Application and Analysis of Helical Piers in Frozen Ground

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Application and Analysis of Helical Piers in
Frozen Ground

• He Liu, Ph.D., P.E.
• Daniel Schubert, P.E.
• Hannele Zubeck, Ph.D., P.E.
• Sean Baginski

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Applications and Advantages

• Helical piers have been used for above
• ground water and wastewater transmission
• lines
• Helical piers have a great potential for use
• in remote villages and facilities
• Advantages not only because they provide
• stable foundations but also because of their
• light weight and fast installation time.

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Helical Pier for Utilidor in St. Michael
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Helical Pier Used for Boardwalk Bridge in
Tuntutuliak
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• Helical piers are widely used in soft soils,
  however, no data nor design procedures exist
  for frozen ground applications.
• Problems related to frozen ground
• include the risk that the piers will fail
• during the installation and long-term
• deformation due to frozen ground creep.

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Study Objectives

• Helical pile stress distribution during
  installation
• Helical pile stress distributions under axial
  load
• Pile displacement and soil stress
• under axial loading
• Long term pile displacement (creep)
• Results will compare with tests
• in CRERL

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Method of Analyses

• To investigate the behavior of helical pier
• foundations in frozen ground, and
• To develop design and installation guidelines,
• Finite Element Analysis (FEA) models are
• developed in this study.
• The scope of work includes developing FEA
• models to simulate the force-deformation
• relationships in the pier and the stress-strain
• relationships in the surrounding frozen soil.

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Installation Strength Analysis
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Installation Failure Model

• FEA model
• shell elements
• Torque 90 kip-in
• Restraint
• At pipe bottom and leading edge

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Bilinear Yield Criteria
Y 50 ksi E 29,000 ksi ET 1450 ksi This
yield criterion allows for both elastic and
plastic deformation of the steel.
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Von Mises Stress on Helix
Stress, ksi
0.0 1 2 4 8 16 32 64
Torque 90 kip-in
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von Mises Stress on Helix
Stress, ksi
8.1 16.1 24.2 32.2 40.3 48.3 56.4 64.4 72.5

• Yield occurs near the corner
• Consistent with real failures
• FEA can provide the accurate
• maximum torque.

Torque 90 kip-in
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Soil Stress and Deflection Analysis
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Soil-Helix Model

• 2 helix configuration
• Diameter 50
• Depth 180
• Helix
• 3-1/2 shaft
• 10 diameter
• 1/2 steel plate
• 30 spacing

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Drucker-Prager Circular Cone Yield Surface
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Vertical Displacement in Soil
- Shallow Model
2 helix, Axial Load 20 kips
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Vertical Stress in Soil
- Shallow Model

• 2 helix, Axial Load 20 kips

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Deep Model
Axial Load 20 kips
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Deep Model
Vertical Stress in Soil, Axial Load20 kips
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• In case of two helix plates, three times
• diameter apart each other
• Soil reaction pressure below the bottom plate
• is 3-4 times larger than that of the top plate
• The bottom plate takes gt70 of the total load
• The reasons are
• - Steel shaft is very stiff between two
• plates, almost no shortening
• - Soil deformation between two plates
• is mainly controlled by the steel
• deformation

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Sub-modeling Detailed Analysis
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Sub Model from the Shallow Model

• Submodel from Large Model
• Soil - 15 diameter
• Depth 10
• Helix
• 3-1/2 shaft
• 10 diameter
• 1/2 thickness

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Vertical Stress in Soil Below Helix
- Sub Model
Stress, psi
-19.5 -17.0 -14.5 -11.9 -9.4 -6.9 -4.4 -1.9 0.0
Axial Load20 kips
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Vertical Soil Stress - Sub Model
Axial Load20 kips
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Helical Pier Stress Sub Model

• Biaxial-bending
• behavior
• Information for
• welding

von Mises stress in Helix, Axial Load 20 kips
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Frozen Ground Creep Analysis
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Creep Equation - for
the Shallow Model
where
n 3 ?e equivalent stress ?cu?
38 kPa at -0.15?C.
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Creep Model Results Shallow Model

• Displacement vs. Time
• 2 Year at 0.15o C
• Secondary Creep
• Axial Load
• 7 kips
• Soil frozen silt
• ? 31o c 5 psi
• ? 130 pcf

• Nonlinear analysis
• Time consuming

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Conclusions

• Helical piers have a great potential for use in
  remote villages and facilities.
• The FEA model results will increase understanding
  of helical piers in various soil conditions as
  well as provide insight into design and
  installation considerations.
• Soil stress is not uniformly distributed under
  helix. Further refinements in design procedures
  are necessary.
• Creep analysis indicates linear secondary
  settlement. It will provide valuable information
  for use the piers in frozen ground.

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