Trench rescue training evolutions often involve digging in good soil conditions with fairly stable, straight walls. The tactics for making these trenches safe for rescuers to enter are elementary, and may not adequately prepare you for an actual incident.
A trench incident requiring emergency response usually means that the soil was poor and most likely experienced some type of collapse. Should this failure have been caused by a lip slide or slough-in, the potential for a large void space exists, and may complicate the shoring techniques you learned. Although the strategy of shoring the trench will remain the same, the tactics may involve digging into our toolbox for a forgotten instrument: Low- and medium-pressure air cushions have found a new home in the world of technical rescue as a rapid way to fill these voids.
Air Cushion Basics
Air cushions are often referred to as “high lift” bags because they are available in inflated heights from 17–48 inches, depending on the manufacturer. Standard uses include the lifting and support of aircraft wings, landing gear and large vehicles, as well as any situation where there’s a large space between the ground and the affected surface. Larger towing and recovery companies often carry them to assist in righting overturned vehicles and may carry even larger cushions than the typical sizes listed here.
Two types of air cushions are available for trench rescue: low-pressure and medium-pressure, ranging in diameter from 24–48 inches. Their load bearing is rated depending on the amount of air pressure the cushion can handle. Low-pressure cushions are typically 7–8 psi (0.5 bar) with a maximum lift of 3,000–13,000 lbs., depending on the size utilized. Medium-pressure cushions are 14.5 psi (1 bar), or double the low pressure, which is mirrored in their lift performance (6,000–26,000 lbs.).
The physical size of the bag and the maximum pressure will dictate how much air the cushion will need to inflate. This could range from 8–122 cubic feet per bag (a typical 60-minute air cylinder contains 88 cubic feet). However, since we are trying to fill a void, we rarely fully inflate these cushions for trench operations. An over-inflated cushion can make shoring operations even more difficult. Tip: Keep a spare air bottle nearby or use an air supply cart.
Air hoses and cushions typically do not have shutoffs like high-pressure air bags. Instead, the air controller may need to remain in place during their use. Each cushion has its own built-in pressure-relief valve.
The cushions are typically made from neoprene, with the medium-pressure cushions often incorporating an aramid fiber such as Kevlar. Because the pressures inside either type of cushion are low, repairs can be made quickly and simply if a puncture occurs. Loops made of nylon webbing allow for attachment of ropes, which can help position the cushion. Some manufacturers now offer alternatively designed cushions specifically for trench applications. These cushions will inflate as a cube instead of a cylinder shape, allowing for more surface contact between the panel and the cushion.
Why Air Cushions?
Shoring constructed for a trench operation is designed to pressurize the panels, which hold back the soil, mitigating potential momentum in the soil. However, voids behind the shoring can allow for additional soil collapse, which in turn may cause the shoring to fail if it has nothing to press against. This could result in additional victim or rescuer injury.
That’s where air cushions come in. They can be used to fill a void created by a slough-in or a lip slide in an area of the trench. Large air cushions can be placed to create an entire “false” wall, which will allow the rescue team to install their trench panels as straight as possible and turn a difficult shoring operation into a manageable, safer one (Photo 2). Multiple cushions or a combination of air cushions and lumber working together may be needed for large collapses (Photo 3).
Air cushions can also be used in more advanced situations such as the collapse of a corner in an intersecting trench (Photos 4 and 5). Note: Deploying air cushions in intersecting trenches requires coordination and should only be done by those who’ve received proper training.
Air cushion installation is fairly simple, but may require some adjustments as the operation progresses. Pneumatic or hydraulic shoring is best for this operation, but timber shoring can be used if no other options exist. If timber shoring must be used, also carry an Ellis screw jack; when attached to the timber, it will make quick adjustment possible.
The following procedure is for a single air cushion in a non-intersecting trench.
- Identify the area that requires the cushion, and select the most appropriate-size cushion available to you.
- Tie a rope to the webbing loop, lower the cushion into place and tie it off to an anchor.
- Place your trench panels (this may require an off-side set to the air cushion side, see Photo 6).
- Install your shoring, but only pressurize the struts to half the manufacturer’s recommended pressure (or hand-tight on an Ellis screw jack). If the struts are equipped with a lock collar, then lock the collar.
- Slowly and carefully, inflate the air cushion until the void space is taken up and contact is made between the soil and the panel. You may not be able to actually see this happen, so monitor the trench panel for deflection.
- Once deflection is noted, cease inflation and fully pressurize your shoring per the manufacturer’s recommendations. Secure your shoring with nails, but keep in mind that an air cushion is behind your panel. Your shoring is now pressurizing the trench panel and the air cushion, and transmitting all that pressure into the soil, holding it back from further collapse.
- Monitor the pressure in the air cushion during the operation and re-inflate as needed. Chances are you won’t see more than 1–2 psi on the controller gauge.
When everything is installed properly, the rescuers inside the trench should feel as if the panels are right up against the walls. Looking from the outside of the trench in, it should appear that the panels are set straight and cleanly. Removal of the air cushion is completed in the reverse order of the installation.
Remove the shoring, then the panels, then the air cushion. Should you deflate the air cushion first, the walls of the trench may push in on your shoring, making it difficult to remove, or possibly shifting the panels uncontrollably. This could result in injury or cause a secondary collapse.
Although air cushions are certainly nothing new to the fire service, they are quickly becoming standard equipment for trench rescue teams everywhere. Mutual-aid departments with this old tool should be on your resource list, especially if your team doesn’t carry them. Early or automatic mutual aid for air cushions will help ensure effective and safe shoring installation.
The bottom line: In proficient hands, air cushions are simple to operate and maintain. Training, size-up and coordination are all key factors in a successful trench rescue operation involving air cushions.
Trench Rescue Definitions
Lip slide: Collapse of the top 2 feet of the trench where the wall intersects the ground. Can be caused when the dirt that was removed from the trench (spoil pile) slides into the trench and pushes the lip with it.
Slough-in: Collapse of a trench wall in such a fashion that an overhang remains, dangerous and difficult to shore.
Off-side set: Trench panel is placed on the opposite wall from where it will be set. Using ropes or lumber, the bottom of the panel is placed in the toe of the trench where it will be set. The top of the panel is then brought over into position.