Geoscience Reference
In-Depth Information
•
This suggests that these methods are not applicable for rocks classified as low to
extremely low strength (Is (50)
<
0.3MPa).
22.6 Compression capacity of rock for a shear zone
failure mode
•
This condition applies for closely spaced joints (S
<
B).
•
A Terzaghi type general bearing capacity theory is used with the following
parameters:
-
φ
γ
The soil properties - cohesion (c), angle of friction (
) and unit weight (
).
-
The footing geometry - embedment (D
f
) and width (B).
•
However, the shape factors for square and circular footings are different, as well
as the bearing capacity factors.
•
The bearing capacity factors for rock are derived from wedge failure conditions,
while the slip line for soils are based on an active triangular zone, a radial shear
zone and a Rankine passive zone.
Table 22.6
Bearing capacity equation.
Consideration
Cohesion
Embedment
Unit weight
Comments
Bearing capacity
N
c
N
q
N
These factors are non dimensional
γ
factors
and depend on
φ
. See next Table
Ultimate Bearing
1.00 c N
c
D
f
N
q
0.5
BN
Strip footing (L/B
10)
+
γ
+
γ
=
γ
capacity (q
ult
)
1.05 c N
c
Strip Footing (L/B
5)
+
=
1.12 c N
c
+
Strip Footing (L/B
=
2)
1.25 c N
c
D
f
N
q
0.8
BN
Square Footing
+
γ
+
γ
γ
1.2cN
c
D
f
N
q
0.7
BN
Circular Footing
+
γ
+
γ
γ
•
∼
Most shallow rock foundations have D
f
0 (ie at the rock surface) and the
embedment term becomes zero irrespective of the N
q
value.
•
The unit weight term is usually small due to the width (B) term and is usually
neglected except in the case of high frictional rock, ie
50
◦
.
φ ≥
22.7 Rock bearing capacity factors
•
These bearing capacity factors have been based on wedge theory. It is different
from the bearing capacity factors of soils.
Table 22.7
Bearing capacity factors (from graphs in Pells and
Turner, 1980).
Friction angle
Bearing capacity factors
N
c
N
q
N
γ
φ
◦
0
4
1
0
10
6
2
1
20
8
4
5
30
15
9
15
40
25
20
45
50
50
60
160
60
110
200
1000
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