Subsurface drainage

Kelappaji College of
Agricultural Engineering & Technology
Kerala Agricultural University, Tavanur-679 573
Chethan B J
MTech(soil and water engineering)

Classification of subsurface drainage
subsurface
drainage
Interceptor
drainage
open
ditches
Buried
drains
Relief
drainage
open
ditches
Buried
drains

Relief drainage
 When the system in a waterlogged area ,removal of the excess water and
gives relief to the land, this system is called relief drainage.
 It may be open or closed(buried) type.
 These drains are used where the groundwater does not flow appreciable and
where it is impossible to intercept the flow effectively.
 This is system used where land having flat with low gradient.
 Relief drains are usually planned in series in a gridiron or herring bone

Open ditches in relief drainage system
oThey are used for large areas which have mild slope or completely slope.
oOpen ditches can carry both surface and subsurface water.
oIt is constructed with higher depth to serve and therefore are capable of
carrying wide range of flow.

Buried drains in relief drainage system
oAny conduit buried in the soil and allowing collection of drainage water
perforations or open joints and disposal of the same through the outlet is called
buried drain.
oThe drain may be clay tiles, concrete, metal, PVC pipes.

Interceptor drainage
 If the system intercepts the water coming from other areas and divert it suitably to save a certain
part of the area getting from waterlogged ,then it is called interceptor drainage.
 The drains are placed perpendicular to the direction of ground water flow.
 Interceptor drains are very effective in controlling the water table for a considerable distance
below or downslope from the drain, but do not have any appreciable influence on the upslope
region.
 Extensive ground water studies are required before deciding about location and depth of ditches and
drains.
 These system are recommended where there is substantial ground water flow from a source like
canal,reservoir,or irrigated areas at higher elevations.
 It may be closed or open type

Open ditches in interception drainage system
oThis system is same as that of open ditches in relief drains where function is to
carry both surface and subsurface water.
oIt is constructed at the base of the slope to intercept the surface and groundwater
flow.

Buried drains in interception drainage system
oThe high water table occur locally due to unusual subsurface formations or
ground water conditions or changes in topographic features.
oThis condition is occur in the large stream valleys or in the gradient is more. In
such area seepage from uplands is more.
oIf open ditches are constructed to intercept the flow ,it may be damaged by
flood flows, causing erosion or channel change.

Subsurface drainage methods
1. Tile drains
2. Mole drains
3. Drainage wells
4. Deep open drains
5. Combination drainage system

1.Tile drains
Tile drainage are a subsurface drainage method consist of short length pipes
(30-90cm)installed at particular depth from land surface.
The pipes are made of concrete or burnt clay.
After digging the trench to the desired depth the pipes are held end to end
without any jointing.
They are covered with an envelope material in certain cases and soil is
backfilled.
Water enters the tile drains through the opening available between the pipes.
A network of tile line is laid with a grade will remove the subsurface water.

1.1Elements of tile drain system
Consists of :
1. Layout of the system
2. Depth and spacing of the drain
3. Size and grade of the tile lines
4. Material of tiles
5. Envelope materials and accessory structures
6. Installation of tile drains

1.11 Considerations for the selection of a layout
o Matching of layout of (subsurface drainage) with irrigation and road layout.
o The layout has to "fit" into the layout of the existing infrastructure.
o The level of the outlet of the subsurface drainage system (determined by the drain depth,
lengths and slopes) has to be at or above the highest water level in the main drain system.
o Pumping of the system.
o Available drainage materials.
o Installation equipment.
o Subsoil conditions.

Different layouts of tile drainage system:
a) Natural system/Random system
b) Parallel system
c) Herringbone system
d) Grid iron system

1.11 Layout of tile drainage system
Drainage system comprises of laterals, main drains and an outlet
Laterals
Main drains
Outlet

Random system
This type used where there are scattered
wet areas in a field somewhat isolate from
each other
Tile line are laid more or less at random to
drain these wet areas
In most cases the tile main follows the
largest natural depression in the field, and
submains and laterals extend to the
individual wet areas.

Herringbone system
Consist of parallel laterals that enter
the main at an angle ,usually from both
side.
This system is used where main and
submain lines in a narrow depression.
This system is used where lateral are
long and area requires thorough
drainage.

Grid iron system
Similar to herringbone except that
laterals enter the main from only one
side
It is used on flat regularly shaped fields
and on uniform soils
It is more economical than herringbone
because the number of junction and
double drained areas are reduced

b) Parallel system:
• In this system the laterals are located perpendicular
to main drain.
• Flat land with regular shape and with soils of
uniform permeability can be drained by this method.

1.12. Depth and spacing of tile drains
Depth and spacing are closely inter related
Depends on:
Texture of soil
Hydraulic conductivity
Types of crops grown
Extend of surface drainage

Outlet conditions
Topography of land
Salt content of soil
Agronomic practices
Drain depth: From ground surface to the bottom of the tile, usually 1-1.5m
Tiles are placed above impermeable layer

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
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

1.13 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

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

1.14 Material of tiles
Clay, Concrete, PVC/Plastic pipes, Bituminous fiber or
steel
Pipe material should be:
Resistant to weathering and freezing
Have high density
Have sufficient strength to withstand static
and dynamic loads
Have uniformity in shape and wall thickness

1.5. 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

Coarsest material is placed immediately
over the tile
Minimum thickness of envelope: 7.5cm

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

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
A man can enter and work within it
Placed about 60 cm below ground surface
Covered by concrete block

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

Fig: Blind inlet
Fig: Surface inlet

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

1.6 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 outlet

2.Mole drainage systems
o A mole drain is an unlined underground drainage channel, formed by pulling a solid
object, usually a solid cylinder with a wedge-shaped point at the end, through the soil
at the proper slope and depth, without a trench having to be dug.
o Mole drains are produced in the subsoil by means of mole plough. These are pipe less
drains.
o Mole drainage is applied only under very specific conditions, mainly in stable clayey
soils.
o The effect of mole drainage is a rapid removal of excess water from the surface layers,
rather than at controlling the water table as such.

o The mole drains have a life span of only a few years(usually 10-15 years) and
have to be renewed frequently.
o Depth of mole drain vary from 45-120cm depending upon moling equipment
o The length of mole drain is decided by length of affected area, soil type,
availability of outlet etc. Safe length vary from 20-80 metre. If soil condition
permits, the maximum length can be about 200 m.
o Smaller diameter have longer life than larger diameter. Diameter varies from 7.5
to 15 cm.
o Mole drains are closer spacing than tile drains. A spacing of 2 to 5 metres is
usually followed.

o Safe gradient of 0.2-3% are given. Too low a velocity prolongs saturation of
the drain and high velocity causes erosion.
o Stagnation of water in the mole drain will weaken the walls ,leading to
deterioration of the channel.
o Mole drain finally discharges into an open ditch and therefore, last portion of
the mole should be provided with the pipe.

o Mole outlet protection comprises of a 1-2m long pipe inserted into the
mole channel

o Clean gravel of size 3-5mm are suitable.
o Ideal time for laying mole drain is soon after harvest of the crop.
o Extensively practiced in England, Western Europe and New Zealand.

2.1 Factors affecting life mole drains
1. Structural stability of subsoil.
2. Amount and intensity of rainfall.
3. Temperature variations.
4. Method of forming mole drain.
5. soil moisture content at the time of formation of mole drain.
6. Diameter of drain.

3.Drainage wells
It is also called as vertical drainage.
A tube well drainage system consists of a network of tube wells to lower the
water table, including provisions for running the pumps, and surface drains to
dispose of the excess water.
Tube well drainage is used in areas with a high soil permeability and
preferably fresh groundwater that can be reused for irrigation.
The system is operation and maintenance intensive and requires a continuous
diesel or electrical power supply.

3.1 Drainage by wells is feasible only under certain
condition as mentioned below
 Aquifer condition: the area to be drain should be underlain by an aquifer,
pumping from which will adequately lower the water table.
 Water quality: the underground water should be of satisfactory quality. Thus
the pumped water could be used for irrigation.
 Soil condition: there should be any intervening layers of low hydraulic
conductivity impending the movement of water.

3.2 Multiple well system
Multiple 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.
Wells are arranged in various patterns. Isolated groups or continuous pattern.
Suction lines of each well is connected to a common pump.

4.Deep open drains
o It is used for subsurface drainage.
o It uses interceptor drains commonly as
their length required is generally less than
relief drains.
o The capacity of interception drain
estimated using Darcy’s law.
o Land area required need constant
maintenance

5.Combination of surface and subsurface drains system
o Transport of drainage over long distances is done by open drains.
o A combination of open drain and tile drain has to be used for the
effective drainage of an area.
o Tile drain are initially costly but they do not take away from the
cultivation.
o Combination of vertical drainage and surface drainage also used for
effective drainage.

Disadvantages of subsurface drains
Require high initial cost.
Requires steeper gradient.
Repair works are costly and inconvenient.
Only seepage water is removed.
Construction is difficult.

Subsurface drainage

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