Soil properties and state of the soil - Geotechnical Investigation and Design Tables - page 82

Table 7.8 Typical friction angle of granular soils.

Type Description/state Friction angle (degrees)

Cohesionless Soft sedimentary (chalk, shale, siltstone, coal) 30-40

Compacted Hard sedimentary (conglomerate, sandstone) 35-45

Broken rock Metamorphic 35-45

Igneous 40-50

Cohesionless Very loose/loose 30-34

Gravels Medium dense 34-39

Dense 39-44

Very dense 44-49

Cohesionless Very loose/loose 27-32

Sands Medium dense 32-37

Dense 37-42

Very dense 42-47

Cohesionless Loose

Sands Uniformly graded 27-30

Well graded 30-32

Dense
Uniformly graded

37-40

Well graded 40-42

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Particle shape (rounded vs angular) also has an effect, and would change the above
angles by about 4 degrees.

•

When the percentage fines exceed 30%, then the fines govern the strength.

•

Refer Figure 5.1.

7.9 Effective strength of cohesive soils
•

The typical peak strength is shown in the table.

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Allowance should be made for long term softening of the clay, with loss of effective
cohesion.

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Remoulded strength and residual strength values would have a reduction in both
cohesion and friction.

Table 7.9 Effective strength of cohesive soils

Type Soil description/state Effective cohesion (kPa) Friction angle (degrees)

Cohesive Soft - organic 5-10 10-20

Soft - non organic 10-20 15-25

Stiff 20-50 20-30

Hard 50-100 25-30

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Friction may increase with sand and stone content, and for lower plasticity clays.

•

When the percentage coarse exceeds 30%, then some frictional strength is present.

•

In some cases (eg cuttings) the cohesion may not be able to be relied on for the
long term. The softened strength then applies.

•

Refer Figure 5.2.