OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at https://www.osha.gov.

October 29, 1991 [Reviewed May 31, 2018]

Mr. Paul Berebitski
Director, Contractor/Government Relations
Indiana Constructors, Inc.
101 West Ohio Street, Suite 750
Indianapolis, Indiana 46204

Dear Mr. Berebitsky:

Your September 25 letter to Mr. Pat Finn requesting an interpretation of the Occupational Safety and Health Administration excavation standards has been forwarded to this office for response.

In regard to your question concerning stacked trench boxes, please be advised that when trench shields are stacked in a deep trench, they do not all have to be designed to resist the forces calculated for the deepest part of the trench. The standard only requires that the shields be used at a depth not greater than that for which they are designed. To facilitate proper stacking in deep excavations, a design or system should be used to prevent the shields from being misplaced in the trench when shields of different ratings are used. Designing the shields so they will fit together only one correct way, or using a marking system that readily identifies the maximum depth each individual shield can be placed, are examples of systems that could be used to prevent such misplacement.

Thank you for your interest in occupational safety and health. We hope you find this information helpful. OSHA requirements are set by statute, standards, and regulations. Our letters of interpretation do not create new or additional requirements but rather explain these requirements and how they apply to particular circumstances. This letter constitutes OSHA's interpretation of the requirements discussed. From time to time, letters are affected when the Agency updates a standard, a legal decision impacts a standard, or changes in technology affect the interpretation. To assure that you are using the correct information and guidance, please consult OSHA's website at https://www.osha.gov. If you have further questions, please feel free to contact the Directorate of Construction at (202) 693-2020.*


Patricia K. Clark, Director
Directorate of Compliance Programs

*[This letter has been modified (non-substantive changes) on May 31, 2018, and reflects current OSHA regulations and policies.]

September 25, 1991

Mr. Pat Finn
Chief of Construction
U.S. Department of Labor
Room N3468
200 Constitution Avenue, N.W.
Washington, D.C. 20210

Dear Mr. Finn:

We are a trade association for highway and utility contractors and one of our members has raised the following question regarding use of trench boxes in compliance with the OSHA 1926 standards.

The question pertains to stacking of trench boxes in a deep trench. When two boxes are stacked, does the top box need to be rated for the full depth of the trench or does it need to be rated only for the depth to which it is placed.

For example, if a contractor is working in a trench that is 10 feet deep, the contractor could stack two six-foot high boxes on top of one another. The bottom box would need to be rated for a depth of at least 10 feet in that particular type of soil. Since the bottom of the top box would only be four feet below the ground surface, would the top box need to be rated for a depth of 10 feet, or would a rating for a depth of four feet be sufficient?

We would appreciate a written response as soon as possible. Thank you very much.


Paul Berebitsky
Contractor/Gov't Relations