Seepage Analysis Civil Engineering (CE) Notes | EduRev

Civil Engineering SSC JE (Technical)

Civil Engineering (CE) : Seepage Analysis Civil Engineering (CE) Notes | EduRev

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Chapter 6 Seepage Analysis

SEEPAGE PRESSURE AND SEEPAGE FORCE
Seepage pressure is exerted by the water on the soil due to friction drag. This drag force/seepage force always acts in the direction of flow.
The seepage pressure is given by Seepage Analysis Civil Engineering (CE) Notes | EduRev
where,Ps = Seepage Pressure gw = 9.18 km/m3.

here, h = head loss and

 Seepage Analysis Civil Engineering (CE) Notes | EduRev

(ii) Fs =hAgw

where, Fs = Seepage force.

(iii) fs =igw
where , fs = Seepage force per unit volume.
i =h / z
where, i = Hydraulic gradient.
NOTE: Ps, Fs and fs acts in the same direction i.e, in the direction of flow.

EFFECTIVE STRESSES:
Seepage Analysis Civil Engineering (CE) Notes | EduRev 
-The pore water pressure (u) is the pressure caused due to the water molecules present in the pores or voids of the soil. It is given by the relation:
Seepage Analysis Civil Engineering (CE) Notes | EduRev
Where,γw=Unit weight of water
Pore water pressure can also be designated as neutral pressure or neutral stress because of the fact that it cannot resist the shear forces in the soil mass.
Note: When the pressure is equal to atmospheric pressure, the pore water pressure is taken as zero.

-The effective stress is given by the formula:
Seepage Analysis Civil Engineering (CE) Notes | EduRev
For saturated soil, Effective stress is given by
Seepage Analysis Civil Engineering (CE) Notes | EduRev

QUICK SAND CONDITION
Quicksand condition is the floatation of particles of cohesionless soil, like fine gravel and sand, due to vertical upward seepage flow. As sand boiling occurs, the bearing capacity and shear strength of the cohesionless soil decrease and the agitations of soil particles become apparent.
How Quicksand Condition Occurs?
Quicksand condition occurs when seepage pressure, which acts in the upward direction, overcomes the downward direction pressure due to submerged weight of soil, and the sand grains are forced apart. The result is that the soil has no capability to support a load.

The soil that experiences quicksand condition would lose shear strength and bearing capacity. The shear strength of cohesionless soil depends on the effective stress. The shear strength is given by:
Seepage Analysis Civil Engineering (CE) Notes | EduRev
The effective stress is given by the following expression:
Seepage Analysis Civil Engineering (CE) Notes | EduRev

Seepage Analysis Civil Engineering (CE) Notes | EduRev
Taking G=2.67, and e=0.67, the result of equation 9 is equal to one.

Thus, the effective stress becomes zero for the soil with above values of G and e, when the hydraulic gradient ‘i’ is unity, i.e. head causing the flow is equal to the length of the specimen.

LAPLACE EQUATION OF TWO DIMENSHIONAL FLOW AND FLOW NET:

(i)
 Seepage Analysis Civil Engineering (CE) Notes | EduRev

where, f = hydraulic head or head difference between upstream and downstream level or head loss through the soil.

(ii) 
Seepage Analysis Civil Engineering (CE) Notes | EduRev

...2D Laplace equation for
Homogeneous soil. where, f = kx H and f = ky . H for lsotropic soil,

 Seepage Analysis Civil Engineering (CE) Notes | EduRev

SEEPAGE DISCHARGE (q)

 Seepage Analysis Civil Engineering (CE) Notes | EduRev

where, h = hydraulic head or head difference between upstream and downstream level or head loss through the soil.

shapefactor = Seepage Analysis Civil Engineering (CE) Notes | EduRev

where,Nf = Total number of flow channels N1/2 = Total number of flow lines.

 N= Ny - 1
where, Nd = Total number equipotential drops.
Nf = Total number equipotential lines.

Hydrostatic pressure = U = hwgw
where, U = Pore pressure
hw = Pressure head
Hw = Hydrostatic head - potentialhead 

  • Seepage Pressure

Ps =h 'gw
where
Seepage Analysis Civil Engineering (CE) Notes | EduRev

Seepage Analysis Civil Engineering (CE) Notes | EduRev

  •  Exit gradient

 Seepage Analysis Civil Engineering (CE) Notes | EduRev

where, Size of exit flow field is b × b.
and Seepage Analysis Civil Engineering (CE) Notes | EduRev

is equipotential drop.

PHREATIC LINE
It is top flow line which follows the path of base parabola. It is a stream line. The pressure on this line is atmospheric (zero) and below this line pressure is hydrostatic.

 (a) Phreatic Line with Filter

Seepage Analysis Civil Engineering (CE) Notes | EduRev

 Seepage Analysis Civil Engineering (CE) Notes | EduRev

Phreatic line ( Top flow line). ¯
Flows the path of base parabola
CF = Radius of circular arc = Seepage Analysis Civil Engineering (CE) Notes | EduRev
C = Entry point of base  parabola

F = Junction of permeable and impermeable   surface
S = Distance between focus and directrix = Focal length
FH = S

(i) q = ks
where, q = Discharge through unit length of dam.
(ii)

 Seepage Analysis Civil Engineering (CE) Notes | EduRev

(iii)
Seepage Analysis Civil Engineering (CE) Notes | EduRev® For 2D

Seepage Analysis Civil Engineering (CE) Notes | EduRev® For 3D

(b) Phreatic Line without Filter

 Seepage Analysis Civil Engineering (CE) Notes | EduRev

(i) For a< 30º

 Seepage Analysis Civil Engineering (CE) Notes | EduRev

(ii) For a> 30º

 q = k a sin a tan a

and 

 Seepage Analysis Civil Engineering (CE) Notes | EduRev

For neglecting the flow of sand of down stream. Some Filte is used.
Remember:-

 Seepage Analysis Civil Engineering (CE) Notes | EduRev
Seepage Analysis Civil Engineering (CE) Notes | EduRev

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