Construction sites, sewer line installations and replacements and utility repairs may warrant opening up a trench in the earth to complete these tasks.Many times, these operations are done safely and with the proper equipment in place to ensure that workers are protected while accomplishing their tasks. If corners are cut and safety is compromised, however, disaster can strike, resulting in a trench collapse that can seriously injure or kill a worker. Whether or not your fire department provides technical rescue in your community, the odds are high that your department will be dispatched to an incident described above, resulting in one or more people stuck within the confines of a trench collapse. This article focuses on tasks and responsibilities that the initial-arriving units should accomplish to ready the incident for the technical rescue team’s arrival.
A little about trenches
A trench can be defined as a temporary excavation in which the length of the bottom exceeds the width of the bottom and is deeper than it is wide. A trench is made up of many parts: the walls; the lip, which is the top two feet of the wall; the belly, or center portion, of the trench wall; the toe, or the bottom two feet of the trench wall; and the floor, or base. Although not part of the trench, the “spoil pile” is the pile of soil that has been removed from the earth to create the trench. It should be a minimum of two feet from the lip of the trench.
Trenches are dug for a variety of reasons; some of which include storm sewers, sanitary lines and utility services. But all of them share some common safety rules.
Any trench that is deeper than five feet needs installed shoring for support. Any trench that can potentially reach depths of 20 feet requires the use of an engineer-designed trench-stabilization system. If none of these options exist, then it may be necessary to dig the trench walls down to the “angle of repose” to avoid a potential collapse. This angle slopes the walls of the trench away from the floor, based on the type of soil encountered in the trench collapse. For example, when dealing with loose type C soil, it may be necessary to dig the walls down to an angle of repose of almost 34 degrees, equaling a significant trench slope starting 15 feet from a 10-foot-deep trench. These operations are not done by hand; heavy equipment and valuable time is needed to accomplish this, which is why it is extremely rare to see these operations at a rescue scene.
Why trenches collapse
Trenches collapse for a variety of reasons. The primary reason for most collapses is the absence of proper trenching equipment. Most times, the absence of proper shoring is a result of workers’ compromise of safety in the spirit of speed in the trench; many times, it is met with disastrous results. Some trenches collapse due to their location in previously disturbed soil.
Intersecting trenches are more prone to collapses because they are open at two sides at the corners. A narrow right of way aids in the collapse due to limited room for spoil pile placement, which should be at least two feet from the lip of the trench. Vibrations near the trench are also blamed for collapses. If a trench is open for a prolonged period, it will dry the water out of the soil, which reduces the cohesiveness of the soil and cause a collapse. Mother Nature is damaging to a trench, with rain water and snow, not to mention winds that will cause collapses. It is important to be able to recognize the potential causes to avoid collapses, including secondary ones, when working in and around trenches.
Some soil types are prone to collapses. Type C soil, or “running soil,” lacks cohesiveness and fails early in a trench. Type B soil, or “saturated soil,” has running water seeping from it and is an indicator of weak areas of the trench walls. Type A soil, or “compact soil,” has a definite cohesiveness and resists collapse well, almost as well as stable rock.