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Shaft Friction Coefficient ALFAs

The coefficient reducing the shaft friction αs differs based on the applied method and the type of soil. The values of this coefficients are built into the program according to EN 1997-2 and NEN 6743 standards.

The values for sands and sands with gravel are listed in the following table:

Piles

NEN 6743

αs [-]

EN 1997-2

αs [-]

prefabricated driven piles or steel piles

0.010

0.010

Franki piles

0.014

0.012

driven wooden piles

0.012

0.012

vibrating or vibropressed

0.012

0.012

cast in place screw piles

0.009

0.009

prefabricated screw piles

0.009

0.009

cast in place screw piles with additional grouting

0.006

0.006

prefabricated screw piles with additional grouting

0.006

0.006

steel tubular piles

0.0075

0.0075

Continuous Flight Auger piles (CFA)

0.006

0.006

bored piles or piles sheeted by bentonite suspense

0.006

0.006

bored piles with steel casing

0.005

0.005

For very coarse-grained sands and gravels the above values are reduced in both methods by a reduction coefficient (coarse-grained sand 0.75, gravel 0.5).

For peat the value of αs = 0 is considered.

For clay and silt the values of αs according to the EN 1997-2 are listed in the following table:

Type of soil

qc [MPa]

αs [-]

clay

> 3

< 0.030

clay

< 3

< 0.020

silt

< 0.025

For clay and silt the values of αs according to the NEN 6743 are listed in the following table:

qc [MPa]

αs [-]

> 1

0.035

< 1

0.0 depth to quintuple of pile diameter

0.025 depth from 5 to 20 multiple of pile diameter

0,035 depth over 20 multiple of pile diameter

If the LCPC (Bustamante) method is used the shaft friction coefficient αs is used depending on the tip resistance qc (orientation values are available in the following table).

Orientation values of the shaft friction coefficient αs based on the cone tip resistance qc

LCPC (Bustamante) Soil type

Cone stress (tip resistance)

qc [MPa]

αs

for piles of type "A"

αs

for piles of type "B"

Maximum shaft resistance

[kPa]

Clay

< 1

0.033

0.033

15

1 < qc < 5

0.025

0.011

35

5 < qc

0.017

0.008

35

Sand

qc< 5

0.010

0.008

35

5 < qc < 12

0.010

0.005

80

12 < qc

0.007

0.005

120

Type "A" includes these types of technology of installation of piles:

  • screw (cast in place, prefabricated, cast in place with additional grouting, prefabricated with additional grouting, CFA piles, bored or piles sheeted by bentonite suspense)

Type "B" includes these types of technology of installation of piles:

  • driven (prefabricated or steel, wooden)
  • Franki piles
  • vibrating
  • steel tubular
  • bored with steel casing

When using Schmertmann method, coefficient αs reducing shaft friction according to Tomlinson is considered. Values used in program are derived from following graph mentioned in publication M. J. Tomlinson: Pile Design and Construction Practice (1994).

Literature:

Tomlinson M. J.: Pile Design and Construction Practice, 4th edition, Taylor and Francis, 1994, ISBN 0 419 18450 3.