Civil Engineering Reference
In-Depth Information
Q
(
B
6
e
)
250 [2
(6) (0.3)]
__________
________________
q
B
2
2
2
238 kN/m
2
[Eq. (8.7)]
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
B
2 m
N
c
3.2 for spread footing, using Fig. 8.8 with
T
/
B
2.5/2
1.25 and
c
2
/
c
1
0
Using the Terzaghi bearing capacity equation to calculate
q
ult
results in
q
ult
s
u
N
c
(
1
0.3
B
L
)
1.3
s
u
N
c
(1.3) (50 kN/m
2
) (3.2)
208 kN/m
2
__
And finally the factor of safety is calculated as follows:
FS
q
ult
q
208 kN/m
2
___
_________
208 kN/m
2
0.87
Solution for Spread Footing Using Fig. 8.9.
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
B
B
2
e
2
2 (0.30)
1.4 m
Fig. 8.9
L
L
2 m
moment only in B direction of footing
T
2.5 m i.e., total thickness of unliquefiable soil layer minus footing
embedment depth
3 m
0.5 m
2.5 m
50 kN/m
2
c
1
s
u
50 kPa
upper cohesive soil layer
c
2
0 kPa
0 kN/m
2
liquefied soil layer
N
c
3.2
for spread footing, using Fig. 8.8 with
T
/
B
2.5/2
1.25 and
c
2
/
c
1
0
Using the Terzaghi bearing capacity equation to calculate
q
ult
gives
q
ult
s
u
N
c
(
1
0.3
B
)
1.2
s
u
N
c
(1.2) (50 kN/m
2
) (3.2)
190 kN/m
2
___
L
Q
ult
q
ult
B
L
(190 kN/m
2
) (1.4 m) (2 m)
530 kN
