Construction trenches: a poorly understood confined space

Did you know? Trenches deeper than 1.30m are considered dangerous. As with other confined spaces, operators must be trained in working within a trench and work in pairs with a trench supervisor, also known as a lookout. This article provides an overview of the various risks that can be found in trenches, as well as the prevention and protection measures that must be implemented to avoid accidents.

Trenches are a narrow, deep excavation used for laying or repairing utilities, foundations, or other related work. They are commonly encountered in the following situations:

- Installation or repair of underground networks (water, gas, electricity, telecommunications). - Sanitation and drainage work.

- Point or continuous foundations.

- Road or rail infrastructure works.

Their presence is very frequent in the field of civil engineering and involves significant specific risks, particularly in relation to their depth and the stability of the ground.

Regulatory framework to be taken into account

The Labour Code, through its articles R4511-8 to R4511-14, sets out the conditions for opening excavations and the obligations of protection against the risks of collapse.

Article R4534-1 and its successors emphasize the importance of preventing collapses during trenching activities. It is also essential to conduct a risk assessment, adapt working methods, and train and inform workers.

Normative references and technical guides such as INRS ED 6084, OPPBTP technical sheets, CARSAT circulars and NF EN 13331 standards provide recommendations and specifications to ensure safety when working in trenches.

Risks associated with working in trenches

The main risks include wall collapse, falls from height, hazardous atmospheres, accidents related to machinery and equipment, material slides and adverse weather conditions.

Several factors can aggravate these risks, such as the nature of the soil, the depth and width of the trench, the presence of vibrations and prolonged exposure. In order to ensure the presence or absence of these risks and to put in place the organizational, technical and human preventive measures for the smooth running of operations, an excavation permit is required. This permit must include most of the sections that would be found in a permit to enter confined spaces.

Among the multitude of risks that can exist in trenches, it is important to focus on toxic, corrosive, and flammable gases and vapors that are heavy (gas density greater than 1) and volatile. Toxic and corrosive vapors are likely to quickly poison operators who are not equipped with FFP3 masks with suitable cartridges and multi-gas detectors calibrated to the substances to be monitored. Flammable vapors and gases are likely to cause explosions if their concentration is in the explosive range (see article on ATEX). To avoid this, the use of an explosimeter seems more advisable than ever.

Collective and individual means of protection

Collective protection against collapse consists of the use of shoring or shielding, the sloping of walls, appropriate signage and marking, secure access devices, regular monitoring (by a specifically designated lookout), monitoring of atmospheres and management of material storage.

For personal protection, personal protective equipment (PPE), safety harnesses and communication devices are required.

Staff training and occupational medicine

It is imperative to provide specific training on collapse risks, good working practices in trenches, identifying signs of instability, emergency evacuation procedures and first aid. Some of these elements are covered in "Intervener / Capacity Supervisor" training (also called "confined space training"). Appropriate medical monitoring is also recommended to assess workers' physical fitness to work in this type of environment, detect medical conditions and prevent risks related to working conditions.

Essential recommendations

In conclusion, it is essential to accurately assess risks, put in place effective signage, ensure secure access, train staff and organize regular medical monitoring.