These misunderstandings can put workers in danger.
As a customer training specialist for the Trench Safety group of United Rentals, I’ve noticed that many contractors share some misunderstandings about protective systems for trenches and other excavations. These can lead to excavations that aren’t safe to enter. Here are six of the most common points of confusion.
Misconception: Only one type of protective system can be used for a particular job.
The real story: OSHA doesn’t require contractors to use a particular protective system in a given situation. The standards say there are many options you have to protect workers and it’s up to the competent person to choose the system that makes the most sense in terms of what’s feasible, what’s the most economical and what works best with the equipment that’s on the jobsite.
Misconception: Trench boxes, aka trench shields, can prevent cave-ins.
The real story: Trench boxes are designed to protect workers within the shield by withstanding the earth pressure imposed on it. However, shields are passive systems in that they don’t apply support to keep a trench wall from collapsing.
Misconception: You can classify a soil as Type C and still use benching or aluminum hydraulic shores as a protective system.
The real story: This problem comes up because many contractors have a company policy of classifying all soils on their projects as Type C soil. (Choosing the “worst case” soil classification helps them limit their liability and/or make the job of the competent person easier.) Once the competent person classifies the soil as Type C, they can’t use benching or aluminum hydraulic shores (per OSHA’s Appendix D) even if the soil is the better Type A or Type B. Companies can’t have it both ways.
One exception to this rule: Aluminum hydraulic shores can be used in Class C-60 soils, using the manufacturer’s tabulated data in place of OSHA’s Appendix D.
Misconception: When you’re using aluminum hydraulic shores to prevent cave-ins, you have to put some type of sheeting — plywood, Finn form or half-inch steel plate — behind the shores.
The real story: As long as you don’t have sloughing or raveling of the soil, you may be able to pressurize the hydraulic shores right against the dirt walls. This option typically would be used when you’re working on a short-term excavation and with more stable soil types.
Misconception: You can use just one aluminum hydraulic shore and consider this an engineered shoring or protective system.
The real story: With hydraulic shores, you can never use just one shore. If you’re using OSHA’s shoring data (Appendix D), OSHA always calls for a minimum of three shores. Some manufacturer’s tabulated data allow you to use just two shores for some applications.
Bottom line: The workers always have to be working in a safe zone, the area between two shores.
Misconception: Once my excavation or trench goes beyond the 20-foot depth, I need to hire an engineer.
The real story: For certain applications that is true, and site-specific engineering always requires a professional engineer’s stamp. But contractors often think they need to hire an engineer when they’ve gone below 20 feet even if they are using a manufacturer’s system. That’s often not the case. Many of these systems already have the engineering done by the manufacturer, showing depths of 20 feet or greater. The tabulated which has been developed by a professional engineer, serves as the engineer’s stamp. The confusion arises because all of OSHA’s data on systems such as wood timbers, hydraulic shoring, sloping and benching stops at a depth of 20 feet. However, there are a lot of systems out there that can go past 20 feet without an additional professional engineer’s stamp.
It is important to note that the information regarding protective systems for trenches and other excavations included in this article should not be a substitute for site-specific advice, instruction or supervision. Consulting with experts regarding specific circumstances, applicable rules and regulations is always recommended.
Kris Graham is a customer training specialist in United Rentals’ Trench Safety group.
A fatal trench cave-in at a company he worked for in the past had a big impact on his career. He joined United Rentals’ Trench Safety group in 1997 and helped developed the excavation training program United Rentals uses today.