Trench shields (also called trench boxes or trench sheets) are steel or aluminum structures used for protecting utility workers while performing their duties within a trench and avoid cave-ins. They are customarily constructed with sidewalls of varying thicknesses held apart by steel or aluminum spreaders.
Spreaders can be interchanged to match the width of the trench. The different materials and building designs lead to a variety of depth ratings, i.e. the depth of a trench that the shield can withstand a collapse without buckling. Depth ratings are determined by registered professional engineers.