In this article we will discuss about:- 1. Introduction to Tile Drain Systems 2. Layout of Tile Drainage Systems 3. Depth and Spacing 4. Capacity 5. Grades 6. Size 7. Materials 8. Loads on Drain Pipes 9. The Ditch Conduit Formula 10. Testing of Tiles 11. Bedding Conditions 12. Accessories 13. Installation 14. Blinding and Backfilling 15. Outlets for the Drains.
Introduction to Tile Drain Systems:
The design of a tile drain system consists in determining-
(1) Layout of the system;
(2) Depth and spacing of the drains;
(3) Size of the drains;
(4) Material of the tiles;
(5) Grade of the tile lines;
(6) Envelope materials and accessory structures; and
(7) Outlets.
Layout of Tile Drainage Systems:
The different types of tile drainage layout followed are –
(1) Random or natural,
(2) Herringbone,
(3) Gridiron, and
(4) The interceptor drains.
In an area it is possible to use one or a combination of the systems depending upon the site conditions.
The random or the natural system is used for draining isolated patches when the entire area does not need drainage. It is thus economical. The herringbone system is used in areas where a main line or submain could be laid in the low area and laterals could be drawn from both sides. The land in this system is well drained.
The gridiron system is similar to the herringbone system except that the laterals enter the main only from one side. The system could be more economical than the herringbone system as less number of junctions are used.
The interceptor or the cut-off drain is placed at the upper edge of a wet area to intercept the subsurface seepage. Open drains are also used for the purpose but require considerable land area.
Depth and Spacing of Tile Drains:
The depth and spacing of tile drains are closely interrelated and depend upon the hydraulic conductivity of the soil, kind of crop, extent of surface drainage, outlet conditions and the agronomic practices followed.
The depth of the tile drains should be such that midway between the drains the watertable should be at a satisfactory depth. Where an impermeable layer is encountered the tiles should be placed above such layer, except when the impermeable layer occurs at very shallow depth. If the tiles are located below the impermeable layer, backfilling of the trenches should be done with permeable soil.
The depth and spacing are determined using different requirements in humid and arid regions. In humid areas, as during heavy rainfall watertable will rise to near the surfaces the rate of drop of the watertable is important. In arid regions under irrigation the consideration is to maintain a minimum depth of the watertable for optimum crop growth.
In general, the deeper the drains the wider the spacing and less number of drains are needed. On the other hand, deeper drains will result in greater installation costs and may cause outlet problems.
The following table gives the general depth and spacing that could be adopted under different conditions:
Capacity of Tile Drains:
After the spacing of the tile drains is determined, the discharge through the tile drains can be calculated either by using the Hooghoudt’s approach for steady state conditions or the USBR’s approach for the unsteady state conditions.
In Hooghoudt’s approach, the maximum rate of flow per unit length of the drain is given by-
The design flow of tile drains is estimated under humid and arid conditions using different approaches. Under humid conditions, the main purpose of the tile drains is to remove the excess rainfall appearing as subsurface flow.
The concept of drainage coefficient is used for determining the capacity of the tile drains. Table 18.3 gives the values of the drainage coefficient for field crops. These values vary depending upon the local soil, crop and surface drainage conditions and some experience is necessary for deciding the value of the drainage coefficient.
In determining the capacity of the tile line the area actually drained by that particular line is to be considered. Additional allowance is to be made where there are surface inlets into the tile line or side hill seepage.
In irrigated conditions, an estimate of the deep percolation from irrigation and other sources like unlined canals etc. is to be made for determining the tile flow. In some areas empirical relations in the following form are developed for estimating tile flow –
In Coachella valley of California (U.S.A.) the values of a and b were estimated to be 0.028 and 0.75 respectively, when A is given in acres and Q in cubic feet per second.
Grades of Tile Drains:
Tile drain grades should be such that no sedimentation occurs in the tiles. A minimum velocity of 45 cm per second is generally recommended. A desirable working grade is 0.2 per cent while higher grades can be adopted.
Higher grades result in higher velocities which may cause un-determining of the tiles. These recommendations are for the laterals. The grades of the main lines are to some extent depend upon the outlet selected. Grades upto 2 to 3 per cent were found to be satisfactory for the main lines.
Size of Tile Drains:
The size of the tile drains is determined using the maximum expected flow and the grade. Manning’s formula is used for the design. The value of n in Manning’s formula recommended is 0.011 for concrete tiles. If other materials are used, a suitable value of n can be chosen from Table 18.4. Using Manning’s formula, the following relationship is developed for the size of the tiles flowing full – 
For practical purposes, the nearest available commercial size is selected. In order to take care of sedimentation and also of any misalignment during installation, a size slightly larger than the calculated size is selected. In case of concrete or clay tiles sizes of 10 cm, 12.5 cm and 15 cm are used.
Example:
In a subsurface drainage system, the laterals were laid out 50 m apart and the 200 m long and have a grade of 0.3.
(i) If the drainage coefficient of the area is 2 cm, what size tiles have to be used?
(ii) If the drainage coefficient is increased to 3 cm, what will be the spacing of the laterals?
Solution:
Materials for Tiles:
Concrete, burnt clay and PVC are the materials used for tiles. As the tiles installed will be there once for all, good quality materials should be used for the tiles. Concrete tiles are easy to manufacture and can be transported without breakages.
Concrete tiles can also be locally made using simple moulds made of mild steel sheets. Concrete 1:2:4 proportions should be used and the tiles should be properly cured. If the concrete tiles are to be placed in acid or alkali soils, the tiles should be specially made using cement with specific chemical characteristics in order to prevent the decay of the tiles with time.
Clay tiles should be well burnt and without any defects. Good clay tiles have a distinct ring when lapped with a metal object. Clay tiles have the advantage of being cheaper than concrete tiles and also can be used in acid and alkaline soils. However they are difficult to transport without breakages and will not be able to withstand as much load as concrete tiles can take.
Plastic pipes introduced around 1960 are becoming increasingly popular for tile drainage. The common materials are the polyvinyl chloride (PVC) and polythelene, of which PVC is more popular. PVC pipes are somewhat more resistant to outside pressure than polythelene pipes and are also cheaper.
Plastic pipes come in both smooth and corrugated types. The smooth pipes are rigid and they come in standard lengths. The corrugated pipes are flexible and they come in coils. The corrugated pipes require less plastic material per unit length and have greater resistance to outside pressure than smooth pipes.
They can conveniently be used with the tile laying machines. They however have greater hydraulic resistance than smooth pipes and hence larger sized pipes are needed to drain the same amount of water.
Circular holes in the corrugations allow the water enter into the tile drain. Around 20 holes per 30 cm length are generally provided. The use of envelope materials increases the effective diameter and hence fewer perforations per unit length will be needed to get the desired flow.
Loads on Drain Pipes:
Drain pipes have to bear the load of the soil above them. Drains located at shallow depths (less than 1 m) are subjected to both the soil weight and any other load resulting due to machinery. Drains located at deeper depths are subjected to soil loads only.
For estimating the loads on drain tiles, two formulae are in vogue. These are the Ditch Conduit Formula for the narrow trenches and the Projecting Conduit Formula for wide trenches. If the ditch is more than 2 or 3 times the outer diameter of the tile then it is considered as a wide trench and otherwise a narrow trench.
The Ditch Conduit Formula:
This formula is used for calculating the load on pipes in narrow trenches and is given by –
Where Cc = load coefficient for project conduits and Bc = outside diameter of the conduit. The load coefficient Cd and Cc in the above formulae are functions of the frictional coefficients of the soil, the height of the fill (H) and the width of the ditch or the width of the conduit as the case may be.
In case of ditch conduits it is assumed that the density of the soil filled is less than that of the original soil. The sides of the ditch offer an upward frictional resistance to the fill material and as such the load on the conduit is less than the weight of the soil directly above it.
But in case of projecting conditions, the load on the conduit is greater than the weight of the soil directly above it, for the reason that the shearing forces due to greater settlement of soil on both sides are downward rather than upward.
Values of Cd and Cc are given in Fig. 18.15. Transition widths are those when the loads computed by both the equations are the same.
Drain tiles should be installed such that the loads on them does not exceed their crushing strength.
Testing of Tiles:
There are two methods in use for testing the strength of concrete or clay tiles. The first one more commonly used is known as the three-edge bearing test. The equipment for this test consists of two wooden blocks on which the tile is placed.
A point load which can be increased is applied on top of the tile. This load is applied either by a mechanical jack or a hydraulic system. The load is gradually increased and the load at which the tile breaks is noted.
The second method is known as the sand bearing test in which the procedure is the same but the arrangement is different. In this method the bottom of the tile is supported by a bed of sand. This represents the conditions found under normal installation procedures more closely than the three edge bearing test.
The crushing strength found by the sand bearing method is nearly 1.5 times higher than the strength found by the three edge method.
The testing of plastic pipes used for tiles is done differently. Plastic pipes do not get crushed like clay or concrete tiles but instead get deformed under load. Deformation is the characteristic that is measured in case of plastic pipes.
Bedding Conditions for Tile Drains:
The bedding conditions of the tiles have an influence on the amount of load which the tiles can take. Fig. 18.17 shows the different bedding conditions and the load factors. The crushing strength as determined by the three edge tests is multiplied by the load factor to determine the load that could be supported. While calculating the loads a safety factor is taken into consideration. The calculated loads are increased by 50 per cent as a safety factor.
Accessories for Tile Drain Systems:
For the tile drainage systems to function satisfactorily several accessories are needed.
These are:
1. Envelope Materials:
In soils where there is a chance of heavy movement of soil particles into the tile drains and consequently clogging them, filter materials are placed around the tiles. The filter or envelope materials prevent the entry of the relatively coarser particles into the tiles.
In addition, the envelope materials serve other purposes as well. They act as bedding material and thus allow the tiles to take greater loads. They also increase the effective diameter of the tiles and allow quantities of water to flow into the tiles.
A variety of materials have been used as envelope materials in tile drains. These consist of gravel, coarse sand, organic materials like corn cobs, straw etc. In case of continuous plastic tile lines in the Netherlands coir matting has also been used as envelope material. Among these, gravel has been the most widely used material because of its efficiency and also of permanent nature.
The gravel envelope provided should prevent the movement of the soil particles into the tile drains. The gravel envelope should be designed for this purpose taking into consideration the nature of the soil around the tile drains. The criteria followed for the design are based on the same principles followed for the design of gravel envelopes for wells.
However, the flow towards the well is faster than the flow towards drains and as such the design criteria are slightly different. The United States Bureau of Reclamation recommends the following criteria for the design of gravel envelopes –
The gravel should be clean, free from organic matter and bentonite clays.
2. Manholes and Sedimentation Basins:
These are vertical structures installed at regular intervals along the tile lines to help in inspection of the tile lines and cleaning the accumulated sediment. These are constructed using either concrete or brick masonry and are of dimensions enough for a person to enter. These could be installed at junctions of tile lines. There are no general guidelines for their spacing but decided upon local conditions.
The top of the manhole or the sedimentation basin can be kept at ground level or below ground level. Ground level structures may interfere with farming operations. If the top of the manhole is located below the ground level, it is located about 60 cm below and covered by a concrete block. In this case it will not be obstruction to farming operations.
3. Inlet to Tile Drains:
Inlets to tile drains are used to admit surface runoff into the tile lines. These could also be used for flushing the tile lines. The two types of inlets which are used are the blind inlets and the surface inlets.
Blind inlets are cheaper and do not interfere with farming operations but they are likely to get clogged. Surface inlets have provision for preventing the trash and other suspended materials entering the tile drains.
Installation of Tile Drains:
Installation of the tiles consists in digging the trench, laying the tiles to the prescribed grade, putting the envelope material and backfilling the soil. Installation procedures depend upon the availability of labour, types of the tiles used and the soil conditions. Installation of the tiles should start from the outlet as this ensure disposal of the drainage water during installation.
Digging of the trenches can be done either by manual methods or by using trenching machines. Manual methods use ordinary excavating tools. After the trench is excavated tiles are laid manually.
In case of machines used for tile drain construction the operation of digging, laying the tiles, filling the envelope material and backfilling is done in one single operation. The operation of these machines is faster when continuous plastic pipes are laid as tiles.
In case of machines used for title drain construction the operation of digging, laying the tiles, filling the envelope material and backfilling is done in one single operation. The operation of these machines is faster when continuous plastic pipes are laid as tiles.
Latest machines use laser techniques for maintaining the grades of tile lines. The tile lines used are continuous PVC pipes with slots. The excavation of the trench, laying the tile line and placing the envelope materials are carried out in one operation.
1. Establishing Grade:
Establishing the proper grade for laying the tiles is very important. The two methods that are used are the line and the sight method. In the line method a string or thin wire is used to set the grade whereas in sight method the grade is established by sighting over two targets.
The bottom of the trench should be dug parallel to the line of sight or the string which represents the required grade. Wooden frames are fixed on the ground at regular intervals to serve as lines of sight or measurement points (Fig. 18.23).
2. Laying the Tile:
The concrete or clay tiles are laid at the bottom of the trench at the proper grade. In case the envelope material is used, it is partly filled in the trench, the tiles are put, the grade checked and rest of the envelope materials is filled in.
Blinding and Backfilling:
After the tiles and envelope material are put, the trenches are filled with top soil for about 15 to 20 cm carefully. This is known as blinding and helps in providing permeable soil around the tile and also keeps the tiles in position.
Backfilling of the trenches is done with the excavated soil either manually or by machines. When the soil contains stones care should be taken while blinding and backfilling operations so that the tiles do not get broken.
Outlets for the Drains:
Functioning of the tile drainage system depends on the proper functioning of the outlet.
The requirements of a satisfactory outlet are:
(1) Provide free flow of drainage water,
(2) Discharge the outflow without any damage to the tile or erosion,
(3) Prevent the entrance of flood waters,
(4) Protect the end or the tile line, and
(5) Keep out rodents.
The principle types of the outlets used are the gravity outlet and the pump outlet. The topography of the land is the main factor that dictates the choice of the outlet. In the gravity outlet water flows out of the drainage system by gravity into an open drain, stream, a pond or any other facility that disposes the water.
When disposing the water the outlet pipe should be above the water level by about 30 cm so that there is a free fall. It should be ensured that the end of the pipe is not covered by water. Where gravity outlets are not possible, pumped outlets are used. In the pumped outlet, the tile drains lead to a sump and water is lifted from the sump and pumped into a drain or other water disposal facility.
Water level in the sump could be allowed to go above the level of the drains for a short while if crops are not affected consequently. The capacity of the sump and the pump depends upon the area that is being drained.
Evaporation ponds have been used as a means of disposing drainage water in areas where any types of outlets are not possible. They are also used in situations where the discharge of drainage water into streams is objectionable from water quality point of view.
Water is collected in ponds of suitable size and allowed to evaporate. Water quality aspects of subsurface drainage waters need to be monitored for their use and disposal.