OVERVIEW OFHELICAL PIERS

History

Terminology

Installation

Advantages/Disadvantages

Axial Capacity

Helix Spacing

Bearing/Pullout Capacity

Torque Verification

HELICAL PIERS

History of Helical Piles(Alexander Mitchell, 1836)

History of Helical Piles(U.S. Patents)

0

5

10

15

20

25

30

35

40

45

50

1860

-186

9

1870

-187

9

1880

-188

9

1890

-189

9

1900

-190

9

1910

-191

9

1920

-192

9

1930

-193

9

1940

-194

9

1950

-195

9

1960

-196

9

1970

-197

9

1980

-198

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1990

-199

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2000

-200

9

US

Pa

tan

ts

Marine Agriculture Utilities Construction1. 1830-1875, Marine Era

(Moorings, Lighthouses)

1

2

3

4

2. 1875-1920, Aggriculture Era(Fences)

3. 1920-1985, Utility Era(Guy Anchors, Pipelines)

4. 1985-Present, Construction Era(Mobile Homes, Retaining Walls,

Underpinning, Piling)

Periods of Use

Patent Category

Common Terms

Central Shaft

Pier Cap

Extensions

Coupling

Lead Section

Helical Blades

Pitch

Pilot Point

Machine Installation

Advantages Low noise and vibrations Small, maneuverable

installation equipment Removal and replacement Ease of transport Compression or tension All-weather installation Rapid installation Does not produce drill spoil Immediate post tensioning Shorter bond length Do not require casing

Disadvantages Cannot penetrate very hard

rock (SPT N>100) Concrete and construction

debris is problematic Difficult in cobble and

boulders Ease of installation means

contractor pre-qualification important

Slender shaft makes lateral bracing imperative

Axial Capacity

Limit States:

Cylindrical Shear& Individual Bearing

Helix Spacing

Most efficient spacingwhere:

Cylindrical Shear= Individual Bearing

Generally occurs at2 to 4 Helix

Diameters

Bearing Capacity

Traditional Soil Mechanics:

Individual Bearing

Q = R2(1.3cNc+Nq)

Cylindrical Shear

Q = 2RL(c+Ktan) +

R2(1.3cNc+ Nq)

Limit State = Cylindrical Shear

or Individual Bearing,whichever is less.

Pullout Capacity

Generally 18% < Bearing Capacity

-40

-30

-20

-10

0

10

20

30

40

-4 -3 -2 -1 0 1 2 3 4

Displacement (in)

Te

st

Lo

ad

(k

ips

)

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

20

40

60

80

100

120

140

160

180

-100 -50 0 50 100

Displacement (mm)

Te

st

Lo

ad

(k

N)

Compression TestPu=36 kips [160 kN]

Tesnion TestPu=30 kips [130 kN]

Hoyt and Clemence (1989)

-91 Full-scale tension load tests

-Torque is statistically a strong verification of capacity

-Capacity:torque ratio depends mostly on hub diameter

Torque Verification

April 2008