Many workplaces contain spaces that are considered "confined" because their configurations hinder the activities of employees who must enter, work in, and exit them. A confined space has limited or restricted means for entry or exit, and it is not designed for continuous employee occupancy. Confined spaces include, but are not limited to underground vaults, tanks, storage bins, manholes, pits, silos, process vessels, and pipelines. OSHA uses the term "permit-required confined space" (permit space) to describe a confined space that has one or more of the following characteristics: contains or has the potential to contain a hazardous atmosphere; contains a material that has the potential to engulf an entrant; has walls that converge inward or floors that slope downward and taper into a smaller area which could trap or asphyxiate an entrant; or contains any other recognized safety or health hazard, such as unguarded machinery, exposed live wires, or heat stress.


Some of the most potentially dangerous issues on worksites involve confined spaces. Unfortunately, workers and supervisors don’t always fully appreciate the hazards or understand the safety practices related to these spaces, especially in trenches.

By definition, a lot of things can be considered a confined space – including a closet in your home. It has limited means of getting in and out, and it’s not designed for continuous human occupancy.

There are two kinds of confined spaces: permit required and non-permit. Of the two, a permit required confined space is the more hazardous. To be considered a permit required confined space, it has a potentially bad atmosphere, no atmosphere, or limited access/egress or other serious hazard that could impact worker safety.

Confined spaces appear frequently on construction sites, and many workers and supervisors don’t realize it. That’s because any trench that’s deeper than four feet meets the definition of a confined space. If you think that’s exaggerating the danger, consider this: if one of your workers were overcome by fumes in a four-foot trench, and you reached in to grab that person and remove them, your face would break the plane of the trench. In all likelihood, you’d then be laying unconscious next to the worker.

Although trenches may seem simple, safety professionals know that any trench requires careful preparation if it becomes deep enough to become a confined space (four feet or deeper), or if there’s a potential for the walls to collapse, either of which requires careful preparation and attention to detail. Among the factors you’ll need to consider are:

Access and egress. The “what if” factor is critical. If somebody does get hurt in a confined space, how will you remove that person? Because you don’t have a vertical opening like a manhole, it can be far more difficult to remove someone, particularly if that person is unresponsive. You’ll have to determine whether the best approach is the entry method, in which you send somebody in after the person, or the non-entry method, which requires the use of rescue equipment and/or a harness. The non-entry method requires considerable thought and professional ability.

Proper equipment. Working in a permit required space/trench may call for a variety of equipment. It may include everything from personal protective equipment to special tools to having rescue equipment on hand. In certain cases, you’ll also need to provide adequate ventilation. And if there is the potential for a fire, entrants may also need intrinsically safe communication to minimize the risk that radio frequency interference could set off an explosion.

Chemical issues. Employers have to be concerned about any chemicals that are used in the trench, but many people are unaware that they need to be just as careful about what’s in the immediate area. Once you identify gases or fumes that could enter the comfined space, check the MSDS to determine their vapor density, which will allow the attendant to predict how they’ll behave. For example, the oxygen we breathe has a vapor density of 1. Acetylene gas has a vapor density of 0.93, which means it’s lighter than air, and will move upward if it escapes. If your confined space has a top, it may displace the breathable air near the ceiling (and create an explosion hazard). In contrast, gasoline has a vapor density of 3.93, making it heavier than air and meaning that it will be lower, near the bottom portion of your space.

Depending on the atmospheric conditions, carbon monoxide may migrate and settle in the lowest place to rest. If your trench is located near a road or busy parking lot, carbon monoxide from vehicle exhaust (or a nearby piece of equipment) could flow into your trench. Because the gas is odorless, you may not be aware that there is a problem until a worker becomes asphyxiated. Similarly, you may encounter contaminated soils (such as a site located on a former gas station), which could require additional ventilation and air monitoring.

Defeating instinct. When someone in a confined space is in trouble, it’s natural for a co-worker to rush in to help. But more than half of the people who are killed in confined-space situations are would-be rescuers going in after someone else. That’s why it’s important to train everyone in the proper steps for rescue and ensure that others don’t go in after an injured worker. The attendant must take control of the situation and be ready to provide details to the rescue professionals so that they can take the correct precautions.

Regarding trenches, cave in has the greatest potential to require rescue. Care has to be taken by the rescuers not to disturb additional soil, neither by equipment nor human intervention. The time of the cave in should be noted, all non-essential personnel removed from the work area, and an area cleared for the rescue team to provide services. Hand tools and equipment located in the trench should be left in place, as a potential marker for where the worker is. A common response by work crew members is to want to free their buddy by using the heavy equipment, but experience has shown that this is one of the wrong things to do to remove injured/buried workers.

Overall, safe confined space entry of trenches/excavations involves 80 percent preparation and 10 percent perspiration.