Shield Length and Shield Strength

Shields are different from shoring systems in several ways, including the fact that shields do not prevent the soil from caving in, but they protect the workers from cave-ins. One place we see this explained in the standard is in the definition of shields.

Shields are defined as:

Shield?(Shield system) means a structure that is able to withstand the forces imposed on it by a cave-in and thereby protect employees within the structure. Shields can be permanent structures or can be designed to be portable and moved along as work progresses.

Shoring systems are defined in Subpart P as:

Shoring?(Shoring system) means a structure such as a metal hydraulic, mechanical or timber shoring system that supports the sides of an excavation and which is designed to prevent cave-ins.

Since shields do not prevent cave-ins, they must be strong enough to handle the loads that the soil imposes on the shield walls. Engineering allows us to be able to anticipate what this loading could be, which is one of the factors that limit the working depth of the shield.

Also, from Subpart P we have the following regulation:

1926.652(a)(2)

Protective systems shall have the capacity to resist without failure all loads that are intended or could reasonably be expected to be applied or transmitted to the system.

How do we know that our shield has the capacity to handle the loading that will be applied to it?

That’s where manufacturer’s tabulated data is helpful. For the purposes of this article, we are going to look at just one factor, shield length, and how that relates to shield strength which is a huge factor in determining the working depth of the shield.

Let’s look at three shields, all having exactly the same characteristics except for shield length.

NOTE: THE FOLLOWING WARNING IS CRITICALLY IMPORTANT TO UNDERSTAND AND HEED.

THE SHIELD DATA IN THIS ARTICLE IS ONLY FROM ONE MANUFACTURER. THIS DOES NOT AT ALL REPRESENT THE WORKING DEPTHS OF SHIELDS MADE BY OTHER MANUFACTURERS, REGARDLESS OF HOW SIMILAR THE SHIELDS APPEAR TO BE. WE MUST ALWAYS REFER THE MANUFACTURER’S TABULATED DATA FOR EACH SHIELD WE USE. IF YOU DO NOT KNOW WHO MANUFACTURED THE SHIELD, YOU MUST FIND OUT OR GET AN ENGINEER TO PROVIDE TABULATED DATA FOR YOU TO USE. THE ENGINEERING FROM MANUFACTURER TO MANUFACTURER WILL DIFFER. YOUR TRENCH SAFETY PROFESSIONAL OFTEN CAN HELP YOU IDENTIFY WHO THE MANUFACTURER IS FOR A SPECIFIC SHIELD.

Back to our article.

?The characteristics which all three of these double wall shields share is height (8’), wall thickness (4”), and vertical pipe clearance (62”).

The difference between the three shields besides the length are the capacity (strength), the weight, and the working depth. Here are the differences for this particular comparison.

The 8’ X 12’ shield capacity is 1,517 psf – weight is 5,290 lbs. working depth in C-60 soil is 28’

The 8’ X 16’ shield capacity is 1,256 psf – weight is 7,400 lbs. working depth in C-60 soil is 24’

The 8’ X 20’ shield capacity is 899 psf – weight is 9,650 lbs. working depth in C-60 soil is 18’

These differences in the shield length are significant and greatly impact decisions critical to safety and getting the work done.

Again, locate the manufacturer’s tabulated data for your specific shield, and know that the working depth for each soil type will play an important role in using the shield safely. Not every shield type necessarily has similar depth ratings.

For more information on this or other excavation safety topics I refer you to www.trenchandexcavationsafety.com