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Practical Advice for Foundation Design – Micropiles for Structural Support - page 5 / 25

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GSP 131 Contemporary Issues in Foundation Engineering

Step 4: Initial Design Considerations

4.1 Section of Spacing. Whether used for small unit loads (e.g., through existing foundations) or higher unit loads (e.g., for a new footing), the minimum spacing should be at least 3 micropile diameters. This criterion was originally developed for driven piles, and it allows for potential deviations in drilling over significant depths and eliminates negative group effects between adjacent micropiles.

4.2 Selection of Length. A minimum bond length may be prescribed in the Contract Documents. However, as for ground anchors, the actual bond stresses mobilized are

sensitive to the construction technique, determined by the Contractor. The satisfactory load testing results prior to

inter al., and so the actual bond bond length must, however, production.

length is often be subject to

If lateral loads are present, analyses will be necessary to establish the required depth to provide fixity. Total length will also be controlled by the depths necessary to resist downdrag and uplift forces, and to provide additional lateral resistance if scour is a consideration.

4.3 Selection of Micropile Cross Section. For preliminary sizing, the micropile cross section will be selected based on a rough estimate of the required structural section capable of resisting the design loads. Also, the use of common casing sizes is preferred to avoid delays associated with material availability. Currently, the most common casing sizes in the U.S. are 141 mm (5½ in) and 178 mm (7 in) with a nominal yield stress of 552 MPa (80 ksi) with the 178-mm (7-inch) casing being the most common. These sizes refer to the outside diameter of the casing. Table 3 provides a summary of material properties for common micropile reinforcement. Other sizes are also available.

4.4 Selection of Micropile Type. A description of the various micropile types (Type A, B, C, and D) is provided in other publications (e.g., Bruce et al., 1997). The selection of the micropile type will usually be left to the discretion of the contractor. As part of the request for bid, however, the Owner must insist that the Contractor provide information on their proposed methods of drilling and grouting. Based on previous project experience, the Owner may wish to disallow certain drilling techniques based on project-specific constraints. For example, the need to limit surface ground movements for a project involving cohesionless ground may preclude the use of certain drilling techniques known to increase the potential for soil caving.

The Owner should provide specific performance criteria (e.g., permissible movements of structures) as part of the bid package so that the Contractor can select an appropriate drilling and grouting procedure, as well as pile stiffness, to satisfy the overall project goals.

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