The development of local infrastructure in communities nationwide has resulted in a large amount of road and utility work. With this work comes opening of soil—and the hazards of possible trench collapse. Workers and would-be rescuers can become caught and, in some cases, killed needlessly in trenches that collapse with little to no warning.
See more below on trench safety standards, how to identify soil, and what happens after the rescue
It has become commonplace for the local emergency responder to be summoned to handle this situation.
Anatomy of a trench
A trench can be defined as a temporary excavation in which the length and the depth of the trench are greater than the width of the floor. For ease of nomenclature, many teams identify an excavation as a hole where the floor’s width is much greater than the actual depth of the trench. Although there are differences between a trench and an excavation, dealing with both can be very dangerous situations.
Trenches are made up of five main parts. The lip is the top two feet of the wall; the belly is the center of the wall; the toe is the bottom two feet of the wall; the wall comprises the lip, belly and toe; the flooralso is known as the base of the trench.
It’s important to identify the parts of the trench during an incident, as each part’s involvement in the collapse can offer various problems in dealing with the stability of the trench.
Size-up for a trench collapse should begin during the response to the scene. Familiarization with your jurisdiction should help you to identify any target hazards that the area might possess and what resources you need to abate them.
Set up
Once on scene, the primary focus should be to regain some sort of control of the incident. It might be necessary to remove would-be rescuers from a dangerous area prior to your rescue operations. This won’t be easy, because emotions might run high, with people trying to help their friends and coworkers.
Furthermore, crowd control is essential. Keep onlookers, the media and nonessential personnel away from the Hot Zone.
During this time, performing accountability of the staff who are on scene prior to the collapse is paramount. After that, it can be determined how many personnel are unaccounted for.
These are reasons why it’s important to set up work zones on scene.
The Hot Zone is the area where the actual trench rescue operations are performed. This area can be as much as five times the depth of the trench.
The Warm Zone is the area where the command post, tool staging, shoring preparation, entry into the Hot Zone and other activities that are associated with the rescue take place.
The Cold Zone is located behind the Warm Zone and is for support operations. This remote area helps to ease load forces and vibrations on the open trench and usually is at least 20 times the depth of the trench away from the Hot Zone.
Next, it’s vital to determine what currently is going on at the incident. Any hazards that are outside of the trench, such as vibrations, traffic congestion, energized utilities and secondary collapse issues, must be addressed.
The area inside of the collapse might provide clues as to the location of a missing worker. A hard hat, a tool belt, pipe strings and grease cans, among other items, can give rescuers an area of proximity as to where the victim might be buried. This would provide an area for rescue operations to begin.
It’s important to consider that, although the victim might not be seen, that doesn’t automatically make the incident a body recovery. The victim might be positioned in a manner that protects the chest and allows for expansion from the weight of the collapsed materials. Pipes, construction materials and other items can provide voids for safe havens for a victim.
Shoring
An operational guideline should be in place, so response to a trench collapse includes the right equipment and personnel to operate on scene. With many departments operating with a minimal staffing pool, it’s imperative that the guideline includes automatic aid departments that are proficient in this area. Staffing is the most precious resource that departments bring to any emergency scene.
There should be considerations for the equipment, too.
Shoring materials include what will be put in place to make the trench safe, including ShorForm, FinForm and EuroForm wooden panels. These panels are a minimum of 1-inch high-grade plywood that’s coated with a phenolic resin that seals and strengthens the sheets to take the abuse of trench work without failing and cracking.
These panels are fitted with uprights, which are 2 x 12-inch wood planks that provide considerable strength to sheeting and serve as an area for cross-bracing to be attached to.
Many times, cross-bracing can include cut dimensional timbers, which will vary in size and diameter, depending on the width and depth of the trench.
Wood shoring materials should be inspected for physical damage from flexing during operations, rot and decay, warping and other defects that might cause the material to fail under pressure.
Other forms of cross-bracing include hydraulic struts and pneumatic struts. Aluminum hydraulic struts are preengineered shoring systems that combine aluminum cylinders and either horizontal or vertical rails to support the sidewalls of a trench. The cylinders are charged with air to extend against the trench walls, set at a specific air pressure to meet the requirements of the soil type. These struts are manually locked into place for maximum compressive strength. The total force on the strut can vary based on the depth of the trench, the compressive force of the soil and additional loads that around the lip of the trench.
No matter the type of equipment, it’s vital that all personnel are well-versed on their use and operation.
Making trench area safe
Approaching the trench end, crews start to drop ground pads and bridging, which help to distribute the weight of personnel and equipment around the lip of the trench. Ground pads are 4 x 8-foot sheets of three-quarter-inch plywood. (The spoil pile side requires 2 x 12-inch planks between the trench lip and the spoil pile.) Ground pads should surround the trench area at least equal to the depth of the trench and should overlap one another to ensure that there are no open areas of soil.
As crew members move the spoil pile farther away from the lip, it might be possible to lay additional ground pads on that side of the trench.
While the ground pads are placed, atmospheric monitoring and supplemental ventilation should be underway. It might be possible to collect methane inside of the trench, which must be removed before a flash fire can occur. Fans that are used for ventilation should be intrinsically safe and be capable of moving a minimum of 1,000 cu. ft. of air per minute.
Type of collapse
Rescuers must identify the type of collapse that occurred. Common types include:
Spoil pile slides occur when the spoil pile slides back into the trench, either by machine operators mistakenly knocking part of the spoil pile back into the trench or because of a surcharge load on the edge if the spoil pile was placed too close to the lip.
Side wall shear occurs when a portion of the wall breaks off and falls into the trench. This is more common in trenches that are open for long periods of time. Signs of impending wall failure include fissure cracks along the edge of the trench.
Slough-in occurs when there’s poor water drainage in the trench soil and the belly of the trench wall falls inward to create a dangerous overhang under the ground pads. This is a tough trench to shore, because the voids that are created by the collapse must be addressed.
Lip-in occurs when the weight of the spoil pile is too close to the lip or when heavy machinery that’s operating in close vicinity to the trench causes the lip to fail, taking everything that’s on the lip (equipment, personnel, etc.) with it into the trench.
Inside of the trench
Once the collapse pattern is identified, crews can begin to shore up the walls of the trench.
A means of egress, such as a ladder or stairway, must be located no farther than 25 feet of lateral distance for rescuers. This is best served by keeping the “two ways out” rule in mind. Place at least two ladders for egress in the trench, one at each end.
Sheeting panel assemblies are laid into place, facing opposite of each other. Once in place, rescuers measure between the panels to install adequate cross-bracing. Cross-bracing will vary with soil type, trench dimension and shape, and type of bracing that will be employed. The bracing is cleated and secured into place, and the process is repeated until there is a safe area to operate to remove the victim.
The next step includes walers, which are large-diameter wood dimensional timbers that support the initial panels and allow the installation of supplemental shoring timbers. The sheer size and weight of the waler requires a lot of personnel to install in a limited amount of room at the floor of the trench. Once the waler system is installed and supplemental shoring is in place, space within the work area can become cramped.
A lot of other things go on inside of the trench.
Patient care will be an ongoing operation, including stabilizing the victim while soil is removed from around the individual.
With the potential for significant injury, it’s vital that ALS is on scene to provide necessary treatment.
Rescuers must navigate around hazards that can’t be removed from the trench, such as pipes, construction equipment and materials. Any encumbrance that’s within the footprint of the trench must be stabilized and secured.
Crews that operate outside of the trench can begin to prepare immobilization and removal equipment, so the victim can be packaged and removed as quickly and efficiently as possible.
Not for the untrained
The scene of a trench collapse is a dynamic, high-risk environment that can result in injury and death to workers and rescuers alike. Therefore, it’s imperative that rescue from these situations only be attempted by emergency personnel who are trained in trench rescue.
Trench Safety Standards
Trenches are dug for numerous reasons, including storm drains, sanitary lines, utility services and repair, and roadway renovation and repair. Although their purpose varies, safety standards must be addressed while the trench is open:
- Any trench or excavation that’s greater than five feet in depth must be shored
- Excavation of a material to a level that’s no greater than two feet below the bottom of the support system is permitted
- Excavated material (also known as the spoil pile) shall not be piled within two feet of the lip of the trench; this often is a significant safety issue that requires a lot of personnel to move the pile immediately
- Personnel shall be protected from the hazards of cave-in while they enter, work in and exit the trench
- Personnel aren’t permitted in shields or trench boxes while they are installed, removed or moved vertically.
- In trenches that are less than five feet deep, shoring might be required if: examination of the trench by a “competent person” suggests a potential collapse situation; vibration from road traffic or machinery can cause a cave-in; the trench has been open for an extended period of time (24 hours); or the trench runs parallel within two feet of a roadway or second open trench
For a complete list of regulations regarding trenches, familiarize yourself with Occupational Safety and Health Administration regulations regarding trenches, mainly 1926.650, 1926.651, 1926.652 and the A, B, C, D and F appendices of 1926 Subpart P. Printing these regulations and carrying them on your apparatus can be worthwhile. They make great reference materials.
Identifying Soil
Identifying the type of soil that’s involved in a trench collapse assists in determining the equipment that’s needed to safely shore the trench. The Occupational Safety and Health Administration defines four categories for most soils.
Stable rock is a natural solid material that can be excavated, but it will remain somewhat intact when exposed to the elements. It can sustain vertical walls up to 90 degrees. This type of material usually requires the aid of a blasting agent to assist in digging in these areas.
Class A soils have an unconfined compressive strength—the load per unit area (square foot) at which a soil fails under compression load—of at least 1.5 tons per square foot. Examples of these soils include strong clay that hasn’t dried out, cemented soils, hardpan soils and clay loam.
Class B soils provide a lower level of stability. They have an unconfined compressive strength of 0.5–1.5 tons per square foot. It isn’t uncommon for water to seep from the trench walls in this type of soil. Class B soils include weak clay soils, dried out unstable rock, previously disturbed soil and granular cohesive soils.
Class C soils are the least stable type of soil. They have an unconfined compressive strength of less than 0.5 tons per square foot. Sandy granular soils, submerged soils and weak clays make up this category.
One way to identify a type of soil is the use of a pocket penetrometer, which is a small measuring device that classifies soils in terms of consistency. A small diameter shaft is pushed into the soil, and the amount of resistance of the force that’s applied is marked on a scale that’s measured in tons per square foot.
A few manual tests also can be performed to help to identify the type of soil that’s in a trench:
Plasticity Test. Mold a moist or damp sample of soil into a ball and attempt to roll it into threads of soil, approximately the thickness of a pencil. If at least a 2-inch length of soil is attained without breaking, the soil is considered cohesive.
Thumb Penetration Test. Take a sample from the top of the spoil pile, which would represent the bottom of the trench, and take a second sample from the bottom of the spoil pile, which would represent the top level of the trench. Take both samples and roll them into a ball. Try to impress your thumb into each ball to see how difficult it is to break the sample apart. If the thumb easily penetrates, the soil is considered unstable; if it takes great effort to penetrate the soil with your thumb, the soil is considered cohesive.
It should be noted that no matter the soil that’s identified, the soil did indeed collapse. Therefore, it is wise to classify any collapse soil as Class C soil for shoring purposes.
After the Rescue
Even after a victim is rescued from a collapsed trench and is receiving definitive medical attention, the scene is far from stabilized. Shoring equipment and tools that were used to make the trench safe must be removed. The process is reversed for removal, but the scene becomes more hazardous, because the trench was open to the elements all the while during the rescue, which might allow for soil to become more likely to collapse. Great care must be taken during demobilization, so all rescuers remain safe during the termination of the incident.
"Technical Rescue: Trench Collapse: On Arrival" originally appeared on Firehouse, an Endeavor Business Media partner site.
