the material of this PPt according to the RTU syllabus

1. WATER RESOURCES ENGINEERING
UNIT – IV
Brijlata Sharma
Assistant professor
JECRC, Jaipur

3. Learning Objectives
1. Causes of Failure
2. Criteria for Safe Design
3. Section of an Earth Dam
4. Seepage Analysis
5. Seepage Control Measures
6. Flow net

4. Causes of Failure
1. Upstream slope failure due to sudden drawdown
2. Failure by excessive pore pressure
3. Downstream slope failure by sliding
4. Failure due to settlement of foundation
5. Failure by sliding of foundation
6. Failure by spreading
1. Hydraulic Failures  40%
2. Seepage Failures  30%
3. Structural Failures  30%

5. Failure due to Tree roots
OverTopping
Gulling
Piping
Foundation
Settlement
Seepage through
Foundation
Sloughing
Toe erosion
Downstream
Sliding

6. Hydraulic Failures

7. Seepage Failures

8. Structural Failures

9. Structural Failures

10. Structural Failures

11. Structural Failures

12. Criteria for Safe Design
1. The upstream and downstream faces are properly
protected against wave action
2. There should not be any possibility of free passage of
water through the embankment or through the
foundation
3. The foundations, abutments, and embankment
must be stable for all conditions of operation
4. The dam as a whole should be earthquake resistant

13. Section of an Earth Dam
1.Top Width
2.Free Board
3.Casing or outer shells
4.Central Impervious Core
5.Cut-off Trench
6.Downstream drainage system

15. Top Width
1. The top width of the dam depends upon:
a) Construction material
b) Height of structure
c) Roadway
2. Empirical Formulae:
a) B = Z/5 +3  for very low
b) B = 0.55 Z1/2 + 0.2 Z for lower dams (<30m)
c) B = 1.65(Z+1.5)1/3  for higher dams (>30m)
Where Z = Height of dam
3. A minimum width of 6m is required for maintenance so that small trucks can
operate on it
4. The berm may be provided for the dam, which are more than 10 m in height.
Minimum berm width may be kept as 3 m.

16. Free Board
Free board is vertical difference between the horizontal crest of the embankment
and reservoir level.
Normal Free Board  Top of the crest to normal reservoir level
Minimum Free Board  Top of the crest to Maximum Reservoir Level during floods
The USBR suggests:
Minimum of 2 m and maximum of 3 m over maximum flood level

17. Casing or Outer Shells
1. The function of casing is to impart stability and protect the
core.
2. The relatively pervious materials, which are not subjected to
cracking on direct exposure to atmosphere, are suitable for
casing.
3. IS: 8826 – 1978 provides recommendations for suitability of soil
used for earth dams

18. Central Impervious Core
1. The core provides impermeable barrier within the body of the dam.
2. Impervious soils are generally suitable for the core (IS 1498 -1970). However soils
having high compressibility & liquid limit, and having organic contents may be
avoided, as they are prone to swelling & formation of cracks.
Following guidelines are recommended for design of core (for Small Dams)
1. The core may be located either centrally or inclined upstream.
2. The minimum top width should be kept 3 m
3. The top level of the core should be fixed at 0.5 m above MWL.
4. The side slopes may be kept 0.5:1 and 1:1.
5. Thickness of core at any section shall not be lesser than 30% (preferably not less than
50 percent) of maximum head of water acting at that section.

20. Cutoff Trench
1. To reduce loss of stored water through foundations and abutments
2. To prevent sub-surface erosion by piping.
The following guidelines may be adopted for design of cut off.
1. The cut off shall be located such that its centre line should be within the base of
impervious core and should be upstream of centre line of dam.
2. The positive cut off should be keyed at least to a depth of 0.4 metre into
continuous impervious sub stratum or in erodable rock formation.
3. A minimum bottom width of 4.0 metre is recommended.
4. Side slopes of at least 1:1 or flatter may be provided in case of over burden while
1/2:1 and 1/4:1 may be provided in soft rock and hard rock respectively.
5. The cut off in the flanks on either side should normally extend up to the top of
impervious core.

21. Downstream Drainage System
To ensure safety of dam, it is very important to handle the seepage water in the dam
so as to maintain the original particles of soils in their place.
The measures commonly adopted for safe disposal of seepage water through
embankment dams are;
1. Toe drain  installed in oldest homogenous dams to prevent softening at d/s
2. Horizontal filter  used in moderate high dams
3. Inclined or vertical filter (chimney filter)  used in higher homogeneous dams
Purpose:
 To reduce pore pressure in the downstream portion of the dame
 To control piping failure

22. Generally, a multi-layer filter or inverted filter is provided in which subsequent layer
becomes increasing coarser than the previous one.
According to Terzaghi, the filter material should fulfill the following criteria:
1. D15 of filter / D85 base material > 4 and < 20
2. D15 of filter / D15 base material < 5
3. The gradation curve of the filter material should be nearly parallel to the gradation
curve of the base material.
Note:
D15 is permeability protection limit, D85 is
piping predicting limit

23. Seepage Analysis
Objective:
To estimate the quantity of seepage (rate of leakage) through dam
using Flow Net
Assumptions:
1. Soil is homogeneous (Coefficient of permeability is constant
everywhere)
2. Soil is isotropic (Coefficient of permeability is same in all
directions)
3. Size of pore spaces do not change
4. Darcy’s law is valid (Flow is laminar) Q=kiA
5. Soil is completely saturated (Degree of saturation is 100%)
6. Hydraulic boundary conditions at entry and exit are known
7. During flow, volume of soil and water remains constant

24. Flow Net
A flow net is a graphical representation of the paths taken by water
in passing through soil.
Characteristics of Flow Net:
1.Flow lines represent flow paths of particles of water
2.Flow lines and equipotential line are orthogonal to each other
3.The area between two flow lines is called a flow channel
4.The rate of flow in a flow channel is constant (∆q)
5.Flow cannot occur across flow lines
6.An equipotential line is a line joining points with the same head
7.The velocity of flow is normal to the equipotential line
8.The difference in head between two equipotential lines is called
the potential
drop or head loss (∆h)
9.A flow line cannot intersect another flow line.
10.An equipotential line cannot intersect another equipotential line

25. Top Flow Line or Phreatic Line
Flow Line
Equipotential Line
Field
Flow Net for an Earth Dam
Phreatic Line is a seepage line separating saturated and unsaturated zones

26. Where k’ = Equivalent
Permeability of the
transformed field

28. Plotting of Phreatic Line
• In order to draw flow net, it is first essential to find
the location and shape of Phreatic
• line or top flow line separating Saturated and
Unsaturated Zones.
• Phreatic line can be located by
1.Graphical Method or Casagrande Method
2.Analytic Method
3.Experimental Method

29. 1. Draw Arc taking C as
centre and CF as Radius
Discharge through the body of dam, q = k s
(for Isotropic Soils)
2. Draw Directrix and
Find Focal Distance S
3. Draw Parabola
curve
(y2= 2sx + s2)

30. Seepage Control Measures
Seepage Control measures are required to prevent adverse effects of
water
percolating through embankment and its foundation.
1. Embankment Seepage Control
a) Toe Filter
b) Horizontal Drainage Filter
c) Percolating Filter d/s of toe
d) Embankment Zoning
e) Chimney Drains extending upwards to embankment
2. Foundation Seepage Control
a) Impervious Cutoff
b) Upstream Impervious Blanket
c) D/s Seepage berms
d) Drainage Trenches
e) Relief Wells

31. 0.3H to 0.4H
H
0.25L to L
L
To keep phreatic line well within the section of embankment and also facilitates drainage

36. Reference
Chapter 10
Irrigation and Water Power Engineering
By Dr. B. C. Punmia, Dr.
Pande Brij Basi Lal, Ashok
Kr. Jain,
Arun Kr. Jain