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Practical Advice for Foundation Design – Micropiles for Structural Support - page 15 / 25

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GSP 131 Contemporary Issues in Foundation Engineering

Also represented in the figure is the range of pile factor values for each micropile type in Table 3.

Finally, it should be noted that contribution of the grout in the micropile has several effects on

this procedure does not take into account the micropile element. The presence of grout in a

the

potential

for

buckling.

The

grout,

whether

located within the casing or included as the bond material around the perimeter of steel, will add to the structural stiffness of the micropile. Furthermore, when the

the

10.000

75-ksi bar

Hollow core bars

1.000

80-ksi casing

Lateral Reaction Modulus, Es (ksi)

0.100

0.010

0.001

Check buckling capacity

36-ksi casing

Use Fy for design

0.1

1

Pile Factor

4

A I 2

E fy 2 10

(in2/kip)

100

1000

Figure 2. Chart for approximate buckling evaluation of micropiles subject to centered loads (Cadden and Gómez, 2002).

grout is used as a drilling fluid, or where it is pressurized as part of the pile installation process, it may significantly increase the stiffness and strength of the surrounding soils. The contribution of the grout to the buckling capacity may be particularly significant for bar and injection bore micropiles where the effect of the increased gross area may be significant when compared to the limited structural section.

Of particular concern in the evaluation of lateral and buckling capacities of micropiles is the location of threaded connections relative to the shear and moment distribution in the pile. Bending test results reported for 178 mm threaded connections by L.B. Foster and Malcolm Drilling Company indicated failure loads of about 1180 kN-m and 1360 kN-m (Stress Engineering Services, 1995, 2003).

Further discussion of the formulation of this analysis, common questions such as the effect of voids penetrated by micropiles, effect of not achieving complete grout return

15

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