Geoscience Reference
In-Depth Information
Rock foundations
22.1 Rock bearing capacity based on RQD
•
The rock bearing capacity is dependent on the rock strength, defects and its
geometry with respect to the footing size.
•
The table below is a first approximation based on RQD, which is a function of
the defects and the strength to a minor extent.
Table 22.1
Bearing pressures (Peck, Hansen and Thorburn, 1974).
RQD (%)
Rock description
Allowable bearing pressures(MPa)
lesser of below values
0-25
Very poor
1-3
25-50
Poor
3-6
UCS
50-75
Fair
6-12
or allowable stress
75-90
Good
12-20
of concrete
90
Excellent
20-30
>
-
This method is commonly used but not considered appropriate for detailed
design.
22.2 Rock parameters from SPT data
•
The SPT values in rock are usually the extrapolated values, as driving refusal
would have occurred before the given values.
Table 22.2
Rock parameters from SPT data.
Strength
Symbol
Point load index
Extrapolated SPT value
Allowable bearing
is (50) (MPa)
(N
o
)
60
capacity
Extremely low
EL
<
0.03
60-150
500 kPa to
Very low
VL
0.03 - 0.1
1.5 MPa
Low
L
0.1-0.3
Medium
M
0.3-1.0
100-350
1 to 5 MPa
High
H
1.0-3.0
250-600
Very high
VH
3.0-10
500
5 MPa
>
>
Extremely high
EH
>
10
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