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Behavior of Helical Piers in Frozen Ground

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Behavior of Helical Piers in Frozen Ground Powered By Docstoc
					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
                                  1
      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.
                                             3
Helical Pier for Utilidor in St. Michael
Helical Pier Used for Boardwalk Bridge in Tuntutuliak
• 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.
                                         11
                        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
                                                   14
                 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.
                                                  15
16
      Installation Failure Model
• FEA model:
  shell elements
• Torque = 90 kip-in
• Restraint:
  At pipe bottom and
  leading edge



                               17
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.
                 18
Von Mises Stress on Helix



                              Stress, ksi
                                      0.0
                                      1
                                      2
                                      4
                                      8
                                      16
                                      32
                                      64




         Torque = 90 kip-in
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
     Torque = 90 kip-in    maximum torque.
21
                            Soil-Helix Model
•   2 helix configuration
•   Diameter = 50”
•   Depth = 180”
•   Helix
    –   3-1/2 shaft
    –   10” diameter
    –   1/2” steel plate
    –   30” spacing

                                           22
Drucker-Prager Circular Cone Yield Surface   23
Drucker-Prager Circular Cone Yield Surface
Drucker-Prager Circular Cone Yield Surface

      The material constant
     The material constant is: is:
            2 sin  
               2 sin
      
     
          3 (3( sin  )  )
            3 3  sin
      the angle internal friction = 31 =
     the angle of of internal friction  31

      The material yield parameter
     The material yield parameter is: is:
             6c6c cos 
                  cos 
      =
     y y =
            3 (3 ( sinsin  )
               3 3  )
      the cohesion value for for the element =
     ccthe cohesion value the element = 5 psi 5 psi

      The yield criterion
     The yield criterion is: is:                  24
                                   1   1
25
Vertical Displacement in Soil
                                       - Shallow
                                         Model




       2 helix, Axial Load = 20 kips
                                     - Shallow
Vertical Stress in Soil                Model




     2 helix, Axial Load = 20 kips       27
ecSD e n e Deep
r l i sa t2l
 t
 i  l pm      i
              x
V ao i lc e- h - Model




               Axial Load = 20 kips
                                              Deep Model




                       s 




Vertical Stress in Soil, Axial Load=20 kips         29
• 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 >70% 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
                                                   30
31
      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


                                      32
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 Load=20 kips
Vertical Soil Stress - Sub Model




        Axial Load=20 kips    34
  Helical Pier Stress – Sub Model




•Biaxial-bending
 behavior
•Information for
 welding
     von Mises stress in Helix, Axial Load =20 kips   35
36
Creep Equation
                   - for the Shallow Model
                                 n
                    e      
           e  c 
                          
                             
                     cu   
  where:
            e  .01 yr 1
           
           n=3
           e = equivalent stress
           cu = 38 kPa at -0.15C.
   Creep Model Results – Shallow Model

• Displacement vs.                                              Time (days)
                                           -3
  Time                               -3.1
                                                0   100   200   300      400   500   600   700


• 2 Year at –0.15o C                 -3.2




                       Displacement (cm)
                                     -3.3
• Secondary Creep                    -3.4


• Axial Load                         -3.5

                                     -3.6

  = 7 kips                           -3.7

                                     -3.8
• Soil – frozen silt                 -3.9


•  = 31o c = 5 psi                        • Nonlinear analysis
                                           -4



•  = 130 pcf                              • Time consuming
                                                                                                 38
                                   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.
                                                           39
40

				
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posted:9/1/2011
language:English
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