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Practical Advice for Foundation Design – Micropiles for Structural Support - page 22 / 25





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GSP 131 Contemporary Issues in Foundation Engineering

  • 2.

    Corrosion protection must extend 15 feet below corrosive material

  • 3.

    Reinforcing steel corrosion protection must extend a minimum of 5 feet into casing

  • 4.

    Minimum 1 inch in soil and 0.5 inch in rock. If protective coatings (epoxy, galvanization, or


encapsulation) are provided in compression, minimum cover may be 0.25 inches in soil or rock. Not recommended for permanent or cyclic tension loads.


Seismic Considerations.

5.6.1 General.

An increasingly common application of micropiles is in seismic

retrofit. Current practice for seismic design of bridge foundations is provided Division 1-A of AASHTO (2002) and these generally apply to micropiles.


Results from centrifuge and finite element studies reported as part of the FOREVER (2003) project indicate that vertical micropiles in groups do little to resist horizontal seismic deformations due to their relatively low stiffness. Under seismic loading, however, inclined micropiles were shown to have a reduced “underground” bending moment as compared to vertical micropiles, but significantly higher negative bending moment at the pile head. In current U.S. practice, the concern over excessive bending moments being mobilized during the design seismic event has resulted in some concerns on the use of inclined piles for seismic retrofit projects. As reported in Chapter 5 of the FOREVER (2003) project, the Eurocode (Eurocode EC 8, 1994) indicates that inclined piles should not be used for transmitting lateral loads to the soil, but if such piles are used, they must be safely designed to carry axial and bending loads. Proponents for the use of micropiles argue that by inclining the piles and thus creating a very stiff system, it is likely that damage that may result from an earthquake will occur within the pile cap, which can be easily fixed. This issue remains unresolved at this time in the U.S. and additional research, like that being performed under FOREVER (2003), is required.

5.6.2 Load Sharing with Existing Foundations. Applications in which micropiles are used to retrofit an existing deep foundation requires that load sharing between existing foundation elements and the micropiles be evaluated. For example, in order to design additional piles to retrofit existing pile foundations damaged by the Mw 6.9 Hyogo-ken Nanbu earthquake in Japan, the following concepts were applied according to the damage of the existing piles (Japan Public Works Research Institute (JPWRI), 2002):

  • Where existing piles were damaged by the earthquake, compression loads are shared by existing piles and the micropiles, but tension, lateral, and moment loads are carried by the new micropiles.

  • Where existing piles were only slightly damaged, all design loads were shared by the existing piles and the new micropiles.

For the retrofit design of an old or deteriorated foundation or for a foundation in which the original structural design details and specifications are uncertain, design seismic loads should be assumed to be carried by the micropiles only. Load sharing between existing and new foundations should only be considered if the Owner has sufficient information on the existing foundation system (i.e., as-built drawings, load


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