The Definition of Trench Box

Definition of Trench Box

A trench box, also called a trench shield, is a steel or aluminum structure placed inside an excavation to protect workers from cave-ins. It is made of two side walls held apart by spreaders or struts. Unlike shoring, a trench box does not stop the trench walls from moving. Instead, it gives workers a protected space so that if soil collapses, it lands on the box rather than on them.

Trench boxes are one of the main protective systems under OSHA's excavation standard, alongside sloping, benching and shoring. They are widely used for pipe, sewer, water main and utility work because they can be dragged or lifted along the trench as work progresses. They are a core control for the caught-in hazard a trench collapse creates. See trench safety and caught-in/between hazard.

OSHA Requirements for Trench Boxes

When a Protective System Is Needed

Under 29 CFR 1926.652(a), each worker in an excavation must be protected from cave-ins by an adequate protective system unless the excavation is made entirely in stable rock, or is less than 5 feet deep and a competent person finds no indication of a potential cave-in. Excavations 20 feet deep or more need a protective system designed by a registered professional engineer.

Design and Use

• Tabulated data: Shield systems must be designed, built and used according to the manufacturer's tabulated data or a design approved by a registered professional engineer, and a copy of that data must be kept at the jobsite.
• Rated depth and soil: Each box is rated for a maximum depth in a given soil type. Using a box deeper than its rating, or in weaker soil than it was designed for, is a common violation. See soil classification.
• Digging below the box: Under 1926.652(g)(2), earth may be excavated up to 2 feet below the bottom of a shield only if the shield is designed for the forces at the full depth of the trench and there are no signs of soil loss from behind or below it.
• Moving the box: Workers may not be inside a shield while it is being installed, removed or moved vertically.
• Lateral movement: Shields must be installed in a way that restricts lateral or other hazardous movement if sudden lateral loads occur.

Access and Other Site Rules

Trench boxes work alongside the general excavation rules in 29 CFR 1926.651. A ladder, stairway or ramp is required in trenches 4 feet or deeper so that no worker has to travel more than 25 feet laterally to reach it. Spoil and equipment must be kept at least 2 feet from the trench edge. A competent person must inspect the excavation and protective system daily before work starts, as needed during the shift, and after rainstorms or other events that increase hazards.

Good Practices for Using Trench Boxes

• Check the rating first: Compare the box's data plate and tabulated data with the planned depth and soil type before setting it.
• Inspect the box: Look for bent walls, cracked welds, damaged spreaders and missing pins before each use.
• Protect the gap above: Where trench walls rise above the box, slope or bench the soil above it so material cannot fall in on workers.
• Stay inside the protected area: Workers should not stand at the open ends or between the box and the trench wall.
• Use the ladder: Enter and exit by ladder, not by climbing the box walls or spreaders.
• Stack only as designed: Use stacked boxes and extensions only in configurations listed in the manufacturer's data.
• Plan the moves: Rig the box at its designed lifting points and keep everyone clear while it is dragged or lifted.

Example of Trench Box

A utility contractor is replacing a water main in a 9-foot-deep trench through sandy soil that the competent person has classified as Type C. Before setting the box, the foreman checks the manufacturer's tabulated data and confirms the box is rated for that depth in Type C soil. A ladder is placed inside the box, and spoil is piled on the far side of the trench, well back from the edge.

Midway through the job, a pipe fitter asks to dig 3 feet below the bottom of the box to reach a service connection. The foreman stops the request, because the standard limits digging below a shield to 2 feet and only when the box is designed for the full trench depth. The crew instead adds a box extension, resets the system and completes the connection inside the protected area.

Frequently Asked Questions: Trench Box

Is a trench box the same as shoring?

No. Shoring pushes against the trench walls to keep them from moving. A trench box does not support the walls; it shields workers from soil that collapses.

At what depth is a trench box required?

OSHA requires a protective system, such as a trench box, sloping or shoring, for excavations 5 feet deep or more unless they are entirely in stable rock. Shallower trenches may also need protection if a competent person sees signs of a possible cave-in.

Can workers stand outside a trench box?

Not in an unprotected part of the trench. The box only protects the space inside its walls, so workers must stay within it unless the rest of the trench is protected another way.

Who decides whether a trench box can be used?

A competent person must inspect the excavation and confirm the box is suitable for the soil and depth, using the manufacturer's tabulated data or an engineer's design.