1. OVERVIEW OF
HELICAL PIERS

2. HELICAL PIERS
History
Terminology
Installation
Advantages/Disadvantages
Axial Capacity
Helix Spacing
Bearing/Pullout Capacity
Torque Verification

3. History of Helical Piles
(Alexander Mitchell, 1836)

4. History of Helical Piles
(U.S. Patents)
50
45 Periods of Use
Patent Category
1. 1830-1875, Marine Era
Marine Agriculture Utilities Construction
(Moorings, Lighthouses)
40
2. 1875-1920, Aggriculture Era
(Fences)
35
3. 1920-1985, Utility Era
(Guy Anchors, Pipelines)
4
30
4. 1985-Present, Construction Era
US Patants
(Mobile Homes, Retaining Walls,
Underpinning, Piling)
25
3
20
15
10 2
1
5
0
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
86
87
88
89
90
91
92
93
94
95
96
97
98
99
00
-1
-1
-1
-1
-1
-1
-1
-1
-1
-1
-1
-1
-1
-1
-2
60
70
80
90
00
10
20
30
40
50
60
70
80
90
00
18
18
18
18
19
19
19
19
19
19
19
19
19
19
20

5. Common Terms
Central Shaft
Pier Cap
Extensions
Coupling
Lead Section
Helical Blades
Pitch
Pilot Point

6. Machine Installation

7. 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

8. 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

9. Axial Capacity
Limit States:
Cylindrical Shear
& Individual Bearing

10. Helix Spacing
Most efficient spacing
where:
Cylindrical Shear
= Individual Bearing
Generally occurs at
2 to 4 Helix Diameters

11. 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,

12. Pullout Capacity
Generally 18% <
Bearing Capacity
Displacement (mm)
-100 -50 0 50 100
40 180
160
30 140
120
100
20
80
Compression Test
60
Test Load (kips)
Pu=36 kips [160 kN]
Test Load (kN)
10 40
20
0 0
-20
-10 Tesnion Test -40
Pu=30 kips [130 kN]
-60
-80
-20
-100
-120
-30 -140
-160
-40 -180
-4 -3 -2 -1 0 1 2 3 4
Displacement (in)

13. Torque Verification
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
April 2008