Trench and Excavations

A trench is an excavation that is deeper than it is wide and less then 15 feet wide. OSHA has published regulations dealing with trenches in 29 CFR Part 1926. This standard regulates the construction and occupation of trenches over 5 feet deep and shallower trenches with special hazards. Illinois Department of Labor (IDOL) has adopted 29 CFR 1926 as the state regulation.

OSHA requires that Escape routes, Air quality monitoring and other protective measures be utilized at all trench excavations.

OSHA statistics are riddled with workers who have been disemboweled or decapitated by backhoes, and would- be rescuers are frequently buried in secondary collapses. In fact, as many as 65% of all deaths in trench cave- ins are would-be rescuers. Trench rescues are a risk in all jurisdictions, however are not common occurrences.

Trench Hazards

Virtually all of the hazards associated with trench rescues are hidden from the untrained rescuer. Although some of the hazards are man-made and can thus be avoided or corrected, the majority of hazards occur naturally in all trenches and cannot be eliminated except to prepare for failure. The following is a discussion of the more dangerous elements of trench rescues.

Secondary collapses

By far the most lethal hazard in trench rescues is the likelihood of secondary collapse. Once a collapse has occurred, the remaining sections of trench wall will have much less stability .The fallen section of dirt typically leaves a larger section unsupported, which in-turn fails, which leaves an even larger section unsupported, which also fails. Unfortunately, these secondary collapses seem to occur just about the time that a rescuer is attempting to dig out the initial victim and are responsible for the high number of rescuer deaths. The four types are illustrated below. Slough-in, sidewall-in, shear-in, and spoil-in.

Trenches dug too deep or too wide

OSHA provides guidelines for the general construction of trenches up to 20 feet deep and 15 feet wide. Excavations beyond these dimensions require special engineering by a Registered Professional Engineer (RPE).

Unprotected trenches

It is easy to get fooled into entering an unprotected trench to rescue a worker who has fallen, or is ill. Just because the fire department is there doesn't mean that the trench will remain intact while you make the rescue. The trench should be properly protected before fire personnel are permitted to enter.

OSHA REGULATIONS

OSHA has published regulations dealing with trenches in 29 CFR Part 1926. This standard regulates the construction and occupation of trenches over 5 feet deep and shallower trenches with special hazards. These regulations apply to rescuers as well as workers. Illinois has adopted OSHA regulations through the Illinois Department of Labor .

Rescues are usually long-term operations

People who are buried in cave-ins are usually "frozen" in the position they were in when the collapse occurred. Often arms and legs are bent at odd positions and in many cases, the victim has suffered fractures. Victims cannot merely be pulled-out from under the dirt. Therefore, the victim must be completely uncovered before he can be removed from the trench. Attempting to "yank" a victim will only result in additional injury, and will not release the victim. Most rescues require as much as 4-10 hours to complete.

Other factors that will effect trench stability

• Many other factors must be considered which will effect trench stabilit . These are:

• Exposure to the elements

• Superimposed loads

• Underground utilities

• Unsupported structures (surface encumbrances)

• Water

Buried utilities

Trenches are excavated to allow for the installation of various utilities. Since most utilities run in easements, it is almost certain that some form of underground utility will be encountered during excavation.

Soil Classification

The more cohesive and uniform the soil is, the more stable it will be. Soils range from stable rock, to clay, to sand, and various mixtures of the same. Soils, which are granular in nature, will readily collapse and will often make it impossible to even dig what we would commonly think of as a trench. Clay is the most stable soil to deal with, and will often stand freely for some period of time. Unfortunately, it is also the soil responsible for most of the fatal collapses. Workers become accustomed to working in clay without incurring much trouble until that one time when it doesn't hold.

OSHA classifies soils as Class-A through Class-C. Class-A soil is the most stable, and will include some form of clay. Class-C soil is extremely unstable and will be comprised of either granular soils like sand, or wet soil of any type.

Wet soils

Wet soils of any type are dangerous due to the added weight of the water, the loss of friction due to the moisture, and the mechanics of the movement of the water through the soil. Trenches which have been dug below the water table will "draw" water into the open hole. Water seeping into the trench will undermine the trench walls pulling them into the trench. This will lead to wall collapse, entrapment and/or drowning of anyone in the trench.

Layered soils

Trench walls will often expose layers of different soils. These layers especially if sloping toward the trench, will allow one layer to slide on another and often leads to collapse.

Fissured soil

Fissures ( cracks) which are visible in the trench walls or in the soil surrounding the trench indicate soil, which is likely to cave-in shortly.

Previously disturbed soils

The most stable soils are those which have gone undisturbed for thousands of years. Each grain of soil was dropped or blown into its position creating a "locking" effect between grains. Once the earth has been disturbed, it is impossible to return it to its original stability.

Vibration

Vibration is extremely destructive to trench stability .Vibration will speed-up the collapse of the walls, and will magnify any other factors, which are effecting the trench. The most common sources of vibration are the digging operations themselves and vehicles passing by. Other sources to consider are trains, processing operations from nearby industry , and motors running near the trench. Vibration is particularly dangerous at rescue scenes, but is usually a factor which can often be controlled. Rescuers must be careful to not add vibration to a scene by careless use of vehicles, generators, or other equipment.

The weight of dirt

A single cubic foot of dirt can weigh as much as 145Ibs., and will average about 100 lbs. per cubic foot. A typical small cave-in involves about 1.5 cubic yards of dirt, or about 4,000 lbs. An average man covered with two feet of soil may be under some 3,000 lbs. of soil, about 1,000 lbs. of which may be on his chest area.

The speed that dirt can move.

Studies have shown that trench walls often collapse in less than l/lOth of a second. A well-intentioned rescuer who thinks that he can climb out, or can be pulled-out by a rope before a collapse catches him, is inviting trouble.

Sloping

Sloping involves cutting back the sides of a trench to an angle at which the earth will no longer slide. OSHA defines sloping angles for each class of soil. The angle, which is sought, is referred to as the "angle of repose" and is merely the angle at which the soil will no longer slide.

Shielding

Shielding involves the use of extremely strong metal boxes, which have been engineered to withstand the pressure of the earth for the size trench that is being worked in. Trench shields (also called trench boxes) are strong enough to protect a worker from cave-ins as long as the worker stays within the shield. Trench shields must be installed no more than 2 feet of the bottom of the trench and even with or above the trench lip.

Shoring

Shoring is a method of protecting the worker by constructing a support system within the trench, which will pressurize the trench walls, enough to create "arches" of support, which will hold-up the trench walls. Shoring is designed to be strong enough to stop the walls from starting to move, but is not designed to be strong enough to stop moving dirt.