Do It Your Way and Safe at the Same Time

I recently received a question regarding sloping at the open end of a trench shield that was being used on a pipeline project.  The manufacturer’s tabulated data was unclear on the issue and an inspector was requiring that the sloping adhere to the OSHA Appendix B for sloping and benching.

This is a prime example of where a hazard analysis for every construction operation pays dividends.  As mentioned above, the tabulated data was unclear and OSHA is also unclear on this issue.  For the contractor, there’s a significant financial impact, as the engineers_cornercost associated with sloping at the leading edge could be substantial if the slope has to be 1:1 or even 1.5:1.  When working with a 1:1 to or 1.5:1 slope, the distance of unsupported side trench wall outside the shield leads to wall collapse of the trench and the excavator may end up too far from the shield to service the pipe laying operation.

In this particular operation the trench is being excavated and the excavator reaches out and pulls the trench shield forward.  This excavation is defined as a trench since OSHA categorizes a trench as an excavation that is deeper than it is wide.  Additionally for this particular review, the hazard created is that the soil bank at the excavation end could cave into the shield where workers are standing (there were no adjacent structure integrity concerns for this project).  In this instance, it is possible for a hazard analysis conducted at the jobsite just prior to the trenching work to safely deal with the condition.

Typically in trench product operations like these utilizing trench shields, trench end protection is not used because the following conditions usually exist:

  • The shield is not pulled tight to the slope hinge (it’s typically 2-ft to 6-ft away),
  • The excavation line at the end of the trench shield is not vertical (there is typically some degree of slope depending on the soil condition),
  • Soil arching occurs at the ends of a trench cave-in at the middle of the trench span, similar to the way hydraulic shores prevent a cave-in between the horizontal and vertical locations of the shore, and
  • The time the trench excavation is left open is short (typically 1 hour or less).

There are limits to when these conditions are predictable; however, historical hydraulic shore spacing gives an indication of what may be reliable assumptions for the project. In Type A & Type B soils the allowable horizontal spans are 6-ft to 8-ft and in Type C-60 soils the spans are 4-ft to 6-ft depending on the excavation’s depth. These same horizontal spacing benchmarks should not be exceeded at the ends of a shield trench.

Given the fact that soil arching is occurring at the end of the trench, the degree of sloping may be able to be reduced for specific applications. Vertical walls should never be used but 0.25:1 in Type A &B soils, and 0.5:1in Type C soils may work depending on specific jobsite conditions. The hazard analysis that takes place at the start of the trench excavation work should establish the sloping based on the jobsite conditions and after the excavation work begins the competent person should observe the sloping to confirm that it is not failing. At the end of a crew’s shift or if the trench end is going to be open for extended periods of time, the sloping should be increased if workers are going to continue working inside the shield.

It is also important to remember that with shielding operations workers must always enter, exit and work inside the shield and never use the trench leading or trailing slopes for access or egress.

What’s commonly become referred to as Type C-80 soils in practice, prove difficult for trench shields as the soil has a tendency to be less cohesive and will typically flow into the ends. In this case the shield is dug into the soil and a leading edge plate should be attached to the front of the shield to prevent the soil from flowing in onto the workers according to manufacturer’s tabulated data and or site specific engineering.

 

About the Author: Joe Turner, P.E. serves as National Trench Safety’s Director of Engineering, Research and Product Development.  Mr. Turner is one of the most recognized figures in the trench safety industry, having provided trench safety plans for the last 20 years.  Among his many accomplishments, is the book Excavation Systems, Design, Planning and Safety, which was published by McGraw-Hill in 2008 and is still used today as a reference for many students and professionals regarding proper engineering techniques.

DISCLAIMER: the information contained in this article is provided for general and illustrative purposes only and is not to be considered Site Specific and or designated engineering for any project or work zone, nor is it to be used or consider to be tabulated data, technical data, advice and or counsel to be used on any jobsite.  Each project is different and is the responsibility of the employer’s designated Competent Person to make decisions upon what systems and methods may be used in compliance with the federal and local regulations, manufactures tabulated data, engineered drawings and other plans.