Geoscience Reference
In-Depth Information
•
A different bearing strength applies for all of the above, for a rock with similar
rock strength. This is presented in the Tables that follow.
•
When RQD - -> 0, one should treat as a soil mass and above concepts do not apply.
•
These failure modes form the basis for evaluating the rock bearing capacity.
22.4 Compression capacity of rock for uniaxial failure mode
•
This is a Uniaxial Compression Failure condition (S
<
B).
•
The table applies for a open vertical to sub-vertical joints.
Table 22.4
Ultimate bearing capacity with failure in uniaxial compression.
Failure mode
Strength range
Design ultimate strength
Uniaxial compression with RQD
70%
15% to 30% UCS
Use 15% UCS
<
Uniaxial compression with RQD
70%
30% to 80% UCS
Use 30% UCS
>
•
Factors of Safety to be applied to shallow foundations.
•
For deep foundations, piles have the effect of confinement, and the Design Ultimate
Strength
∼
Allowable Bearing Capacity.
•
An alternative approach to this uniaxial failure condition is presented below.
22.5 Ultimate compression capacity of rock for shallow
foundations
•
<
This applies for the uniaxial compression failure mode ie open joints with S
B.
•
2 c tan (45
◦
+ φ
It uses the Ultimate Bearing Capacity
=
q
ult
=
/2). This is the Mohr
σ
=
Coulomb Failure criterion for the confining stress
0.
3
•
The table assumes the cohesion, c
=
10% q
u
(Chapter 9) for all RQD Values.
•
This applies to shallow foundations only, and a factor of safety is required for the
allowable case.
Table 22.5
Ultimate bearing capacity (using above equation from Bell, 1992).
Angle of friction
q
ult
(kPa) using q
u
values 1 MPa-40 MPa
Low
Medium strength
High
Very high
1MPa
5
10
20
40MPa
30
◦
0.2
0.8
1.5
3.1
6.1
40
◦
0.2
1.1
2.2
4.4
8.7
50
◦
0.3
1.6
3.1
6.3
13
60
◦
0.5
2.4
4.8
9.7
19
•
The ultimate capacity seems unrealistically low for values of low strength rock,
ie where q
u
=
1MPa. However it is approximately consistent for 15% UCS
(RQD
<
70%) given in the previous Table.
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