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M
H
1
R
z
P
x
2
H
2
D
E
x
1
z
c
P
W
2
1
2
C
G
A'
F'
F
H
W
1
A
x
3
x
4
B
Figure 10.10
PROPERTIES
c
u
30 kPa
H
1
20 m
18 kN/m
3
H
2
10 m
10 kN/m
3
H
6 m
w
144 m
2
Area
ABGF
x
1
4.5 m
42 m
2
Area
GCDEF
x
2
13 m
80
o
x
3
11 m
R
22.80 m
x
4
3 m
Remark
For a partly submerged slope, the water pressure at the slope forms a
contribution to the resistance moment. This is accounted for by considering the fact
that the water weight of section A'BF' balances that of section BGF' (see Fig
10.10). Therefore, by excluding in the driving moment of the water in soil under
the free water level, i.e. ABGF, accounts for the water pressure effect at the slope.
Hence, taking the submerged unit weight
w
for that zone incorporated this. For
the remainder, the saturated soil above the free water level, the total unit weight
applies. This is done in case 1. Case 2 shows that when the water level drops, the
water pressures at the slope vanish, and the stability factor decreases significantly.
-
application 10.3
The stability factor of a translational slide of a slope, from which groundwater
flows out under angle
, is
F
= (tan
/ tan
)
)(1
(
w
/
)(
tan(
)
)tan(
)
+
) +1). If
the soil is isotropically permeable
= 1, then,
F
seems independent of the
permeability. Explain why that is correct.
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