Geoscience Reference
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dry
h
h c
groundwater table
h
z
saturated
H
u
'
uu
' +
'
Figure 4.8 Groundwater pressure and effective stress
For the present case, the increase of vertical effective stress (in the saturated
zone) becomes
v ' = (1-0.42)x10x1 = 5.8 kPa. The increase of effective stress
causes compression, which results in ground settlement at the surface (land
subsidence). The average soil elasticity is estimated, using (3.1b) and adopting C c =
0.005
E = 1.5
v '/ ((1 -n ) C c )
1.5(½ H
' ) / ((1 -n ) C c ) = 0.75 H (1 -n )(
-
w )/ ((1 -n ) C c )
= 0.75 H (
-
w )/ C c = 0.75x10(18.9-10)/0.005 = 13350 kPa
The corresponding local vertical strain due to the vertical effective stress
increase becomes
v =
v '/E = (5.8 / 13350) = 0.00043
The settlement S equals the induced strain over the entire saturated soil layer
height (the drying zone itself contributes for 50%)
S = ( H - ( h
½
h ))
v = (10 - (1
0.5)) 0.00043 = 3.6 mm
which is practically insignificant.
However, for a soft clay layer with similar dimensions and a compression index
C c = 0.1, the corresponding settlement due to 1 m groundwater table lowering
would be in the order of 7 cm, which is not insignificant.
For submerged soil a change in the free water level would not give rise to any
change in effective stress, since there is no Archimedes effect. Is this also valid,
when the pore water contains gas bubbles? What happens in clay soil with large h c
reaching the surface, after lowering?
application 4.2
Consider an open end tube placed partly in unsaturated soil (Fig 4.9). At time t =
0, it is quickly filled with a water volume V = h 0 A m 3 , which will infiltrate in the
soil (the air in the pores can escape at the tube tip without hindrance). The time t e
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