A trench is a type of excavation or depression in the ground that is generally deeper than it is wide (as opposed to a swale or a bar ditch), and narrow compared with its length (as opposed to a simple hole or pit).^[1]

In geology, trenches result from erosion by rivers or by geological movement of tectonic plates. In civil engineering, trenches are often created to install underground utilities such as gas, water, power and communication lines. In construction, trenches are dug for foundations of buildings, retaining walls and dams, and for cut-and-cover construction of tunnels. In archaeology, the "trench method" is used for searching and excavating ancient ruins or to dig into strata of sedimented material. In geotechnical engineering, trench investigations locate faults and investigate deep soil properties. In trench warfare, soldiers occupy trenches to protect them against weapons fire and artillery.

Trenches are dug using manual tools such as shovel and pickaxe or heavy equipment such as backhoe, trencher, and excavator.

For deep trenches, the instability of steep earthen walls requires engineering and safety techniques such as shoring. Trenches are usually considered temporary structures that are backfilled with soil after construction or abandoned after use. Some trenches are stabilized using durable materials such as concrete to create open passages such as canal and sunken roadways.

Natural 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 oceanic trenches, are created by geological movement of tectonic plates. Some oceanic trenches include the Mariana Trench and the Aleutian Trench.^[2][3] The former geoform is relatively deep (approximately 10 kilometres (6.2 mi)), linear and narrow, and is formed by plate subduction when plates converge.^[4]

In the civil engineering fields of construction and maintenance of infrastructure, trenches play a major role. They are used for installation of underground infrastructure or utilities (such as gas mains, water mains, communication lines and pipelines) that would be obstructive or easily damaged if placed above ground. Trenches are needed later for access to these installations for service. They may be created to search for pipes and other infrastructure whose exact location is no longer known ("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 the steep walls.

An alternative to digging trenches is to create a utility tunnel. Such a tunnel may be dug by boring or by using a trench for cut-and-cover construction. The advantages of utility tunnels are the reduction of maintenance manholes, one-time relocation, and less excavation and repair, compared with separate cable ducts for each service. When they are well mapped, they also allow rapid access to all utilities without having to dig access trenches or resort to confused and often inaccurate utility maps.

An important advantage to placing utilities underground is public safety. Underground power lines, whether in common or separate channels, prevent downed utility cables from blocking roads, thus speeding emergency access after natural disasters such as earthquakes, hurricanes, and tsunamis.^[5]

In some cases, a large trench is dug and deliberately preserved (not filled in), often for transport purposes. This is typically done to install depressed motorways, open railway cuttings, or canals. However, these large, permanent trenches are significant barriers to other forms of travel, and often become de facto boundaries between neighborhoods or other spaces.

Trenches have often been dug for military purposes. In the pre-firearm era, they were mainly a type of hindrance to an attacker of a fortified location, such as the moat around a castle (this is technically called a ditch). An early example of this can be seen in the Battle of the Trench, a religious war, one of the early battles fought by Muhammad.^[6]

With the advent of accurate firearms, trenches were used to shelter troops. Trench warfare and tactics evolved further in the Crimean War, the American Civil War and World War I, until systems of extensive main trenches, backup trenches (in case the first lines were overrun) and communication trenches often stretched dozens of kilometres along a front without interruption, and some kilometres further back from the front line. The area of land between trenches in trench warfare is known as "No Man's Land" because it often offers no protection from enemy fire. After WW1 had concluded, the trench became a symbol of WW1 and its horrors.

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  Allied troops entrenched in the Battle of Tuyutí.
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  Soldiers in a trench during the Gallipoli Campaign of World War I.
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  The Boer trench at the Battle of Magersfontein contributed to the surprise defeat of the Highland Brigade on 11 December 1899 during the Second Boer War.
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  Soldiers in a trench on the Western Front in World War I in winter.
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  Soviet soldiers inside a trench during the Siege of Leningrad at World War II
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  Iranian soldiers inside a trench during the Iran–Iraq War
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  A Ukrainian soldier, equipped with an anti-drone gun, in a trench during the 2022 Russian invasion of Ukraine.

Trenches are used for searching and excavating ancient ruins 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, often arranged 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.^[7]

The safety concerns associated with trenching and excavation activities can vary significantly depending on the specific processes involved and the environmental conditions of the work site. Although nearly all trenching and excavation work is unique in some way, the most significant and common hazard is related to cave-ins or sidewall failures that can result in partial or full entrapment. While the exact weight can vary, one cubic yard of dirt has the potential to weigh in excess of 3,000 pounds, depending on the soil composition and moisture content^[8]. Additional hazards include hazardous atmospheres, falling loads, and falls from height.

With the high potential of injuries during trenching and excavation activities, in the United States the Occupational Health & Safety Administration outlines strict guidelines to follow when performing these types of activities in 29 CFR 1926 Subpart P - Excavations. Throughout this subpart, OSHA specifies that the safety planning and implementation process be conducted by a competent person.

Competent Person:

A competent person is someone designated by a company or organization as an individual who is capable of making decisions related to the associated job tasks. It is important to note that although OSHA does not have very strict guidelines defining what makes someone a competent person, those individuals are expected to have credible training or experience that enables them to identify related hazards and take immediate action to eliminate or control hazardous conditions.^[9]

According to 29 CFR 1926.651(k)(1) competent persons are also required to conduct daily inspections of the excavation site at the beginning of the work day and as needed throughout the shift.

Underground Utilities:

Before the start of any trenching or excavation activities, underground utilities must be located and identified by the appropriate authority having jurisdiction. This helps ensure that sewer, telephone, fuel, electrical, water, and other underground lines are not struck or damaged.^[10]

In the United States, 811 is the universal phone number that can be called to request that underground utilities be marked. The amount of time required to complete this process varies by state, but the request must be made several days in advance before the project begins. Depending on state statutes, which also vary, marked utilities still have a tolerance zone because the marked location may vary from the actual underground position.

For example, in North Carolina, General Statute 87-117 requires that if the diameter of the facility is known, the tolerance zone extends one-half of the known diameter plus 24 inches on either side of the designated center line. If the diameter of the facility is not known, the tolerance zone is 24 inches on either side of the outside edge of the mark indicating the facility. For subaqueous facilities, a clearance of 15 feet on either side of the indicated facility is required.^[11]

Soil Classification:

In regard to trenching and excavation activities, soil classification is arguably one of the most important processes because it serves as the basis for many of the safety-related decisions applied at a specific worksite. Soil classification must be completed before any sloping or benching systems are designed. OSHA states in OSHA 29 CFR 1926 Subpart P – Appendix A (c)(2) that soil classification must be based on at least one visual analysis and at least one manual analysis. Listed below are the four general classifications of soil and their definition as listed by OSHA.

• Stable Rock: Natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed
• Type A Soil (Most Stable): Cohesive soils with an unconfined compressive strength of 1.5 ton per square foot (tsf) (144 kPa) or greater. Examples of cohesive soils are: clay, silty clay, sandy clay, clay loam and, in some cases, silty clay loam and sandy clay loam.
• Type B Soil: Cohesive soil with an unconfined compressive strength greater than 0.5 tsf (48 kPa) but less than 1.5 tsf (144 kPa); or Granular cohesionless soils including: angular gravel (similar to crushed rock), silt, silt loam, sandy loam and, in some cases, silty clay loam and sandy clay loam.
• Type C Soil (Least Stable): Cohesive soil with an unconfined compressive strength of 0.5 tsf (48 kPa) or less; or Granular soils including gravel, sand, and loamy sand; or  Submerged soil or soil from which water is freely seeping; or Submerged rock that is not stable; or Material in a sloped, layered system where the layers dip into the excavation on a slope of four horizontal to one vertical (4H:1V) or steeper.
  • One common practice in the construction industry is to classify all soil as Type C instead of risking misclassification. This approach is taken to maximize safety and account for the many changing variables, especially environmental conditions.

Protective Systems:

Some of the most important aspects of the competent person’s job in relation to trench and excavation safety include knowing when and what types of safety equipment and precautions are required. OSHA requires that all employees be protected from cave-ins. The only exceptions are when the excavation is made entirely of stable rock or when the excavation is less than 5 feet (1.5 meters) deep and a competent person has determined that there is no potential for a cave-in. One important point to note is that, depending on the situation, a protective system may still be required even if the excavation is less than 5 feet (1.5 meters) deep. This is why the training and knowledge of the designated competent person are so important. Listed below are the four general methods/systems used to protect employees from cave-ins.

• Benching: A protective method used to prevent cave-ins by excavating the sides of a trench or excavation into a series of horizontal steps or levels, usually separated by vertical or nearly vertical surfaces.
• Sloping: A protective technique that prevents cave-ins by cutting the walls of an excavation back at an angle that slopes away from the trench.
• Shoring: A protective system that uses supports, such as aluminum hydraulic equipment or other structural supports, to hold the sides of an excavation in place and prevent soil movement or collapse.
• Shielding: Most commonly associated with trench boxes, this system protects workers inside a trench by using a protective structure designed to withstand soil movement and cave-ins.

Access & Egress:

For trenching and excavation sites, it is important to provide a safe way for workers to enter and exit the trench. For trenches that are 4 feet (1.2 meters) or more in depth, an approved means of egress such as a stairway, ladder, ramp, or other safe method must be provided. This means of egress should be positioned so that workers do not have to travel more than 25 feet (7.62 meters) laterally to reach one of these exit methods.^[12]

Additional Trenching & Excavation Safety Topics to Consider:

• Hazardous Atmospheres: Natural or artificially occurring conditions inside an excavation that could affect the safety and health of workers. Examples include hazardous chemicals, nearby landfills, or exhaust fumes from equipment operating in the area.
• Water Intrusion and Accumulation: When excavations have the potential for water accumulation or other engulfment hazards, 29 CFR 1926.651(h)(1) requires that a method be in place to control the hazard. If proper controls are not implemented, work in the excavation cannot proceed.
• Spoil Piles: Excavated soil and equipment must be kept at least 2 feet (0.6 meters) away from the edge of the excavation to reduce the risk of cave-ins or materials falling into the trench.
• Fall Protection: Excavations greater than 6 feet (1.8 meters) in depth must have a method of fall protection to prevent workers from falling into the excavation. A common example is the installation of guardrails.
• Nearby Structures: Excavations located near buildings, structures, or roadways should involve consultation with an engineer to ensure that the excavation does not cause undermining or create structural safety concerns.

• Trenching and Excavation (a NIOSH Safety and Health Topic, Centers for Disease Control and Prevention)
• Trench Safety Awareness (a NIOSH Publication, Centers for Disease Control and Prevention)