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Protective systems are methods of protecting workers from cave-ins of material that can fall or roll into an excavation, or from the collapse of nearby structures. As mentioned earlier, if an excavation is less than 5 feet deep, OSHA does not require a 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 protective measures. It is up to the planners of the construction project and the competent person on site to determine which systems will work best. 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. Sheeting is a type of shoring system that keeps the earth in position. It can be driven into the ground or work in conjunction with a shoring system. 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 (sometimes called Finland form) is used in conjunction with strutted systems such as hydraulic or timber shoring. These strutted systems are also referred to as active systems. The most frequently used strutted system involves aluminum hydraulic shores which are lightweight, re-usable and installed and removed completely from above ground.
A shield, also known as a trench box, is another common protective system used by contractors. Trench boxes are not designed to prevent cave-ins, but rather serve to "shield" workers within the structure should a cave-in occur. This is an excellent choice when placing continuous installations, as in pipe laying The box is 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. Never stack shields that are not designed for that purpose, and do not stack shields from different manufacturers, as they may not be compatible.
- 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.
- Shielding should always be used according to manufacturer’s tabulated data.
With both shoring and shielding, workers are only protected as long as they stay within the confines of the system.
Sloping and benching are another means of protecting workers from cave-in hazards. Sloping is a method of cutting back the trench walls at such an angle that there is little chance of collapse. This is referred to as an "angle of repose", and must be suitable to the type of soil.
Simple Slope Type A
Benching is a process of stepping off the earthen walls of an excavation.
Sloping can be used as a system by itself or in conjunction with benching.
Sloping and Benching Together
In the real world, there are very few applications where sloping and/or benching can be used. Why? Most often, the luxury of available space is the first consideration. Many excavations are dug in right-of-ways where the presence of other utilities and traffic become major considerations. Moreover, for every cubic yard of soil that is removed, it is very likely that nearly the same amount of material must be put back, and compacted as well.
Simple Slope in Type C Soil
If the location to be excavated has been previously disturbed, as it frequently is along a right-of-way, the soil type will very likely be classified as "C". With Type C soil, the excavation walls must be sloped back on each side of the excavation one and one half feet for every foot of depth.
Add all these factors up and it soon becomes clear: sloping, even in conjunction with benching, may be desirable-but not always very practical and economical.
A competent person must be familiar with the various sloping and benching configurations available, should that be the choice for protecting workers. In sloping and benching, important points to remember are the "weakest link" in determining what type of soil is supporting what type. If type C is supporting type B or any other type of combination, the sloping and benching configuration chosen must be in accordance with the OSHA standard. Refer to Appendix A of this workbook for the various conditions in which sloping and/or benching can be used.