Protective Systems - Trench Safety

Protective Systems - Trench Safety

Volume 3, Issue 1

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2010 Texas Summit

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Pipelines: Your safety


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Trenching and excavation safety

Part four: Protective Systems

By David V. Dow

(top) Multiple steel trench shields in use on a

deep sewer project.

(above) Aluminum trench shields work

particularly well with rubber-tired backhoes.

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


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


There are five options for providing

a protective system:



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


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.



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.


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.



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


To summarize, by having a properly

trained and authorized “Competent

10 Vol. 3, Issue 1

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


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

(below) Vertical hydraulic shores are being used on this electrical duct-line




We’re committed to to your safety.

How about you?

Vol. 3, Issue 1


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