Screw-Piles, Helical Anchors and Soil Mechanics – Where are We?

Kansas City ASCE Geotechnical Seminar January 10, 2014

Presented by

Dr. Alan J. Lutenegger, P.E. Professor of Civil Engineering University of Massachusetts

and Executive Director

International Society for Helical Foundations (ISHF) www.helicalfoundations.org

The Complexity of Design

Single-Helix or Multi-Helix? “Tapered” or Uniform Helices? Close or Large Helix Spacing? Square-Shaft or Round-Shaft?

Compression or Tension? Sand or Clay?

Plain Shaft or Grouted Shaft? Embedment Length?

Etc.

Topics

1. Uplift of Shallow Single-Helix Screw-Piles 2. Grouted Shaft Helical Piles

3. Efficiency of Multi-Helix Anchors 4. Installation Disturbance

1. Uplift Behavior of Shallow Single-Helix Screw-Piles

Uplift of Single-Helix Screw-Pile vs. Drilled Shaft Uplift Load (lbs.)

0 2000 4000 6000 8000 10000 12000

Dis

plac

emen

t (in

.)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

12 in. Dia. x 10 ft. Concrete Pier6 5/8 in. Screw-Pile with 12 in. Helix @ 4 ft.

Round-Shaft Screw-Piles

1.Pipe Pile Installed by

Rotation Rather than Driving

2.Both Shaft and Helix are Engaged During Installation

and Loading

Traditional Failure Mechanism of Uplift of Shallow Plate “Anchors”

Uplift Factors for Plate Anchors in Sand

Shaft Contribution is Traditionally Neglected for Shallow Uplift

Uplift of Shallow Screw-Piles with Same Helix Diameter but Increasing Pipe Diameter

Load (lbs)

0 4000 8000 12000 16000 20000 24000 28000

Dis

plac

emen

t (in

.)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

2.875'' Pipe with 12'' helix 4.5'' Pipe with 12'' helix 6.625'' Pipe with 12'' helix

Uplift of Pipe vs. Screw-Pile

Load (lbs)

0 5000 10000 15000 20000 25000

Dis

plac

emen

t (in

.)

0.00

0.50

1.00

1.50

2.00

2.50

6 5/8 in. Plain Pipe at 8 ft. 6 5/8 in. Instant Foundation w/12 in. Helix at 8 ft.

More Realistic Model for Uplift?

1. Shaft Resistance Along Pipe

2. Local BC Failure Above Helix

3. No Propagation of Failure Surface to Ground Surface

Parametric Analysis Considering Changes in Helix Diameter & Pipe

Diameter

In Uplift: “Effective” Helix Area = Gross Helix Area – Pipe Area

Effective Area/Gross Helix Area 0 = Plain Pipe; 1 = All Helix (No Pipe)

Theoretical Load Distribution (@ ultimate) of Helical Pipe Pile in Uplift

Effective Helix Area/Total Helix Area (%)0 20 40 60 80 100

% T

oatl

Cap

acity

Fro

m H

elix

0

20

40

60

80

100H/D = 4

Theoretical Load Distribution of Helical Pipe Piles in Uplift for Different Lengths

Effective Helix Area/Total Helix Area (%)0 20 40 60 80 100

% T

oatl

Cap

acity

Fro

m H

elix

0

20

40

60

80

100H/D = 4 H/D = 8H/D = 16H/D = 32

Comparison Between Conventional & Modified Theory

Observed Ultimate Capacity (lbs)0 5000 10000 15000 20000

Pred

icte

d U

ltim

ate

Cap

acitr

y (lb

s)

0

5000

10000

15000

20000

Current Theory - "Wedge" BreakoutHelix + Pipe Shaft ResistanceObserved = Predicted

Where are We Headed?

1. Refinement of Model for Shallow Uplift of Screw-Piles to Consider Shaft Resistance

2. Separation of Shaft and End Capacity at

Different Levels of Load

1. Summary We Need to Rethink How Round Shaft Screw-Piles

(Helical Pipe Piles) Behave in Uplift

1. Design the Helix Capacity Using Traditional Bearing Capacity Theory

2. Design the Shaft Capacity Using Traditional Approaches for Pipe Piles (e.g., Alpha or Beta

Methods) with Consideration for Pipe Plugging

2. Grouted Shaft Helical Piles

A Screw-Pile or Helical Anchor with a Central Steel Shaft Surrounded by PC Grout

Helical Pulldown Micropiles (HPMP)

& Helical Cast-in-Place Displacement Piles

(HCIPDP) (Similar to ACIPD Piles)

• What is the Contribution of the Shaft to Total Capacity? In Compression? In Tension?

• Can We Estimate Shaft Capacity from Installation Parameters?

HPMP (< 6 in.)

& HCIPDP (> 6 in.)

Exhumed Grouted Shaft

Helical Micropile

Is a Grouted Shaft Helical Pile/Anchor a Structural Element

or a Geotechnical Element?

Depends!

Typical Design Situations

Soil Contribution to Capacity Plain vs. Grouted Shaft Helical Pile

HPMPs in Centralia, Mo. (SS5 8/10/12 w/ 6 in. x 14 ft. Grout Column)

Axial Load (kips)0 10 20 30 40 50 60 70 80

Dis

plac

emen

t (in

.)

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

Ungrouted MicropileGrouted Micropile

Contribution of the Grouted Shaft Axial Load (kips)

0 5 10 15 20 25 30

Dis

plac

emen

t (in

.)

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

Grouted Shaft - Calculated by Subtraction

HPMPs in Farmington, Mo. (SS5 10/12/14 5 in. x 45 ft. Grout Column)

Axial Load ( kips)

0 25 50 75 100 125 150 175 200 225

Dis

plac

emen

t (in

.)

0.00

0.50

1.00

1.50

2.00

2.50

Ungrouted MicropileGrouted Micropile

Contribution of the Grouted Shaft Axial Load ( kips )

0 20 40 60 80 100 120 140 160

Dis

plac

emen

t (in

ches

)

0.00

0.50

1.00

1.50

2.00

2.50

Grouted Shaft - Calculated by Subtraction

Installation Torque in Farmington, Mo. Installation Torque ( ft-lbs)

0 1000 2000 3000 4000 5000 6000D

epth

(ft)

0

10

20

30

40

50

60

Ungrouted MicropileGrouted Micropile

Cumulative Installation Torque Cumulative Installation Torque (ft-lbs)

0 50x103 100x103 150x103 200x103 250x103D

epth

(ft)

0

10

20

30

40

50

60

Ungrouted MicropileGrouted Micropile

Load Distribution in Deep Foundations (% End vs. % Side)

Depends on: Pile Type & Use

Installation Method Geometry (L/D)

Soil Type Stratigraphy

Load Level (Relative to Ultimate) End and Side Don’t Develop Capacity at the Same Rate

Reese et al. 1976

Drilled Shaft in Very Stiff Clay D = 2.5 ft.; L/D = 9.1

At Qult

36.8% End Bearing; 63.2% Side Resistance At Qult/2

5.7% End Bearing; 94.3% Side Resistance

Observed Distribution @ Qult

L/D0 20 40 60 80 100 120

% L

oad

from

Pile

Tip

at Q

ult

0

20

40

60

80

100

Driven Piles - Sand(Coyle & Castello 1986)Driven Piles - Clay(Tomlinson 1957)

Where are We Headed? HCIPDP Similar to ACIPD Pile

Shaft Capacity Related to Installation Energy

2. Summary

We Need to Rethink How Grouted Shaft Helical Micropiles and Helical Displacement Piles Behave or How We Want Them to Behave

The Grouted Shaft May be the Most Important Element of the Pile;

The Lead Helical Section May be Just a Construction Expedient

Design Shaft Capacity Using Conventional Geotechnical Approach – e.g., Auger-Cast Piles & Considering Grout Take

Installation Energy Appears like it Might be one Approach to

Validating Shaft Capacity

3. & 4. Efficiency of Multi-Helix Anchors & Installation Disturbance

Why do We Suspect that Colinear Multi-Helix Piles/Anchors May not be 100% Efficient?

The Traditional Design Approach

e.g., Canadian Foundation Manual

Qh = Ah(suNu + γDbNq + 0.5γBNγ)

What’s Important in This Equation? Sands: Ø’ & γ

Clays: su

Qt = ∑Qh

Where Might we Expect Installation Disturbance and a Reduction in

Efficiency?

“Structured” Soils “Cemented” Soils “Sensitive” Soils

Dense Sands All Soils?

Single, Double and Triple Helix Anchors in Sand (Clemence et al. 1994)

Efficiency of Multi-Helix Anchors in Sand

NUMBER OF HELICES1 2 3 4 5

EFF

ICIE

NC

Y (%

)

30

40

50

60

70

80

90

100

s/D = 1.5s/D = 3.0Clemence et al. (1994)s/D = 3.0

Torque Profiles in Sand (Clemence et al. 1994)

Torque (ft-lbs)0 1000 2000 3000 4000 5000

Dep

th (f

t)

0

2

4

6

8

10

12

14

16

18

20

22

Single DoubleTriple

Torque/Torquesingle

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Dep

th (f

t)

0

2

4

6

8

10

12

14

16

18

20

22

R1/1 R2/1 R3/1

Archival Data in Clay s/B = 2.6 - 4.5 (36 in. Spacing)

Number of Helices

1 2 3 4 5 6

Effic

ienc

y (%

)

30

40

50

60

70

80

90

100

A = 8 in. E = 10 in. J = 11.3 in. N = 13.5 in. S = 15 in. Average

Square-Shaft Single- & Multi-Helix - Clay

Torque (ft.-lbs.)0 500 1000 1500 2000

Dep

th (f

t.)0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

SS5-12SS5-12/12SS5-12/12/12

Torque/Torquesingle

0 1 2 3 4 5 6

Dep

th (f

t.)

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Ratio 1/1 Ratio 2/1 Ratio 3/1

Round-Shaft Single- & Multi-Helix - Clay

Torque (ft.-lbs.)0 1000 2000 3000 4000

Dep

th (f

t.)

0

2

4

6

8

10

12

14

16

18

20

22

24

RS2875-12RS2875-12/12RS2875-12/12/12

Torque/Torquesingle

1.0 1.5 2.0 2.5 3.0 3.5 4.0

Dep

th (f

t.)

0

2

4

6

8

10

12

14

16

18

20

22

24

Ratio 1/1Ratio 2/1Ratio 3/1

Vane Shear Tests Over Round-Shaft and Square-Shaft

Single-Helix Anchors in Clay

Undrained Shear Strength (psf)0 1000 2000 3000 4000 5000 6000

Dep

th (f

t)

0

2

4

6

8

10

12

14

16

18

20

Undisturbed PeakUndisturbed RemoldedRS2875-12 SS5-12

Vane Shear Tests Over Square-Shaft Single- Double- and Triple-Helix

Anchors

Undrained Shear Strength (psf)0 1000 2000 3000 4000 5000 6000

Dep

th (f

t.)

0

2

4

6

8

10

12

14

16

Undisturbed PeakSS5 12SS5 12/12 SS5 12/12/12

Canadian Foundation Manual

Qh = Ah(suNu + γDbNq + 0.5γBNγ)

For Clays We Need to Adjust su for Installation Disturbance if Everything Else is Constant

For Sands We May Need to Adjust Ø’

Or We Might Assign a “Local Efficiency” for Each Helix

Monitoring Installation

Installation Torque Installation Advance (rev/ft)

Installation Speed

“Disturbance Factor”

DF = (Rotations per Advance)/(Advance/Pitch)

For Ideal or “Perfect” Installation of Screws with a 3 in. Pitch

DF = 4/4 = 1

Measured Disturbance Factor -Clay

Disturbance Factor 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Dep

th (f

t.)

0

1

2

3

4

5

6

7

8

9

10

Pile-1 RS 450-14Pile-2 RS 450-14

Advance (Rotations/ft.)2 4 6 8 10 12 14

Dep

th (f

t)

0

1

2

3

4

5

6

7

8

9

10

Pile-1 RS 450-14Pile-2 RS 450-14

Load Test Results from Previous Installations

Load (lbs.)

0 5000 10000 15000 20000 25000 30000

Dis

plac

emen

t (in

.)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Pile-1Pile-2

Where are We Headed? For Clays We Might Relate Available Strength to DF

Disturbance Factor1.0 1.5 2.0 2.5 3.0 3.5 4.0A

vaila

ble

She

ar S

treng

th R

atio

(su/s

upea

k)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Low SensitivityHigh Sensitivity

Skempton (1950) Referring to triple-helix screw-piles in compression;

“…For Mr. Morgan’s double and triple screw-cylinders, it was necessary to recognize that the clay beneath the upper screws had been remoulded by the passage of the first screw. However, the whole of the volume of the clay contributing to the bearing capacity of the upper screws would not be fully remoulded and, as a rough approximation, it could be assumed that the average shear strength of the volume of clay was equal to:

cp2 = c – ½(c – cr)

3. & 4. Summary

Installation of Screw-Piles and Helical Anchors Causes Disturbance to the Soil

The Degree of Disturbance will Depend on a Number of

Factors, Including: Soil Initial State

Sensitivity Installation Quality

Summary

1. The Behavior of Screw-Piles and Helical Anchors is More Complex that has Been Considered

2. The Failure Mechanisms Need to Consider the Specific Geometry and Soil Behavior

3. Installation Disturbance is Real and Should be Considered in Design

4. Design Methodologies will Need to Change to Reflect These Considerations

5. Installation Monitoring of both Torque and Advance is Essential

ISHF www.helicalfoundations.org