The document discusses the design and construction of marginal embankments and guide banks for irrigation projects. It also discusses potential failure modes of weirs and barrages built on permeable foundations, including piping, undermining from subsurface flow, and scouring from surface flow. It describes Bligh's theory and Lane's weighted creep theory for designing impervious floors to prevent uplift and piping. Precautions that can be taken include increasing the impervious floor thickness, providing sheet piles, and using energy dissipation structures.

1. 07. Diversion Headworks-3
B.Sc. Civil Engineering 8th Semester
Muhammad Ajmal (PhD)
Lecturer
Agri. Engg. Deptt.
CE-402 Irrigation Engineering
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2. Marginal Embankments or dykes
 The marginal embankments or dykes are earthen embankments which are constructed parallel to
the river bank on one or both the banks according to the condition.
 The top width is generally 3 to 4 m and side slope is generally 1 ½ : 1 to 2: 1.
 The height of the embankment depends on the highest flood level.
 A suitable margin is provided between the toe of the embankment and the bank of the river.
 To resist the effect of erosion on the embankment, wooden piles are driven along the river
banks throughout the length of dyke.
 The length of the dyke is protected by boulders pitching with cement grouting and the
downstream side is protected by turfing.
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3.  The Marginal Bunds are constructed for the following purposes.
(a) It prevents the flood water or storage water from entering the surrounding area.
(b) It retains the flood water or storage water within a specified section.
(c) It Protects the towns and village from devastation during the heavy flood.
(d) It protects valuable agricultural lands.
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Marginal Embankments or dykes

4. Marginal Embankments
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5. Guide Bank
 When a barrage is constructed across a river which flows through the alluvial soil, the guide
banks must be constructed on both the approaches to protect the structure from erosion. It is
an earthen embankment with curved head on both the ends.
 Functions of the Guide Bank
 It protects the barrage from the effect of scouring and erosion.
 It controls the tendency of changing the course of the river.
 It controls the velocity of the flow near the structure.
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6. Guide Bank
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7. Causes of Failure of weir or barrage on permeable foundation
 The combined effect of surface flow and subsurface flow may cause the failure of the weir or
barrage.
(i) Failure due to subsurface flow
(a) By piping or undermining:
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8. By Piping or Undermining
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9. Causes of Failure of weir or barrage on permeable foundation
(a) By piping or undermining:
 Video
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10. Causes of Failure of weir or barrage on permeable foundation
(a) By piping or undermining:
 The water from the upstream side continuously percolates through the bottom of the
foundation and emerges at the downstream end of the weir or barrage floor.
 The force of percolating water removes the soil particles by scouring at the point of
emergence.
 As the process of removal of soil particles goes on continuously, a depression is formed
which extends backwards towards the upstream through the bottom of the foundation.
 A hollow pipe like formation thus develops under the foundation due to which the weir or
barrage may fail by subsiding. This phenomenon is known as failure by piping or
undermining.
 Video
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11. By Uplift Pressure
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12. Causes of Failure of weir or barrage on permeable foundation
(b) By uplift Pressure: The percolating water exerts an upward pressure on the foundation of the
weir or barrage. If this uplift is not counterbalanced by the self weight of the structure, it may fail
by rupture.
2. Failure by Surface Flow:
(a) By Hydraulic Jump: When the water flows with a very high velocity over the crest of the
weir or over the gates of the barrage, then hydraulic jump develops. This hydraulic jump causes a
suction pressure or negative pressure on the downstream side which acts in the direction of uplift
pressure. If the thickness of the impervious floor is not sufficient, then the structure fails by
rupture.
(b) By Scouring During floods: The gates of the barrage are kept open and the water flows with
high velocity. The water may also flow with very high velocity over the crest of the weir. Both the
cases can result in scouring effect on the downstream and on the upstream side of the structure.
Due to scouring effect on the downstream and on the upstream side of the structure, its stability
gets endangered by shearing.
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13. By Hydraulic Jump
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14. By Hydraulic Jump
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15. By Scouring During floods
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16. Precautions Against Failure
 The following precautions can be taken to prevent failure.
(a) The length of the impervious layer should be carefully designed so that the path of the
percolating water is increased consequently reducing the exit gradient.
(b) Sheet piles should be provided on the upstream side and the downstream side of the
impervious floor to increase the length of percolating water so that the uplift pressure is
considerable reduced.
(c) The thickness of the impervious floor should be such that the weight of the floor is
sufficient to counterbalance the uplift pressure.
(d) Energy dissipater blocks like friction blocks, impact blocks, should be provided.
(e) Inverted filter should be provided with concrete blocks on the top so that the percolating
water does not wash out the soil particles.
(f) Deep foundation like well foundation should be provided for the barrage piers
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17. Precautions Against Failure
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18. Increasing Floor Thickness
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19. Bligh’s Theory
 Design of Impervious Floor for Sub surface
 The foundation seepage may cause damage to structure by Uplift pressure and Piping.
 A number of theories have been developed to control this problem. Like
(1) Bligh’s Creep Theory (2) Lane’s Weighted Creep Theory and (3) Khosla’s Theory
(1) Bligh’s Creep Theory:
 According to this theory the percolating water follows the outline of the foundation base
of the hydraulic structure.
 Similarly the water percolating to creep along the base profile of the apron with the sub
soil proportional to the length of its travel and is known as length of creep.
 It is assumed that the gradient is constant through the impervious length of the apron. This
length is the sum of horizontal and vertical length of creep and the loss of head is
proportional to the length of the creep.
 If H is the total head loss between the U/S and D/S and length of the creep is L, then:
Note:
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20. Bligh’s Theory
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Source: B.C Punmia et al.

21. Bligh’s Theory
 Design Criteria
 Safety against piping
 The length of creep should be sufficient to provide a safe hydraulic gradient according to the
type of the soil.
 Thus the creep length is given by; L = CH, C = 1/c = coefficient of creep
(1) Safety against uplift pressure
Note:
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22. Bligh’s Theory
 Safe “C” values for different soils
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Type of soil Value of C (C =1/c)
Boulder+ Gravel + Sand 4 to 6
Gravel + Sand 9
Fine sand 15
Very Fine sand and silt 18
Coarse sand 12
Clay soil 1.6 to 3 (Hard is 1.6 soft is 3)

23. Bligh’s Theory Limitations
 Bligh made no distinction between horizontal and vertical creep.
 Did not explain the idea of exit gradient - safety against undermining cannot simply be
obtained by considering a flat average gradient but by keeping this gradient will be low
critical.
 No distinction between outer and inner faces of sheet piles or the intermediate sheet piles,
whereas from investigation it is clear, that the outer faces of the end sheet piles are much
more effective than inner ones.
 Losses of head does not take place in the same proportions as the creep length.
 The uplift pressure distribution is not linear but follow a sine curve.
 Bligh’s does not specify the absolute necessity of providing a sheet pile at D/S where as it is
essential to have a deep vertical cut off at D/S end to prevent undermining.
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24. Lane’s Weighted Creep Theory
 Based on statistical investigation of as many as 278 dams, weirs, and barrages across the
globe, Lane observed that vertical creep is more effective than the horizontal creep.
 He modified the Bligh’s creep theory by evolving ‘Lane weighted creep theory’.
 According to this theory, the weighted creep length (Lw) is given by;
𝐿𝑤 =
1
3
𝑙 + 𝑉
l = sum of all horizontal contacts and all sloping contacts having slope less than 45o
V = sum of all vertical contacts and all the sloping contacts steeper than 45o
Hence for Previous Figure 12.6 (b)
Lw =
1
3
𝑙 + 2𝑑1 + 2𝑑2
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25. Lane’s Weighted Creep Theory
 To ensure safety against piping, Lane suggested that the weighted creep length must not be
less than the following:
𝐿𝑤 = 𝐶𝑤𝐻
Where
𝐿𝑤 = weighted creep length
𝐶𝑤= Lane’s creep coefficient, the value of which depends on the type of soil [Table 12.2, Punmia]
 Lane’s theory though an improvement over the Bligh’s creep theory, is empirical and lacked the
background for a rational basis of design.
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 Example
 Figure 12.8 shows the sections of a hydraulic structure founded on sand. Calculate the average
hydraulic gradient. Also, find the uplift pressure at points 6, 12, and 18 m from the u/s end of the
floor and find the thickness of the floor at these points.
Solution : Note

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Quiz#2