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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
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