Construction work can be dangerous, but underground construction has proven to be the most dangerous. The hazards associated with excavations include cave-ins as well as other issues.
Cave-in fatalities continue to occur because those excavating fail to provide an adequate protective system for the workers in the excavation. There are several reasons for this mistake. Some do not fully understand the situation and are operating on a misunderstanding of soil and what causes a cave in. For example, there are those who think they can dig a trench and get their work done before a cave-in occurs. Others think they will be able to just move out of the way if the trench wall begins to fail. These assumptions are dangerous, but what is more difficult to understand is that there are those who are aware of the consequence of being in a cave-in yet still choose to take the risk.
What every employee working in or around excavations should know is that all soil types will eventually collapse, and no one knows when that cave-in will happen. There are many factors involved that could initiate a cave in and many of them are invisible to the observer.
An inadequate protective system can spell disaster, as well. For example, if someone is planning to slope the trench side back as a means to prevent a cave-in, that sloping system must be adequate, or it will not be safe. In other words, if the slope is too steep for the given soil type, a cave-in can still occur. This can happen when someone is in a hurry or in those situations where there isn’t enough room to slope adequately.
Why is a cave-in dangerous? Well, it goes back to the weight of the soil. The force of impact when soil hits the human body is usually more than a body can withstand. The result could be broken bones, internal injuries, and asphyxiation. However, even those buried by soil who somehow avoid the above-mentioned injuries can still die of something called “crush syndrome.” This phenomenon occurs when the weight of the soil on the victim’s body restricts blood circulation, much like a tourniquet. In these situations, if circulation is not restored quickly enough, the isolated body tissues begin to die, and the blood becomes toxic. If this continues long enough, once the buried person is finally uncovered from the soil and circulation is restored, the victim may experience organ failure or even death when the toxic blood and chemicals begin to recirculate. The physical problems associated with crush syndrome can vary with the individual, but one potential result is a heart attack.
The study of why and how cave-ins occur begins with an understanding of soil mechanics. This includes the important concept of the “adjacent area.” The adjacent area is defined as the area out from the edge of the excavation a distance that is equal to the depth of the excavation. The adjacent area potential hazards include anything that is in the way or anything that can fall into the excavation. Therefore trees, boulders, sidewalks, pavements, and other structures such as sign bases, walls, and utility poles must be supported or removed because the excavation work can cause them to become unstable.
The weight of objects in the adjacent area contributes to the downward forces and are called “surcharge loads.” Whenever an excavation is opened, these downward pressures find relief by caving into the void made by the excavation.
In addition to surcharge loads, there are other site conditions in the adjacent area which also help to cause a cave ins. These include fissures, previously disturbed soils, vibrations, water and soil layers.
A protective system is required if the excavation is 5 ft or deeper and is also required if it is less than five feet deep if any collapsing soil or structure can hit an employee. Keep in mind that under state law a protective system may be required at four feet, so check your state and local laws for these requirements.
In conclusion, the takeaway is to know that all excavated soil walls will eventually fail due to natural and man-made reasons. Gravity alone is enough to cause a cave-in, but the other influences only serve to make it happen more quickly. An adequate protective system will save lives when used correctly.