Excerpt
2
1.1 - Bligh's Creep Theory
To prevent internal erosion and particle migration, control of seepage pressures and velocities
must be given due consideration in the design of hydraulic structures.
The percolation length (seepage) for a foundation can be determined by using various methods.
There are number of methods available to analyze the problem on seepage and uplift pressure, and one of
which is Bligh's theory of creep. Other methods are Lane's Method, Kosla's Theory and Flow nets.
Based on Bligh's theory, that along the bottom contour of the structure, the water creeps, and the
percolation length (seepage) can be determined.
1
1.2 Concept of the Theory:
The water which percolates into the foundation creeps through the joint between the profile of the
base of structure and the subsoil.
2
The seeping water comes out at the downstream end. Then, water travels along the vertical,
horizontal or inclined path without making any distinction.
3
The head of water lost in the path of percolation is the difference of water levels on the upstream
and the downstream ends. The imaginary line which joins the water levels on the upstream and the
downstream end is called a hydraulic gradient line.
4
(Refer to Figure 1)
Figure 1: Illustration of Bligh's creep theory (Patterned from: Santosh Kumar Garg,
Irrigation Engineering and Hydraulic Structures, 2006)
1
Santosh Kumar Garg, Irrigation Engineering and Hydraulic Structures, Khanna Publishers, Delhi:2006, p 553
2
Shreyasi Sen, "Bligh's Creep Theory for Design of Weir on Permeable Foundation", Your Article Library,
http://www.yourarticlelibrary.com/water/water-engineering/blighs-creep-theory-for-design-of-weir-on-permeable-
foundation/61166/
3
Ibid
4
"Bligh's Creep Theory for Seepage Flow", The Constructions of Deductions, 2015,
https://yamannvinci069.blogspot.com/2015/08/blighs-creep-theory-for-seepage-flow.html
3
1.3 Summary of Bligh's assumptions:
5
The percolating water creeps along the base profile of the structure, which is in contact with the
subsoil.
Line of creep is the path of percolation along the base of the structure.
Creep length is the total length of the path traversed by the percolating water. Total length
covered by the percolating water till it emerges out at the downstream end.
The length of the line of creep includes the vertical distances along both sides of cutoff walls or
curtain of sheet piling (if any), as well as horizontal distance along the apron.
The head loss per unit length of creep (hydraulic gradient) is proportional to the distance of the
point from the upstream of the foundation (constant).
The limitation of this theory is that it does not differentiate between the horizontal and vertical creeps in
estimating the exit hydraulic gradient.
The Creep length, L
6
The hydraulic gradient or the loss of head per unit length of creep is,
Refer to Figure 1:
where:
b = the total horizontal distance
d1; d2; d3 are length of piles
H = total height of water upstream
Based on the assumptions, the following conclusions are derived:
· For any point, the head loss is proportional to the creep length.
· As the hydraulic gradient is constant, if L
1
is the creep length up to any point, then head loss up to
any point will be (H/L) L
1
and the residual head at this point will be (H - (H/L) L
1
).
· Considering the cutoffs, the head losses will be:
(H/L) 2d
1
, (H/L) 2d
2
and (H/L) 2d
3
· The reciprocal of the hydraulic gradient (L/H) is known as Bligh's coefficient of creep, C= L/H.
5
"Bligh's Creep Theory", http://www.aboutcivil.org/bligh%27s-creep-theory-of-hydraulic-structures.html
6
Santosh Kumar Garg, Irrigation Engineering and Hydraulic Structures, Khanna Publishers, Delhi:2006, p 554
L= b + 2d
1
+ 2d
2
+ 2d
3
3
2d
2
2d
1
2d
b
H
L
H
4
1.4 Safety against piping:
7
In order to ensure that the structure is safe againt piping, the following should be taken into
consideration:
· The creep length should be sufficient to provide a safe hydraulic gradient according to the
type of soil (L = CH)
· Bligh recommended certain values of C for different soils presented in Table 1.
· The hydraulic gradient (H/L) should be equated to 1/C
H/L = 1/C (for the soil)
Table 1: Bligh coefficient of creep C and safe hydraulic gradient
Type of soil
Value of C
Safe
Hydraulic Gradient
Light sand &mud (River Nile)
18
1/18
Fine Micaceous sand
15
1/15
Coarse grained sand
12
1/12
Sand mixed with boulder and
gravel; and for loam soil
5 to 9
1/9 to1/5
Gravel
5
1/5
Source:
Shreyasi Sen, "Bligh's Creep Theory for Design of Weir on Permeable Foundation", Your Article Library,
http://www.yourarticlelibrary.com/water/water-engineering/blighs-creep-theory-for-design-of-weir-on-permeable-
foundation/61166/
1.5 Safety against uplift pressure:
8
· The uplift pressure (residual seepage head) at that point is the ordinate of the subsoil hydraulic
gradient line above the bottoms of the floor at any point.
Figure 2. Uplift Pressure Diagram (Source:
Author's own work)
7
Ibid
8
M. R. Kabir, ECONOMICAL & PHYSICAL JUSTIFICATION FOR CANAL, Chapter 5, http://www.uap-
bd.edu/ce/Handouts/CE-461/Doc/Chapter-5.pdf
5
If h' is the uplift pressure head at a point under the floor, the pressure intensity is,
9
This pressure is to be resisted by the weight of the floor with thickness (t) and density
m
(for
concrete = 2400 kg/m
3
).
The downward force per unit area due to the weight of the floor is:
Therefore, equating
which gives:
Where:
S
m
is the relative density of the floor material. Thus, we can write,
and for the thickness of the floor,
10
where h is the pressure head (ordinate of hydraulic gradient) measured above the top of floor, and
(S
m
-1) is submerged specific gravity of the floor material.
A safety factor of 4/3 to 3/2 can be applied in the design of thickness:
Assuming a value of
Sm= 2.24,
Then,
t 1.08 h to 1.2 h
9
Santosh Kumar Garg, Irrigation Engineering and Hydraulic Structures, Khanna Publishers, Delhi:2006, p 555
10
"Bligh's Creep Theory for Seepage Flow", The Constructions of Deductions, 2015,
https://yamannvinci069.blogspot.com/2015/08/blighs-creep-theory-for-seepage-flow.html
h
g
P
t
g
W
m
'
gh
t
g
m
t
S
t
h
m
m
t
t
S
t
h
m
1
1
m
m
S
h
S
t
h
t
1
2
3
1
3
4
m
m
S
h
to
S
h
t
6
For design efficiency, the following notes should be taken into consideration:
- The design will be economical if the greater part of the creep length (impervious floor) is
provided upstream of the weir where nominal floor thickness would be sufficient.
- The downstream floor has to be thicker to resist the uplift pressure. However, a minimum floor
length is always required to be provided on the downstream side from the consideration of surface
flow to resist the action of fast flowing water whenever it is passed to the downstream side of the
weir.
- The provision of maximum creep length on the upstream side of the barrier structure also reduces
uplift pressures on the portion of the floor provided on the downstream side of the barrier.
- A vertical cutoff at the upstream end of the floor reduces uplift all over the floor.
- According to Bligh's theory, a vertical cutoff at the upstream end of the floor is more useful than
the one at the downstream end of the floor.
1.6 Limitations of Bligh's Theory
11
12
13
Bligh made no distinction between horizontal and vertical creep.
The theory holds good as long as horizontal distance between cut-offs or pile lines is greater than
twice their depth.
No distinction is made between the effectiveness of the outer and inner faces of sheet piles and
short and long intermediate piles.
Later investigations have shown that the outer faces of the end piles are much more effective than
the inner ones.
Intermediate piles of shorter length than the outer ones are ineffective except for local
redistribution of pressure.
No indication on the significance of exit gradient.
Average value of hydraulic gradient gives idea about safety against piping.
For safety purposes, the exit gradient must be less than critical exit gradient.
The loss of head is proportional to creep length (assumption) is not true and actual uplift pressure
distribution is not linear, but it follows a sine curve.
Bligh did not specify the absolute necessity of providing a cut-off at the downstream end of the
floor, whereas it is absolutely essential to provide a deep vertical cutoff at the downstream end of
the floor to prevent undermining.
11
Shreyasi Sen, "Bligh's Creep Theory for Design of Weir on Permeable Foundation", Your Article Library,
http://www.yourarticlelibrary.com/water/water-engineering/blighs-creep-theory-for-design-of-weir-on-permeable-
foundation/61166/
12
"Bligh's Creep Theory", http://www.aboutcivil.org/bligh%27s-creep-theory-of-hydraulic-structures.html
13
13
Fetene Nigussie, Hydraulic Structures II-Lecture Note, https://www.scribd.com/document/305701128/Chapter-4-PART-1
Excerpt out of 16 pages
- Quote paper
- Florante Jr Poso (Author), 2017, Bligh's & Lane's Theory of Seepage, Munich, GRIN Verlag, https://www.grin.com/document/373936
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