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Practical Advice for Foundation Design – Micropiles for Structural Support - page 2 / 25

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GSP 131 Contemporary Issues in Foundation Engineering

ADSC (International Association of Foundation Drilling), and DFI (The Deep Foundations Institute), and the work products of the IWM (International Workshop on Micropiles) meetings in Seattle (1997), Ube (1998), Turku (2000), Lille (2001), Venice (2002), and Seattle again (2003). This brief presentation of a practical design

approach is drawn from course to be taught to

a new manual being prepared as a basis for the various Departmentsof Transportation

a micropile short

nationwide.

It

follows the multi-step approach shown in Table 1, and high capacity pile shown schematically in Figure 1.

relates

to

the

typical

composite

Table 1. Design steps for micropiles used for structural foundations.

Step 1. Step 2. Step 3.

Evaluate Feasibility of Micropiles Review Available Information and Geotechnical Data Develop Applicable Loading Combinations

Step 4.

Initial Design Considerations

  • A.

    Micropile spacing

  • B.

    Micropile length

  • C.

    Micropile cross section

  • D.

    Micropile type (i.e., Type A, B, C, or D)

Step 5.

Step 6.

Step 7.

Final Design

  • A.

    Geotechnical strength limit states

    • 1)

      Bond length design

    • 2)

      End bearing

    • 3)

      Group effects for axially loaded micropiles

    • 4)

      Uplift resistance of single micropiles and micropile groups

    • 5)

      Downdrag

  • B.

    Structural strength limit states

    • 1)

      Axial compression of cased length

    • 2)

      Combined axial compression and bending of cased length

    • 3)

      Axial tension of cased length

    • 4)

      Axial compression and tension of uncased length

    • 5)

      Lateral resistance of single micropiles

    • 6)

      Lateral resistance of micropile groups

    • 7)

      Buckling of micropiles

  • C.

    Other structural considerations

    • 1)

      Grout to steel bond capacity

    • 2)

      Ultimate structural capacity

    • 3)

      Micropile to footing connection

  • D.

    Service limit states

    • 1)

      Micropile settlements

    • 2)

      Micropile group settlement

    • 3)

      Lateral deflection of micropile groups

  • E.

    Corrosion Protection

  • F.

    Seismic Considerations

Develop Load Testing Program and Construction Monitoring Requirements – not covered in this paper Finalize Contract Documents (i.e., Drawings and Specifications) – not covered in this paper

2

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