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Land drains
The purpose of a land drain is to allow water in wet or swampy ground to rapidly drain away or to relieve hydrostatic pressure. They are subterranean linear structures which are laid to a fall which should be as steep as practicable. They are used in agriculture and in building construction sites. Modern land drains take the form of a perforated or discontinuous (i.e. open-jointed) pipe. Typically, the land drains conduct the surplus water to an open ditch or natural water source.
==In agriculture==
Traditionally, land drains were formed in clay soils and peats by excavating a trench and forming a "tunnel" using flat stones. This was very labour-intensive but could be often be done using free materials at hand. Typically they were two to three feet (600mm-900mm) below the surface. Agricultural land drains have to be installed sufficiently deep to avoid plough damage. In 1843 in England short earthenware pipes were first used laid edge to edge. The earliest type consisted of a "u" shaped trough onto which a flat lid was placed. Later the extruded clay pipe was developed. These are still used. These can be laid in an excavated trench, or a horizontal hole is formed in the ground using a mole plough and the pipes are forced in by means of a hand or mechanical press. By this means, heavy wet soils, bogs and swamps could be rendered amendable to agriculture. Virtually all crops need a well drained soil to grow well.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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