Geoscience Reference
In-Depth Information
9.12 Rock shear strength and friction angles of specific
materials
•
The geologic age of the rock may affect the intact strength for sedimentary rocks.
•
The table assumes fresh to slightly weathered rock.
•
More weathered rock can have significantly reduced strengths.
Table 9.12
Typical shear strength of intact rock.
Origin
Rock type
Shear strength
Cohesion (MPa)
Friction angle
◦
Sedimentary - soft
Sandstone (triassic), coal, chalk,
1-20
25-35
shale, limestone (triassic)
Sedimentary - hard
Limestone, dolomite, greywacke
10-30
35-45
sandstone (carborniferous),
Limestone (carborniferous)
Metamorphic - non-foliated
Quartzite, marble, gneiss
20-40
30-40
Metamorphic - foliated
Schist, slate, phyllite
10-30
25-35
Igneous - acid
Granite
30-50
45-55
Igneous - basic
Basalt
30-50
30-40
9.13 Rock shear strength from RQD values
•
The rock strength values from RQD can be used in rock foundation bearing
capacity assessment.
Table 9.13
Rock mass properties (Kulhaway and Goodman, 1988).
RQD (%)
Rock mass properties
Design compressive strength
Cohesion
Angle of friction
0-70 (Very poor to fair)
0.33 q
u
0.1 q
u
30
◦
70-100 (Good to excellent)
0.33-0.8 q
u
0.1 q
u
30-60
◦
•
q
u
=
UCS
=
Uniaxial Compressive Strength of intact rock core.
•
When applied to bearing capacity equations for different modes of failure (refer
later chapters), the design compressive strength seems to be high. Chapter 22
provides comparative values.
9.14 Rock shear strength and friction angles based on
geologic origin
•
The geology determines the rock strength.
•
Values decrease as the weathering increases.
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