Quick Study
What happens
if the direction
if the upward
of the seepage
seepage force
force is
was
equal
vertically
in
up?
magnitude to the effective weight of the soil?
the force
Instead
ofvector
θ being
diagram
as shown
would
θbefore:
= look
90° something like this:
θ
Resultant Body Force or
Stress, σ’
θ =Effective
90°
Seepage
So instead of Craig’s vector diagram looking
like Force
this: iwV :
Effective Weight  ’b2 :
It’s alive!
This is known as a QUICK condition.
The effective stress drops to 0 and the soil
In old English, quick means alive.
cannot support anything that won’t float on
The
soil
looks
like
its
boiling
and
appears
to
be
water!
“alive”.
IT’S ALIVE!!
Quick Study
If a quick condition develops, the resultant
Where would you expect to see seepage
body force = 0 and iwV = ’V
vertically upwards?
the hydraulic
The
Near
seepage
the downstream
forces
gradient
to
the surface
soil
that
right
produces
of the
the
a quick
sheet
pile wall (downstream
seepage
condition
vectors
is calledare
the
side) will
almost
critical
completely
hydraulic
all have
upward vertical
vertical
gradient,
(no
ic: horizontal
components
component)
γ' Gs  1
ic 
γw

1e
check out eqn. 1.21 on pg 20 in Craig
Quick Study
d
2
d
This depth
The
Model
mass
studies
has
of embedment
ahave
volume
shown
of:of a
sheetthe
that
pilesoil
wall
mass
extends
extending
from
downstream
over
this depth,
soil d
surface
and half
to
the bottom
this
distanceoffrom
the sheet
the piling
piling.
is
most prone to the quick
condition.
It is designated, d as shown.
d
1 2
V  d  1  2 d
2
Quick Study
d
2
h = 0
∆s
d
hm
Δh
ie 
Δs
For
At
Hence,
A
Factor
surface
sands,
the
ofaDC,
average
Safety
factor
the average
of
hydraulic
against
Safety
head, hmboiling
gradient
heaving
against
(quick
is
forestimated
seepage
condition)
at the from
surface
at theDC
midpoint
to
adjacent
can
AB
beis:
expressed
of
to line
the DC
sheet
using
from
piling
the
theis
flow hydraulic
expressed
exit
net. as the
gradient,
average
ie
hm gradient,
critical
over
thehydraulic
last
element
(AEFG):
ic
i

At surface mAB the excess
d
divided by this average
total head has been
gradient, im:
dissipated (h = 0).
ic ic
Fheave
boil  
ie im
Quick Study
The
Consider
elevation
the head
equilibrium
at DC is
of
–d
the
if forces
the downstream
acting on the
freesoil
water
mass surface
ABCD with
is at
unit
ABweight
sat.
zm  - d
d
2
d
γ d
2
1
2 sat
The average
poreof
water
total weight
ABCD is
pressure
on CD is:
sat x the Volume
of (h
ABCD
m+d)w
The boundary water force
on CD is the area of the
PWP distribution on CD:
1
2 d(hm  d) γw
½d(hm+d)w
Quick Study
To find the Resultant Body
Force of ABCD:
d
2
if γ' γsat - γw , then γsat  γ'γw
1
d
2
2
2
2
2
1
1
1
1

(
γ
'

γ
)d

γ
'
d

γ
d
γ
d
w
2
2
2 w
2 sat
3
1
2
d(hm  d)γw  1 h
2
 d   wd
m w
2
1
2
 '  γ' d  γwd  hmγwd  γwd
1
2
2
2
1
2
1
2
1
2
 '  γ' d  21 hmγwd
1
2
2
2
or
Quick Study
The
average
hydraulic
Now,
consider
the equilibrium
gradient
for seepage
of the forces
acting from
on the
DC
AB is: ABCD
hm with
soiltoskeleton
i

m
bouyant unit weight
d ’.
d
2
d
1
2
γ' d2
Jm  hm wd
1
2
The seepage
on of
ABCD
effectiveforce
weight
is:
ABCD is ’ x the Volume
of
2
h
d
m
ABCD
Jm   w
 1 hm w d
d
2
2
The resultant body force of ABCD is:
 '  γ' d  21 hmγwd Look familiar?
1
2
2
Quick Study
So, if σ’ = 0 as with a quick condition, then
0  γ' d  21 hmγwd
1
2
1
2
2
γ' d  21 hmγwd
2
A Factor of Safety against heaving (quick
condition) can then be expressed.
γ'
Generally,
The
stabilizing
a 1factor
force
ofissafety
the effective
is the ratio
weight
of
2
γforce
icforces
γdestabilizing
'd
stabilizing
of
theFsoil
forces
mass.
over
seepage
w
2 γ'd The
 1


 is the
destabilizinghforce.
h
γ
d
h
γ
im
m
m
w
m
w
2
d
Quick Study
dfilt
’filt
Well,
If the
you
factor
unit
could
weight
of
increase
safety
of thethe
is
effective
makingmaterial
the
weight
engineer
the
nervous,
filter
is of
’filt
andsoil
it is
adjacent
you could
placed
toto
aalways
depth
the sheet
redesign
of dpile
the
filt, then
wall
structure
the
by
extra
adding
and
effective
arepeat
filterweight
the is
material
process...
density).
w’
= ’filt x(high
dfilt and
the new
factor of safety would be:
Not an option?
γ'd  w'
F 1
2 hm γw d
1
2
2