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 physically 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 away 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 will fail early in a trench. Type B soil, or “saturated soil,” will have running water seeping from it and will be an indicator of weak areas of the trench walls. Type A soil, or “compact soil,” will have a definite cohesiveness and resist collapse well, almost as well as stable rock. It is important to recognize that all types of soil are capable of collapsing without warning. It is also possible that you will encounter multiple types of soil in a trench wall; this is considered a “layered soil,” and is the most dangerous to work in or around, due to its makeup.
The strength of the soil
Identifying the strength of the soil is critical to the operation. Rescue teams and engineers have tools and devices such as pentrometers that are used to measure compressive soil strength. Although most departments do not carry such instruments, a simple field test a responder could use is referred to as a “Thumb Penetration Test.” In that test, a responder takes a sample from the top of the spoil pile, to represent the bottom of the trench, and a sample of the bottom of the spoil pile, to represent the top of the trench, and compresses the soil into a ball. The responder then tries to press a thumb into the sample and make an impression that stays. Additionally, a plasticity test can be performed on the soil. A rescuer takes a sample of soil and tries to roll the sample into small, thin pencil-like strips of soil; any soil that is over the width of the palm can be considered to have positive cohesiveness. But remember why the department is there – the trench has already failed.
Setting up the scene
First-arriving units must set the tone for successful mitigation; this happens upon arrival. If a trench rescue team has not been summoned on the initial alarm, it must be requested to respond immediately. Other on-scene operations should include the following:
• Size-up – Identify the cause of the collapse, and how many victims are trapped. Take into consideration what the entrapment is; is it soil or additional materials? Many times, pipes, vaults, machinery and other materials fall into the trench, which can result in added mechanisms of entrapment to the victims.
• Primary assessment – Determine who is in charge at the trench site (sometimes referred to as the “trench boss”) and keep that person at your side. The trench boss can be a source of information in relation to accountability of missing workers, language barriers, purpose for the trench, equipment needs and potential hazardous material presence.
• Victim locators – Identify some clues about where a victim MAY be buried. While there may be a general area as to where the victim may be located, the position of the victim can vary based on the actions the victim was performing prior to the collapse. Look for clues such as tool buckets, lunch boxes, pipe joint materials, flag stakes or engineer hubs (used to identify the location of a trench), laser targets and personal protective equipment such as hard hats, protective eyewear and work gloves.
• Secondary assessment – Consider who is responding and what special units may be needed on scene. These include vacuum trucks, hazmat teams, local utility providers and ambulances, and they should be requested early. This operation will be manpower-intensive; be sure to have plenty of responders in the tactical reserve to replace rescuers that are performing support functions (cutting stations and spoil pile removal, for example). Managing traffic and crowd control will be of importance. Be sure local law enforcement is notified of the need to re-route traffic within the affected area and to keep spectators from becoming additional victims. A good rule of thumb is to start the “hot zone” a minimum of 100 feet from the incident.
By the time these tasks are completed, the rescue teams and forward Incident Support Team (IST) personnel should be arriving on-scene. This does not relieve the initial-alarm companies from the incident, because many tasks will need additional personnel to help, such as laying ground pads, ventilation, atmospheric monitoring and other support functions that will need to be completed. All resources will be needed to keep the scene safe, efficient and successful.
Emergency response personnel will be called to these incidents in their jurisdiction, regardless of whether they can provide the necessary skills to perform victim removal. Since they will most likely be first on scene, it is vital that the steps taken by the initial companies are enacted to ensure the highest probability for success on the rescue scene. Having these tasks completed prior to the arrival of the rescue unit will allow the specialists to go right to work on victim removal, increasing the potential for success.
- This article was originally published in the September 2012 issue of Firehouse Magazine. You can see all of the Firehouse Magazine articles online by subscribing here.