Download - Adg1 19-10-11
DAMS AND BARRAGES
Irrigation projects
Requirement :
Source of water – River (Surface water), Underground water
Source of water – River (Surface water), Underground water
Cultivable area – Area suitable for irrigation
Good Agriculture practice
Cultivable area – Area suitable for cultivation
Area sown – Area where seeds are placed
Command area – Area suitable for cultivation and receive water
Irrigable area – Area irrigated receiving water
Water is a natural resource and precipitation flow along natural contours
Water is available through surface flows – Rivers and Tanks
Through ground water resources – wells, springs
Civilization started first near water source – that is man settled near water source
The Mohenjodaro is the city built 5000 years ago.
The city was destroyed and rebuilt not less than seven times.
Indus Valley Civilization is first known one where man constructed cities
Indus Valley Civilization is similar to Nile Valley Civilization, Tigris Civilization.
Recent excavations show the city is fairly big
Dwelling houses are many in number
The construction is with brick
Big houses have two or more storyies
There are big structures containing large pillered halls
Streets of the city are wide and straight
Nagarjuna Konda where civilization survived near Krishna river
Civilization extended to distant places far away from rivers
Man through his ingenuity able to bring water by gravity to even parched deserts by constructing diversion structures or storage reservoirs
Now it is also possible to lift waters to a greater heights by lift schemes and by canal system to the higher fields
Harnessing of surface water for irrigation / water supplyRivers are of two categoriesi. Perennial Riversii. Non Perennial RiversPerennial river – water flows throughout the year – Ganga, Brahmaputra (North Indian Rivers)Non Perennial river – water flows only for few months – Godavari, Krishna (South Indian Rivers)Irrigation works are classified asi. Diversion works – Weir, Barrageii. Storage work – Dams
In diversion work, on obstruction called Weir or an anicut is constructed across the river to raise the bed.
Before Obstruction
After Obstruction
This rise in bed results raise in water level.
With increased water level, waters are diverted
There will be no storage and function only when there is water in level
Sometimes shutters will be provided to store the water
Shutters in raised position Shutters in fallen position
This rise in bed results raise in water level.
With increased water level, waters are diverted
There will be no storage and function only when there is water in level
Sometimes shutters will be provided to store the water
The elements of diversion work are
i. Body wall
ii. Head sluice
Storage work – Solid Barrier of considerable height across the river at suitable location – Dam
A reservoir (big tank) is formed in the river valley
Components of storage work
A non over flow section
A spill way to dispose off flood water
Head sluices on either side to tap water from reservoir
River sluices to dispose off early floods canal system
Types of DamsRigid dams and Non Rigid dams
Rigid dams are rigid like rock – constructed with masonry, concrete, steel etc.
Example : Concrete Dam, Masonry Dam, Timber Dam and Steel Dam
Non Rigid Dams – constructed with soft materials
Earth Dam, Rock Fill dam
Classified as per material used -
Example : Concrete Dam constructed with concrete
Masonry Dam constructed with masonry
Likewise Timber Dam and Steel Dam
Earthen dam with soil
Rockfill dam with rocks
Concrete Dams constructed with concrete
Masonry Dams constructed with masonry
Likewise Timber and Steel Dam
Timber DamSteel Dam
Rubber Dam
Classification of Dams Gravity Dam – External forces are resisted by weight of the dam
N.S. Dam (1957-74)(Reservoir of 408.24 TMC)
Buttress Dam – External forces are resisted by multiple arches built in between the piers
Buttress Dam – Himayat Sagar, Hyderabad
Earth Dam – is an Engineering structure
• Designed and built with available soils
• Earth Dam with one type of soil – Homogenous section
Casing soils – Gravelly soils
Gravelly clays (G.C), Sandy clays (C.S) and Silty sand (S.M)
Hearting soils – Clayey soils
Clay of high plasticity (C.H), Clay of low plasticity (C.L), Clay of intermediate plasticity (C.I)
Earth Dam is economical – as materials are available at site
Foundations are simple – can be constructed almost on the ground
Only cut off trench have to be keyed to impervious zone, if impervious zone is deeper, a sheet pile or a slurry wall be taken and sometimes curtain grouting can be taken up
Earth Dam consists of
Main section
Cut off trench
Up stream rivetment and down stream turfing
Filters – Rock Toe – Toe drain
Spill way have to be with Rigid material
Earth Dam settlement – 10% due to shrinkage
Rock Fill Dam
Rock Fill Dam consists of boulders
Sluicing by sand to fill up the voids
The impervious membrane of the upstream face
Rock fill dam settles by 20 – 25%
Normally the construction of dams will be in the river with flowing water
Area has to be cleared i.e. water has to be diverted so that, excavation can be taken up and bed is exposed for work
Water will be diverted and area of work has to be dry
The temporary structure to divert the water is called Cofferdam
Sometimes, wells will be sunk, upstream and downstream to divert the water
Sometimes, a tunnel will be constructed to divert the water. It is called a diversion tunnel.
Construction of Rigid Dams
Area is cleaned and over burden removed
Area is divided into blocks
Blasting will be taken till sheet rock or approved strata is reached
Grouting the area – Consolidated Grouting – Pressure Grouting
Construction of Earth Dams
Area is cleaned by removing trees and shrubs
The top soil to be scarified to 15 cm depth
Cut off trench excavated
Impervious soils are placed with optimum moisture content and consolidated in the cut off trench
Layers should not be more than 30 cm
Barrow Area
Barrow area to be identified so that specified soils are available
Close to be dam site
Top surface to be scrapped
Barrow pits not more than 25 m x 10 m x 11/2m
Should be away from dam by 10 times height of the upstream and 2 time height from down stream
Consolidated by roller till Proctors Density of 100 is achieved
After filling cut off trench main earth dam section will be taken
Filters are provided as and when the dam is raised
Dam have to be constructed with extra height of 10% or specified to take care of shrinkage
Construction aspects of Dams
i. Whenever a dam has to be constructed, the approach roads connecting to the nearest town have to be formed so that the site is approachable for materials and men comfortably for the work.
ii. On the down stream side of the proposed site, a bridge will also be constructed to have the connectivity between the banks.
iii. Frequent inspections are required. A guest house and quarters for the staff have to be constructed.
iv. The area where the dam will be constructed have to be marked.
v. Vegetation has to be removed
vi. The line indicating the width of the dam at the foundation level has to be marked
vii. For work convenience, the area will be divided into blocks. Since the excavation has to be carried out to the required depths where good rock is available. This requires blasting operations.
viii. For good progress, earth work excavation, blasting and concreting have to be in a phased manner. Initially, three blocks will be taken and excavation in the first block will be started and after completing the excavation, the machinery will be subjected to next block.
ix. Now the blasting operations which require to reach the foundation level will be taken. The blasting will be started in the first and second block and the debris will be deposited far away without creating any hindrance for the future works
x. Then the blasting operations will be continued to the next blocks
xi. In the first block, grouting operations will be started both consolidation and curtain grouting and the area will be made ready for laying the concrete
xii. It has to be ensured that before laying the concrete, there should not be any blasting operations at the adjacent two blocks that is when concreting has to be started in the first block, the blasting operations in the second and third blocks have to be completed. Similarly, when concrete operations have to be started in second block, the blasting operations for the third and fourth blocks have to be completed. The blasting in no way should disturb the already laid young concrete.
xiii. In this sequence, earthen excavation, blasting of the rock, concreting of the dam will be proceeded.
xiv. If the river is very big, the operation will be started from both sides.
xv. Once the concreting operation is started, it is only the concrete
which can go uninterruptedly.
xvi. Curing arrangements has to be made for the concrete and all precautions for concreting have to be taken up
xvii.During floods, the work will be put to stop and all precautions has to be taken that there should not be any damage to the concrete already laid and the concrete that is already laid is not disturbed by
the flood water.
xviii.All machinery have to be shifted to safe place well before the floods
xix. Any protection works required for the main work of dam has to be planned and completed before the on set of floods.