Defensive trenches

A trench is a type of excavation or depression in the ground that is generally deeper than it is wide (as opposed to a wider gully, or ditch), and narrow compared with its length (as opposed to a simple hole).

In geology, trenches are created as a result of erosion by rivers or by geological movement of tectonic plates. In the civil engineering field, trenches are often created to install underground infrastructure or utilities (such as gas mains, water mains or telephone lines), or later to access these installations. Trenches have also often been dug for military defensive purposes. In archaeology, the "trench method" is used for searching and excavating ancient ruins or to dig into strata of sedimented material.

Some trenches are created as a result of erosion by running water or by glaciers (which may have long since disappeared). Others, such as rift valleys or more commonly oceanic trenches, are created by geological movement of tectonic plates. Some oceanic trenches include the Mariana Trench and the Aleutian Trench. The former geoform is relatively deep (approximately 10 kilometres (6.2 mi)), linear and narrow, and is formed by plate subduction when plates converge.

In the civil engineering field of construction or maintenance of infrastructure, trenches play a major role. They are used to place 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. They may also be created later 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'). Finally, trenches may be created as the first step of creating a foundation wall. Trench shoring is often used in trenchworks to protect workers and stabilise embankments.

...
Wikipedia