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 Test piles ~ 1 m 1. Hole cored 2. Pile assembled and placed in hole 3. Pile grouted under low pressure Production piles ~ 30 m long 1. Assembled pile with roller bit attached used to drill hole 2. Left in place and grouted under high pressure

7. Allowable stress design: P allow = α  f ' c  A grout + β  f y x A steel Methodf y max (MPa) αβ Allowable Load (kN) AASHTO (Service load design)5500.40.472000 Chicago Building Code2000.40.4 (1) 800 Massachusetts Building Code410.33 (3) 0.4 (2) 1400

8. Vulcan Quarry, McCook, IL.

9. On site preparation of micropiles

10. Micropile 1 Micropile 2 Micropile 3

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

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

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. ComputedObserved No.Allowable structural load (kN) Davisson allowable load with FS = 2 (kN) Allowable load for 13 mm movement (kN) (1)(2)(3) 11630880138020003800 2156080013204001200 315608001320>2225>4450 4156080013201000not 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.