Diversion head works ajitha miss
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Transcript of Diversion head works ajitha miss
DIVERSION HEADWORKSDIVERSION HEADWORKS
CANAL HEADWORKSCANAL HEADWORKS
Canal head works– Structures/works constructed across river and at the
head of the off taking canal
Canal head works
Diversion head works
To raise water level in river and divert the required quantity
Storage head works
To store water on u/s of river and divert the required quantity
DIVERSION HEADWORKSDIVERSION HEADWORKS
Purposes
– Raises water level in the river
– Regulates supply of water into the canal
– Controls the entry of silt into the canal
– Provides some storage for a short period
– Reduces the fluctuations in the level of supply in river
TYPES OF DIVERSION HEAD WORKSTYPES OF DIVERSION HEAD WORKS
1. Temporary diversion head works
– Consists of a bund constructed across river to raise the water level in the river and will be damaged by floods
2. Permanent diversion head works
– Consists of a permanent structure such as a weir or barrage constructed across river to raise water level in the river
LOCATION OF CANAL HEAD WORKSLOCATION OF CANAL HEAD WORKS
Depends on the stages of flow (reaches) of river
(i) Rocky stage
(ii) Boulder stage
(iii) Trough stage or alluvial stage
(iv) Delta stage
Both rocky and delta stages are not suitable for location of diversion head works
SUITABLE SITE FOR DIVERSION SUITABLE SITE FOR DIVERSION HEAD WORKSHEAD WORKS
Having selected the reach of the river, selection suitable site in accordance with the following considerations
1. As far as possible, a narrow, straight, well defined channel confined between banks not submerged by the highest flood
2. Should be possible to align the off taking canal in such a way that command of its area is obtained without excessive digging
3. Materials of construction such as stone, sand etc. should be available in the vicinity of the site
4. Site should be accessible by rail or road
SUITABLE SITE FOR DIVERSION SUITABLE SITE FOR DIVERSION HEAD WORKSHEAD WORKS
COMPONENTS OF DIVERSION COMPONENTS OF DIVERSION HEADWORKSHEADWORKS
1. Weir or Barrage 2. Divide wall or divide groyne 3. Fish ladder 4. Pocket or approach channel 5. Under sluices or scouring sluices 6. Silt excluder 7. canal head regulator 8. River training works such as marginal bunds and guide
bunds
COMPONENTS OF DIVERSION COMPONENTS OF DIVERSION HEADWORKSHEADWORKS
WEIRWEIR
Weir is a structure constructed across river to raise the water level and divert the water into the canal
Weir aligned at right angle to the direction flow
Shutters are provided at the crest of the weir so that part of raising up of water is carried out by shutters
According to the material used for construction and certain design features
1. Masonry weirs with vertical drop walls
2. Rock fill weirs with sloping aprons
3. Concrete weirs with a downstream glacis
CLASSIFICATION OF WEIRSCLASSIFICATION OF WEIRS
MASONRY WEIR WITH VERTICAL MASONRY WEIR WITH VERTICAL DROPDROP
Weir consists of– Impervious horizontal floor or apron
– A masonry weir wall with either side vertical; or both faces inclined; or u/s face vertical and d/s face inclined
– Curtain walls or cutoffs or piles are provided at the u/s and d/s ends of the floor
– Block protection at the u/s end and graded inverted filter at the d/s end
– Longing aprons or pervious aprons after block protection graded filter
MASONRY WEIR WITH VERTICAL MASONRY WEIR WITH VERTICAL DROPDROP
ROCKFILL WEIRS WITH SLOPING ROCKFILL WEIRS WITH SLOPING APRONSAPRONS
Weir consists of
– A masonry weir wall
– Dry packed boulders laid in the form of glacis or sloping aprons
– Some intervening core walls
ROCKFILL WEIRS WITH SLOPING ROCKFILL WEIRS WITH SLOPING APRONSAPRONS
CONCRETE WEIRS WITH CONCRETE WEIRS WITH DOWNSTREAM GLACISDOWNSTREAM GLACIS
Floor made of concrete
Sheet piles of sufficient depth provided at the u/s and d/s ends
Sometimes intermediate piles are also provided
Hydraulic jump is developed at the d/s slope due to which considerable amount of energy is dissipated
Suitable on pervious foundations
CONCRETE WEIRS WITH CONCRETE WEIRS WITH DOWNSTREAM GLACISDOWNSTREAM GLACIS
BARRAGEBARRAGE
Crest is kept at a low level
Raising up of water level is accomplished by means of gates
During floods, these gates are raised and clear off the high flood level
CAUSES OF FAILURES OF WEIRS ON CAUSES OF FAILURES OF WEIRS ON PERMIABLE FOUNDATIONSPERMIABLE FOUNDATIONS
Causes of failures
– Due to seepage or subsurface flow
– Due to surface flow
Due to subsurface flow– Piping or undermining
– By uplift pressure
Due to surface flow
– By suction due to hydraulic jump
– By scour on the u/s and d/s of the weir
CAUSES OF FAILURES OF WEIRS ON CAUSES OF FAILURES OF WEIRS ON PERMIABLE FOUNDATIONSPERMIABLE FOUNDATIONS
DESIGN OF IMPERVIOUS FLOOR FOR DESIGN OF IMPERVIOUS FLOOR FOR SUBSURFACE FLOWSUBSURFACE FLOW
Bligh’s creep theory
Khosla’s theory
BLIGH’S CREEP THEORYBLIGH’S CREEP THEORY Design of impervious floor or apron
– Directly depend on the possibilities of percolation in the porous soil on which the apron is built
Bligh assumed that– Hydraulic gradient is constant throughout the
impervious length of the apron– The percolating water creeps along the contact of base
profile of the apron with the sub-soil, losing head enroute, proportional to the length of its travel
– Stoppage of percolation by cut off (pile) possible only if it extends up to impermeable soil strata
Bligh designated the length of travel as ‘creep length’ and is equal to the sum of horizontal and vertical length of creep
BLIGH’S CREEP THEORYBLIGH’S CREEP THEORY
If ‘H’ is the total loss of head, loss of head per unit length of creep (c),
c-percolation coefficient
Reciprocal of ‘c’ is called ‘coefficient of creep’(C)
BLIGH’S CREEP THEORYBLIGH’S CREEP THEORY
Design criteria
(i) Safety against piping
Length of creep should be sufficient to provide a safe hydraulic gradient according to the type of soil
Thus, safe creep length,
Where, C= creep coefficient=1/c
BLIGH’S CREEP THEORYBLIGH’S CREEP THEORY
Design criteria
(ii) Safety against uplift pressure
Let ‘h’’ be the uplift pressure head at any point of the apron
The uplift pressure = wh’
This uplift pressure is balanced by the weight of the floor at this point
BLIGH’S CREEP THEORYBLIGH’S CREEP THEORY
If, t =thickness of floor at this point G = specific gravity of floor material Weight of floor per unit area
=
BLIGH’S CREEP THEORYBLIGH’S CREEP THEORY
BLIGH’S CREEP THEORYBLIGH’S CREEP THEORY
LIMITATIONS OF BLIGH’S THEORYLIMITATIONS OF BLIGH’S THEORY
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. Also the uplift pressure distribution is not linear but follow a sine curve
Bligh did not specify the absolute necessity of providing a cutoff at the d/s end
LIMITATIONS OF BLIGH’S THEORYLIMITATIONS OF BLIGH’S THEORY
LANE’S WEIGHTED CREEP THEORYLANE’S WEIGHTED CREEP THEORY
An improvement over Bligh’s theory
Made distinction between horizontal and vertical creep
Horizontal creep is less effective in reducing uplift than vertical creep
Proposed a weightage factor of 1/3 for horizontal creep as against the 1 for vertical creep
KHOSLA’S THEORYKHOSLA’S THEORY
Dr. A. N. Khosla and his associates done investigations on structures designed based on Bligh’s theory and following conclusions were made
– The outer faces of sheet piles are much more effective than inner ones and the horizontal length of floor
– The intermediate sheet piles, if smaller in length than the outer ones were ineffective
– Undermining of floors started from the tail end. If hydraulic gradient at exit is more than the critical gradient, soil particles will move with water and leads to failure
– It is absolutely essential to have reasonably deep vertical cutoff at the d/s end to prevent undermining
KHOSLA’S THEORYKHOSLA’S THEORY
Khosla and his associates carried out further research to find out a solution to the problem of subsurface flow and provided a solution
– Khosla’s theory
– Considered the flow pattern below the impervious base of hydraulic structures on pervious foundations to find the distribution of uplift pressure on the base of the structure and the exit gradient
KHOSLA’S THEORYKHOSLA’S THEORY
KHOSLA’S METHOD OF KHOSLA’S METHOD OF INDEPENDENT VARIABLESINDEPENDENT VARIABLES
A composite weir section is split up into a number of simple standard forms
The standard forms(a) A straight horizontal floor of negligible thickness with a sheet pile either at the u/s end or at the d/s end of the floor
(b) A straight horizontal floor of negligible thickness with a sheet pile at some intermediate point
(c) A straight horizontal floor depressed below the bed but with no vertical cutoff
KHOSLA’S METHOD OF KHOSLA’S METHOD OF INDEPENDENT VARIABLESINDEPENDENT VARIABLES
These standard cases were analyzed by Khosla and his associates and expressions were derived for determining – The residual seepage head (uplift pressure) at key points
(key points are the junction points of pile and floor, bottom point of pile and bottom corners of depressed floor)
– Exit gradient
– These results are presented in the form of curves
KHOSLA’S METHOD OF KHOSLA’S METHOD OF INDEPENDENT VARIABLESINDEPENDENT VARIABLES
The curves gives the values of Φ (the ratio of residual seepage head and total seepage head) at key points
The directions for reading the curves are given on the curves itself
The curves are for specific cases only In actual practice
– consider the assembled profile with piles at u/s end, d/s end, intermediate point, floor has some thickness and slope
– combination of simple profiles needs to be considered– Corrections need to be applied
1. Correction for thickness of floor
2. Correction for mutual interference of piles
3. Correction for slope of the floor
(i) Straight floor of negligible thickness with pile at u/s end (ii) Straight floor of negligible thickness with pile at some
intermediate point (iii) Straight floor of negligible thickness with pile at d/s end The pressure obtained at the key points from curves are then
corrected for (i) Thickness of floor (ii) Interference of piles (iii) Sloping floor
CORRECTION FOR THICKNESS OF FLOORCORRECTION FOR THICKNESS OF FLOOR
Pressure at actual points C1 and E1 can be computed by considering linear variation of pressure between point D and points E and C
When pile is at u/s end, Correction for
Pressure at
For the intermediate pile,
Correction for
Correction for
When pile at d/s end,
Correction for
CORRECTION FOR THICKNESS OF FLOORCORRECTION FOR THICKNESS OF FLOOR
Percentage correction for mutual interference of piles (C)
d- depth of pile on which the effect of another pile of depth D is required to be determined
D- depth of pile whose effect is to be determined on the neighbouring pile of depth d
CORRECTION FOR MUTUAL CORRECTION FOR MUTUAL INTERFERENCE OF PILESINTERFERENCE OF PILES
This correction is positive for points in the rear and subtractive for points in the forward direction of flow
For example, if we want to find the interference of pile no. 2 on pile no.1, the correction will be positive as point C is on rear side of pile 2
CORRECTION FOR MUTUAL CORRECTION FOR MUTUAL INTERFERENCE OF PILESINTERFERENCE OF PILES
CORRECTION FOR SLOPECORRECTION FOR SLOPE
The % pressure under a floor sloping down is greater than that under a horizontal floor
The % pressure under a floor sloping up is less than that under a horizontal floor
Correction is plus for down slopes and minus for up slopesSlope (vertical/horizontal) Correction (%)
1 in 1 11.2
1 in 2 6.5
1 in 3 4.5
1 in 4 3.3
1 in 5 2.8
1 in 6 2.5
1 in 7 2.3
1 in 8 2.0
The corrections given table are to be further multiplied by the proportion of horizontal length of slope to the distance between the two pile lines in between which the sloping floor is located
The slope correction is applicable only to that key points of pile line which is fixed at the beginning or end of the slope
CORRECTION FOR SLOPECORRECTION FOR SLOPE