The data dictates what you can and cannot safely do.
Manufacturer’s tabulated data typically consist of tables, charts and other information created by a registered professional engineer that guides competent persons in choosing, assembling and installing a trench protective system, including a trench box. Installing a trench box in a manner that is not in accordance with the manufacturer’s tabulated data can put workers inside the trench box at risk.
In short, using the manufacturer’s tab data is critical. In 1926 Subpart P, OSHA stipulates the following:
“Design of support systems, shield systems, or other protective systems that are drawn from manufacturer’s tabulated data shall be in accordance with all specifications, recommendations, and limitations issued or made by the manufacturer. Deviation from the specifications, recommendations, and limitations issued or made by the manufacturer shall only be allowed after the manufacturer issues specific written approval.”
James Zerr, Southwest district manager for United Rentals Trench Safety, has worked with many contractors to help ensure they are using trench boxes according to the manufacturer’s tabulated data. Zerr offered the following advice based on issues he’s seen first-hand in the field.
Read the tab data closely. Check it for all the variables that can impact the use of a trench box, including the depth rating for various types of soil and the maximum allowed spreader width.
Monitor conditions frequently. Since site conditions can change quickly, be sure the competent person assesses them continuously to ensure that the trench box is still being used within the limits of the tabulated data. For example, when a trench box went in, the competent person may have determined that the soil was type B. But different circumstances, such as rain or surface water draining into the trench, may change that soil designation to type C. The trench box that initially worked well may no longer be appropriate for that job. “A lot of responsibility falls on the competent person,” Zerr noted.
Be prepared to change trench boxes. Do not assume the same trench box will serve throughout the project. “On certain jobs, the contractor might need multiple boxes. They might start with an aluminum box and then have to change to a steel box because there’s more water, or the trench gets deeper,” said Zerr.
Classify the soil properly. To get the maximum use of a trench box, take the time to accurately classify the soil. “A lot of companies just classify everything as type C soil. That really restricts the kind of boxes you can use because it limits your depth ratings,” said Zerr.
Understand what depth rating to follow. When using multiple protective systems, be sure to use trench boxes with the right depth rating. If a contractor has a trench with a vertically sided lower portion that is 18 feet deep, with a 12 feet deep sloped embankment at the top, the stacked boxes must be rated for 30 feet. “It’s the overall depth that counts; it doesn’t matter if they slope the top,” said Zerr.
Know when you need to get a plan from an engineer. “Contractors will sometimes put plates behind a box, if the box itself is not tall enough for the trench. Or, they may put plates or sheeting at the ends of a box,” Zerr noted. Since this is not allowed by the tabulated data, a contractor must get a registered professional engineer to approve of that usage. Another example is suppose the tabulated data allows for spreader length of up to 20 feet, but the contractor wants to use 24 feet spreaders to go wider. An engineer would need to calculate all factors – load, deflection over a span, and other issues – to see if that would be appropriate. The contractor would need the engineer to provide written approval before proceeding.
Keep the tabulated data on the worksite. Tabulated data is a simple, generic shoring plan. OSHA requires that tabulated data be kept on the worksite for the competent person to review. Consider that tab data is to a trench box as a safety data sheet is to a chemical. If you use a chemical, the safety data sheet must be available. The same is true with tab data when using a box.
Contractors sometimes get complacent about consulting and following the tabulated data requirements for trench boxes, according to Zerr – and that is a dangerous habit. “Everybody believes that since they’ve been doing this for so long nothing is ever going to happen. Then something does happen and it’s too late,” he said.
As a best practice, teams should consult experts before beginning a trench or excavation project. The information above is for general informational purposes only, the trench experts in United Rentals’ United Rentals’ Trench Safety group are ready to assist when the competent person on a worksite has a question about using tab data with trench boxes. Understanding the requirements – and when a custom-engineered system is required – can prevent a tragedy in a trench.