Trench boxes are engineered steel or aluminum structures that are used to help protect workers who work inside trenches. These boxes are also known as sewer boxes, manhole boxes, trench shields, or tap boxes. Digging trenches is a necessary part of 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 trench boxes, are where the work is primarily done.

Trenches 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.

Trench work is extremely hazardous. In the United States, hundreds of people are killed yearly and many more injured due to trench cave-ins. As a result, protective measures are generally taken to prevent workers in the trenches from being trapped or seriously injured in the event of trench wall collapse. One method for protection that has proved to be effective is the use of trench boxes to support trench walls.

A trench box is comprised of two plates, also known as shields, that are often made of steel. These plates 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 trench box sits at the bottom of the trench while the top edge of the box extends above the trench top.

Trench 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 wall failure, the soil is held back by the outer walls of the trench box.

Trench boxes can be permanent or portable. The portable ones are pulled along the trench by a backhoe or other mechanisms using chains, cables, and hooks as the work progresses. Permanent trench boxes can't be moved down the line of a trench, and must be removed once the work in that area is completed.

Pre-manufactured trench boxes are available for purchase, or they may be custom-built for a specific job. The design and materials used in making a trench box can vary and typically depend on a variety of factors including the depth and soil of a trench. In the United States, the Occupational Safety and Health Administration (OSHA), a governmental agency, sets safety and depth rating regulations for the construction of trench boxes or shields. 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 determine a box’s depth rating and stamp their approval on the boxes.