A trench is a type of excavation or depression in the ground. Trenches are generally defined by being deeper than they are wide (as opposed to a wider gully or ditch), and by being narrow compared to their length (as opposed to a simple hole).
A number of areas exist in which trenches play a significant role like a ditch
Trenches have long been used to carry water. Trenches can be used for draining
purposes, leading water away from a swamp
that is to be dried out. Likewise they can be used for irrigation
purposes, directing water into dry areas. Both uses generally require a slope for the water to flow down.
Archeologists may use the 'trench method', pioneered by Dame Kathleen Kenyon
, for searching and excavating
or to dig into strata
of sedimented material to get a sideways (layered) view of the deposits - with a hope of being able to place found objects or materials in a chronological order. The advantage of this method is that it destroys only a small part of the site (those areas where the trenches, oftenarranged in a grid pattern, are located). However, this method also has the disadvantage of only revealing small slices of the whole volume, and modern archeological digs usually employ combination methods.
Trenches are a natural feature in many landscapes. Some are created by rivers
in flow (which may have long since fallen dry), others are features created by geological movement, such as oceanic trenches
. The later form is relatively deep, linear and narrow, and is formed by plate subduction
In the civil engineering
field of construction or maintenance of infrastructure, trenches play a major role. They may be created to search for pipes and other infrastructure that is known to be underground in the general area, but whose exact location has been lost ('search trench' or 'search slit'). They are also used to underground easily damaged and obstructive infrastructure or utilities
(such as gas mains
, water mains
or telephone lines
). A similar use for higher bulk would be in pipeline transport
. Finally, trenches may be created as the first step of creating a foundation wall
While trenches have often been dug as defensive measures, in the pre-firearm eras, they were mainly a type of hindrance for an attacker of a fortified location, such as the moat around a castle (this is technically called a ditch).
Only with the advent of accurate firearms, and the tactics that evolved in World War I and the Crimean War, did the use of trenches as positions for the defender of a fortification become common, though the Māori of New Zealand were known to have used it earlier in their Pā fortifications in the late 19th Century. The military usage evolved very quickly in the First World War, until whole systems of extensive main trenches, backup trenches (in case the first lines were overrun) and communication trenches had been developed, often stretching dozens of kilometres along a front without interruption, and some kilometres further back from the opponents' lines.
- Trenches are often used for mass graves, sometimes even dug by prisoners about to be executed (see, for example, the Holocaust novel Night).
- Sunken trenches may be combined with a wall on one of their sides to form a ha-ha, a type of hidden fence.