Introduction To Dams (FCE 511)
Introduction To Dams (FCE 511)
Introduction To Dams (FCE 511)
Introduction to Earth Dams
The procedures taken in the design and
construction of earth dams include:
Thorough pre-design and preconstruction
investigation of the dam foundation conditions and of
the construction materials and design of dams.
Application of engineering skills and techniques
to design
Carefully planned and controlled construction
Carefully designed and installed
instrumentation and monitoring of the completed
dams
Earth dams
Overtopping 30 Hydrological
h
d Pervious sand gravel
w
Impervious rock
Partial cutoff
A partial cutoff that does not go all the way to
the foundation,
It is designed to a proportion of the depth to the
rock or to impermeable layer.
The reduction in area is not proportional to the
reduction in the flow. Thus the reduction cannot be
estimated from the flow equation
The action of the partial cut off is similar to that
of an obstruction of in a pipe.
Experiments have shown that a 50% cutoff
results in 25% reduction of the seepage while an 80%
cutoff results in 50% reduction in the seepage.
Sheet piling and Slurry trench
•Sheet piling
An expensive method of cutting of the seepage
through the foundation of an earth dam.
Seepage continues to pass through the sheet pilling
interlocks. Can be used sometimes in conjunction with
the cutoff trenches. The sheet piles cannot be
performed in cobbles and boulders
•Slurry trench
This is a trench excavated and filled with concrete
below the impervious layer. The trench is kept in
position by placing bentonite before concreting to form
Grouting
Improves the stability and reduces the permeability of
pervious foundations under dam walls.
Grouting materials include:
• Cement – water
• Cement – bentonite – water
• Bentonite
Grouting is usually an expensive process and it
should be allowed after extensive testing and
evaluation.
Upstream blankets and embankment toes
Upstream blankets
These are usually made of same material as the
impervious core material. In effect the path of the
underseepage is increased and hence the loss of water is
reduced.
•Downstream embankment toes
•Reduce uplift pressures at the exit of the dam and
readily permit discharge and prevent piping of the fines
•Convey the discharge
Achieved by
•Extending the downstream zones
•Pervious downstream shells or by use of horizontal
blankets or use of toe drains
Embankment Design
Embankment slopes
Design parameters include gradation, composition and
corresponding behavior of the soils under different
conditions of saturation and loading.
The stress – strain relationships can be very complex.
Design of earthfill dam embankment depends largely
on successfully designed, constructed and well
performing dams.
σ τ
σ3
θ
Reservoir area
•Assess the siltation
•Assess the conservation measures being undertaken
in the neighborhood of the dam
•What is the state of the fence of the reservoir fence
for the fenced reservoirs?
AOB
Ndakaini dam
Crest 2045 masl
11000
Full storage level 2041 masl
2030 masl 2.25
3000 1 2025 masl
Draw-off tower 4000
2015 masl 1
3
1.5
1 Downstream shoulder
3000 5
2.25
upstream shoulder 1 1 2005 masl
2000 masl Core
70000
3.5
4000
1
10000 Filter drain
2.75
1
Drainage layers
culvert 2.25
1 1985
intake Drainage blanket 5000 masl
2.2
1
outlet
Original Ground level
Draw-off pipe
1 Protective Core 2
gravel 300mm 1
10m stand off pipe Hand placed
Draw off riprap300mm sand filter
pipe Toe drain
1000
trough
5000
Birica Dam, Nyeri district Dam
Crest
7000
Gross freeboard
1500
500
7000
3 Homogeneous embankment 1
5000
1000
sand filter
4000
1000
5000