Civil Engineering Reference
In-Depth Information
Using the Terzaghi bearing capacity equation to calculate
q
ult
yields
q
ult
cN
c
s
u
N
c
(50 kN/m
2
) (5.5)
275 kN/m
2
[Eq. (8.6
a
)]
And finally the factor of safety is calculated as follows:
q
ult
275 kN/m
2
___
_________
FS
q
80 kN/m
2
3.4
Solution for Strip Footing Using Fig. 8.9.
To calculate the factor of safety in terms of a
bearing capacity failure for the strip footing, the following values are used:
Q
P
50 kN/m
for strip footing
e
M
Q
5 kN
m/m
__
__________
50 kN/m
0.10 m
for middle one-third of footing,
e
cannot
exceed 0.17 m, and therefore
e
is within middle one-third of footing
B
B
2
e
1
2 (0.10)
0.8 m
Fig. 8.9
T
2.5 m i.e., total thickness of unliquefiable soil layer minus footing
embedment depth
3 m
0.5 m
2.5 m
c
1
s
u
50 kPa
50 kN/m
2
upper cohesive soil layer
c
2
0 kPa
0 kN/m
2
liquefied soil layer
N
c
5.5
using Fig. 8.8 with
T
/
B
2.5/1.0
2.5 and
c
2
/
c
1
0
Using the Terzaghi bearing capacity equation to calculate
q
ult
gives
q
ult
cN
c
s
u
N
c
(50 kN/m
2
) (5.5)
275 kN/m
2
[Eq. (8.6
a
)]
Q
ult
q
ult
B
(275 kN/m
2
) (0.8 m)
220 kN/m
And finally the factor of safety is calculated as follows:
FS
Q
ult
Q
220 kN/m
___
_________
50 kN/m
4.4
Solution for Spread Footing Using Eq. (8.7).
To calculate the factor of safety in terms
of a bearing capacity failure for the spread footing, the following values are used:
Q
P
500 kN
for spread footing
e
M
Q
150 kN
m
__
__________
500 kN
0.30 m
for middle one-third of footing,
e
cannot
exceed 0.33 m, and therefore
e
is within middle one-third of footing
Converting
Q
to a load per unit length of the footing yields
500 kN
_______
Q
2 m
250 kN/m
