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Practical Advice for Foundation Design – Micropiles for Structural Support - page 16 / 25





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GSP 131 Contemporary Issues in Foundation Engineering

to fill an annulus space, as well as several example calculations and case histories, can be found in the complete White Paper developed by the ADSC Micropile Committee and available through the ADSC Technical Library (Cadden and Gómez, 2002).

    • 5.3

      Other Structural Design Considerations.

      • 5.3.1

        Grout to Steel Bond Capacity. The bond between the cement grout and the

reinforcing steel bar allows the composite action of the micropile, and is the mechanism for transfer of the pile load from the reinforcing steel to the ground. Typical ultimate bond values range from 1.0 to 1.75 MPa for smooth bars and pipe, and 2.0 to 3.5 MPa for deformed bars (ACI 318).

In the majority of cases, grout-to-steel bond does not govern the pile design. The structural or geotechnical pile capacity typically governs.

As is the case with any reinforcement, the surface condition can affect the attainable bond. A film of rust may be beneficial, but the presence of loose debris or lubricant or paint is not desirable. Normal methods for the handling and storage of reinforcing bars apply to micropile construction. For the permanent casing that is also used to drill the hole, cleaning of the casing surface can occur during drilling, particularly in granular soils. Adequate cleaning of casing to be re-used in cohesive soils is essential to avoid reduction bond capacity.

5.3.2 Ultimate Structural Capacity. Micropile load testing may be carried to loads up to and, in some cases, greater than two times the design load. For load testing, maximum test loads should not exceed 80 percent of the ultimate structural capacity of the micropile, which is given by:


[ g r o u t c ' f = 8 5 . 0

× Agrout

  • +

    f yca sin g

× Aca sin g

  • +

    f ybar

× Abar


(Equation 9)


=[f yca sin g

× Aca sin g

  • +

    f ybar

× Abar


(Equation 10)

For some designs, the verification test pile(s) may require larger pile casing and reinforcing bar than the production micropiles. The resulting stiffer micropile can adequately confirm the grout-ground bond strength for production micropiles, but

will likely not provide representative structural displacement behavior. deflections measured for proof-tested micropiles at 100 percent of the design will need to be relied upon to provide best estimate of structural deflections.

The load

5.3.2 Micropile to Footing Connection. Unless a single micropile is used, a pile cap

(footing) is necessary all the micropiles in accordance with the ACI 318.

to spread the structure loads and any overturning

the group. Reinforced concrete pile AASHTO Standard Specifications for

caps are Highway

moments designed Bridges

to in or


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