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Recommended Values of Skin Friction

The table is based on field and laboratory tests.

Skin friction: Classig drilled nails

qs,k [kPa]1) according to EA Pfähle

Cohesionless soils

Cohesive soils

Rocks

qc 2) [MN/m2]

cu 3) [kN/m2]

qu,k 4) [MN/m2]

7,5

15

25

60

150

250

0,5

5

20

55 -80

105 - 140

130 -170

30 - 40

50 -65

65 - 85

70 - 250

500 -1000

500 - 2000

Notes:

1) Determination of skin friction should be part of geological survey

2) Cone resistance, CPT test (according to DIN EN ISO 22476-1)

3) Total cohesion of soil

4) Uniaxial compression strength

Literature:

EA-Pfähle, ISBN: 978-3-433-03005-9

Values in the table based on soil and rock classification.

Estimated bond strength of soil nails in soil and rock (source: Elias a Juran, 1991)

Material

Construction method

Soil / rock type

Ultimate skin friction qs [kPa]

Rock

Rotary drilled

Marl / limestone

300 - 400

Phyllite

100 - 300

Chalk

500 - 600

Soft dolomite

400 - 600

Fissured dolomite

600 - 1000

Weathered sandstone

200 - 300

Weathered shale

100 - 150

Weathered schist

100 - 175

Basalt

500 - 600

Slate / hard shale

300 - 400

Cohesionless soils

Rotary drilled

Sand / gravel

100 - 180

Silty sand

100 - 150

Silt

40 - 120

Piedmont residual

40 - 120

Fine colluvium

75 - 150

Driven casing

Sand / gravel

-low overburden

-high overburden

190 - 240

280 - 430

Dense moraine

380 - 480

Colluvium

100 - 180

Augered

Silty sand fill

20 - 40

Silty fine sand

55 - 90

Silty clayey sand

60 - 140

Jet grouted

Sand

380

Sand gravel

700

Fine - grained soils

Rotary drilled

Silty clay

35 - 50

Driven casing

Clayey silt

90 - 140

Augered

Loess

25 - 75

Soft clay

20 - 30

Stiff clay

40 - 60

Stiff clayey silt

40 - 100

Calcareous sandy clay

90 - 140

Note: Convert values in kPa to psf by multiplying by 20.9. Convert values in kPa to psi by multiplying by 0.145.