Page 19 - Trench and Excavation

Chapter 3 - Protection Systems

As the previous section showed, the collapse of a trench can produce tragic consequences for
the workers inside. To help protect the workers, OSHA has established acceptable means of
trench protection: sloping and benching shield systems, timber shoring, and aluminum shoring.
CFR 1926 Subpart P contains Appendices B, C, and D, which cover these protective system
categories respectively. These appendices are mandatory only if they are used.

Protective System a method of protecting employees from cave-ins, from material that could
fall or roll from an excavation face, into an excavation, or from the collapse
of adjacent structures. The protective systems available are: Shielding,
Sloping or Benching, and Shoring.

All trenches must be evaluated by a Competent Person to determine the protective system need
and type. Protective systems are required for all trenches whose depth is greater than 5 feet.
Further, these systems may be required for trenches less than 5 feet deep if the Competent
Person determines that there is a significant collapse hazard.

The systems discussed will be applicable only for trenches under 20 feet in depth. If the trench is
deeper than that, the protective system must be designed and/or approved by an RPE.

The protective systems are designed around two basic ideas: Preventing and Protecting. Each
idea has its own merits and liabilities. Regardless of the theory behind it, the protective system
employed must be sufficient to protect workers in the trench. If it is a constructed system, it must
have strength sufficient to withstand the loads placed upon it.

Additional Requirements for Support Systems for Trench Excavations

Excavation of material to a level no greater than 2 feet (.61 m) below the bottom of the members
of a support system shall be permitted, but only if the system is designed to resist the forces
calculated for the full depth of the trench, and there are no indications, while the trench is open,
of a possible loss of soil from behind or below the bottom of the support system.

Shielding Systems

Shield (shield system) a structure that can withstand the forces imposed
on it by a cave-in, and, thereby, protect the employees within the structure.
Shielding systems are probably the most frequently used form of trench
protection. They offer excellent protection that can be quickly put in place
with a minimum of disruption of the area surrounding the trench.

Shielding systems are designed to be moved as the trench is being
excavated and successive sections of pipe are laid. Because of this, the trench needs to be
approximately 4 inches wider than the shield. Obviously, since the walls of the shield are not in
constant contact with the walls of the trench, the trench walls gain no strength from the presence
of the shield system. The purpose, therefore, of the shield system is not to prevent a wall collapse,
but rather, to protect the workers when it does collapse.

The shield must be able to withstand the potential stresses placed upon it and must be installed
in a way which prevents lateral movement during an impact. The strength of the shield system

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