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Helical Pile Design Verification - Pipe with Casing & Grout

3'' Pipe with External Casing and Grout with 8''-10''-12'' Helixes


A. Project description


This example presents a detailed example for calculating the ultimate axial capacity of a helical pile according to Vesic 1974 and Meyerhoff/Hansen method. Table 1 below presents assumed soil properties, Table 2 summarizes the soil stratigraphy, while Table 3 describes assumed helical pile properties.


Table 1: Soil properties


Table 2: Stratigraphy (Boreholes)


Table 3: Helical pile properties

We will examine three cases:


  • The pile as described in the previous table

  • The use of an external casing (Diameter: 8'' , length: 12 ft)

  • The use of an external casing as described above and 6'' grout extended to 8 ft from the pile bottom.


Soil Properties and Model in HelixPile


Pile Section Properties and Helix Configuration


B. Ultimate bearing capacity calculations – Cylinder failure method


B1. Manual calculations


For the cylinder latteral pressures factor, we will use the Mitch – Clemence method:


K = 0.09e(0.08fr) = 1.366


Table 4:  Ultimate shear stress line force calculations.


Figure: Ultimate shear stress line force on cylinder body.


Cylinder strength: Fcylinder = A1 + A2 = 14120.725 + 9867.31 = 23988.035 lbs = 23.99 kips


Vesic method


Tip: Fult = 2.45 kips (compression)


Plate 1: Fult = 15.24 kips (compresion)


Plate 3: Fult = 32.95 kips (tension)


So, the utimate cylinder compression capacity is Fult, comp =  23.99 + 2.45 + 15.24= 41.68 kips


The utimate cylinder tension capacity is Fult, tension =  23.99 + 32.95 = 56.94 kips


Meyerhoff/Hansen


Tip: Fult = 3.32 kips


Plate 1: Fult = 20.62 kips


Plate 3: Fult = 44.59 kips


So, the utimate cylinder compression capacity is Fult, comp =  23.99 + 3.32 + 20.62 = 47.93 kips


The utimate cylinder tension capacity is Fult, tension =  23.99 + 44.59 = 68.58 kips


Ultimate shaft capacity calculations:


  • Case I (no grout, no external casing)


Table 5: Shaft resistance calculation parameters – Case I


Table 6: Shaft resistance calculations – Case I


Fshaft = A1+A2+A3+A4 = 7957.66 lbs = 7.96 kips


Figure: Shaft resistance diagram


  • Case II (external casing 8'' , no grout)

 

Table 7: Shaft resistance calculation parameters – Case II


Table 8: Shaft resistance calculations – Case II


Figure: Shaft resistance diagram


Fshaft = A1+A2+A3+A4+A5 = 10496.89 lbs = 10.5 kips


  • Case III (external casing 8'' , grout 6'')

Table 9: Shaft resistance calculation parameters – Case III


Table 10: Shaft resistance calculations – Case III


Figure: Shaft resistance diagram

 

Fshaft = A1+A2+A3+A4+A5+A6 = 17573.71 lbs = 17.57 kips


Table 11: Shaft and cylinder tension capacity – Vesic method


Table 12: Shaft and cylinder compression capacity – Vesic method


Table 13: Shaft and cylinder tension capacity – Meyerhoff/Hansen method


Table 14: Shaft and cylinder compression capacity – Meyerhoff/Hansen method


B2. Calculations with HelixPile


Figure: Compression and tension cylinder method results in HelixPile (Vesic method – Case I)


Figure: Compression and tension cylinder method results in HelixPile (Meyerhoff/Hansen method – Case I)


Figure: Compression and tension cylinder method results in HelixPile (Meyerhoff/Hansen method – Case II)


Figure: Compression and tension cylinder method results in HelixPile (Vesic method – Case III)


Figure: Compression and tension cylinder method results in HelixPile (Meyerhoff/Hansen method – Case III)


Table 15:  Comparison between manual calculations and HelixPile results.



 

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