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Last edited 05 Sep 2019
A trench is a hollow cut into the ground, usually with parallel sides and which is typically deeper than it is wide. Trenches were used in the First World War as a means of conducting combat as they offered a degree of protection and cover from enemy fire.
In construction, trenches are typically excavated for:
- Laying or getting access to services.
- Constructing foundations such as strip foundations. For more information see: Strip foundation.
Trenches may be dug manually or by mechanical excavators, with spoil typically loaded onto dump trucks to be taken for disposal, or re-used elsewhere on site. They need to be constructed with care and must be adequately supported to prevent the sides from caving in, thereby causing a hazard to life, as well as resulting in unnecessary costs and delays to the project.
For shallow trenches in firm ground, open timbering is usually used – whether made of timber or steel trench struts – in conjunction with poling boards. For softer ground, the number of poling boards is increased and they are arranged at closer intervals. Where the trench is deeper than 1.8m, or in loose soil, the sides are usually close-boarded.
In deep trenches, especially where they are likely to be left open for any length of time, the pressures exerted by the trench sides onto the boarding are often considerable and there is the possibility of the sides caving in if not adequately supported. They may also fill with water following heavy rain which may cause structural problems and require a pump to evacuate the water.
The construction of diaphragm walls often requires relatively narrow trenches to be cut – typically 500mm – 1,000mm wide and to depths of 50m and more. Excavation is typically undertaken using rope-suspended mechanical or hydraulically-operated grabs.
Preventing such deep, narrow trenches from caving in would be difficult using physical means such as struts and sheeting. Therefore, to provide support (before placing reinforcement and pouring the concrete) a bentonite slurry is poured into the trench. The slurry has thixotropic properties which means it is load bearing and so keeps the trench sides from collapsing. As the concrete is piped into the bottom of the trench, the bentonite is gradually displaced.
For more information see: Diaphragm wall.
For more information see: Temporary works.
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