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Practical Advice for Foundation Design – Micropiles for Structural Support - page 13 / 25

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GSP 131 Contemporary Issues in Foundation Engineering

inclined micropiles should not be used where the potential for ground settlement around the inclined micropile (e.g., downdrag) is a possibility.

5.2.7 Buckling of Micropiles.

Micropile capacity is frequently dictated by the

structural micropile

strength of the element, rather grout and surrounding soils.

than by the geotechnical bond between the Therefore, it is reasonable to believe that,

where

very

soft

soils

or

voids

overly

the

bearing

strata,

buckling

may

potentially

control the load-carrying capacity of buckling of steel piles driven to rock,

a micropile. To address concerns regarding Bjerrum (1957) published results of buckling

tests load

and related them to the methods available at the time. He presented results of tests performed on piles with a variety of sections, including bars, rails, and H-

sections.

He concluded that even very soft soils

could

provide

enough lateral

restraint

to prevent buckling of most pile sections.

The issue of buckling of micropiles has been the subject of attention of several researchers, including Mascardi (1970, 1982) and Gouvenot (1975). Their results seem to support Bjerrum’s conclusion that buckling is likely to occur only in soils with very poor mechanical properties such as peat and soft clay. Experiments carried out by CalTrans (Brittsan and Speer, 1993) on high capacity micropiles installed through a very thick (33 m) deposit of San Francisco Bay Mud, and case histories of rock-socketed micropiles in karst (Cadden et al., 2001, Gómez et al., 2004) have further shown that micropiles can be successfully applied in a variety of “difficult” subsurface environments.

It cannot be inferred, however, that buckling in micropiles will never occur. Buckling of piles is a complex soil-pile interaction problem that involves the pile section and elastic properties, soil strength and stiffness, and the eccentricity of the applied load.

Equation 7 can be used to estimate the critical load, Pcr, of a pile (Bjerrum, 1957):

P c r =

π 2 EI

l2

+

Esl 2 π2

(Equation 7)

where:

E

=

I

=

L

=

Es

=

modulus of elasticity of the pile material [force/area] minimum moment of inertia of the pile [length4] “unsupported” length of the pile [length] modulus of lateral reaction of the soil [force/area],

i.e.,

slope

of

p-y

diagram (not to be confused with modulus of subgrade reaction).

The term “unsupported” refers to the portion of the pile that is only subject to the lateral restraint provided by the soil. The first term of Equation 7 corresponds to Euler’s equation for buckling in columns. The second term reflects the contribution of the lateral restraint provided by the soil. Theoretically, buckling should only be a

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