PRACTICE AND RESEARCH

ON

MICROPILE GROUPS AND NETWORKS

Prof. François SCHLOSSER

ENPC - CERMES

2nd LIZZI lecture Tokyo IWM August 2004

PRACTICE AND RESEARCH ON MICROPILE

GROUPS AND NETWORKS

1) Development in micropile construction

2) Examples of micropile groups under vertical loading

3) Behaviour and design of A-shape micropiles under horizontal loading

4) Micropiles in liquefiable soils

RECENT DEVELOPMENT

IN

MICROPILE CONSTRUCTION

1) Types of micropiles tested in the Forever project

2) Driven and grouted micropile

3) Self -drilling injected micropile

French Bored Micropile Classification

GROUTING WITH THE “TUBE A MANCHETTE”

( Type IV : Repeated and Selective Injection)

Main type of micropile tested at the FOREVER experimental site:

Fontainebleau sand (Dr = 0.5)

Boring, Grouting by gravity (Type II )

Iia : complementary grouting from the top

iib : complementary grouting from the bottom

Main parameter : and qs (side friction stress)

R-SOL MICROPILE

Soil

Micropile type

pl (MPa)

qs (kPa) measured

qs (kPa) Bustamante&Doix

chart Fontainebleau loose sand (Forever)

II h II b IV (R-Sol) IV(Ischebeck)

0.4 0.4 0.4 0.4

52 52 72 72

45 – 50 45 – 50 90 – 100 90 – 100

Loose clayey

sand (Saint Etienne)

IV

1.3

375

180

Sand and gravel Weathered clay (Rueil)

II II

3.8 1.8

225 135

375 200

Comparative Results

EXAMPLES OF MICROPILE GROUPS UNDER VERTICAL LOADING

1) Reinforced foundation of the Uljin nuclear power plant

2) Micropiled raft withstanding water uplift pressures at A86 urban highway in Rueil (near Paris)

3) Foundation reinforcement of the old Pierre bridge in Bordeaux

ULJIN NUCLEAR POWER PLANT

Micropile reinforced foundation in the fault zone

DESIGN METHODOLOGY:

1) Equivalent homogeneous material assumed for the plug of reinforced faulted rock .

2) Elastic isotropic FEM calculations for determining the required modulus Ev.

3) Homogeneization of a group of micropiles in 1 D deformation using the results of load tests on isolated micropiles.

DESIGN OF THE PILE GROUP

HOMOGENEIZATION METHODS

1-D method : (Blondeau et al., 1987)

2-D and 3-D methods : (de Buhan et al., 1995-7)

RUEIL MICROPILED RAFT

Traction load test

Reinforcement : tube Ø = 89 mm e = 9.5 mm

Borehole : Ø = 125 mmSoil : alluvium + chalkTotal length : L = 19 mFree length : Lf = 4 m

GOUPEG METHOD

( Hybrid model taking into account the micropile interaction )

z

c

r

i

j

Z

X

Y

Pile I Pile J

Piz

Piy

w0 proper settlement w induced settlement

u or v induced displacement u0 or v0 proper displacement

Pix

p

y

Facteur p

Facteur y

0

101

Jj

JjJjJj u

uuuCoef

1) Load transfer functions model (GOUPIL - LCPC) : p-y, t-z, q-z ( Frank, 1983 – Degny and Romagny, 1987)

2) Mindlin’s equations for evaluating the micropile interaction (O’Neill, 1977)

Settlement

(mm)

Load (kN)

Comparison between measured and calculated load - settlement curves of the micropiles of the group

FOUNDATION REINFORCEMENT OF THE PIERRE BRIDGE IN BORDEAUX

_______

Movement of the water level

( sea tide, river flow)

_______

Wooden piles :

B = 0,30 m s/B = 4

Micropiles : B = 0,22 m s/B = 10

MICROPILES :

• Bored micropiles• Reinforced tube (178/154 mm)• Type IV (injection with “tube à manchettes”) in the marl• Type II ( global injection at low pressure ) in the masonry• Measured load transfer 5 to 20 %

STABILIZATION OF THE SETTLEMENTS AFTER MICROPILES INSTALLATION

BEHAVIOUR AND DESIGN OF A-SHAPED MICROPILES UNDER HORIZONTAL LOADING

1) Results of the Forever full scale load tests

2) St Maurice anti-noise wall

2) Slope stabilization at the site of the Millau viaduct

FOREVER RESULTS

THE THREE A-SHAPED NETWORKS

HORIZONTAL LOADING

Bearing capacity of the networks

largely exceeding

bearing capacity of the group

Horizontal loading test of an A-shaped micropiles network

ST MAURICE ANTI-NOISE WALL

1,75 m

,50 m

,25 m

,40

m

,40

m

3,50

m

2,84 m 4,00 m

4,00

m

34,21 NGF

35,10 NGF

20 °

13,5 °

0,65 m0,60 m

LATERAL LOADING TEST

0

50

100

150

200

250

300

350

400

450

0 5 10 15 20 25

déplacement de la tê te (mm)

micropieu vertical

micropieu inc liné

Vertical

Inclined( traction)

MICROPILE LOAD – DISPLACEMENT CURVES

Displacement at the top (mm)

Axi

al f

orce

at

the

top(

kN

)

-0,0005

0

0,0005

0,001

0,0015

0,002

0,0025

0,003

0 30 60 90 120 150

micronivelle (avant)

micronivelle (arrière)

inclinomètre (micropieu vertical)

inclinomètre (micropieu incliné)

comparateurs (face avant)

0

0,01

0,02

0,03

0,04

0,05

0 30 60 90 120 150ro

tati

on

(ra

d)

micronivelle (avant)

micronivelle (arrière)

inclinomètre (micropieu vertical)

inclinomètre (micropieu incliné)

comparateurs (face avant)

ROTATION ( R ) VERSUS APPLIED LOAD ( F )

F (kN)

R(rad)

R : rotation of the footing

F : (lateral) load applied to the wall

Applied load ( kN)

Measurement

Goupeg 1

Goupeg 2

Rotation (rad)

COMPARISON BETWEEN MEASUREMENTS AND GOUPEG CALCULATIONS

SLOPE STABILIZATION AT THE MILLAU VIADUCT SITE

LANDSLIDE ON THE SOUTH WORKS TRACK (2001)

A – SHAPED MICROPILES

• Type 3 (global injection)

• Tube : Ø = 157/178 mm

• Inclination : = 20°

• Borehole diameter :B = 0.30 m

• Spacing : s = 1.70 m

• Soil : alluvium, altered marl, marl

Global safety factor against sliding :

F = 1.13 with q = 46 kPa

F0 = 1.00 with q = 0

SOIL – MICROPILES INTERLOCKING EFFECT

Present design methods do not take it into account ( Ce 1 )

Energy equation :

K.H2 = 4k + G.V

F = K . M = k . ( f) = G . ( f)

f < f Plasticity of the soil without flow, then failure at = f

f = f Failure

f > f Plasticity of the micropiles without flow, then failure at = f

MICROPILES IN LIQUEFIABLE SOIL

1) Behaviour of single micropiles and micropile groups in liquefiable soil (FEM modelling)

2) Behaviour of A-shaped micopiles in liquefiable soil (centrifuge modeling)

F.E.M. MODELLING

p = 0,3

E = 24 000 MPap

= 2 500 kg/m 3p

Micropieu

10 m

5 m

z

16 m

Sol = 1 900 kg/m 3

sat

k = 0,1 cm/s

ü g

Surface libre (p = 0)

q = 0)f

Base rigide et imperméable (u = 0, z

Déplacements et pression interstitielle équivalents

16 m

4 m

15 m

Shahrour and Ousta. (1997,1998)

Shahrour, Sadek, Ousta. (2001)

0 0,5 1 1,5

0

0,2

0,4

0,6

0,8

1

t=1 s t=2 st=4 st=5 s

z/L

p

'v0

z L

(b)

0 0,5 1 1,5

0

0,2

0,4

0,6

0,8

1

z/L

p

'v0

z

SOIL LIQUEFACTION WITH AND WITHOUT MICROPILE

0 10 20 30 40

0

0,2

0,4

0,6

0,8

1

Mmax (kN.m)

zL

P1

P2

P3

2*2

Isolé

3*3

Bending moment

üg=0,1g, ksol=0,1 cm/s, f=2 Hz

0 10 20 30 40

0

0,2

0,4

0,6

0,8

1

Mmax (kN.m)

zL

P1

P2

P3

2*2

Isolé

3*3

Bending moment

0 10 20 30 40

0

0,2

0,4

0,6

0,8

1

Mmax (kN.m)

zL

P1

P2

P3

2*2

Isolé

3*3

0 10 20 30 40

0

0,2

0,4

0,6

0,8

1

Mmax (kN.m)

zL

P1

P2

P3

2*2

Isolé

3*3

Bending moment

üg=0,1g, ksol=0,1 cm/s, f=2 Hz

3D

3D

1 2

3

3D

3D

3D

3D

3D

3D

1 2

3

3D

3D

1 2

3

3D

3D

MICROPILE GROUPS IN LIQUEFIABLE SOIL (FEM)

CENTRIFUGE MODELLING ON A-SHAPED MICROPILE GROUPS

Juran I. ( New York Polytechnic University)

20 m

2.5 m8 m

2 m

50 g

Accelerometer (Acc) Piezometer (PW) LVDT (Lateral Displacement)

LV 4 LV 5

LV 3

LV 2

LV 1

PW2

PW1

Acc 3

Acc 4

Acc 5

Acc 6

Slightly Cemented Sand

1.5 m

2.5 m

Acc in

PW4

PW5

Acc 9

Acc 7

20 m

2.5 m8 m

2 m

50 g

Accelerometer (Acc) Piezometer (PW) LVDT (Lateral Displacement)

LV 4 LV 5

LV 3

LV 2

LV 1

PW2

PW1

Acc 3

Acc 4

Acc 5

Acc 6

Slightly Cemented Sand

1.5 m

2.5 m

Acc in

PW4

PW5

Acc 9

Acc 7

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0 5 10 15 20 25Time, sec

Ac

ce

lera

tio

n,

g

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0 5 10 15 20 25Time, sec

Ac

ce

lera

tio

n,

g

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0 5 10 15 20 25Time, sec

Ac

ce

lera

tio

n,

g

amax = 0,4 g at the base

MAX. ACCELERATION IN THE VICINITY OF THEA-SHAPED MICROPILES

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

0 5 10 15 20 25

Time, sec

Ex

ce

ss

Po

re P

res

su

re,

kP

a

PW- 2, free field 2x2 Networkdepth = 4.0 m

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

0 5 10 15 20 25

Time, sec

Ex

ce

ss

Po

re P

res

su

re,

kP

a

PW- 2, free field 2x2 Networkdepth = 4.0 m

EXCESS PORE PRESSURE IN THE VICINITY OF THE A-SHAPED MICROPILES

ru = u / σ’v0

Max. value = 0,6

compared to 1 in

free field.

CONCLUSIONS

1) The main parameter in micropile vertical bearing capacity is the side friction stress qs. Grouting with the “tube à manchettes” gives the best results.

2) Present design methods of micropile groups are conservative, leading to Ce 1. They neglect the soil/micropiles interlocking.

3) A-shaped micropiles (elementary networks) quite well withstand lateral loading. In slope stabilization, interlocking has a beneficial effect on the global safety.

4) In seismic events A-shaped micropiles are well withstanding horizontal movements and thus prevent liquefaction of saturated loose sands.

THANK YOU

FOR

YOUR ATTENTION !

Micropile and nail launcher

using compressed air

( Myles and Bridle, 1991 )

Metallic bar

Energy at the tip

Relatively large penetration

Apparently good lateral friction, but further research needed

RAILWAY EMBANKMENT STABILIZED BY MICROPILES :

Additional settlement due to the installation of the micropiles

1,75 m

,50 m

,25 m

,40

m

,40

m

3,50

m

2,84 m 4,00 m

4,00

m

34,21 NGF

35,10 NGF

20 °

13,5 °

0,65 m0,60 m

LATERAL LOADING TEST

-2

0

2

4

6

8

10

12

14

16

0 100 200 300 400

effort (kN) pr

ofon

deur

sous

la se

mel

le (m

)

AXIAL LOAD ON VERTICAL PILEForce (kN)

Depth

z (m)

( compression )

-2

0

2

4

6

8

10

12

14

16

0 100 200 300 400

AXIAL LOAD ON INCLINED MICROPILE

Depth

z (m)

Force (kN) ( traction )