Trench rescue: The importance of approach
How to use ground padding and bridging in your rescue operation
There are a lot of variations on how to approach a trench and make the area safe to commence shoring and rescue operations. I think a consensus is developing amongst some of the technical groups regarding what that approach should look like.
As I unload some of this info here, it is important to underscore that these recommendations are based on solid foundational engineering and safety concepts. These are not revolutionary new concepts; they are just commonly dismissed or overlooked.
Trench rescue ground padding and bridging
To provide a quick overview, we are going to focus on two basic trench rescue considerations: ground padding and bridging. Please note I said and, not or. These are not absolutes, as every rescue can present challenges that require unique solutions. However, I advocate that these concepts be the norm when possible, and, if you deviate from the norm, you should have a strong understanding of the risks and potential consequences.
We will establish that the preliminary requirements for trench access and rescue are being completed – atmospheric monitoring, ladders every 25 feet maximum, dewatering, safety assessments, and ventilation/lock-out tag-out, soil and trench assessments.
Ground padding: Ground padding is most commonly half-inch x 4 feet x 8 feet sheets of plywood. They can also be prepared in a narrower variety, such half-inch x 2 x 8 for the spoil pile side, and they often have a grit coat applied to them to offer some slip resistance. Other diameters, such as three-quarter-inch, may also exist in some caches, but the goal is to apply a load-distribution surface to the ground around the trench that is lightweight but durable and tough enough for rescuer traffic.
Developing a process for approaching the trench can pay huge dividends in efficiency and safety. Here is an example of a ground pad placement process:
- Identify the confirmed (or most likely) location of the victim from a remote and stable location. This is often done from a slight distance at the end wall of the trench. The assessor can bring a ground pad with them to the end wall for the initial visual inspection if the pads are already on scene.
- Once the location is established, from the non-spoil pile side or the most accessible side of the trench, stand back the full 8 feet if possible and slide the panel into position so that the center of the panel is centered on the victim location. This results in the panel running perpendicular lengthwise to the trench.
- Add two additional panels side by side to each side of the initial panel set. This results in five parallel panels running perpendicular lengthwise to the trench.
- On the spoil side, slide in your narrower panels from the end-walls if the pile is encroaching on the trench. The spoil side may require more versatility depending on proximity to the trench and access.
The goal is to end up with 20 feet length of trench surrounded by ground padding. This can be done in less than 2 minutes with a well-trained and adequate crew of trench rescuers.
Here is the golden nugget with applying some formality to how you place your ground pads: The ground pads have now created the template for your panel sets. This means the panels that you will eventually drop into the trench should marry up with the edges of your ground pads. The first panels will go in on the first centrally located ground pad, and the additional panels will grow out in both directions laterally. The ground pad placements also lay the foundation for your bridging.
Bridging: Before we delve into details, let’s first embrace that bridging is the most overlooked aspect of approaching the trench. Many practitioners feel comfortable operating on ground pads alone. However, ground pads reduce point loading the ground surface and trench lip to a certain degree, but they do not provide any definitive safety measure to prevent rescuers from ending up in the bottom of the trench if a secondary collapse should occur.
Bridging is exactly what you think – any structurally sound material(s) that provides both parallel and perpendicular bridges to work on around the trench. These may be built out of 2 x 12s framed out with 2 x 6s or 2 x 4s, or they may be ground ladders laid flat with 2x material set on the rungs. They may also be larger dimension timber, such as 4 x 4s with 2x’s on top. Every cache should be capable of providing at least four 16-foot-long bridges and/or four 12-foot-long bridges.
All of these combinations are acceptable and usually depend on the responding agency’s storage and deployment capabilities. If you are responding in just apparatus, without a support trailer of some type, then you’re probably not going to have 16-foot-long bridges prebuilt and ready.
Here is a system for applying bridges:
- Establish the perpendicular bridges that run across the trench. In most scenarios, we want to end up with at least three pairs of panels set into the trench. This secures a 12-foot-long section of the trench.
- By using your ground pad system as a template, set your perpendicular bridges just outside of the three primary ground pads. This means the center ground pad placed at the victim, with the additional ground pads on each side. This bookends a 12-foot section of the trench with a little bit of wiggle room. Using 16-foot bridges here will accommodate most trench widths and allow the ends of the bridges to be a decent distance away from the trench lips.
- On the non-spoil side, apply a single tear of cribbing for rigid bridges and a couple layers of cribbing for more flexible bridging. The goal is to elevate the bridging just enough that when it is loaded with rescue personnel, it does not come into contact with the ground anywhere near the trench. On the spoil pile side, the bridging can often be set on earth shelves that are quickly dug into the spoil pile.
- Once the two perpendicular bridges are placed, we are then ready to set the parallel bridges. All of this can be set and pushed across from the access with the easiest and safest access and this entire process is applied with personnel at least a few feet away from the trench lip on ground pads. Set the first bridge ON TOP OF the perpendicular bridges and push it across to the opposing side of the trench. This can be done with long-handled tools, such as pike poles or specialized trench tools that also manipulate pneumatic and hydraulic shore. A trench boss or operational leader should be positioned at the end wall to direct proper positioning of the bridge. Once the far bridge is placed, then the near side bridge can be placed and pushed into position. It should also be properly positioned through directions from the trench boss.
A note about proper positioning: If bridging is placed properly, it provides templates for all your trench panel placements. Panels should always be placed with as little deflection as possible. The straighter the panels, the less likelihood of panel displacement or shoring displacement in the event of secondary collapse.
It is also imperative to understand that shoring engineering is based on straight and plumb calculations and orientation, and pneumatic and hydraulic baseplates only allow for 15–20 degrees of maximum deflection before they have edge contact only with the strongbacks and may underperform, fail prematurely or travel.
To keep our panels straight, we want to make sure that the inside edges of our bridging line up directly above the bottom inside corners of the trench. This typically results in void space behind the panels as they progress upward from the bottom. The shores can be very lightly set to the panels, and then backfilling can commence with soil, cribbing, trench bags or surface buttresses. The result is a very clean shoring system with near vertical panels and proper shoring.
Safe and supportive shoring
I strongly encourage that practitioners follow each of these steps. This entire approach process can be accomplished in well under 4 minutes. If the initial template is safe and supports the shoring system placement, the shoring operation is significantly faster, safer and stronger than applying free-standing panels with no structured plan of attack.
Stay safe and train hard!