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Practical Advice for Foundation Design – Micropiles for Structural Support - page 7 / 25

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GSP 131 Contemporary Issues in Foundation Engineering

materials not creep susceptible, the quality control is very good, and a high level of

load testing is conducted.

TYPE A

TYPE B

TYPE C

TYPE D

35-70

35-95

50-120

50-145

50-120

70-190

95-190

95-190

70-145

70-190

95-190

95- 240

95-215

120-360

145-360

145-385

95-265

120-360

145-360

145-385

95-190

95-310

120-310

120-335

205-550

N/A

N/A

N/A

515-1,380

N/A

N/A

N/A

1,035-2,070

N/A

N/A

N/A

520-1,725

N/A

N/A

N/A

1,380-4,200

N/A

N/A

N/A

Table 4. Summary of typical αbond

values (grout-to-ground bond) for preliminary

micropile design and feasibility evaluation.

Silt & Clay (some sand) (soft, medium plastic) Silt & Clay (some sand) (stiff, dense to very dense) Sand (some silt) (fine, loose-medium dense) Sand (some silt, gravel) (fine-coarse, med.-very dense) Gravel (some sand) (medium-very dense) Glacial Till (silt, sand, gravel) (medium-very dense, cemented) Soft Shales (fresh-moderate fracturing, little to no weathering) Slates and Hard Shales (fresh-moderate fracturing, little to no weathering) Limestone (fresh-moderate fracturing, little to no weathering) Sandstone (fresh-moderate fracturing, little to no weathering) Granite and Basalt (fresh-moderate fracturing, little to no weathering)

SOIL / ROCK DESCRIPTION

GROUT-TO-GROUND BOND NOMINAL STRENGTHS (KPA)

Type A: Type B: Type C:

Type D:

Gravity grout only Pressure grouted through the casing during casing withdrawal Primary grout placed under gravity head, then one phase of secondary “global” pressure grouting Primary grout placed under gravity head, then one or more phases of secondary “global” pressure grouting

End bearing is only a realistic consideration for moderately loaded micropiles founded on competent rock (i.e., act as simple struts). The design can be conducted as for end bearing shafts, in the absence of site-specific experience. The capacity of such piles is usually controlled by the structural capacity of the micropile (i.e., steel and grout).

Group effects in compression are related to an efficiency factor, as for other pile systems. Type A piles are assumed to have a factor of 1, provided the center-to- center spacing is at least 3 diameters. For Type B, C, and D micropiles, the drilling and pressure grouting processes may, in reality, create an efficiency factor greater than 1, due to the improvement of the soil between piles, as first demonstrated by Lizzi (1982). This effect is more prominent in granular soils than in cohesive soils. To the authors’ knowledge, however, this potential advantage has not yet been

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