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.) 0.0 0 2000 4000 6000 8000 10000 12000 0.5 Displacement (in.) 1.0 1.5 2.0 2.5 12 in. Dia. x 10 ft. Concrete Pier 6 5/8 in. Screw-Pile with 12 in. Helix @ 4 ft. 3.0
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) 0.0 0 4000 8000 12000 16000 20000 24000 28000 0.5 Displacement (in.) 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) 0.00 0 5000 10000 15000 20000 25000 0.50 Displacement (in.) 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 % Toatl Capacity From Helix 100 80 60 40 20 H/D = 4 0 0 20 40 60 80 100 Effective Helix Area/Total Helix Area (%)
Theoretical Load Distribution of Helical Pipe Piles in Uplift for Different Lengths % Toatl Capacity From Helix 100 80 60 40 20 H/D = 4 H/D = 8 H/D = 16 H/D = 32 0 0 20 40 60 80 100 Effective Helix Area/Total Helix Area (%)
Comparison Between Conventional & Modified Theory 20000 Predicted Ultimate Capacitry (lbs) 15000 10000 5000 Current Theory - "Wedge" Breakout Helix + Pipe Shaft Resistance Observed = Predicted 0 0 5000 10000 15000 20000 Observed Ultimate Capacity (lbs)
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) 0.00 Axial Load (kips) 0 10 20 30 40 50 60 70 80 0.25 Displacement (in.) 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Ungrouted Micropile Grouted Micropile
Contribution of the Grouted Shaft 0.00 Axial Load (kips) 0 5 10 15 20 25 30 0.25 Displacement (in.) 0.50 0.75 1.00 1.25 1.50 Grouted Shaft - Calculated by Subtraction 1.75 2.00
HPMPs in Farmington, Mo. (SS5 10/12/14 5 in. x 45 ft. Grout Column) Axial Load ( kips) 0.00 0 25 50 75 100 125 150 175 200 225 0.50 Displacement ( in.) 1.00 1.50 2.00 Ungrouted Micropile Grouted Micropile 2.50
Contribution of the Grouted Shaft Axial Load ( kips ) 0.00 0 20 40 60 80 100 120 140 160 Displacement ( inches ) 0.50 1.00 1.50 2.00 Grouted Shaft - Calculated by Subtraction 2.50
Installation Torque in Farmington, Mo. Installation Torque ( ft-lbs) 0 0 1000 2000 3000 4000 5000 6000 10 20 Depth ( ft) 30 40 50 60 Ungrouted Micropile Grouted Micropile
Cumulative Installation Torque Cumulative Installation Torque (ft-lbs) 0 0 50x10 3 100x10 3 150x10 3 200x10 3 250x10 3 10 20 Depth (ft) 30 40 50 Ungrouted Micropile Grouted Micropile 60
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 Q ult 36.8% End Bearing; 63.2% Side Resistance At Q ult /2 5.7% End Bearing; 94.3% Side Resistance
Observed Distribution @ Q ult 100 % Load from Pile Tip at Q ult 80 60 40 20 Driven Piles - Sand (Coyle & Castello 1986) Driven Piles - Clay (Tomlinson 1957) 0 0 20 40 60 80 100 120 L/D
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 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 Q t = Q h
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 100 90 EFFICIENCY (%) 80 70 60 50 s/d = 1.5 s/d = 3.0 Clemence et al. (1994) s/d = 3.0 40 30 1 2 3 4 5 NUMBER OF HELICES
Torque Profiles in Sand (Clemence et al. 1994) 0 0 2 Single Double Triple 2 R1/1 R2/1 R3/1 4 4 6 6 8 8 Depth (ft) 10 12 Depth (ft) 10 12 14 14 16 16 18 18 20 20 22 0 1000 2000 3000 4000 5000 Torque (ft-lbs) 22 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Torque/Torque single
Archival Data in Clay 100 s/b = 2.6-4.5 (36 in. Spacing) Efficiency (%) 90 80 70 60 50 A = 8 in. E = 10 in. J = 11.3 in. N = 13.5 in. S = 15 in. Average 40 30 1 2 3 4 5 6 Number of Helices
Square-Shaft Single- & Multi-Helix - Clay 0 0 1 1 2 2 3 3 4 4 5 5 6 6 Depth (ft.) 7 8 9 Depth (ft.) 7 8 9 10 10 11 11 12 13 14 15 16 SS5-12 SS5-12/12 SS5-12/12/12 0 500 1000 1500 2000 12 13 14 15 16 Ratio 1/1 Ratio 2/1 Ratio 3/1 0 1 2 3 4 5 6 Torque (ft.-lbs.) Torque/Torque single
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
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 Vane Shear Tests Over Square- Shaft Single- Double- and Triple-Helix Anchors Depth (ft.) 2 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)
Canadian Foundation Manual Q h = A h (s u N u + γd b N q + 0.5γBN γ ) For Clays We Need to Adjust s u 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 0 0 1 Pile-1 RS 450-14 Pile-2 RS 450-14 1 Pile-1 RS 450-14 Pile-2 RS 450-14 2 2 3 3 Depth (ft) 4 5 6 Depth (ft.) 4 5 6 7 7 8 8 9 9 10 2 4 6 8 10 12 14 Advance (Rotations/ft.) 10 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Disturbance Factor
Load Test Results from Previous Installations Load (lbs.) 0.0 0 5000 10000 15000 20000 25000 30000 0.5 Displacement (in.) 1.0 1.5 2.0 2.5 Pile-1 Pile-2 3.0 3.5
Where are We Headed? For Clays We Might Relate Available Strength to DF 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
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: c p2 = c ½(c c r )
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