16. Seepage Terminology
Stream line is simply the path of a water molecule.
From upstream to downstream, total head steadily decreases
along the stream line.
concrete dam
impervious strata
soil
datum
hL
TH = h TH = 0 L
17. Seepage Terminology
Equipotential line is simply a contour of constant
total head.
concrete dam
impervious strata
soil
datum
hL
TH = h TH = 0 L
TH=0.8 hL
18. Flownet
A network of selected stream lines and equipotential
lines.
concrete dam
impervious strata
soil
curvilinear
square
90Âș
19. Quantity of Seepage (Q)
f
Q = kh âŠ.per unit length normal to the plane
d
N
L N
# of flow channels
# of equipotential drops
concrete
dam
impervious strata
hL
head loss from upstream to
downstream
20. Heads at a Point X
= h
Elevation head = -z
Pressure head = Total head â Elevation head d
TH = hL TH = 0
concrete
dam
impervious strata
hL
datum
z
X
Total head = hL - # of drops from upstream x Dh
Dh
L
N
21. Piping in Granular Soils
datum
concrete
dam
impervious strata
soil
hL
At the downstream, near the dam,
i h exit D
Dh = total head drop
Dl
l
= D the exit hydraulic gradient
22. Piping in Granular Soils
If iexit exceeds the critical hydraulic gradient (ic), firstly
the soil grains at exit get washed away.
This phenomenon progresses towards the upstream, forming a
free passage of water (âpipeâ).
datum
concrete
dam
impervious strata
soil
hL
no soil; all water
23. Piping in Granular Soils
Piping is a very serious problem. It leads to downstream
flooding which can result in loss of lives.
Therefore, provide adequate safety factor against piping.
concrete
dam
impervious strata
soil
F = i
c
piping i
exit
typically 5-6