weirs on permeable foundations

WEIRS ON PERMEABLE

FOUNDATIONS(WATER RESOURCES ENGINEERING – II)

UNIT – VI

Rambabu Palaka, Assistant ProfessorBVRIT

Diversion Headworks

Learning Objectives

1. Causes of Failures of Weirs on Permeable

Foundations

2. Find Uplift Pressure and Thickness of Floor using

Bligh’s Creep Theory

Lane’s Weighted Creep Theory

Khosla’s Theory

Application of Correction Factors

3. Launching Apron

Causes of Failures of Weirs on Permeable Foundations

Causes of Failure:

1. Due to Seepage or Sub-surface Flow

a) Piping or Undermining

b) Rupture of Floor by Uplift Pressure

2. Due to Surface Flow

a) By Suction due to Hydraulic Jump

b) By Scour on the u/s and d/s of the weir

Design of Impervious Floor

Directly depended on the possibilities of percolation in porous subsoil

Water from upstream percolates and creeps (or travel) slowly through weir base

and the subsoil below it.

The head lost by the creeping water is proportional to the distance it travels (creep

length) along the base of the weir profile.

The creep length must be made as big as possible so as to prevent the piping

action. This can be achieved by providing deep vertical cut-offs or sheet piles

1. Bligh’s Creep Theory (1912)

2. Lane’s Weighted Creep Theory (1932)

3. Khosla’s Theory (1936)

Bligh’s Creep Theory (1912)

Assumptions:

1. Hydraulic Gradient is constant throughout the impervious length of the apron.

2. Creep Length is the sum of horizontal and vertical creep.

3. Stoppage of percolation by cut off or sheet pile possible only if it extends up to

impermeable soil strata.

Creep Length:

Coefficient of Percolation:

(Loss of head per unit length of creep)

Coefficient of Creep:


Design Criteria:

a) Safety against Piping

Safe Creep Length, L = C.H

where C = Coefficient of Creep = 1/c

b) Safety against Uplift Pressure

Floor Thickness,

where

h = Ordinate of Hydraulic Gradient Line measures above the top of floor

ρ = Specific Gravity of Floor Material

Nominal Thickness of 1 m

Nominal Thickness of 1.5 m

L2 = L – l1 – (b + 2d1 + 2d2)

b


d1 = HFL – Max. Scour Depth d2 = HFL after Retrogation – Max. Scour Depth

DESIGN


Limitations:

1. No distinction between horizontal and vertical creep.

2. Holds good so long as horizontal distance between the pile lines is greater than

the twice their depth

3. Did not explain about Exit Gradient

4. 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.

5. Losses of head does not take place in the same proportions as the creep length.

Also the uplift pressure distribution is not linear but follow a sine curve

6. Bligh does not specify the absolute necessity of providing a sheet pile at

downstream which is essential to prevent undermining or piping.

Lane’s Weighted Creep Theory (1932)

From the analysis of 200 dams all over the world, Lane’s concluded that horizontal

creep is less effective in reducing uplift than vertical creep. Therefore, he suggested

a factor of 1/3 for horizontal creep against 1 for the vertical creep

Assumptions:

1. Slopes steeper than 450 are taken as Verticals (d)

2. Slopes less than 450 are taken as Horizontals (l)

Creep Length:


Khosla’s Theory (1936)

After studying a dam failures constructed based on Bligh’s theory, Khosla came out

with the following;

1. Outer faces of end sheet piles were much more effective than the inner ones

and the horizontal length of the floor.

2. Intermediated piles of smaller length were ineffective except for local

redistribution of pressure.

3. Undermining of floor started from tail end.

4. It was absolutely essential to have a reasonably deep vertical cut off at the

downstream end to prevent undermining.


Horizontal Floor with negligible small thickness:


Special Cases:

Straight horizontal floor of negligible thickness with

1. Pile at upstream ends.

2. Pile at downstream end.

3. Pile at intermediate points.

4. Depressed below the bed

(with no cutoff)

Where

b = Length of weir foundation

d = depth of pile

Φ = Percentage of Pressure at a

given point

H = Height of Water

Example:

PD = (ΦD /H) 100

Khosla’s Curve for Exit Gradient


Most designs do not confirm to elementary profiles (specific cases). In actual cases,

we may have a number of piles at upstream level, downstream level and

intermediate points and the floor also has some thickness.

Method of independent variable:

This method consists of breaking up a complex profile into a number of simple

profiles. The pressures obtained at the key points by considering simple profile are

then corrected for the following:

1. correction for the thickness of floor

2. correction for mutual interference of piles

3. correction for slope of the floor.

U/S Pile

Intermediate Pile:

D/S Pile:

Where

C = Percentage of Correction to

be applied the pressure head

Inverted Filter and Launching Apron

Inverted Filter:

An inverted filter is provided immediately at the end of d/s impervious apron to

relieve the pressure.

Approximate length is 1.5 d2

Launching Apron:

After the inverted filter, a launching apron is provided to protect the d/s pile from

scour holes progressing in the u/s direction.

Approximate length is 2.5 d2

A similar launching apron is provided to the u/s side with a length equal to 2d1

Previous Questions

1. Describe with the help of suitable sketches Bligh's creep theory for the safe

design of apron in an irrigation work

2. How does Lane’s theory differ from Bligh’s Creep Theory

3. Discuss Khosla's theory for design of weirs on permeable foundations,

Enumerate the various corrections that are needed in its application

4. Discuss utility and limitations of Khosla`s theory

5. Explain salient features of Khosla’s theory and how it is used in the design of

permeable foundations

6. Compare the Bligh’s and Khosla’s theories for the design of impervious floor

Reference

Chapter 12

Irrigation and Water Power EngineeringBy Dr. B. C. Punmia,

Dr. Pande Brij Basi Lal,

Ashok Kr. Jain,

Arun Kr. Jain

weirs on permeable foundations

Education