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 1 st 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?
2. Characterization of Soil Parameters 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
Traditional Design Model
Q h = A h (s u N u + γd b N q + 0.5γBN γ ) What s Important in This Equation? Sands: Ø & γ Clays: s u
Evaluation of Ultimate Capacity (Traditional Soil Mechanics Approach) Single-Helix Clay Undrained TSA Q H = s u N c A H Sand Drained ESA Q H = N q σ v A H
Multi-Helix Most Literature Says: Q T = Q HI In Uniform Soils with Same Size Helices Q T = N x Q HI??????????????
Now Include Shaft Resistance for Round Shafts Q T = Q HI + Q S Q S = f s A S TSA f s = s u α ESA f s = βσ v
Other than Compositional Characteristics, Most Soil Parameters are Not Unique Including s u and φ Clay Undrained Shear Strength: but which s u?? Sand - N q from φ : but which φ and which N q?
s u in Clay from Different Tests
N q Chart from Popular Book; but φ is Not Unique φ TC ; φ TE ; φ PS ; φ DS ; Curvature of Envelope, etc.
3. Understanding Effects of Installation Disturbance (Related to 2.) Somewhat Unique to Helical Piles and Anchors but Important for Many Deep Foundations We Need to Evaluate How Contractor Installation May Affect Soil Parameters
Structured Soils Cemented Soils Sensitive Soils Dense Sands All Soils?
Tension Loading of 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 0 0 2 2 4 4 6 6 8 8 Depth (ft.) 10 12 14 Depth (ft.) 10 12 14 16 16 18 20 22 RS2875-12 RS2875-12/12 RS2875-12/12/12 18 20 22 Ratio 1/1 Ratio 2/1 Ratio 3/1 24 0 1000 2000 3000 4000 Torque (ft.-lbs.) 24 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Torque/Torquesingle
Efficiency in Soft Clay & Stiff Clay 100 100 Efficiency (%) 90 80 70 60 50 SS5-12 SS5-12 RS2875-10 RS2875-12 RS350-12 Trend Efficiency (%) 90 80 70 60 50 RS2875-10 RS2875-12 RS350-12 SS5-10 Trend 40 40 30 30 20 1 2 3 4 20 1 2 3 4 Number of Helices Number of Helices
0 2 Vane Shear Tests Over Round-Shaft and Square-Shaft Single-Helix Anchors in Clay Depth (ft) 4 6 8 10 12 14 16 18 20 Undisturbed Peak Undisturbed Remolded RS2875-12 SS5-12 0 1000 2000 3000 4000 5000 6000 Undrained Shear Strength (psf)
0 2 Vane Shear Tests Over Square-Shaft Single- Doubleand Triple-Helix Anchors in Clay Depth (ft.) 4 6 8 10 12 14 16 Undisturbed Peak SS5 12 SS5 12/12 SS5 12/12/12 0 1000 2000 3000 4000 5000 6000 Undrained Shear Strength (psf)
What About the QUALITY of the Installation?
High Quality vs. Poor Quality Installation in Clay
0 Good and Poor Quality Installation 1 Depth (ft.) 2 3 4 5 6 CP1 Good CP3 Good CP4 Bad CP5 Bad 7 8 9 10 3 6 9 12 15 Revolutions Per Ft.
0 1 2 3 0 1 2 3 RS2875 SCG RS2875 P Depth (ft.) 4 5 6 RS2875 SCG RS2875 P Depth (ft.) 4 5 6 7 7 8 8 9 9 10 3 4 5 6 7 8 9 No. Revolutions per ft. 10 0 500 1000 1500 2000 2500 3000 Torque (ft.-lbs.)
Consequence of Poor Installation 25000 20000 Load (lbs.) 15000 10000 5000 RS2875 SCG RS2875 P 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Displacement (in.)
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
0 1 2 Depth (ft.) 3 4 5 6 RS2875 SCG RS2875 P 7 8 9 10 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 Installation Disturbance Factor
For Clays We Might Want to Relate Available Strength to IDF Available Shear Strength Ratio (s u /s upeak ) 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Disturbance Factor 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 c p2 = c [½(c c r )]; where c p2 = operational undrained shear strength; c = peak undrained shear strength; c r = remolded undrained shear strength
4. Understanding Role of Shaft for Large Round Shaft Screw- Piles and Helical Anchors Somewhat Unique to Screw-Piles and Helical Anchors but Important for Many Deep Foundations We Need to Understand How Design Load is Carried
Transfer Load To Helix? Provide a Component of Load Capacity?
Influence of Shaft 30000 25000 Uplift Load (lbs) 20000 15000 10000 5000 2.875 in. Pipe with 12 in. Helix 4.5 in. Pipe with 12 in. Helix 6.625 in. Pipe with 12 in. Helix 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Displacement (in.)
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 @ Q ult % Load from Pile Tip at Q ult 100 80 60 40 20 Sand - Coyle & Castello (1986) Clay - Tomlinson (1957) Sand - Randolph et al. (1994) 0 0 20 40 60 80 100 120 L/D
Load Tests to Failure on Helical Pile and Adjacent Plain Driven Pipe Pile
20000 16000 Upolift Load (lbs.) 12000 8000 4000 2.875 in. x 8 ft. Plain Pipe 2.875 in. Pipe x 8 ft. with 12 in. Hleix 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Displacement (in.) Q 20 = 16,400 lbs.; Q 10 = 13,200 lbs. Q 10 /Q 20 = 0.80 Δ @ Q 10 /2 = 0.18 in. @ Q 10 Q shaft = 2600 lbs.; Q helix =10,600 lbs.
20000 16000 Uplift Load (lbs.) 12000 8000 4000 4.5 in. x 8 ft. Plain Pile 4.5 in. Pipe x 8 ft. with 12 in. Helix 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Displacement (in.)
Silty Sand 4.5 in. Pipe Shaft 30000 25000 4.5 in. x 8 ft. Plain Pipe 4.5 in. x 8 ft. Pipe with 12 in. Helix Uplift Load (lbs.) 20000 15000 10000 5000 0 0.0 0.5 1.0 1.5 2.0 2.5 Displacement (in.)
Q ult = f (Soil Properties & Pile/Anchor Geometry) T = f (Soil Properties & Pile/Anchor Geometry) Q ult = TK t But K t Depends on a Number of Factors Because Torque Depends on a Number of Factors
Q ult = TK t 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. 3 rd 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