The seepage pressure is given by
PS = hγω where, Ps = Seepage pressure
γω = 9.81 kN/m3
Here, h = head loss and z = length
(ii) FS = hAγω where, Fs = Seepage force
(iii) where, fs = Seepage force per unit volume.
i = h/z where, I = Hydraulic gradient.
Quick Sand Condition
It is condition but not the type of sand in which the net effective vertical stress becomes zero, when seepage occurs vertically up through the sands/cohesionless soils.
Net effective vertical stress = 0
where, ic = Critical hydraulic gradient.
2.65 ≤ G≤ 2.70 0.65 ≤ e ≤ 0.70
- To Avoid Floating Condition
Laplace Equation of Two Dimensional Flow and Flow Net: Graphical Solution of Laplace Equation
where, ∅ = Potential function = kH
H = Total head and k = Coefficient of permeability
… 2D Laplace equation for Homogeneous soil.
where, ∅ = kX H and ∅ = ky H for Isotropic soil, kx= ky
Seepage discharge (q)
where, h = hydraulic head or head difference between upstream and downstream level or head loss through the soil.
- Shape factor =
where, Nf = Total number of flow channels
= Total number of flow lines.
where, Nd = Total number equipotential drops.
= Total number equipotential lines.
- Hydrostatic pressure = U =
where, U = Pore pressure hw = Pressure head
hw = Hydrostatic head – Potential head
- Seepage Pressure
Ps = h'γw where,
- Exit gradient,
where, size of exit flow field is b x b.
and is equipotential drop.
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
Phreatic line (Top flow line).
Follows the path of base parabola
CF = Radius of circular arc =
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.
(b) Phreatic Line without Filter
(i) For ∝ < 30°
q = k a sin2 ∝
(ii) For ∝ > 30°
q = k a sin ∝ tan ∝ and
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