What Are Trench Boxes?

Trench boxes are steel or aluminum structures that serve the dual purpose of extending a trench’s stable life and helping to keep workers safe while completing jobs. Depending on location and main purpose, these boxes may also be known as sewer boxes, manhole boxes, trench shields, or tap boxes. These are almost always used in the construction context, typically as workers are first pouring foundations but also as they need to make repairs underground to things like wiring or piping. Digging trenches is a necessary part of most construction and engineering work. When digging for a building’s foundation, laying pipe or telephone lines, installing valves, or doing any number of underground jobs, trenches, and often more specifically the boxes within those trenches, are where the work is primarily happening.

The physical construction of the average box is usually pretty straightforward. Most are primarily comprised of two plates, also known as “shields,” that are typically made of steel. The shields line the trench walls and are held up by spreaders. These spreaders, also known as braces, are typically beams or bars that are placed perpendicular to the steel plates that hold up the trench walls. The plates and the braces are bolted or welded together. The lower edge of the box sits at the bottom of the trench while the top edge of the box extends above the trench top. In addition to adding stability, this feature also makes it easier to get in and out, at least in most cases.

Most of the time, the boxes are designed to allow workers to do their jobs within the plates of the box. As a result, the spreaders or braces are placed as out of the way as possible. In the event of a trench failure, the soil is held back by the box's outer walls.

Trench boxes can be permanent or portable. The portable ones are generally pulled along the trench by a backhoe or other mechanisms using chains, cables, and hooks as the work progresses. Permanent boxes can't be moved down the line of a trench. Their permanency is usually more about locational stability than anything else, though; they’re almost always removed once the work is complete, but getting them out often takes more effort.

Pre-manufactured boxes are often available for purchase, though depending on the job it might make more sense to arrange for them to be custom built. A lot depends on the specifics of the trench and the nature of the project. The design and materials used in making a trench box can vary and typically depend on a variety of factors including the depth of soil and that soil’s composition.

One of the biggest benefits of these boxes is to improve the stability of a trench as a whole. Trenches typically have something called a stand-up time. This is the amount of time that a trench will last from the time it’s dug to the time its walls start to cave in. Stand-up time can be as short as a few seconds to as long as several months, depending on factors such as depth, soil type, water content in the soil, weather conditions, and activity around the trench. A properly placed and fitted box will usually give workers more control over the trench’s stability and overall stand-up time.

Even with the safety precautions of today’s modern construction firms, trench work has the potential to be extremely hazardous. In the United States alone, hundreds of people are killed yearly and many more injured due to trench cave-ins. Specially-designed boxes are often used as a means of adding protection and mitigating the risk that workers could be trapped or seriously injured in the event of trench wall collapse. Accidents still can happen, but with boxes they’re usually much less likely.

It’s often the case that government safety authorities set safety and depth rating regulations for the construction of trench boxes or shields, usually in the name of assuring quality and standardizing industry best practices. The depth rating determines the depth of a trench for which a box can effectively withstand the forces imposed by a potential cave-in. Registered, professional engineers typically determine a box’s depth rating and stamp their approval on the boxes.