Field Loading Test
on Micropile Foundation
regarding the Effect of Prestress
IWM2003 in Seattle
Kinya Miura:
GeoMechanics Group,
Faculty of Engineering,
Toyohashi University of Technology
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
bearing capacity by micropiles
qv
FT-MP-Test
R=
Qv/As
qv
FT-Test
MP-Test
+
The ratio R was calculated from three types of loading tests:
calling “Index of Network Effect”
Improvement Ratio, R
Improvement observed in
laboratory loading tests
2.4
2.0
MP-FD-Test
S-R-Type, n=8, =0deg.,
L=100 mm
1.6
Dense
1.2
R=1
0.8
Medium
10%
5%
Relateve Displacement,
Loose
20%
Sv/D
Confining effect
by micropile group

Confining effect is enhanced by positive dilatancy
of ground material, but not by negative dilatancy.
Prestress (tension) in MPs

To overcome the disadvantage of negative
dilatancy, the idea of prestress occurred.
Laboratory Loading Tests
Model footing for applying prestress
Normalized Vertical Load,
qv (kPa)
Test results for medium dense sand ground
150
k = 0.0 Medium ground
0.57
0.41
0.36
0.17
100
Total Load carried by FT-PSMP
0
-50
Vertical Load on Micropiles
Qmp/Af (kPa)
-Qmp0/Qmpmax=0
50
0
2
4
6
8
10
150
100
-Qmp0/Qmpmax=0
50
0.57
0
-50
0
2
4
6
8
Load by micropile group
start with negative values
10
Base Pressure
qv - Qmp/Af (kPa)
150
0.57
100
0.41
0.36
0.17
50
0
-50
-Qmp0/Qmpmax=0
0
2
4
6
8
10
Vertical Displacement, Sv (mm)
Load by footing base
base pressure is enhanced by
the prestress.
Test results for medium dense
sand ground
150
k = 0.0 Medium ground
Sv/B = 5%
100
qf
Normalized Vertical Load, qv (kPa)
50
qv
Qmp/Af
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
150
Sv/B = 10%
100
qv
Bearing capacity was improved
even under small displacement
and on non-dilative soil ground
qf
50
Qmp/Af
0
150
0.0
0.1
0.2
0.3
0.4
0.5
Sv/B = 20%
0.6
qv
100
qf
50
Qmp/Af
0
0.0
0.1
0.2
0.3
0.4
0.5
Prestresss Ratio, -Qmp0/Qmpmax
0.6
Ratio R was greater than 1.3
Field Loading Tests
The boring log and the Nvalue 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)
The micropile was 3 m long,
100 mm in diameter, with a
steel core-rod of 32 mm in
diameter
(a) S-MP
(b) FT-MP/FT-PSMP
Application of prestress with jacks
Loading in vertical direction
(FT-MP/FT-PSMP test)
Loading in horizontal direction
(sectional view)
Setup for horizontal loading
Load, P (kN)
FT loading test
4
3
2
200
200
160
150
120
100
80
50
40
0 0
1
Elapsed time, t (hour)
0
0
Residual deformation
Elastic deformation

50
100
150
200
Pmax = 210 kN, Smax = 181.43 mm
250
Load, P (kN)
S-MP loading test
120
160
200
5
4
3
2
40
30
32
24
24
18
16
12
8
6
0 0
1
Elapsed time, t (hour)
6

Pmax = 30 kN, Smax = 28.69
mm
12
18
24
30
0
0
Settlement, S (mm)
80
Load, P (kN)
Settlement, S (mm)
40
Settlement, S (mm)
5
250
Settlement, S (mm)
Loading test on FT and MP
Residual deformation
Elastic deformation
8
16
24
32
40
Load, P (kN)
Vertical loading test
on FT-MP and FT-PSMP
Load, P (kN)
FT-MP loading test
240
360
180
240
120
120
60
0 0
2
0
0
Py = 240 kN
Pult = 560 kN
120
240
360
480
600
Load, P (kN)
Elapsed time, t (hour)
Settlement, S (mm)
60
120
180
FT-PSMP loading test
240
300
Pmax = 600 kN
Smax = 227 mm
Py = 240 kN
Pult = 600 kN
8
6
4
600
300
480
240
360
180
240
120
120
60
0 0
2
Elapsed time, t (hour)
0
0
Settlement, S (mm)
4
480
Smax = 243 mm
Residual deformation
Elastic deformation
Load, P (kN)
6
300
Residual deformation
Elastic deformation
120
240
360
480
600
Load, P (kN)
60
Settlement, S (mm)
8
600
Settlement, S (mm)
Pmax = 560 kN
120
180
240
300
Effect of prestress on bearing
capacity of FT with MP group
Load (kN)
0
100
200
300
400
500
600
700

0
50
Settlement, S (mm)

100
FT-PSMP
150

200
250
FT
300
FT+8MPs FT-MP
The bearing capacity of
FT reinforced with MPs is
remarkably increased.
The bearing capacity of
prestressed MP group (FTPSMP) is 7% higher than
FT-MP group.
Settlement of MP group is
reduced 16.4% due to
prestress (FT-PSMP).
Horizontal loading test
on FT-MP and FT-PSMP
Pmax = 30 kN
2
1
16
18
12
12
8
6
4
0 0
Elapsed time, t (hour)
6
0
0
6
max = 27.45 mm
12
18
24
30
Load, P (kN)
Horizontal
displ, S (mm)
12
18
FT-PSMP horizontal loading test
24
30
5
4
3
2
1
Elapsed time, t (hour)
Pmax = 30 kN
max = 13.84 mm
30
20
24
16
18
12
12
8
6
4
00
6
12
18
24
30
0
0
Horizontal
displacement, S (mm)
3
24
Residual deformation
Elastic deformation
Load, P (kN)
4
20
Horizontal
displacement, S (mm)
5
30
Horizontal
displ, S (mm)
Load, P (kN)
FT-MP horizontal loading test
6
Residual deformation
Elastic deformation
12
18
24
30
Load, P (kN)
Effect of prestress on horizontal
movements
Horizontal load (kN)
0
5
10
15
20
25
30

0
Horizontal displacement (mm)
5

10
15
FT-PSMP
20

25
FT-MP
30
The effect of prestress on
horizontal movement
control was significant.
The lateral movement was
increased rather linearly
with load in the nonprestress group (FT-MP).
y = 13.84 mm in the
prestressed group (FTPSMP).
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 nonprestressed 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