
Angle of internal friction for a given soil is the angle on the graph (Mohr's Circle) of the shear
stress and normal effective stresses at which shear failure occurs.
Angle of Internal Friction, f, can be determined
in the laboratory by the Direct Shear Test or the Triaxial Stress Test.

Typical relationships for estimating the angle of internal friction, f, are as follows:
Empirical values for f, of granular soils
based on the standard penetration number, (from Bowels, Foundation Analysis).
SPT Penetration, NValue
(blows/ foot)

f (degrees) 
0

25  30

4

27  32

10

30  35

30

35  40

50

38  43

Relationship between f, and
standard penetration number for sands, (from Peck 1974, Foundation Engineering Handbook).
SPT Penetration, NValue
(blows/ foot)

Density of Sand

f (degrees) 
<4

Very loose

<29

4  10

Loose

29  30

10  30

Medium

30  36

30  50

Dense

36  41

>50

Very dense

>41

Relationship between f, and
standard penetration number for sands, (from Meyerhof 1956, Foundation Engineering Handbook).
SPT Penetration, NValue
(blows/ foot)

Density of Sand

f (degrees) 
<4

Very loose

<30

4  10

Loose

30  35

10  30

Medium

35  40

30  50

Dense

40  45

>50

Very dense

>45

Other soil and soil related properties are listed below:
Angle of Internal Friction
Bearing Capacity Factors
Cohesion
External Friction Angle
Factor of Safety
Lateral Earth Pressure Coefficients
Modulus of Vertical Subgrade Reaction
Soil Unit Weights
Young's Modulus or modulus of elasticity
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