and predicted compression behaviour for three sample micropiles.

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Figure 4. Measured and predicted compression (a) and tension (b) behaviour of the micropiles tested in the site of Senigallia (AN).

REFERENCES

AGI – Italian Geotechnical Association (1984), Raccomandazioni sui pali di fondazione (in Italian). Bustamante, M. & Doix, B. 1985. Une méthode pour le calcul des tirants et des micropieux injectés, Bull. Liaison Lab. Ponts et Chaussées, Paris, 140, nov-dèc 1985 – Ref. 3047, 75-92. Buzzi, O. Fityus, S. Sasaki, Y. & Sloan, S. 2008. Structure and properties of expanding polyurethane foam in the context of foun-dation remediation in expansive soil, Mechanics of Materials 40, 1012-1021. Mandolini, A. 1995. Prove di carico su pali di fondazione, Hev-elius, Benevento (in Italian). Misra, A. & Chen, C.H. 2004. Analytical solution for micropile design under tension and compression, Geotechnical and Geologi-cal Engineering 22, 199-225. Valentino, R. & Stevanoni, D. 2010. Micropiles made of rein-forced polyurethane resins: load tests and evaluation of the bearing capacity, Electronic J. of Geot. Engineering 15, H, 895-912. Valentino, R. Romeo, E. & Misra, A. 2013. Mechanical aspects of micropiles made of reinforced polyurethane resins, Geotechnical and Geological Engineering 31, 2, 463-468.

Axial pile forces in piled embankments,field measurements

Forces axiales dans les piles dans les remblais empilées,mesures sur le terrain

S.J.M. van Eekelen*12, A. Bezuijen31 and A.F. van Tol21

1 Deltares, Delft, Netherlands2 Delft University of Technology, Delft, Netherlands

3 Ghent University, Ghent,* Corresponding Author

ABSTRACT Several measurements were carried out in a basal reinforced piled embankment in the Netherlands. The present paper focuseson the influence of truck passages on the axial forces in the piles. The changes in axial forces in the piles were measured using two systems:(1) optic fibres attached to a square steel tube pile, measuring pile strains at ten positions along the pile length and (2) the total pressure onthe pile cap with total pressure cells. Additionally, the axle loads of passing trucks and the load on the subsoil between the piles were meas-ured. The measured changes in pile strains show that most truck load is transported to the subsoil by friction along the pile shafts. Compari-son between measurements and calculations show that the truck wheel loads are spread stronger than assumed by Boussinesq.

RÉSUMÉ Plusieurs mesures ont été effectuées dans un remblai renforcé basal empilés dans les Pays-Bas. Le présent document met l'ac-cent sur l'influence des passages de camions sur les forces axiales dans les piles. Les changements dans les forces axiales dans les piles ontété mesurées à l'aide de deux systèmes: (1) des fibres optiques attachés à un pieu à tube d'acier carré, mesurant souches de pile à dix posi-tions le long de la longueur de la pile et (2) la pression totale sur le capot de pile avec un total cellules de pression. En outre, les charges paressieu des camions de passages, et la charge sur le sous-sol ont été mesurées. Les changements mesurés dans les souches de poils montrentque plus la charge du camion est transporté vers le sous-sol par frottement le long des arbres de poils. Comparaison entre les mesures etcalculs montrent que les charges de roues de camion doivent être répartis plus que prévu par Boussinesq.

1 INTRODUCTION

Several researchers have published about research onpiled embankments. Only a limited number of thempresented measurements of load distributions andpile responses during the passage of a train or a truck(Van Eekelen et al., 2010, Van Duijnen et al., 2010).This is for example of importance because the truckloads need to be converted into a uniformly distribut-ed load to be able to design the construction.

This paper reports axial load and strain measure-ments on a steel tube pile in a basal reinforced piledembankment. These measurements were carried outusing optic fibres at 10 different depth levels andwith total pressure cells on top of the pile caps. Theresponse of the piles during truck passages will bepresented in the present paper.

2 WOERDEN FIELD TEST

A motorway exit in Woerden, the Netherlands, wasreconstructed. Part of the new road was built on a ba-sal reinforced piled embankment. The road construc-tion started in April 2010, and the road was opened inJune 2010.

The subsoil consists of a ca. 17 m thick layer ofvery soft clay. The average undrained shear strengthcu along the pile shaft can be determined from theCPT in Figure 2: top-down until NAP -10 mcu average (qc-u)/15 = 13 kPa, until NAP -13 mcu 17 kPa, where u is the pore pressure in kPa.

Figure 1 shows the piled embankment at the moni-toring location. The system consisted of precast pileswith average 2.24x2.26 m2 centre-to-centre (CTC)-spacing and square 0.75x0.75 m2 prefcast concrete

Geotechnical Engineering for Infrastructure and Development

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pile caps with smoothly rounded edges. On top of thepile caps lies 0.05-0.16 m sand. The geosynthetic re-inforcement (GR) lies directly on the sand. The com-pacted fill consists of broken recycled constructionmaterial (0-40 mm), such as concrete and bricks.

A layer of PET geotextile 600/50 lies upon thesand, perpendicular to the road axis. A layer of PETgeogrid 600/50 lies on top of that along the road axis.On top of the fill lies 0.25 m asphalt granular materi-al mixture and 0.18 m asphalt. Van Eekelen et al.(2012, 2015a,b,c) describe the monitoring program.The present paper presents measured axial pile loads.

square pile cap 0.75x0.75 m2 circular pile cap 0.84 m

1:20

2.17

driving lane

690 691 692 693 694 695

hard shoulder

2.22 2.23 2.31 2.06

sand

fill1.80

692

693

686

687

688

asphalt gravel mixture

asphalt

691

682

geosynthetic (GR)

2.11

Figure 1. Monitoring area. Piles are precast piles except piles 693and 695, which are square steel tube piles.

3 MEASUREMENTS

3.1 Pile strains

Four long glass fibres, with each 10 strain sensors,were attached to each side of pile 693 to measure pilestrains. The fibres were protected by casting them in-to a resin. The purpose of the optic fibres was to beable to calculate the bending pile moments in the pilefrom the measured pile strains. Therefore, pile 693

was not a precast concrete pile, but a square steeltube pile, specified in Table 1. This was done to besure about the section modulus of the pile, which isnecessary to calculate the pile moments.

Table 1. Square steel tube pile 693, properties and optic fibres.

Width b mm 300

b

b

d

optic fibres

Thickness d mm 8Length l m 24Area Atube mm2 9275Young’smodulus

E N/mm2 2.10E+05

Weight G kg/m 72.8Momentof inertia

Ix=Iy mm4 1.31E+08

Sectionmodulus

Wx=Wy mm3 8.75E+05

Depth pile cap surface NAPa m -1.36Asphalt surface NAPa m +0.50Distance to neighbouring piles:

North m 2.31West m 2.21South m 2.23East m 2.28

a NAP: Dutch reference level

Due to problems with the optic fibre measure-ments, only the changes in pile strains due to truckpassages can be presented in the present paper.

-30

-25

-20

-15

-10

-5

0

0246810

dept

hN

AP

(m)

friction ratio (%)

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

-30

-25

-20

-15

-10

-5

0

0.000.250.500.75

cone resistance qc (MPa)

local friction (Mpa)

local friction (Mpa)cone resistance qc (MPa)

top pile caps in monitoring area

Figure 2. Typical CPT at the monitoring location

3.2 Axle loads of passing trucks

The weight of the axles of passing trucks was meas-ured using weigh-in-motion devices (WIM) withpressure sensors, installed in the asphalt at two loca-tions. Each WIM measured the truck velocity andconfiguration and the weight of each axle twice.

Table 2 gives the measured geometry and weightdistribution of the considered trucks. These meas-urements were carried out on June, 12th, 2012, whichis more than two years after opening the road. Theground water on that day was at 0.06 m below thepile cap surface of pile 693. It should be noted thatthe standard deviation of the four axle load meas-urements is ca. 25% of the measured load. This largestandard deviation is probably caused by a limitedaccuracy of the WIM devices.

Table 2 Considered trucks.

1339a 1339b 1354 1355 1405a 1405bvelocity km/h 58 56 49 47 45 64Lengthtruck

m 16.15 11.00 9.77 15.94 18.00 8.78

Standarddeviationlength

mm 17 -a 17 16 9 11

Numberof axles

- 6 3 2 4 5 3

Distanceto axle 1

axle 2 m 2.62 1.70 5.74 3.69 3.80 4.18axle 3 m 3..91 5.15 10.36 7.66 5.95axle 4 m 9.64 12.31 13.36axle 5 m 10.94 14.66axle 6 m 12.44

Totalweight

kN 283 360 189 307 768 298

Standarddeviationtotalweight

kN 67 -a 47 73 163 86

Weightaxle 1 kN 68 132 62 62 89 94axle 2 kN 56 124 127 69 163 120axle 3 kN 55 104 104 195 85axle 4 kN 35 72 176axle 5 kN 35 145axle 6 kN 35a total weight and configuration of truck 1339b has been pro-

vided by truck owner.

3.3 Load distribution

The total load on pile 693 (and pile 692, not present-ed in the present paper) was measured on the pilecap, below the geosynthetic reinforcement, as de-scribed by Van Eekelen et al. (2012, 2015a,b). Forthis purpose, the square pile cap was replaced by acircular one. This made it possible to apply a using atotal pressure cell (TPC) with the same diameter(0.85 m) as the pile cap. However, a smaller woodenplate of 0.75 m diameter had to be applied betweenthe TPC and the pile cap because of the relativelystiff edges of the large TPC.

Converting the pressure (in kPa) measured withinthe TPC into a force (in kN) can be done two ways:using the TPC diameter (maximum interpretation) orusing the diameter of the wooden plate (minimum in-terpretation). Figure 3 to Figure 7 give both results.Figure 9 gives the average of these two values.

The pressure underneath the GR at locations G23and G38 (Figure 1) was also measured with TPCs.

4 RESULTS MEASUREMENTS

Figure 3 to Figure 7 show the measured axle loads,the changes in pile strains and axial forces N.

An off-set (non-zero start value) was applied forthe pile strains. This makes it possible to see the dif-ferences between the different strain gauges.

The maximal axial force increments N shown inthe figures were calculated from the maximal meas-ured pile strain increments, using N = EAtube ,where the values of E and Atube are given in Table 1.

The measurements show that deeper strain gaugesmeasure smaller pile strain responses to the passingtrucks; the truck load is almost completely trans-ferred to the soft soil around the pile by means offriction along the pile shaft. Only a very limitedamount of the pile load reaches the toe of the piles.

These measurements should agree with the un-drained shear strength cu (kPa) of the subsoil, whichwas determined in section 2. Consider truck 1339b.The measured average strain along the pile shaft is 4microstrain. For a 20 m long pile this gives an elon-gation of -4 10-6 20 = -0.08 mm. We have seen thatthe friction behaviour along the pile shaft is dominantto the pile toe, and therefore it can be assumed that 1mm displacement of a pile activates 50% of the max-

1213

pile caps with smoothly rounded edges. On top of thepile caps lies 0.05-0.16 m sand. The geosynthetic re-inforcement (GR) lies directly on the sand. The com-pacted fill consists of broken recycled constructionmaterial (0-40 mm), such as concrete and bricks.

A layer of PET geotextile 600/50 lies upon thesand, perpendicular to the road axis. A layer of PETgeogrid 600/50 lies on top of that along the road axis.On top of the fill lies 0.25 m asphalt granular materi-al mixture and 0.18 m asphalt. Van Eekelen et al.(2012, 2015a,b,c) describe the monitoring program.The present paper presents measured axial pile loads.

square pile cap 0.75x0.75 m2 circular pile cap 0.84 m

1:20

2.17

driving lane

690 691 692 693 694 695

hard shoulder

2.22 2.23 2.31 2.06

sand

fill1.80

692

693

686

687

688

asphalt gravel mixture

asphalt

691

682

geosynthetic (GR)

2.11

Figure 1. Monitoring area. Piles are precast piles except piles 693and 695, which are square steel tube piles.

3 MEASUREMENTS

3.1 Pile strains

Four long glass fibres, with each 10 strain sensors,were attached to each side of pile 693 to measure pilestrains. The fibres were protected by casting them in-to a resin. The purpose of the optic fibres was to beable to calculate the bending pile moments in the pilefrom the measured pile strains. Therefore, pile 693

was not a precast concrete pile, but a square steeltube pile, specified in Table 1. This was done to besure about the section modulus of the pile, which isnecessary to calculate the pile moments.

Table 1. Square steel tube pile 693, properties and optic fibres.

Width b mm 300

b

b

d

optic fibres

Thickness d mm 8Length l m 24Area Atube mm2 9275Young’smodulus

E N/mm2 2.10E+05

Weight G kg/m 72.8Momentof inertia

Ix=Iy mm4 1.31E+08

Sectionmodulus

Wx=Wy mm3 8.75E+05

Depth pile cap surface NAPa m -1.36Asphalt surface NAPa m +0.50Distance to neighbouring piles:

North m 2.31West m 2.21South m 2.23East m 2.28

a NAP: Dutch reference level

Due to problems with the optic fibre measure-ments, only the changes in pile strains due to truckpassages can be presented in the present paper.

-30

-25

-20

-15

-10

-5

0

0246810

dept

hN

AP

(m)

friction ratio (%)

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

-30

-25

-20

-15

-10

-5

0

0.000.250.500.75

cone resistance qc (MPa)

local friction (Mpa)

local friction (Mpa)cone resistance qc (MPa)

top pile caps in monitoring area

Figure 2. Typical CPT at the monitoring location

3.2 Axle loads of passing trucks

The weight of the axles of passing trucks was meas-ured using weigh-in-motion devices (WIM) withpressure sensors, installed in the asphalt at two loca-tions. Each WIM measured the truck velocity andconfiguration and the weight of each axle twice.

Table 2 gives the measured geometry and weightdistribution of the considered trucks. These meas-urements were carried out on June, 12th, 2012, whichis more than two years after opening the road. Theground water on that day was at 0.06 m below thepile cap surface of pile 693. It should be noted thatthe standard deviation of the four axle load meas-urements is ca. 25% of the measured load. This largestandard deviation is probably caused by a limitedaccuracy of the WIM devices.

Table 2 Considered trucks.

1339a 1339b 1354 1355 1405a 1405bvelocity km/h 58 56 49 47 45 64Lengthtruck

m 16.15 11.00 9.77 15.94 18.00 8.78

Standarddeviationlength

mm 17 -a 17 16 9 11

Numberof axles

- 6 3 2 4 5 3

Distanceto axle 1

axle 2 m 2.62 1.70 5.74 3.69 3.80 4.18axle 3 m 3..91 5.15 10.36 7.66 5.95axle 4 m 9.64 12.31 13.36axle 5 m 10.94 14.66axle 6 m 12.44

Totalweight

kN 283 360 189 307 768 298

Standarddeviationtotalweight

kN 67 -a 47 73 163 86

Weightaxle 1 kN 68 132 62 62 89 94axle 2 kN 56 124 127 69 163 120axle 3 kN 55 104 104 195 85axle 4 kN 35 72 176axle 5 kN 35 145axle 6 kN 35a total weight and configuration of truck 1339b has been pro-

vided by truck owner.

3.3 Load distribution

The total load on pile 693 (and pile 692, not present-ed in the present paper) was measured on the pilecap, below the geosynthetic reinforcement, as de-scribed by Van Eekelen et al. (2012, 2015a,b). Forthis purpose, the square pile cap was replaced by acircular one. This made it possible to apply a using atotal pressure cell (TPC) with the same diameter(0.85 m) as the pile cap. However, a smaller woodenplate of 0.75 m diameter had to be applied betweenthe TPC and the pile cap because of the relativelystiff edges of the large TPC.

Converting the pressure (in kPa) measured withinthe TPC into a force (in kN) can be done two ways:using the TPC diameter (maximum interpretation) orusing the diameter of the wooden plate (minimum in-terpretation). Figure 3 to Figure 7 give both results.Figure 9 gives the average of these two values.

The pressure underneath the GR at locations G23and G38 (Figure 1) was also measured with TPCs.

4 RESULTS MEASUREMENTS

Figure 3 to Figure 7 show the measured axle loads,the changes in pile strains and axial forces N.

An off-set (non-zero start value) was applied forthe pile strains. This makes it possible to see the dif-ferences between the different strain gauges.

The maximal axial force increments N shown inthe figures were calculated from the maximal meas-ured pile strain increments, using N = EAtube ,where the values of E and Atube are given in Table 1.

The measurements show that deeper strain gaugesmeasure smaller pile strain responses to the passingtrucks; the truck load is almost completely trans-ferred to the soft soil around the pile by means offriction along the pile shaft. Only a very limitedamount of the pile load reaches the toe of the piles.

These measurements should agree with the un-drained shear strength cu (kPa) of the subsoil, whichwas determined in section 2. Consider truck 1339b.The measured average strain along the pile shaft is 4microstrain. For a 20 m long pile this gives an elon-gation of -4 10-6 20 = -0.08 mm. We have seen thatthe friction behaviour along the pile shaft is dominantto the pile toe, and therefore it can be assumed that 1mm displacement of a pile activates 50% of the max-

van Eekelen, Bezuijen and van Tol

Geotechnical Engineering for Infrastructure and Development

1214

imum pile shaft friction (NEN, 2011). Thus -0.08mm elongation approximately activates 4% of themaximum pile shaft friction. The maximum activatedshaft friction is equal to the measured maximum axi-al force: 12 kN (Figure 4) as the pile toe barely feelsthe truck passage. The maximum pile shaft friction

will therefore be: 12 kN/4% = 300 kN. The area ofthe pile shaft is 20 m 4 0.3 m = 24 m2. The undrainedshear strength cu lies thus around 300 kN / 24 m2 =12 kPa, which indeed is in the range of cu 13 kPathat was determined in section 2.

12 3 4

5

200

150

100

50

0 0 3 6 9 12

axle

load

s(kN

)

location of axles (m)

6-24-22-20-18-16-14-12-10

-8-6-4-20

0 2 4 6 8 10 12 14

dept

hre

lativ

eto

top

ofpi

le(m

)

change in axial force in pile (kN)

TPC A+B min interprTPC A+B max interprEA×DELTAepspile + pile cap

Figure 3. Truck 1339a: a. axle loads b. measured pile strains c. axial pile force at the maximum value of the highest strain gauge,(results of TPCs and measured pile strains).

1

location of axles (m)

200

150

100

50

0 0 3 6 9 12 15

axle

load

s(kN

)

23

-24-22-20-18-16-14-12-10-8-6-4-20

0 2 4 6 8 10 12 14

dept

hre

lativ

eto

top

ofpi

le(m

)

change in axial force in pile (kN)

TPC A+B mininterprTPC A+B maxinterprEA×DELTAepspile + pile cap

Figure 4. Truck 1339b: a. axle loads b. measured pile strains c. axial pile force at the maximum value of the highest strain gauge,(results of TPCs and measured pile strains).

100

50

0

axle

load

s(kN

)

location of axles (m)

21

0 3 6 9 12 15

3 4

location of axles (m)

150

100

50

00 3 6 9 12 15

1 2

axle

load

s(kN

)

truck 1354

truck 1355

-24-22-20-18-16-14-12-10

-8-6-4-20

0 2 4 6 8 10 12 14

dept

hre

lativ

eto

top

ofpi

le(m

)

change in axial force in pile (kN)

1354 TPC A+Bmin interpr1354 TPC A+Bmax interpr1354EA×DELTAeps1354EA×DELTAeps1355 TPC A+Bmin interpr1355 TPC A+Bmax interpr

Figure 5. Truck 1354 and 1355: a. axle loads b. measured pile strains c. axial pile force at the maximum value of the highest straingauge, (results of TPCs and measured pile strains).

1215

imum pile shaft friction (NEN, 2011). Thus -0.08mm elongation approximately activates 4% of themaximum pile shaft friction. The maximum activatedshaft friction is equal to the measured maximum axi-al force: 12 kN (Figure 4) as the pile toe barely feelsthe truck passage. The maximum pile shaft friction

will therefore be: 12 kN/4% = 300 kN. The area ofthe pile shaft is 20 m 4 0.3 m = 24 m2. The undrainedshear strength cu lies thus around 300 kN / 24 m2 =12 kPa, which indeed is in the range of cu 13 kPathat was determined in section 2.

12 3 4

5

200

150

100

50

0 0 3 6 9 12

axle

load

s(kN

)

location of axles (m)

6-24-22-20-18-16-14-12-10

-8-6-4-20

0 2 4 6 8 10 12 14

dept

hre

lativ

eto

top

ofpi

le(m

)

change in axial force in pile (kN)

TPC A+B min interprTPC A+B max interprEA×DELTAepspile + pile cap

Figure 3. Truck 1339a: a. axle loads b. measured pile strains c. axial pile force at the maximum value of the highest strain gauge,(results of TPCs and measured pile strains).

1

location of axles (m)

200

150

100

50

0 0 3 6 9 12 15

axle

load

s(kN

)

23

-24-22-20-18-16-14-12-10-8-6-4-20

0 2 4 6 8 10 12 14

dept

hre

lativ

eto

top

ofpi

le(m

)

change in axial force in pile (kN)

TPC A+B mininterprTPC A+B maxinterprEA×DELTAepspile + pile cap

Figure 4. Truck 1339b: a. axle loads b. measured pile strains c. axial pile force at the maximum value of the highest strain gauge,(results of TPCs and measured pile strains).

100

50

0

axle

load

s(kN

)

location of axles (m)

21

0 3 6 9 12 15

3 4

location of axles (m)

150

100

50

00 3 6 9 12 15

1 2

axle

load

s(kN

)

truck 1354

truck 1355

-24-22-20-18-16-14-12-10

-8-6-4-20

0 2 4 6 8 10 12 14

dept

hre

lativ

eto

top

ofpi

le(m

)

change in axial force in pile (kN)

1354 TPC A+Bmin interpr1354 TPC A+Bmax interpr1354EA×DELTAeps1354EA×DELTAeps1355 TPC A+Bmin interpr1355 TPC A+Bmax interpr

Figure 5. Truck 1354 and 1355: a. axle loads b. measured pile strains c. axial pile force at the maximum value of the highest straingauge, (results of TPCs and measured pile strains).

van Eekelen, Bezuijen and van Tol

Geotechnical Engineering for Infrastructure and Development

1216

better with the measurements than the load distribu-tion assumed by Boussinesq.

296 306 316 326 336 346 356

-14-12-10-8-6-4-202-1

01234567

19 20 21 22 23distance (m)

axia

lfor

cein

pile

693

(kN

,fro

mto

pop

ticfib

rest

rain

gaug

e)

tota

lloa

dA

+Bon

pile

cap

693,

kN

time (sec)

(A+B)

2.27 m

5.15 m

15.6 m

Axialforce N

Figure 8. Axial force determined from strain measured with opticfibre 0.53 m below pile cap and pile load A+B measured with TPC(minimum interpretation). On top: determination of spreading an-gle of axle load of truck 1339b load.

0

5

10

15

20

25

30

0 5 10 15calc

ulat

edlo

adpe

rpile

forp

ile69

3(k

N/p

ile)

measured load per pile for pile 693 including loadon subsoil (kN/pile)

spreading angle / optic fibresspreading angle / TPC(A+B)measured=calculatedBoussinesq / optic fibresBoussinesq / TPC(A+B)

diameter pilecap d = 0.85m, CTCdistance pilessx=sy=2.25 m,heightembankmentH=1.79 m,virtual height2.27 m, unitweight 18.3kN/m3

Figure 9. Calculated and measured change of total load on pile693 due to truck 1339b. TPC (A+B) is the average of the minimumand maximum interpretation.

6 CONCLUSIONS

Truck passages give changes in axial pile forces in abasal reinforced piled embankment. These changeswere measured using (1) optic fibres attached to thesquare steel tube piles, measuring pile strains at tenpositions along the pile shaft and (2) total pressurecells on the circular pile caps. Additionally, the axleloads, truck configurations and the load on the sub-soil were measured.

The measured pile strain changes show a decreasewith depth. The pile toe barely feels the truck. Thisshows that most truck load is transported to the sub-soil by friction along the pile shafts.

For design purposes it is necessary to convert apassing truck into a uniformly distributed load. Twocalculation methods are described and compared tothe measurements: a Boussinesq-based method andspreading each wheel load with a spreading angle of67o. This value followed from the truck velocity andtime that the truck is felt by the pile. The spreadingcalculations agree better with the measurements thanthe load distribution assumed by Boussinesq.

ACKNOWLEDGEMENT

The authors are grateful for the support in the moni-toring project of the Dutch research program GeoIm-puls, Province Utrecht, The Dutch Ministry of Publicworks, Huesker, KWS Infra, Movares and Deltares.The financial support from Deltares, Naue, TenCateand Huesker for the research on piled embankmentsis greatly appreciated.

REFERENCES

NEN 9997-1: 2011 NL, (2011). Geotechnical Design - part 1: ge-neral Rules. Nederlands Normalisatie Instituut, Delft, Netherlands.Van Duijnen, P.G., Van Eekelen, S.J.M., Van der Stoel, A.E.C.,2010. Monitoring of a railway piled embankment. Proceedings,9 ICG, Brazil, 1461-1464.Van Eekelen, S.J.M, Bezuijen, A. & Alexiew, D., 2010a. TheKyoto Road Piled Embankment: 31/2 Years of Measurements.Proceedings, 9 ICG, Brazil, 1941-1944.Van Eekelen, S.J.M., Jansen, H.L., Van Duijnen, P.G., De Kant,M., Van Dalen, J.H., Brugman, M.H.A., Van der Stoel, A.E.C. &Peters, M.G.J.M., 2010b. The Dutch design guideline for piledembankments. Proceedings, 9 ICG, Brazil, 1911-1916.Van Eekelen, S.J.M., Bezuijen, A., van Duijnen, P.G., 2012. Doesa piled embankment ‘feel’ the passage of a heavy truck? High fre-quency field measurements. Proceedings, EuroGeo 5. Valencia.Digital version: volume 5: 162-166.Van Eekelen, S.J.M., Bezuijen, A. & Van Tol, A.F., 2015a. TheWoerden piled embankment. Part I: long-term measurements. Tobe published in J. Geotech. Geoenviron. Eng.Van Eekelen, S.J.M., Bezuijen, A. & Van Tol, A.F., 2015b. TheWoerden piled embankment. Part II: truck passages and compari-son to analytical calculations. To be published in J. Geotech. Ge-oenviron. Eng.Van Eekelen, S.J.M., Bezuijen, A. & Van Tol, A.F., 2015c. Vali-dation of analytical models for the design of basal reinforced piledembankments. Geotext. Geomemb. 43, 56-81.

Load-settlement behaviour of three pile groups: a case study

Le comportement de trois groupes de pieux vissés : étude d’un cas réel

P.O. Van Impe*1,2, W.F. Van Impe1,2 , A. Manzotti2 and L. Seminck3 1 Ghent University, Ghent, Belgium

2 AGE Consultants bvba, Ghent, Belgium 3 GFS Industries NV, Belgium

* Corresponding Author ABSTRACT The paper presents the case study on the construction of three 48m diameter steel tanks, each founded on a group of 422 dis-placement cast in-situ piles. The three tanks are close enough to each other to induce interaction. The movements of the tank foundations have been monitored during the hydro-testing of the steel tanks, and during the subsequent working stage of the tanks. The bearing layer for the pile group is a 5 m thick stiff sand layer at a depth of about 20m, overlain by a very heterogeneous soft fill containing sand pockets, and underlain by a very thick slightly overconsolidated clay. The authors present some short and long term settlement prediction for the tanks, based on soil parameters derived from CPT on site, and compare this to the measured settlements. The initially derived soil parameters are re-evaluated in order to predict the long term settlement for the full life span of the construction.

RÉSUMÉ Cette contribution décrit le cas réel de trois groupes de pieux servant comme fondations de trois réservoirs de combustibles; chaque réservoir, en acier, se trouvant fondé sur 422 pieux vissés avec refoulement. Les trois réservoirs se trouvent en proximité l’un de l’autre et peuvent donc être considéré se comportant en interaction majeure. Les déformations en trois axes ont été mesurées pendant les épreuves hydraulique de chaque réservoir et en plus durant l’exploitation des réservoirs remplis de combustibles. La couche résistante des groupes de pieux a une épaisseur de 5m de sable très rigide, à une profondeur de 20m, recouverte par de multiples couches ou lentilles minces et très hétérogènes jusqu’à la surface du terrain naturel. Le sol en dessous de la couche portante peut être considéré comme une ar-gile légèrement sur-consolidée sur une épaisseur d’environ 100m. On présente ici la comparaison entre les résultats d’analyse de prédiction des tassements immédiats et à long terme, pour les trois réservoirs en interaction, partant de l’interprétation de multiples essais de pénétra-tion en profondeur d’un côté, et les mesures en fonction du temps de ces tassements des trois réservoirs de l’autre côté. Les estimations des paramètres de rigidité des diverses couches ont été ré-analysées utilisant cette banque de données des tassements mesurés afin de pouvoir prévoir avec plus de confidence les tassements à très long terme.

1 INTRODUCTION

The three tanks (each 33.000 m³) are steel structures of 48m in diameter and a height of 19m. Figure 1 shows the relative location of the tanks on the site in Ostend, Belgium.

The tanks are positioned in a triangular arrange-ment at an interdistance (centre-to-centre) of about 65m. (Tanks 1 and 3 are slightly further apart, see figure 1). They are founded on a 48.8m diameter, 60cm thick reinforced concrete slab, supported by 422 displacement screw piles.

The 460mm diameter displacement screw piles of the Omega type are placed at an interdistance of 2.2 m (centre-to-centre) and reach to a depth of 21.5m. They are designed to each take a maximum design load of 960 kN, including some 180 kN negative skin friction.

We refer to Van Impe et al. (2013), and the dis-cussion by Fellenius (2014), for more details on the design and related pile testing. Both papers also pre-sent some initial settlement predictions.