Geology Reference
In-Depth Information
1
Permanent load (dead) = 3 MN
Variable load (live) = 1 MN. This has to be
factored higher by 1.3 so total load is 4.3 MN
where
α
is an adhesion factor (take as 0.5) and
γ
is a model factor = 1.4
So Rs
¼
4
,
964 kN
1 m
Base resistance
,
Rb
¼
0
:
785
ð
Nc
256
Þ=γ
c
u
= 60 kPa at ground surface
where Nc is a bearing capacity factor (take as 9) and
γ
is the same model
factor = 1.4
Area of shaft, A
s
= 87.97 m
2
So Rb
¼
1
,
292 kN
Assume 'undrained'
shear strength profile
c
u
= 60
+
7
z
kPa
Average c
u
along
pile length = 158 kPa
(
60
+
7
×
14)
Total design compressive resistance
,
Rc
¼
4,964
=
1
:
6
þ
1,292
=
2
:
0
¼
3,749 kN
With partial factors 1.6 on shaft and 2.0 on end bearing, which are
designated in the UK National Annex to BS EN 1997
1 for a bored
pile not subject to test. If tested, factors would be 1.4 and 1.7 respectively
(see Bond & Simpson, 2010 for discussion).
The factored design resistance, 3.749 MN, is less than the fac-
tored compressive load, 4.3 MN, so longer piles would be required.
The traditional, unfactored global FoS would be (4,964 + 1,292) × 1.4 /
4,000, i.e. 2.19, which is lower than would normally be required
following traditional design rules (typically 2.5 to 3.0) in the absence of
extensive preliminary and working pile tests.
-
28
m
c
u
= 256 kPa
(60
+
7
×
28)
Area of base, A
b
= 0.785 m
2
(circular pile)
1
Note that the symbol
γ
has been used in Eurocodes to represent
'
partial factors
'
. This can lead to confusion in that the symbol
γ
is more generally used to represent unit
weight as in
γ
R
for the unit weight of rock.
