DESIGN OF SUBSURFACE DRAINAGE SYSTEM
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Transcript of DESIGN OF SUBSURFACE DRAINAGE SYSTEM
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WELCOME
NAMITHA M RID. No: 2015664502
M.Tech.Land and Water
Management Engineering
TNAU
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DESIGN OF SUBSURFACE
DRAINAGE SYSTEM
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SUBSURFACE DRAINAGEDrains laid deep in the
ground
and covered
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Fig: Subsurface drainage system-sectional view
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Placed at suitable depth and grade below the ground surface
Lowers the capillary surface and
water tableAerates the root zone
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1.Advantages of Subsurface DrainsProvides aeration to root zoneImproves soil structure and maintains soil temperature
Avails land for early cultivationFacilitates easy movement of farm machines
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Removes undesirable salts from
root zoneDecreases chances of flood
hazardsOccupies no surface landSmall capacity drains are
requiredLess maintenance cost
compared to surface drains
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2. Disadvantages of subsurface drainsRequire high initial costRequires steeper gradientRepair works are costly
and inconvenientOnly seepage water is
removedConstruction is difficult
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3. Subsurface drainage methods
Tile drains Mole drainsDrainage wellsDeep open drainsCombination of
tile and mole drains
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4. Investigations for subsurface drainage
Topographic map of the areaData of soil physiochemical
propertiesPosition of ground water
table and its fluctuationQuality of groundwater
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Logs of soil and subsurface material
Crops to be grown and their drainage requirements
Irrigation practices and their requirements
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5. Groundwater studies for planning subsurface drainage3 parameters:
oDepth and fluctuation of groundwater tableoQuality of groundwateroHydraulic conductivity of the subsoil material
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DESIGN OF TILE DRAINAGE SYSTEMConsists of :
1. Layout of the system2. Depth and spacing of the drain 3. Size and grade of the tile lines4. Material of tiles5. Envelope materials and
accessory structures6. Installation of tile drains
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1. Layout of tile drainage system
Drainage system comprises of laterals, main drains and an outlet
Laterals
Main drains
Outlet
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Different layouts of tile drainage system:
a)Natural system
b)Parallel system
c) Herringbone system
d)Grid iron system
e)Random system
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f) Intercepting system
g)Double system
h)Grouping system
i) Composite system
j) Zig zag system
k)Sink hole system
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a) Natural system:
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b) Parallel system:
c) Herringbone system:
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d) Grid iron system:
e) Random system:
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f) Intercepting system:
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g) Double system:
h) Grouping system:
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i) Composite system:
j) Zigzag system:
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2. Depth and spacing of tile drains Depth and spacing are closely inter
related Depends on:
Texture of soilHydraulic conductivityTypes of crops grownExtend of surface drainage
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Outlet conditionsTopography of landSalt content of soilAgronomic practices
Drain depth: From ground surface to the bottom of the tile, usually 1-1.5m
Tiles are placed above impermeable layer,
if any
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Deeper the drain, wider the spacing, lesser the number of drains required
Drain spacing-
’Hooghouts drain spacing formula’
S2 = 4K/R [H2-2hd+2Hd-h2]
where,
d- Depth to the impermeable layer from the drain bottom
h- Height of water in the drain
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H-Height of water in midway between 2 drains
S- Drain spacing
D-Distance from the impermeable layer to the maximum height of water between the drains
K- Hydraulic conductivity
R- Replenishment rate
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3. Size and grade of the tile drains Drains are designed based on
Manning's formula Drains are laid on longitudinal slope:
0.05-3% Working grade: 0.2% Size of tiles pipes: 30-90cm long,
10-15cm diameter
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Diameter of tile drains,
d = 5.3(Dc)0.375 A0.375 S-0.1875
where,
d- Internal diameter of tile drain, cm
Dc – Drainage coefficient, cm/day
A-Drainage area, ha
S- Hydraulic gradient or tile grade, m/m
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4. Material of tiles
Clay, Concrete, PVC/Plastic pipes, Bituminous fibre or steel
Pipe material should be:Resistant to weathering and freezing
Have high densityHave sufficient strength to withstand static and dynamic loads
Have uniformity in shape and wall thickness
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5. Envelope materials and accessory structures
a) Envelope materials: Materials that cover the
drains placed in less pervious strata
Gravel, coarse sand etc. Prevents inflow of soil into
the drains Increases effective drain
diameter
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Coarsest material
is placed immediately over the tile
Minimum thickness of envelope: 7.5cm
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Recommendations for gravel envelope (USBR):
For uniform soils,
D50 of envelope/D50 of soil = 5 to 10
For graded soils,
D50 of envelope/ D50 of soil = 12 to 58
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b) Manholes and sedimentation basins:
Vertical structures Installed at regular intervals
along the tile lines Constructed by concrete or
brick masonry Helps in cleaning and inspection
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A man can enter and work within it
Placed about 60 cm below ground surface
Covered by
concrete block
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c) Inlet to tile drains: Allows water into the drain Two types:
Blind inlet- cheaper but chances of clogging
Surface inlet- have provisions to prevent the trash entering into the drains but costly
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Fig: Blind inlet
Fig: Surface inlet
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d) Outlet for drains:
Water from tile drains are discharged into big size surface drains
Either by gravity or pumping
Gravity outlet- invert level of tile drain is higher than fully supply level of surface drain
Pump outlet- bed level of the outlet is higher tan the discharging tile drain
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e) Installation of tile drains:
Consists of: Excavation of a trench Laying the tile at predetermined
grade, depth and spacing Putting the envelope material
and backfilling the soil Installation of tile should start
from out let
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MULTIPLE WELL SYSTEM
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MULTIPLE WELL SYSTEMMultiple wells are installed in a
common area of influence
Cone of depressions overlap each
other
Results in increased drawdown in each
well
Provides more drainage effect
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Fig: Multiple well system
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Wells are arranged in various patterns
Isolated groups or continuous pattern
Suction lines of each well is connected to a common pump
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