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Practical Advice for Foundation Design – Micropiles for Structural Support - page 9 / 25





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GSP 131 Contemporary Issues in Foundation Engineering

FSg as used in private work is typically 1/0.33, based on historical building codes. FHWA currently allows 1/0.4. FSsteel is taken typically as 0.4 by private bulding codes, and up to 0.47 by FHWA documents.

Consideration of the unsupported length of a micropile can be addressed through the inclusion of the effective length factor, K, and the unsupported micropile length, l. If the micropile is continuously supported along its entire length (i.e., the unsupported length is zero), then Fa in Equation 1 is equal to Fy-steel/FS.

Alternatively, as an example, a design condition for a micropile supported footing may be to assume a certain thickness of near surface soil is removed as a result of scour. This depth of scour would be equivalent to the micropile unsupported length. For micropiles with a nonzero unsupported length, the following equations are used to evaluate Fa for use in Equation 1:

if 0<

Kl t r

= , a c F C

Fy steel FS

× 1

2 t 2Cc r 2 K l

(Equation 2)


Kl t r

, a c F C = >

π 2 Esteel [ ] 2 t r K l F S

(Equation 3)

where Cc =

s t e e l E 2 2 π Fysteel

K L rt Fy-steel = the minimum steel compatibility). = effective length factor (assumed equal to 1.0); = unsupported length of the micropile; = radius of gyration of the steel section only = (I/A)1/2; and yield stress (see subsequent




Strain compatibility between structural components must be checked. For the steel and grout interface, AASHTO (2002) limits the maximum usable concrete compression strain to 0.003. The maximum usable strength of the steel (i.e., 600 MPa) is based on a typical allowable concrete strain of 0.003 (i.e., 200,000 MPa (the Young’s modulus of steel) x 0.003 = 600 MPa). It should be noted, however, that in the cased section, the additional confinement of the casing provides higher grout strength. This may impact the bar strength used in a design. For pipe, this is typically less of a problem due to the lower yield strength of the casing (typically a maximum of 552 MPa.


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