By United Rentals
By United Rentals
Trench boxes, aka trench shields, are used to protect workers from the pressure and weight of soil in the event of a cave-in.
When you are renting a trench box, follow these tips.
Determine whether a trench box is the appropriate system
A trench box may or may not be the best protective system for your trench. OSHA requires a competent person to evaluate the soil, determine whether a protective system is necessary, and determine which type of protective system is best.
“The competent person must have the knowledge and the training to make those decisions, and also the authority from their employer not only to pick the shoring method but also to make sure other conditions around the trench are safe,” said Brett Sondergard, mid-Atlantic district manager for United Rentals Trench Safety.
“Trench boxes are generally a good idea when you have a moving operation, where you are laying pipe like a sewer line that is stretching out over some distance,” said Sondergard. “You can place the trench box where you’re working that hour or that day, and, as you continue to dig the line out, you continue to pull the trench box along.”
Factor in soil type, trench depth and spreader size and clearance
Is the soil you’re working in sandy and wet or clay? How deep will your trench go? These are key conditions to consider when you are choosing a trench box. Check the tabulated data sheets from the trench box manufacturer to see what will work in your situation.
You should also consider the spreader length (which determines the width of the trench box) and clearance.
“The length of the spreaders and the clearance — the distance from the lower spreader to the bottom of the trench box — are important to take into account,” said Sondergard. “If the spreaders for the trench box are too big or not high enough off the ground, they could interfere with the work of laying the pipes.” Spreader clearance is usually about 5 feet, so if your pipe diameter is larger than that, you’ll need special spreaders to increase the clearance.
Consider the size of your excavation equipment<
“If you have a very small machine, you can’t use a big trench box because that machine won’t be able to pick up or move the trench box to the next location,” said Sondergard.
Using a large excavator with a small trench box may not be a good choice either. The excavator bucket may be too large to fit inside the trench box to pull it along, or it may be so heavy that it will damage the trench box.
Decide on steel or aluminum
Trench boxes are made of either aluminum or steel. If you’re using a smaller excavator or zero tail-swing excavator, aluminum might be your better option.
“These machines can dig a good trench, but they can’t pick up much weight, so they won’t work with a traditional steel box. So as an industry, we have moved into a lot more options for aluminum trench boxes, which can be handled by smaller machines,” said Sondergard.
Some contractors don’t like aluminum trench boxes because they don’t have the same depth ratings as equivalent-size steel trench boxes. But, by going with steel, you might be getting more box than you really need.
“You might be able to put a big steel box in the ground, but if you don’t need to be 22 feet down, and your machine isn’t even capable of digging 22 feet down, then why would you want to drag this heavy box around?” Sondergard asked. “An aluminum box of the same size is about 50 percent of the weight. It may be good for only 15 feet, but if that’s all you’re going down, that’s fine.”
Think about the spreader logistics
Contractors don’t always take into account how the various elements involved in the excavation need to work together, according to Sondergard. Suppose a company is installing a 36-inch pipe and orders 4-foot spreaders. That leaves just 6 inches of clearance on either side of the pipe — not enough room for crews to work.
Another example: Someone who is using a 4-foot bucket on an excavator orders trench spreaders that are only 3 feet long, so the crew can’t use the bucket to lower in the pipe or grade the bedding stone because it will not fit inside the box.
“It’s very important that the trench box is conducive to the work that they’re going to do, and the spreader is an important part of that,” said Sondergard.
Consider guardrails and ladders for improved safety and efficiency
Contractors sometimes use trench boxes when they’re building a manhole or another structure that requires workers to stay in the same location for some time. In those situations, it’s a good idea to install fall protection in the form of guardrails that bolt to the top of the trench boxes.
Many companies use fiberglass or aluminum ladders to get their workers down into the trench box; but, if a construction project is moving quickly, the crews may get tired of moving the ladder in and out and stop using it. At that point, they may be tempted to jump into the trench box or ride down in the excavator bucket. A safer option is a ladder that attaches to the side of the trench box and moves along with it.
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