Rock properties - Geotechnical Investigation and Design Tables - page 109

9.15 Friction angles of rocks joints
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At rock joints the friction angle is different from the intact friction angles provided
in the previous tables.

Table 9.15 Typical range of friction angles (TRB, 1990).

Rock class Friction angles range (degrees) Typical rock types

Low friction 20 to 27 Schists, shale

Medium friction 27 to 34 Sandstones, siltstone, chalk, gneiss, slate

High friction 34 to 40 Basalt, granite, limestone, conglomerate

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Effective Rock Friction Angle = Basic Friction angle ( φ ) + Roughness Angle (i).

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Above table assumes no joint infill is present.

9.16 Asperity rock friction angles
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The wavelength of the rock joint determines the asperity angle.

Table 9.16 Effect of asperity on roughness angles, (Patton,1966).

Order of asperities Wavelength Typical asperity angle (i ◦ )

First 500 mm 10 to 15

Second

<

50 to 100 mm 20 to 30

9.17 Shear strength of filled joints
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The infill of the joints can affect the friction angle.

•

If movements in clay infill has occurred then the residual friction angle is relevant.

Table 9.17 Shear strength of filled joints (Barton, 1974).

Material Description Peak Residual

c (kPa) φ ◦ c r (kPa) φ r

Granite Clay filled joint 0-100 24-45

Sand-filled joint 50 40

Fault zone jointed 24 42

Clays Overconsolidated clays 180 12-18 0-30 10-16