Field Loading Test on Micropile Foundation

regarding the Effect of Prestress

Kinya Miura:GeoMechanics Group,Faculty of Engineering,

Toyohashi University of Technology

IWM2003 in Seattle

We conducted two series of loading tests for the effect of prestress.

Model loading tests on the footings reinforced with Prestressed Micropiles

got an idea of prestressing micropiles

Field loading tests on the footings reinforced with Prestressed Micropiles

made sure of the idea

Details of the tests are in the two conference papers distributed

A measure for the improvement of A measure for the improvement of bearing capacity by micropilesbearing capacity by micropiles

+

R =

qv

qv Qv/As

FT-MP-Test

FT-Test MP-Test

The ratio R was calculated from three types of loading tests:calling “Index of Network Effect”

Improvement observed in Improvement observed in laboratory loading testslaboratory loading tests

0.8

1.2

1.6

2.0

2.4

Relateve Displacement, Sv/D5%

L=100 mm

MP-FD-TestS-R-Type, n=8, =0deg.,

Impr

ovem

ent R

atio

, R

R = 1Loose

Medium

Dense

20%10%

Confining effectConfining effect by micropile group by micropile group

Confining effect is enhanced by positive dilatancy of ground material, but not by negative dilatancy.

Prestress (tension) in MPsPrestress (tension) in MPs

To overcome the disadvantage of negative dilatancy, the idea of prestress occurred.

Laboratory Loading Tests

Model footing for applying prestress

Test results for medium dense sand ground

0 2 4 6 8 10-50

0

50

100

1500.57

0.410.360.17

-Qmp0

/Qmpmax

=0

k = 0.0 Medium ground

Nor

mal

ized

Ver

tical

Loa

d,

q v (kP

a)

0 2 4 6 8 10-50

0

50

100

150

0.57

-Qmp0

/Qmpmax

=0

Ver

tical

Loa

d on

Mic

ropi

les

Q

mp/A

f (kP

a)

0 2 4 6 8 10-50

0

50

100

150

0.360.17

0.41

0.57

-Qmp0

/Qmpmax

=0Bas

e Pr

essu

req v -

Qm

p/Af (

kPa)

Vertical Displacement, Sv (mm)

Total Load carried by FT-PSMP

Load by micropile group start with negative values

Load by footing base base pressure is enhanced by the prestress.

Test results for medium dense sand ground

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0

50

100

150

qf

Qmp

/Af

qv

Nor

mal

ized

Ver

tical

Loa

d, q

v (kP

a)

k = 0.0 Medium groundS

v/B = 5%

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0

50

100

150

qf

Qmp

/Af

qv

Sv/B = 10%

0.0 0.1 0.2 0.3 0.4 0.5 0.60

50

100

150

qf

Qmp

/Af

qv

Prestresss Ratio, -Qmp0

/Qmpmax

Sv/B = 20%

Bearing capacity was improved even under small displacementand on non-dilative soil ground

Ratio R was greater than 1.3

Field Loading Tests

The boring log and the N-value of the upper 10 meters is presented

The subsoils were fill, loam, cemented clay, sandy clay, and fine sand, respectively.

The fill, loam and clay were soft and rather uniform mechanically; the N-values are less than 5.

Plan view of test site

Illustration of Micropile (MP)

(a) S-MP (b) FT-MP/FT-PSMP

The micropile was 3 m long, 100 mm in diameter, with a steel core-rod of 32 mm in diameter

Application of prestress with jacks

Loading in vertical direction

(FT-MP/FT-PSMP test)

Loading in horizontal direction

(sectional view)

Setup for horizontal loading

Loading test on FT and MP

0 50 100 150 200 2500

40

80

120

160

200 Residual deformation Elastic deformation

Settl

emen

t, S

(m

m)

Load, P (kN)

FT loading test

5 4 3 2 1 00

50

100

150

200

250

Loa

d, P

(kN

)

Elapsed time, t (hour)

Settl

emen

t, S

(m

m)

200

160

120

80

40

0

Pmax = 30 kN, Smax = 28.69 mm

0 8 16 24 32 400

6

12

18

24

30 Residual deformation Elastic deformation

Settl

emen

t, S

(m

m)

Load, P (kN)

S-MP loading test

5 4 3 2 1 00

8

16

24

32

40

Loa

d, P

(kN

)

Elapsed time, t (hour)

Settl

emen

t, S

(m

m)

30

24

18

12

6

0

Pmax = 210 kN, Smax = 181.43 mm

Vertical loading test on FT-MP and FT-PSMP

0 120 240 360 480 6000

60

120

180

240

300 Residual deformation Elastic deformation

Settl

emen

t, S

(m

m)

Load, P (kN)

FT-MP loading test

8 6 4 2 00

120

240

360

480

600L

oad,

P (

kN)

Elapsed time, t (hour)

Settl

emen

t, S

(m

m)

300

240

180

120

60

0

Pmax = 560 kN

Smax = 243 mm

Py = 240 kN

Pult = 560 kN

0 120 240 360 480 6000

60

120

180

240

300 Residual deformation Elastic deformation

Settl

emen

t, S

(m

m)

Load, P (kN)

FT-PSMP loading test

8 6 4 2 00

120

240

360

480

600

Loa

d, P

(kN

)

Elapsed time, t (hour)

Settl

emen

t, S

(m

m)

300

240

180

120

60

0

Pmax = 600 kN

Smax = 227 mm

Py = 240 kN

Pult = 600 kN

Effect of prestress on bearing capacity of FT with MP group

The bearing capacity of FT reinforced with MPs is remarkably increased.

The bearing capacity of prestressed MP group (FT-PSMP) is 7% higher than FT-MP group.

Settlement of MP group is reduced 16.4% due to prestress (FT-PSMP).

300

250

200

150

100

50

00 100 200 300 400 500 600 700

FT FT+8MPs FT-MP

FT-PSMP

Settl

emen

t, S

(mm

)

Load (kN)

Horizontal loading test on FT-MP and FT-PSMP

0 6 12 18 24 300

4

8

12

16

20 Residual deformation Elastic deformation

Hor

izon

tal

disp

l, S

(mm

)

Load, P (kN)

FT-MP horizontal loading test

5 4 3 2 1 00

6

12

18

24

30

Loa

d, P

(kN

)H

oriz

onta

ldi

spl,

S (m

m)

Elapsed time, t (hour)

30

24

18

12

6

0

Pmax = 30 kN

max = 27.45 mm

0 6 12 18 24 300

4

8

12

16

20 Residual deformation Elastic deformation

Hor

izon

tal

disp

lace

men

t, S

(m

m)

Load, P (kN)

FT-PSMP horizontal loading test

5 4 3 2 1 00

6

12

18

24

30

Loa

d, P

(kN

)H

oriz

onta

ldi

spla

cem

ent,

S (

mm

)

Elapsed time, t (hour)

30

24

18

12

6

0

Pmax = 30 kN

max = 13.84 mm

Effect of prestress on horizontal movements

The effect of prestress on horizontal movement control was significant.

The lateral movement was increased rather linearly with load in the non-prestress group (FT-MP).

y = 13.84 mm in the prestressed group (FT-PSMP).

30

25

20

15

10

5

00 5 10 15 20 25 30

FT-MP

FT-PSMP

Hor

izon

tal d

ispl

acem

ent (

mm

)

Horizontal load (kN)

Conclusions (Laboratory Tests) The bearing capacity of footing is improved by the

interaction between footing, subsoil and micropile group, and induce confining effect on the subsoil.

The prestress is effective to enhance the confining effect, even at the beginning of loading and on non-cohesive soil ground, such as loose sand and soft clay.

Conclusions (Field Tests) The effect of prestress on the improvement of

bearing capacity is significant: vertical bearing capacity was increased by 7% in

prestressed MP group (FT-PSMP) than in the non-prestressed MP group (FT-MP).

Coefficient of subgrade reaction was remarkably improved. (more than twice in both vertical and horizontal directions)

1.86×104 kN/m3 (FT-MP), 3.97×104 kN/m3 (FT-PSMP) in Vertical

1.01×103 kN/m3 (FT-MP), 1.71×104kN/m3 (FT-PSMP) in Horizontal

Thank you for your attention!

End of the presentation