1. Research at Northwestern University: End-bearing Micropiles in Dolomite

2. Outline Introduction Test section details Axial load test results
Axial load distributions
Design implications
Conclusions

3. Participants: TCDI-Hayward Baker, Lincolnshire, IL
Vulcan Quarry, McCook, IL
Northwestern University

4. Objective
To evaluate the axial load transfer characteristics of micropiles embedded in dolomite so that rational design procedures can be developed

5. Overview: Axial load tests in Vulcan Quarry
Four test piles with lengths of 0.6, 1.2, 1.8 and 2.4 m
Piles consist of 178-mm-diameter, 13 mm wall thickness, 550 MPa steel casings filled with 38 MPa grout. Roller bit is welded to bottom.
Axial load distribution determined by vibrating wire strain gages on steel, embedment gages in grout and telltale readings
Two piles were extracted to examine grout-steel and grout-rock interfaces

6. Installation procedures
Production piles ~ 30 m long
Assembled pile with roller bit attached used to drill hole
Left in place and grouted under high pressure
Test piles ~ 1 m
Hole cored
Pile assembled and placed in hole
Pile grouted under low pressure

7. Allowable stress design:
Pallow = α  f 'c  Agrout + β  fy x A steel
Method
fy max (MPa) α β
Allowable Load (kN)
AASHTO (Service load design)
550
0.4
0.47
2000
Chicago Building Code
200
0.4(1)
800
Massachusetts Building Code
410
.33(3)
0.4(2)
1400

8. Vulcan Quarry, McCook, IL.

9. On site preparation of micropiles

10. Micropile 1
Micropile 3
Micropile 2

11. Top - 2
Bottom
Top - 3
Top -4
Top - 1
Rock Conditions

12. Load test frame Reaction anchor transfer beams transfer girder
hydraulic jack
test pile

13. Axial load test results

14. Axial Load vs. Deflection of Micropile 1

15. Axial Load vs. Deflection for Micropile 2

16. Axial Load vs. Deflection for Micropile 3

17. Axial Load vs. Deflection for Micropile 4

18. Summary of load test results
Pile 1 failed at 2000 KN and 4000 KN on second loading, cumulative tip movement = 10 mm (RQD = 22)
Pile 2 failed 800 KN on first loading and 2000 KN on second loading, cumulative tip movement =25 mm (RQD = 0)
Pile 3 did not fail at 4450 KN, tip movement = 2 mm (RQD = 87)
Pile 4 with soft bottom exhibited a plunging failure at 2000 KN

19. Axial load distributions
Determining moduli for composite pile – Fellenius (1989) method
Data

20. Strain Gage Data from Micropile 3

21. Axial Load Distributions for Micropiles 1 and 3

22. Axial Load Distribution of Micropile 2

23. Axial Load Distribution for Micropile 4

24. Mobilized Side Resistance vs Axial Head Deflection

25. Summary of load transfer data
No load transfer in upper 1 m – due to low confinement and poor rock quality
Critical interface was steel/grout; verified from visual observations of extracted piles
Shorter piles (1 and 2) were end-bearing; capacity a function of RQD
Pile 4 with soft bottom had an average unit side resistance approximately equal to that of a smooth bar pulled from concrete (3500 kPa)

26. Allowable structural load (kN)
Computed
Observed
No.
Allowable structural load (kN)
Davisson allowable load with FS = 2 (kN)
Allowable load for 13 mm movement (kN)
(1)
(2)
(3) 1
1630
880
1380
2000
3800 2
1560
800
1320
400
1200 3
>2225
>4450 4
1000
not applicable
(1) – AASHTO
(2) – Chicago Building Code
(3) – Massachusetts Building Code

27. Example: Production pile
Typical length in Chicago: 25 to 30 m
When pile tip moves 2 mm under 4450 KN (like pile 3), design for movements
For 27.5 m long pile:
12.5 mm deformation – 1350 KN capacity
25 mm deformation – 2600 KN capacity
Both greater than 800 KN based on Chicago code

28. Conclusions
Stresses in piles were in excess of those specified in codes without detrimental effects on performance
Steel-grout interface governed axial load transfer behavior along side
No side resistance mobilized in top 1 m of test piles due to low stresses and grout pressures and poor quality rock
Due to relatively high compressibility, allowable axial loads of full-scale piles, founded on competent rock, are determined more rationally from allowable deformation considerations, rather than code-specified allowable stresses.