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Practical Advice for Foundation Design – Micropiles for Structural Support - page 8 / 25

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GSP 131 Contemporary Issues in Foundation Engineering

exploited in North American practice, since this is a very complex issue which so far has not been rationalized to a sufficient degree. Uplift resistance calculations for single piles follow the same logic as for compression piles, given the “skin friction” load transfer mechanism. The value of load testing is particularly relevant in these considerations, especially when nominal safety factors approach 2. The design of groups of piles subject to uplift follows the method described by FHWA (1996) for driven piles (cohesionless soils) and driven piles (cohesive soils) to consider the group performance: these are well recognized and utilized procedures.

5.2 Design for Structural Strength Limit States. Although the structural design of micropiles is sufficiently different from more conventional drilled shafts or driven piles, local construction regulations and/or building codes may indirectly address micropile design and therefore need to be considered by the design engineer. However, the comprehensive structural design of micropiles will likely not be provided in these sources. Efforts are underway by organizations such as ADSC to add micropile-specific code sections in both the AASHTO and International Building Code (IBC).

The calculation of the allowable compression and bending capacity and calculation of the allowable tension capacity for the upper cased length of the micropile is discussed here. Since it is common for the upper cased length of the micropile to be located in a relatively weak upper soil zone, consideration of a laterally unsupported length is

included in evaluation is Buckling of conditions.

determination of the compression capacity of the micropile.

This

consistent

with

methods

used

in

structural

steel

ASD

for

beam-columns.

micropiles

may

be

an

important

consideration

for

specific

project

5.2.1 Axial Compression of Cased Length. The allowable compression load for the cased (free) length is given as

Pcallowable

=

g r o u t g r o u t c F ' S f

× Agrout

+

Fysteel FS grout

(Abar

  • +

    Acasin g

) ×

a F

Fysteel FS ysteel

(Equation 1)

where, f´c FSg A F FS A A F g y-steel y-steel bar casing a = = = = = = =

uniaxial compressive strength of grout factor of safety on grout cross sectional area of grout minimum steel yield stress = factor of safety on grout cross sectional area of bar cross sectional area of casing allowable axial stress

8

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