OVERVIEW OF

HELICAL 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

9

1990

-199

9

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 spacing

where:

Cylindrical Shear

= Individual Bearing

Generally occurs at

2 to 4 Helix Diameters

Bearing Capacity

Traditional Soil

Mechanics:

Individual Bearing

Q = R2(1.3cNc+ Nq)

Cylindrical Shear

Q = 2 RL(c+K tan ) +

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 Test

Pu=36 kips [160 kN]

Tesnion Test

Pu=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