1. 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

2. 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

3. A measure for the improvement of bearing capacity by micropiles
The ratio R was calculated from three types of loading tests:
calling “Index of Network Effect”

4. Improvement observed in laboratory loading tests

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

6. Prestress (tension) in MPs
To overcome the disadvantage of negative dilatancy, the idea of prestress occurred.

7. Laboratory Loading Tests
Model footing for applying prestress

8. Test results for medium dense sand ground
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.

9. Test results for medium dense sand ground
Bearing capacity was improved
even under small displacement
and on non-dilative soil ground
Ratio R was greater than 1.3

10. 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.

11. Plan view of test site

12. 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

13. Application of prestress with jacks

14. Loading in vertical direction
(FT-MP/FT-PSMP test)

15. Loading in horizontal direction
(sectional view)

16. Setup for horizontal loading

17. Loading test on FT and MP
Pmax = 210 kN, Smax = mm
Pmax = 30 kN, Smax = mm

18. Vertical loading test on FT-MP and FT-PSMP
Pmax = 560 kN
Smax = 243 mm
Py = 240 kN
Pult = 560 kN
Pmax = 600 kN
Smax = 227 mm
Py = 240 kN
Pult = 600 kN

19. 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).

20. Horizontal loading test on FT-MP and FT-PSMP
Pmax = 30 kN
max = mm
Pmax = 30 kN
max = mm

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

22. 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.

23. 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

24. Thank you for your attention!
End of the presentation