1. AN INTRODUCTION TO MICROPILES CIVL 141 Spring 2011

2. A Local Situation:

3. A Shallow Footing Definition: D f /B < 2

4. Capacity of Spread Foundations (cohesionless soils)

5. THIS ONLY CONSIDERS THE SHEAR STRENGTH OF THE SOIL; WHAT ABOUT SETTLEMENTS?

6. Settlement-Based Capacity Use a bearing pressure that results in meeting some allowable settlement. – Less than 1 inch of total settlement; or – Less than ½ inch of differential settlement.

7. Settlement-Based Capacity Use a bearing pressure that results in meeting some allowable settlement. – Less than 1 inch of total settlement; or – Less than ½ inch of differential settlement. FOR COHESIONLESS SOILS (SANDS), THIS CRITERIA GENERALLY CONTROLS. THE LOST CAPACITY FROM SHEAR STRENGTH CAN BE SIGNIFICANT.

8. FOR MANY PROJECTS, THERE IS MOTIVATION TO INCREASE THE BEARING LOAD WITHOUT ADDITIONAL SETTLEMENTS OR SIMPLY REDUCE THE EXPECTED SETTLEMENTS ALTOGETHER.

9. HOWEVER, A CHALLENGE EXISTS TO DO SO WHILE MAINTAINING CONSTRUCTION BUDGETS

10. Lower the Bearing Pressure

11. Lower Bearing Pressures Do Not Always Provide Less Settlement q allow = 2,000 psf

12. Lower Bearing Pressures Do Not Always Provide Less Settlement q allow = 4,000 psf

13. More-Often, This is the Observed Design Situation

14. Currently Available Alternatives For Reducing Settlement Densify Bearing Soils – Over-Excavate and Recompact – Rapid Impact Compaction Deep Foundations (drilled shafts or piles) Intermediate Solutions – Rammed Aggregate Piers – Helical Piers – Micropiles

15. Over-Ex and Recompact

18. Micropiles

19. Micropiles are high capacity, small diameter (typically 5" to 12") drilled and grouted in- place piles designed with steel reinforcement to primarily resist structural loading.

20. Micropiles Micropiles are high capacity, small diameter (typically 5" to 12") drilled and grouted in- place piles designed with steel reinforcement to primarily resist structural loading. Micropiles do not necessarily use hollow-core bars but this is what I will present today.

21. Micropile Types

22. Hollow-Core Bar (F y = 75 ksi)

23. Pressure-Grouted Micropiles This discussion will be limited to hollow-core, pressure-grouted micropiles because they can be readily adapted to shallow foundations. Too, flow of grout is ensured over the complete length of the micropile.

24. Structural Limitations Ref: FHWA Manual, Section 5 (Through full-scale tests, elastic buckling does not appear to be a design concern and is not considered by FHWA. (note: IBC uses 0.33 and 0.40 instead of 0.40 and 0.47, respectively) Compression:

25. Structural Limitations Ref: FHWA Manual, Section 5 (note: IBC uses 0.6 instead of 0.55) Tension:

26. Geotechnical Limitations

27. Capacity of a Micropile

28.  grout-soil can be 3 to 10 kips per square foot, found from a test; F typically taken as 2.5; bit diameter is conservative; and L = bond length of micropile.

29. Capacity of a Micropile q tip ; taken as 32 ksf (Reese et al. 1976) F typically taken as 3; R f = 0.6B

30. Example q tip = 32 ksf for medium dense sands; Bit diameter = 0.5 feet; L = 10 feet F is 3 for end bearing and 2.5 for side friction; and R f = 0.6(0.5 feet);  grout-soil = 7 ksf

31. Example P ult = 51 kips

32. Estimating  grout-soil

34. Structural Limitations During Testing

35. Grout-Soil Shear Strength from a Vertical Tension Test 10-ft long bonded zone

36. Grout-Soil Shear Strength from a Vertical Tension Test

37. Word of Caution (or a Strong Reason to Use Hollow-Core Bars)

40. Example situation

41. Assumes no support from soil

42. Example situation Shear in concrete – acceptable?

43. Example situation Negative moment

44. Connection to Spread Footing South Mountain Community College, Phoenix, AZ Double-reinforced footing (grade beam)

45. Connection to Spread Footing

46. Proof loading is typically completed on about 5% of the production micropiles. Loading is completed to 167% of the Design Test Load. Creep test may or may not be completed. Proof Loading

47. Mt. Lemmon Highway Milepost 9.8 Tucson, Arizona Dec. 2009

48. Proof Loading Mt. Lemmon Highway Milepost 9.8 Tucson, Arizona Dec. 2009

49. Proof Loading Mt. Lemmon Highway Milepost 9.8 Tucson, Arizona Dec. 2009