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Practical Advice for Foundation Design – Micropiles for Structural Support - page 19 / 25

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GSP 131 Contemporary Issues in Foundation Engineering

5.4

Service Limit States.

5.4.1 Micropile Vertical Movements.

Specifications must identify the maximum

allowable total movement of the pile head at the design load.

This movement will

comprise elastic and permanent components. Equation 11:

Elastic movement is calculated from

elastic

=

PL AE

(Equation 11)

Whereas P (load) and L (length over which it acts) are known or can be closely estimated, AE (the pile stiffness) is complex due to the:

  • Contribution of the grout to the pile’s stiffness due to the pile acting in compression; and

  • Varying reinforcement (type and length) used in some micropiles, with casing reinforcement in the upper portion of the pile and bar reinforcement in the lower portion.

For estimates of displacement caused by compression loads, the stiffness of the composite section at a given point in the pile can be determined using the formula:

EApile

= [Agrout

× Egrout

]+ [Asteel

× Esteel

]

(Equation 12)

For estimates of displacement caused by tension loads, the stiffness of the composite

section can be determined using the formula:

EApile = [Asteel × Esteel

]

(Equation 13)

The value for Esteel

can be assumed to be 200,000 MPa. The value for Egrout

can be

quite variable, but for design calculations may be estimated as E ( f c ( M P a ) ) 1 / 2 . B a s e d o n e x p e r i e n c e , a r e a s o n a b l e e s t i m a t e o f (MPa) = 4732 × in rock may be grout residual

on the order of 0.5 to 5 mm depending on the degree of weathering, whereas for soils, this value may be as high as 12 mm.

For micropiles bonded in competent soil, it is reasonable to assume that the elastic length is equivalent to the length of the pile above the bond zone and one-half of the length of pile bond length (i.e., half of the minimum design bond length). For micropiles bonded in rock, the full length of the micropile above the top of rock should be assumed for the elastic length. It is noted that the assumption of the lengths over which elastic movements occur can be confirmed via load testing in which loads are incrementally cycled (similar to a ground anchor performance test) to allow for measurement of the elastic movement.

Creep may be a concern in organic soils and clayey soils with a LL > 50% and PI > 20%. Creep deformations are not assessed during the design phase, but are

19

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