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Research at Northwestern University: End-bearing Micropiles in Dolomite

Research at Northwestern University: End-bearing Micropiles in Dolomite. Outline. Introduction Test section details Axial load test results Axial load distributions Design implications Conclusions. Participants : TCDI-Hayward Baker, Lincolnshire, IL Vulcan Quarry, McCook, IL

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Research at Northwestern University: End-bearing Micropiles in Dolomite

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

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

  8. Vulcan Quarry, McCook, IL.

  9. On site preparation of micropiles

  10. Micropile 1 Micropile 3 Micropile 2

  11. Top - 1 Bottom Top - 2 Bottom Bottom Top - 3 Top -4 Bottom 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. 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 1560 800 1320 >2225 >4450 4 1560 800 1320 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.

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