Alan J. Lutenegger, P.E., PhD, F. ASCE Professor Department of - - PowerPoint PPT Presentation

Alan J. Lutenegger, P.E., PhD, F. ASCE

Professor Department of Civil & Environmental Engineering University of Massachusetts

41st IOWA ASCE Geotechnical Conference March 9, 2017

What are Helical Piles and Anchors? Characterization of Soil Parameters Understanding Effects of Installation Disturbance Understanding Roles of Shaft & Helix Torque-to-Capacity Ratios

Section 1802.1 defines a Helical Pile as:

“Manufactured steel deep foundation element consisting of a central shaft and one or more helical bearing plates. A helical pile is installed by rotating it into the ground. Each helical bearing plate is formed into a screw thread with a uniform defined pitch.”

This Technology is Not New

It is Over 180 Years Old 1st Recorded use of Screw-Piles was by Alexander Mitchell (1780-1868) in 1836 for Ship Moorings and was then applied by Mitchell as Foundations for Maplin Sands Lighthouse in England in 1838

Mitchell’s Screw-Pile Specifications for Maplin Sands

Material – Cast Iron Shaft Diameter – 5 in. Screw (Helix) Diameter – 4 ft. Depth Below “Mudline” – 12 ft. Orientation - Vertical

Pier & Bridge Construction

The Industry is Largely Driven by Manufacturers and Contractors

Applications in Iowa Soils?

Not Unique to Helical Piles and Anchors but Needed for all Geotechnical Projects We Need to Evaluate Models Used for Design and Determine Input Parameters

• 2. Characterization of Soil

Parameters

Traditional Design Model

Qh = Ah(suNu + γDbNq + 0.5γBNγ) What’s Important in This Equation? Sands: Ø’ & γ Clays: su

Clay – Undrained TSA

QH = suNcAH

Sand – Drained ESA

QH = Nqσv’AH

Evaluation of Ultimate Capacity (Traditional Soil Mechanics Approach) Single-Helix

Multi-Helix

Most Literature Says:

QT = ∑ QHI

In Uniform Soils with Same Size Helices

QT = N x QHI

??????????????

Now Include Shaft Resistance for Round Shafts

QT = ∑ QHI + QS

QS = fs AS

TSA fs = suα ESA fs = βσ’v

Other than Compositional Characteristics, Most Soil Parameters are Not Unique Including su and φ’

Clay – Undrained Shear Strength: but which su?? Sand - Nq from φ’: but which φ’ and which Nq?

su in Clay from Different Tests

Nq Chart from Popular Book; but φ’ is Not Unique φ’TC; φ’TE; φ’PS; φ’DS; Curvature of Envelope, etc.

Somewhat Unique to Helical Piles and Anchors but Important for Many Deep Foundations We Need to Evaluate How Contractor Installation May Affect Soil Parameters

• 3. Understanding Effects of

Installation Disturbance

(Related to 2.)

“Structured” Soils “Cemented” Soils “Sensitive” Soils Dense Sands All Soils?

Tension Loading

• f Single-Helix

in Clay

Compression Loading of Single-Helix in Clay

Tension and Compression Loading of Multi-Helix in Clay

Round-Shaft Single- & Multi-Helix - Clay

Torque (ft.-lbs.)

1000 2000 3000 4000

Depth (ft.)

2 4 6 8 10 12 14 16 18 20 22 24

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

Torque/Torquesingle

1.0 1.5 2.0 2.5 3.0 3.5 4.0

Depth (ft.)

2 4 6 8 10 12 14 16 18 20 22 24

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

Efficiency in Soft Clay & Stiff Clay

Number of Helices

1 2 3 4 Efficiency (%) 20 30 40 50 60 70 80 90 100

SS5-12 SS5-12 RS2875-10 RS2875-12 RS350-12 Trend

Number of Helices

1 2 3 4

Efficiency (%)

20 30 40 50 60 70 80 90 100

RS2875-10 RS2875-12 RS350-12 SS5-10 Trend

Vane Shear Tests Over Round-Shaft and Square-Shaft Single-Helix Anchors in Clay

Undrained Shear Strength (psf)

1000 2000 3000 4000 5000 6000

Depth (ft)

2 4 6 8 10 12 14 16 18 20

Undisturbed Peak Undisturbed Remolded RS2875-12 SS5-12

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

Undrained Shear Strength (psf)

1000 2000 3000 4000 5000 6000

Depth (ft.)

2 4 6 8 10 12 14 16

Undisturbed Peak SS5 12 SS5 12/12 SS5 12/12/12

What About the QUALITY of the Installation?

High Quality

• vs. Poor

Quality Installation in Clay



“Good” and “Poor” Quality Installation

Revolutions Per Ft.

3 6 9 12 15

Depth (ft.)

1 2 3 4 5 6 7 8 9 10

CP1 Good CP3 Good CP4 Bad CP5 Bad

• No. Revolutions per ft.

3 4 5 6 7 8 9

Depth (ft.)

1 2 3 4 5 6 7 8 9 10 RS2875 SCG RS2875 P

Torque (ft.-lbs.)

500 1000 1500 2000 2500 3000

Depth (ft.)

1 2 3 4 5 6 7 8 9 10

RS2875 SCG RS2875 P



Consequence of “Poor” Installation

Displacement (in.)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Load (lbs.)

5000 10000 15000 20000 25000

RS2875 SCG RS2875 P

“Installation Disturbance Factor”

IDF = (Rotations per Advance)/(Ideal Advance/Pitch) For Ideal or “Perfect” Installation of Screws with a 3 in. Pitch

IDF = 4/4 = 1

Installation Disturbance Factor

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

Depth (ft.)

1 2 3 4 5 6 7 8 9 10

RS2875 SCG RS2875 P

For Clays We Might Want to Relate Available Strength to IDF

Disturbance Factor

1.0 1.5 2.0 2.5 3.0 3.5 4.0

Available Shear Strength Ratio (su/supeak)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Low Sensitivity High Sensitivity

Installation of Helical Piles and Anchors Causes Disturbance to the Soil Behind the Helices The Degree of Disturbance Will Depend on a Number of Factors, Including: Soil Initial State, Sensitivity & Installation Quality

Installation Requires Independent Monitoring

“… 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)]; where cp2 = operational undrained shear strength; c = peak undrained shear strength; cr = remolded undrained shear strength”

Somewhat Unique to Screw-Piles and Helical Anchors but Important for Many Deep Foundations We Need to Understand How Design Load is Carried

• 4. Understanding Role of Shaft

for Large Round Shaft Screw- Piles and Helical Anchors

Transfer Load To Helix? Provide a Component

• f Load Capacity?

Influence of Shaft

Displacement (in.)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Uplift Load (lbs)

5000 10000 15000 20000 25000 30000

2.875 in. Pipe with 12 in. Helix 4.5 in. Pipe with 12 in. Helix 6.625 in. Pipe with 12 in. Helix

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

Distribution of Load in Driven Piles @ Qult

L/D

20 40 60 80 100 120

% Load from Pile Tip at Qult

20 40 60 80 100

Sand - Coyle & Castello (1986) Clay - Tomlinson (1957) Sand - Randolph et al. (1994)

Load Tests to Failure on Helical Pile and Adjacent Plain Driven Pipe Pile

Q20 = 16,400 lbs.; Q10 = 13,200 lbs. Q10/Q20 = 0.80 Δ @ Q10/2 = 0.18 in. @ Q10 Qshaft = 2600 lbs.; Qhelix =10,600 lbs.

Displacement (in.)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Upolift Load (lbs.)

4000 8000 12000 16000 20000

2.875 in. x 8 ft. Plain Pipe 2.875 in. Pipe x 8 ft. with 12 in. Hleix

Displacement (in.)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Uplift Load (lbs.)

4000 8000 12000 16000 20000

4.5 in. x 8 ft. Plain Pile 4.5 in. Pipe x 8 ft. with 12 in. Helix

Silty Sand – 4.5 in. Pipe Shaft

Displacement (in.)

0.0 0.5 1.0 1.5 2.0 2.5

Uplift Load (lbs.)

5000 10000 15000 20000 25000 30000

4.5 in. x 8 ft. Plain Pipe 4.5 in. x 8 ft. Pipe with 12 in. Helix

Qult = f (Soil Properties & Pile/Anchor Geometry) T = f (Soil Properties & Pile/Anchor Geometry)

Qult = TKt

But… Kt Depends on a Number of Factors Because Torque Depends on a Number of Factors

Qult = TKt

An Empirical Equation, not a Theoretical Equation

Measuring Torque -Direct Methods

Installation Torque RPM Crowd Installation Advance (rev/ft.) (Full Depth of Installation)

• 1. Recommendations to Client of Feasibility –

Design-Build

• 2. Recommendations to Client as any other

Traditional Foundation System with Sizes, Allowable Loads, etc.

• 3. Provisions for Installation Monitoring & Load

Testing

• 1. The Behavior of Helical Piles and Anchors is More

Complex than has Previously Been Considered but Follows Basic Soil Mechanics

• 2. Evaluation of Soil Parameters for Design Must

Consider Installation Disturbance

• 3. 3rd Party Installation Monitoring of Torque,

Advance and RPM is Essential

• 4. On Site Load Tests of Production Piles/Anchors is

Important to Validate Contractor Torque-to- Capacity Correlations

Fixed Mast Installation Rigs Automated Installation Monitoring Increased Use of Larger Diameter Round Shafts & Helices