Trench shoring systems are methods of protecting workers from cave-ins of material that can fall or roll into an excavation, or from the collapse of adjacent structures. If an excavation is less than 5 feet deep, OSHA does not require protective systems unless the competent person sees signs of a potential cave-in. (It is important to remember that a wall collapse in a trench four and 1/2 feet deep can still have serious results!) For trenches between 5 feet and 20 feet deep, shoring and sheeting, shielding, sloping and benching are all acceptable trench shoring systems.
It is up to the planners of the construction project and the competent person on site to determine which systems are most appropriate. If an excavation is greater than 20 feet deep, a registered professional engineer must design the protective system. Shoring systems are structures of timber, mechanical, or hydraulic systems that support the sides of an excavation and which are designed to prevent cave-ins.
Various Trench Shoring Systems
Sheeting is a type of shoring system that retains the earth in position. It can be driven into the ground or work in conjunction with trench shoring systems. Driven sheeting is most frequently used for excavations open for long periods of time. Another type of sheeting, in which plates or shoring grade plywood is used in conjunction with strutted systems such as hydraulic or timber shorting. These strutted systems are also referred to as active systems. The most frequently used strutted system involves aluminum hydraulic shores which are lightweight, reusable and installed and removed completely from above ground.
A shield, often referred to a trench shoring box is another common protective system used by contractors, an excellent choice when continuous horizontal installations are contemplated. It’s placed in the trench and dragged along with the progress of the work.
A few important points about shields:
-Personnel should be out of the box and above ground when the shield is being moved.
-You could be caught between the moving box and fixed object(s);
-The top of the shield should extend at least eighteen (18) inches above the level of any materials that could cave or roll into the trench;
-Some shields are designed to be stacked, one on top of another. The forces of a cave-in can literally push a box sideways, causing a crushing hazard. After a box is positioned
for the work, the voids between the box and the trench wall should be filled with excavated material to prevent displacement caused by a cave-in.
-Sloping and benching are another means of protecting workers from cave-in hazards. Sloping is a method of excavating in which the walls of an excavation are laid back to an “angle of repose” suitable to the type of soil.