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Trenching and excavation safety, part four

by David Dow

This is the fourth and final article covering the basics of trenching and excavation safety. To recap the previous articles:

• Part 1 discussed the role of the “Competent Person” on the job site.
• Part 2 presented the importance of following the “General Requirements” section of OSHA’s 29 CFR 1926 Subpart P – Excavations.
• Part 3 covered the soil classification process.

With just one exception, OSHA requires the use of protective systems in all trenches and excavations that are deeper than five feet. The one exception involves trenches or excavations dug in stable rock. In those situations, it is “legal” to work without protective systems. But be aware – stable rock is a very rare type of soil.
If the trench or excavation is less than five feet deep, a protective system may still be required if there is a potential for a cave-in. (Note: In some areas, state laws may require use of protective systems for trenches and excavations that are deeper that four feet. Always check with your state regulations).

There are five options for providing a protective system:

OPTION 1 – SLOPING AND BENCHING

This procedure calls for, first, classifying the soil, and then referring to the charts and illustrations in the OSHA Standard to determine the correct angle of bank.

Type Soil Angle of Bank
Stable Rock – Vertical wall
Type A Soil – ¾ to 1 (H to V), or 53 degrees
Type B Soil – 1 to 1 (H to V), or 45 degrees
Type C Soil – 1 ½ to 1 (H to V), or 34 degrees

OPTION 2 – TIMBER SHORING

This is a system of wooden walers, cross braces, and uprights, that support the walls of a trench or excavation. The OSHA Standard specifies the sizes of timbers, based upon the various types of soil, the depth of the trench or excavation, and the type of timber used to build the structure (oak or Douglas fir). As a practical matter, few contractors or utilities use timber shoring. There are a number of reasons, including limited availability of timber, the need for skilled carpenters to build the system, the time required to build, safety issues, and the easy availability of other, more cost effective, systems.

OPTION 3 – ALUMINUM HYDRAULIC SHORING

These systems have replaced a lot of timber shoring. Developed in California in the late 1950s, there are two types of hydraulic shoring: vertical shores and horizontal walers. Both employ aluminum rails and hydraulic cylinders that apply pressure to the walls of a trench, creating an “arching effect” that prevents the walls from collapsing.

OPTION 4 – TRENCH SHIELDS

These are aluminum or steel structures designed to protect workers by withstanding the forces imposed by a cave-in. Regardless of the material they are made of, the principles are the same.

OPTION 5 – SITE-SPECIFIC ENGINEERED SYSTEMS

Occasionally the charts in the OSHA Standard for sloping and benching, or timber shoring, as well as manufactured systems such as aluminum hydraulic shoring or trench shields, will not work. There may be job-site conditions that exist or other factors that prevent the use of such systems. In these situations, a registered professional engineer must design a system specifically for the job. The engineer has to be registered in the state where the system will be used, and will have to “stamp” the design. There are additional requirements, as well, all of which are covered in the OSHA Standard.

To summarize, by having a properly trained and authorized “Competent Person” on site, following the General Requirements section of the OSHA Standard, carefully classifying the soil, and utilizing protective systems, we can all work safely in trenches and excavations.

Editor’s Note: This is the fourth and final article in this series that discusses specific steps that contractors and utilities can take to help ensure the safety of their personnel. Obviously, this is an overview of the subject. It is impossible to properly cover the topic in just four relatively short articles. Contractors and utilities involved in underground work will need additional training to ensure worker safety.

David V. Dow is co-founder and Vice-President of TrenchSafety and Supply, Inc. Learn more about his company at www.trenchsafety.com.