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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