Geoscience Reference
In-Depth Information
Table 22.12
Typical factors of safety for design of deep foundations for downward loads (Coduto, 1994).
Factors
Good control
Normal control
Poor control
Very poor control
Subsurface conditions
Uniform
Not uniform
Erratic
Very erratic
Subsurface exploration
Thorough
Thorough
Good
Limited
Load tests
Available
Not available
Not available
Not available
Construction inspection
Constant monitoring
Periodic
Limited
None
and testing
monitoring
-
Bored piles constructed with drilling fluids without the ability for even a down
the hole camera inspection - very poor control.
22.13 Ultimate compression capacity of rock for driven piles
•
The Ultimate Bearing Capacity
=
q
ult
=
2q
u
tan
2
(45
◦
+ φ
/2).
•
The design compressive strength
=
0.33-0.8 q
u
(Chapter 9).
•
<
>
The table below uses 0.33 q
u
for RQD
70% and 0.5 q
u
for RQD
70%.
Table 22.13
Ultimate bearing capacity for driven piles (using above equation from Tomlinson, 1996).
Angle of friction
RQD%
q
ult
(kPa) using q
u
values 1 MPa-40 MPa
1 MPa
5
10
20
40 MPa
30
◦
70
0.4
1.9
3.9
7.8
15
<
>
70
0.6
2.9
5.9
12
24
∗
40
◦
70
0.8
3.9
7.9
16
<
>
70
1.2
6.0
12
24
∗
Concrete strength governs
∗
50
◦
70
1.6
8.0
16
Concrete strength governs
∗
<
>
70
2.5
12
25
∗
60
◦
70
3.8
19
Concrete strength governs*
<
>
70
5.8
29*
•
Note this ultimate capacity is significantly higher capacity than the previous table
for shallow foundations.
•
A passive resistance term, tan
2
(45
◦
+
φ
/2), enhances the pile capacity.
•
The capacities are 1 to 8 times the previous table based on low to high friction
angles respectively for RQD
<
70% and 3 to 12 times for the RQD
>
70%.
22.14 Shaft capacity for bored piles
•
The shaft capacity increases as the rock quality increases.
•
Seidel and Haberfield (1995) provides the comparison between soils and rock
capacity.
•
(q
u
P
a
)
1
/
2
.
The shaft adhesion
= ψ
•
=
∼
P
a
atmospheric pressure
100 kPa.
•
ψ =
adhesion factor based on quality of material.
•
=
q
u
Unconfined Compressive Strength of Intact Rock (MPa).
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