Soil stability and sloping in trench excavation

NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.

August 4, 1980

Mr. Jerry Kennebeck
Bituminous Insurance Companies
[Recipient address omitted]

Dear Mr. Kennebeck:

This is in response to your July 3, 1980 letter regarding interpretations of 29 CFR 1926.652, Specific Trenching Requirements. OSHA is aware of the difficulties involved in interpreting this standard and has a study underway that would eliminate these problems.

Unstable soil as defined in 1926.653(q) is soil that cannot be depended upon to remain in place without extra support. Soft material as given in 1926.652(b) is also material that cannot be depended upon to remain in place without extra support. This material would be easily compressible and most likely have a high moisture content. The determination of soil composition is not always easy. The OSHA standards do not specify particular test to be conducted before digging begins. The American Society of Testing Materials (ASTM) publishes a book entitled "Procedures for Testing Soils" which describes the specific procedures for testing soils.

29 CFR 1926.652(c) refers to only hard or compact soil which would be grouped under compacted angular gravels in Table P-1, and therefore would have an approximate angle of repose of 1/2-foot horizontal to a 1-foot rise (1/2:1).

The second sentence of 1926.652(c) has been interpreted to mean that the angle of repose always begins at the bottom of the trench. (See Diagram 1). If the angle of repose is set at 1/2:1, (1910.652(c) does not permit a greater angle for hard or compact soil), the angle would be measured from the toe of the trench on both sides. At the five-foot level, a bench is permitted. This leaves a wedge of soil five feet high and a length at the bench that extends to the sloped angle of repose. If a pipe with a diameter greater than six feet is to be laid in this trench, then the bench must be a minimum of four feet wide.

In regard to your question of sloping in an area where there is a six-foot overlay of average soil above sand, a trench must be sloped as per table P-1 or shored according to table P-2.

The safety of the worker should come first when designing a job operation. The design of excavation and trenching is not simple, as soil stability can change with changing weather. Please make sure that your clients always look at all aspects of a job for hazards before placing the employee into the operation.

This interpretation along with your letter, will be forwarded to our National Office for confirmation. You will be notified by this office upon receipt of confirmation.

Enclosed is a pamphlet entitled "Safety in Trenching and Excavation, Selected References." This may provide you with additional information that you can use.

Thank you for your interest in the field of safety and health.

Sincerely,
Charles E. Adkins
Assistant Regional Administrator
Office of Technical Support