Environmental Engineering Reference
In-Depth Information
Figure 10.10.
Control of “blow-out” or liquefaction by pressure relief wells and weighting berm.
required factor of safety, i.e.
critical gradient
actual gradient
F
UC
(10.5)
1
actua
llgradient
should be at least 3 and preferably 5.
The authors' preference is to use the zero effective stress, F UT definition, because this
allows a better understanding of the ways in which the factor of safety can be improved.
In the event that pore pressures are calculated or known from piezometers to be too
high, the factor of safety can be increased by adding a weighting berm (see Figure 10.10)
or by providing pressure relief wells. The weighting berm is effective by adding to the ver-
tical stress; the pressure relief drains by reducing the pore pressure.
If the original pore pressure at X was hp 1 , the original factor of safety would have been
h
hp
sat
w1
F
(10.6)
UT
If the berm of height h f and unit weight
f was constructed, then
hh
hp
sat
f
f
F
(10.7)
UB
w1
If no berm was provided but instead pressure relief wells were constructed, resulting in
a reduction of pore pressure to
w hp 2 , then
h
hp
sat
F
(10.8)
UW
w2
If both the berm and pressure relief wells were constructed
hh
hp
sat
f f
w2
F
(10.9)
UWB
 
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