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Practical Advice for Foundation Design – Micropiles for Structural Support - page 11 / 25

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GSP 131 Contemporary Issues in Foundation Engineering

Ptallowable

= 0.55 Fybar

× Abar

(Equation 6)

For the uncased portion of the pile, the reinforcing bar yield stress used in the calculations in compression must not exceed 600 MPa.

5.2.5 Lateral Resistance of Single Micropiles. The response of a single micropile to lateral loading at or near the ground surface is commonly evaluated using a “p-y analysis”. This will

  • Determine the necessary penetration of the micropile to carry the loads at the micropile head without undergoing lateral deflection at the ground line which would result in excessive lateral movement of the supported structure;

  • Determine the required micropile diameter, pipe casing and reinforcing steel sizes and strength properties, and grout strength to resist the design bending moment, shear force, and axial load that will be supported by the micropile; and

  • Determine the deformation and rotation of the micropile in order to model the effects of foundation deformation on the performance of the structure.

Procedures for constructing p-y curves for various soil and water table conditions as well as static or cyclic loading conditions are provided in the COM 624P program documentation (Wang and Reese, 1993) and in the documentation for the program LPILE (Reese et al., 2000).

It is specifically noted that the development of a pile model, subsurface profile, and other input parameters for a p-y analysis is equivalent to that for a driven pile or drilled shaft. The reader is referred to FHWA-HI-97-013 and FHWA-IF-99-025 for specific modeling details related to lateral loading analysis.

Furthermore, it is common to find bars and casings installed in coupled sections, each perhaps as short as 1 m. Performance of the coupled steel in compression is not an issue of concern. Tensile and bending stresses, however, have a greater impact on the integrity of the casing at the joint location primarily because of the reduced thickness of the casing over the length of the threaded area. Currently, no specific testing standard exists for evaluating the tension or bending capacity of a threaded casing joint appropriate for micropile applications. If significant tension and/or bending forces are being considered for a micropile design, the Owner should require the contractor to provide data demonstrating the adequacy of the proposed joint detail. Since a common testing method does not exist, these data will need to be reviewed by a qualified engineer. As projects involving vertical micropiles subject to lateral forces become more commonplace, a means to evaluate allowable tensile and bending stresses for threaded joints will become necessary, especially since many casing providers in the U.S. have a slightly different proprietary threading detail.

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