The Theory of Shielding
The employer is responsible for the safe condition of materials and equipment used for protective systems. Defective and damaged materials and equipment can result in the failure of a protective system and cause excavation hazards. To avoid possible failure of a protective system, the employer must ensure that:
1. Materials and equipment are free from damage or defects.
2. Manufactured materials and equipment are used and maintained in a manner consistent with the recommendations of the manufacturer and in a way that will prevent employee exposure to hazards.
3. While in operation, damaged materials and equipment are examined by a competent person to determine if they are suitable for continued use. If materials and equipment are not safe for use, they must be removed from service. These materials cannot be returned to service without the evaluation and approval of a registered professional engineer.
Some trench shields are composed of aluminum or fiberglass. OSHA standards permit the use of a trench shield as long as the protection it provides is equal to or greater than the protection that would be provided by the appropriate shoring system. Employees must know to work only within the protection of the shield. Also, if a slide starts, workers must know that they should not run out of the shield into the path of the slide.
Shield (shield system) means a structure that can withstand the forces imposed on it by a cave-in and thereby protect employees within the structure. Shields can be permanent structures or can be designed to be portable and moved along as work progresses. Shield systems must not be subjected to loads exceeding those that the system was designed to withstand. Shields must be installed in a manner to restrict lateral or other hazardous movement.
Trench boxes are different from shoring because, instead of shoring up or otherwise supporting the trench face, they are intended primarily to protect workers from cave-ins and similar incidents. The excavated area between the outside of the trench box and the face of the trench should be as small as possible. The space between the trench boxes and the excavation side are backfilled to prevent lateral movement of the box. Shields may not be subjected to loads exceeding those which the system was designed to withstand. Trench boxes are generally used in open areas, but they also may be used in combination with sloping and benching. The box should extend at least 18 inches (0.45 meters) above the surrounding area if there is sloping toward excavation. This can be accomplished by providing a benched area adjacent to the box.