Geoscience Reference
In-Depth Information
application 8.1
Consider the situation of application 6.1. The mentioned properties in Table 6.2
have a statistical variation. Since the second (clay) and third (silty clay) layer show
a significant contribution to the settlement, in particular the virginal loading part
(
'
B
)
-b
, only the effect of the variation in parameter
b
is considered in this
example.
If the standard deviation
'
C
/
sd
for the compressibility index
C
c
for the clay layer,
determined by several soil tests, is 0.12 and for the silty clay (loam) layer 0.03,
then the corresponding compressibility index
b
becomes, using (8.3), equal to
0.2413 for the clay and 0.0635 for the loam. The corresponding virginal strain
increment in the clay layer becomes (top)
1 -
(
'
B
)
-
b
=
1 - (116/66)
-0.2413
=
'
C
/
'
B
)
-
b
=
1 - (123/73)
-0.2413
= 0.118, which gives an
average strain of 0.123 and a settlement of 0.123 m. The virginal strain increment
in the silty clay layer becomes (top): 1 -
(
0.127 and (bottom) 1 -
(
'
C
/
'
B
)
-
b
=
1 - (123/73)
-0.0535
= 0.0275
'
C
/
'
B
)
-
b
=
1 - (163/113)
-0.0635
= 0.0194, which gives an average
strain of 0.024 and a settlement (layer
5
m) of 0.120 m.
So, using the 95% confidence level, the contribution of the clay and loam layer
to the total settlement becomes 0.243 m. If the mean values would have been used
this value should be 0.185 m. In this respect the standard deviation of the
settlement
S
, due to the considered effect is about (0.243 - 0.185)/1.64 = 0.035, or
a variation coefficient
V
of 0.035/0.185 = 18.9%. The next step is to define and
apply a proper partial safety factor. For a 95% confidence level such a partial
safety factor would be:
and (bottom): 1 -
(
'
C
/
= 1 + 1.64
V =
1 + 1.64 x 0.189 = 1.33, and so a margin of
33%. In fact, one should include all variation aspects, which will make the
variation probably in the order of 30 to 40%. Obviously, the outcome corresponds
to a safety factor of 1.5 to 1.65.
P = 800 kN (permanent)
Q = 180 kN (variable)
backfill
0.5 m
= 25 kN/m
3
0.5 m
= 18 kN/m
3
w
= 10 kN/m
3
c
u
= 30 kPa
BxB
Figure 8.14 A footing design
application 8.2
The design of a square footing in undrained soil is determined using EC7 (Table
8.2). Values are shown in Fig 8.14, and partial safety factors are given in Table 8.2.
Design choice for the size of the footing
B
= 3.25 m. Other relevant formulas
are, the weight of the foundation and backfill:
G
= (3.25)
2
× (0.5 × 18 + 0.5 × 25) =
227 kN, the design value of vertical actions:
S
d
=
G
(
P
+
G
) +
Q
Q
, the design value
Search WWH ::
Custom Search