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Practical Advice for Foundation Design – Micropiles for Structural Support - page 20 / 25

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GSP 131 Contemporary Issues in Foundation Engineering

checked during creep testing as being within acceptable limits and displaying a stabilizing trend.

5.4.2 Micropile Group Settlement. Micropiles in a group can undergo additional vertical displacement as a result of consolidation of soil layers below the bottom of the micropile group. Where a single pile will transfer its load to the soil in the immediate vicinity of the pile, a pile group can distribute its load to the soil layer below the group. Consideration should be made for this group displacement when the soil below the group is cohesive in nature and subject to consolidation. The method for evaluating pile group settlements is similar to that for driven piles (see FHWA HI- 97-013, 1996). Equations for one-dimensional consolidation are used.

5.5 5.5.1

Corrosion Protection.

General.

Protecting

the

metallic

components

of

a

micropile

against

the

detrimental effects of corrosion is necessary to assure adequate long-term durability of the micropile. The degree and extent of corrosion protection is a function of loading condition, the expected service life of the micropile, the aggressiveness of the ground, the perceived importance of the structure, and consequences of failure. In all cases, it is the responsibility of the Owner to select the corrosion protection for each

micropile.

The conditions promoting corrosion of micropiles include:

  • Low resistivity of ground;

  • High concentration of chlorides or sulfides in ground or groundwater;

  • Too low or too high hydrogen potential (pH) of ground or groundwater;

  • High saturation conditions; and

  • Stray currents.

TEST

UNITS

STRONG CORROSION POTENTIAL/ AGGRESSIVE

MILD TO NO CORROSION POTENTIAL/ NON-AGGRESSIVE

ASTM STANDARD

AASHTO TEST METHOD

pH Resistivity Sulfates Chlorides

ohm-cm ppm(1) ppm

< 4.5, >10 <2,000 >200 >100

5.5 < pH < 10 Greater than 5,000 Less than 200 Less than 100

G51 G57 D516 D512

T 289-91 T 288-91 T 290-91 T 291-91

Tests

listed

Table 5. Criteria for assessing ground corrosion potential.

in

Table

5

are

used

to

Note:

(1) ppm = parts per million.

In general, the ground is classified with a strong corrosion potential or aggressive if any one of the conditions listed in the first column of Table 5 exceeds the limits listed in the third column of the table during the service life of the micropile. In addition, buried structures immediately adjacent to the project having exhibited corrosion or

classify the corrosion potential of the ground.

5.5.2

Evaluation of Soil Corrosion Potential.

20

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