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:
prefabricated driven piles or steel piles
driven wooden piles
vibrating or vibropressed
cast in place screw piles
prefabricated screw piles
cast in place screw piles with additional grouting
prefabricated screw piles with additional grouting
steel tubular piles
Continuous Flight Auger piles (CFA)
bored piles or piles sheeted by bentonite suspense
bored piles with steel casing
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
For clay and silt the values of αs according to the NEN 6743 are listed in the following table:
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)
for piles of type "A"
for piles of type "B"
Maximum shaft resistance
1 < qc < 5
5 < qc
5 < qc < 12
12 < qc
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
- 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).
Tomlinson M. J.: Pile Design and Construction Practice, 4th edition, Taylor and Francis, 1994, ISBN 0 419 18450 3.