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EFFECTS OF SEEPAGE
SEEPAGE AND TUNNELING (PIPING) Seepage occurs when the water
seeps through the tiny soil
pores and finds its way into some bigger cracks. This starts a
process of tunneling where a tiny crack becomes larger and larger
as the water starts moving through it and carrying the surrounding
soil particles away with it.
Eventually the crack widens to the point where the water comes
rushing through the levee and crumbles the entire structure.
SAFETY OF HYDRAULIC STRUCTURES AGAINST PIPING When piping,
heaving or quicksand
occurs, the soil has no bearing capacity, hence it can not
support structures.
Piping or heaving originates in the soil mass when hydraulic
gradient i is greater
than or equal to the critical hydraulic
gradient, icr,
w
cri
'
e
Gsicr
1
1
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TERZAGHI (1922) conducted some model tests with a single row of
sheet piles as shown in Figure and found that the failure due to
heaving (or piping) takes place within a distance of D/2 from the
sheet piles (D is the depth of penetration of the sheet pile).
In order to prevent failure, the weight (W) of the soil prism in
the zone must be greater than the uplifting force due to
seepage
3U
WFS
Factor of Safety against heaving:
Where: W = submerged weight of soil in the heave zone per unit
length of sheet pile U = uplifting fore caused by seepage on the
same volume of soil
HT
D
D 2
H = Total head loss
h2
1. Estimate the average pressure head (h1) at point P along the
base (a-b) of the soil prism of unit thickness.
The variation of pressure over the base is considered to be
parabolic.
STEPS:
3
21
ba hhh
aTa hHh
bTb hHh
U
ha
hb h1
D
2. Determine the actual seepage pressure head (hs) to be
dissipated through the soil prism.
3. Estimate the uplifting force, U
21 hhhs
)1(2
:whereD
A Surface area of the base of soil prism
swsw hD
hAU 2
U
ha
hb h1
D
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The average hydraulic gradient across the prism,
4. Calculate the submerged weight of the soil prism.
D
hi sav
avwavw iDDiD
U 22
1
2
'2
1 2DW
U
ha
hb h1
D
5. Calculate the Factor of Safety:
swh
D
U
WFS
21
'2
1 2
wsh
DFS
' or
waviFS
'
av
cr
i
iFS or
TERZAGHIs Alternate Method for Flow around a Sheet Pile
21
'
HHC
DFS
wo
(Das, 2014)
L
hiexit
Where: h = head loss between the last two equipotential lines. L
= the length of the flow element d
N
Hh
HARZA (1935) investigated the safety of hydraulic structures
against piping. The factor of safety (FS) against piping:
exit
cr
i
iFS
iexit is the maximum exit gradient which can be determined from
the flow net.
A factor of safety of 3 4 is considered adequate for the safe
performance of the structure.
e
Gi s
w
cr
1
1'
Where:
Harza also presented charts for the maximum exit gradient of
dams constructed over deep homogeneous deposits.
B
hCiexit
Example #1
The sheet pile arrangement shown in Figure is to be examined for
adequacy. Determine the factor of safety against piping failure
using a.)Terzaghis method b.) Harzas method
Impermeable
e = 0.61
Gs = 2.67
sat = 20 kN/m3
k = 2.6 x 10-5 m/s
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PREVENTION OF PIPING
To increase the factor of safety against failure, several
methods were recommended:
1. Placing filter material over the danger zone.
2. Lengthening the flow lines, by driving the sheet pile deeper
or by installing sheet piles at one or both ends of a concrete
dams.
Filter
3. Lengthening the flow lines at concrete dams by constructing
upstream or downstream concrete aprons.
EFFECT OF CAPILLARY RISE
Water surface exposed to the atmosphere is under tension, called
capillary tension.
dhc
1
The smaller the capillary tube diameter, the larger the
capillary rise.
In soils, water also rises above the ground water table because
of surface tension. The speed of rise depends on the soil
types:
a.) In clay, capillary rise is slow due to very small pore size
as well as the presence of water bonded to the clay particles.
b.) In sand and silty sand, the rise depends on the: pore size,
particle shape and distribution density, original water content,
viscosity of water
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EFFECTIVE STRESS IN THE ZONE OF CAPILLARY RISE
The effects of capillary rise on pore pressure are:
1. An increase in the density of the soil within the region;
this increases the total stress ().
2. The pore pressure, u is negative throughout the region. The
value at a point in a layer of fully saturated soil by capillary
rise is
If partial saturation is caused by capillary action:
hu w
hS
u w
100
Where: h = height of the point under consideration measured from
the ground table. S = degree of saturation in percent
3. The pore water pressure due to capillary rise varies linearly
with depth, becoming zero at the ground water table.
4. The capillary action has no effect on the pore water pressure
below the ground water table.
Example #2
A soil profile is shown in Fig. Given H1 = 1.83m, H2 = 0.91m, H3
= 1.83m. Plot the variation of total stress, pore water pressure,
and effective stress with depth.
H1
H2
H3