The low permeability of many subsoils, which create a perched water table, is the major reason for artificial drainage in horticultural soils. Clay, clay loam and silty clays, when wetted, become almost impermeable as the clay swells and the cracks close; clay is 'puddled' to form a liner for ponds. This 'top water problem' is dealt with by putting in pipes to intercept the trapped gravitational water. Straight lines of pipes are placed at an even gradient from the highest point to the outfall in a ditch or main drain (see Figure 19.4). The pipes are laid below cultivation depth in a series of parallel lines across the slope to the headland of the area to be drained. Where a valley or the lower areas lie within the area to be drained, a herringbone pattern is used.
Silt traps should be placed at regular intervals to help to service the system at points where there is a change of gradient or direction (see Figure 19.4). The spacing between the lines of pipes depends on the permeability of the soil, a maximum of 5 metre intervals being necessary in clay subsoils. Soil permeability and the land use dictate the depth of the drains which is normally more than 60 cm. Drains should be set deeply in cultivated land where heavy equipment and deep cultivation will not disturb the pipes. Shallow drains can be used where rapid drainage is a high priority and the pipes are not likely to be crushed by heavy
(a) Simple Interceptor Drainage System:
(a) Simple Interceptor Drainage System:
Main Laterals Mole drains or subsoiling drain
Figure 19.4 Drainage
Figure 19.4 Drainage vehicles or severed by cultivating equipment, e.g. gardens and sports grounds. Pipe drainage is usually combined with secondary treatments, such as mole drainage or subsoiling, to achieve effective drainage at reasonable cost. Deep subsoiling improves soil permeability and the pipes carry the water away. Installation costs can be reduced because pipes can be laid further apart. Similarly mole drainage over and at right angles to the pipes enables them to be spaced 50-100 metres apart.
The pipes are made of clay or plastic. The diameter of the pipe depends on the gradient available and the amount of water to be carried when wet conditions prevail. Tiles (clayware) are usually 300 mm pipes either 75 mm or 100mm in diameter (see Figure 19.1). These lead into a ditch or a larger main drain. The tiles are butted tightly together to allow entry of water, but not soil particles. It is recommended that they are covered with permeable fill, usually stones or clinker, to improve water movement into the drains. Plastic pipes consist of very long lengths of pipe perforated by many small holes and usually covered with a rough felt to keep out soil particles.
An outlet into a ditch is very vulnerable to damage and so it should consist of a strong, long pipe set flush in a concrete or brick headwall so that it is neither dislodged by erosion in the ditch nor by people using it as a foothold. Outlet pipes should be glazed to prevent frost damage. Vermin traps should be fitted to prevent pipes being blocked by nests or dead animals (see Figure 19.4).
Mole drainage is very much cheaper than pipe drainage. A mole plough draws a 75 mm ' bullet ' , followed by a 100 mm plug, through the soil at a depth of 500-750 mm from a ditch up the slope of a field or across a pipe drain system with permeable backfill (see Figure 19.4). The soil should be plastic at the working depth so that a tunnel to carry water is created. The soil above should be drier so that some cracks are produced as the implement is drawn slowly along. These cracks improve the soil structure and conduct water to the mole drain. Sandy and stony areas are unsuitable because tunnels are not properly formed or collapse as water flows. Tunnels drawn in clay soils can remain useful for 10-15 years, but in wetter areas their useful life may be nearer 5 or even as little as 2 years.
Sandslitting is used on sports grounds to remove water from the surface as quickly as possible. It involves cutting narrow trenches at frequent intervals in the soil and infilling with carefully graded sand that conducts water from surface to a free draining zone under the playing surface.
Clay pipes well butted up
French drains can be placed around impermeable surfaces, such as concrete hard standings and patios, to intercept the run-off.
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