When Cemented Soil Can Be Classified as Stable Rock

Did I read that right? WHEN CAN CEMENTED SOIL BE CLASSIFIED AS STABLE ROCK?

I will address that statement but I'm going to sneak up on it by making another point first. I once heard an excavation safety competent person instructor clearly say that a Type A soil would not cave in. That is incorrect, it is a dangerous statement, and shows a lack of understanding about soil stability and what is in the OSHA standard. It is hard to believe he was once an instructor.

According to Appendix A, as far as the competent person is concerned, the only soil classification that WILL NOT cave-in is “Stable Rock”. One only has to look at the sloping charts in Appendix B (Table B-1 Maximum Allowable Slopes) to see that there are maximum allowable slopes for Types A, B, and C. The only exception is for Stable Rock.

Or, go to Appendix C or D, or even poke around in any manufacturer’s tabulated data where there are shoring (and shield) references for Type A soil.

To simplify, as far as the competent person is concerned, Types A, B, and C will cave in, and we cannot allow workers in those excavations if the walls are vertical unless we have an adequate protective system in place.

According to the definition of Type A soil in Appendix A, the soil types that qualify as a Type A include cohesive soils with an unconfined compressive strength of 1.5 tsf or greater. Some examples of cohesive soils are provided in the definition. Cemented soils such as caliche and hardpan are listed as well. The definition of Type A soils also includes several site condition limitations such as vibrations, fissures and soil that has been previously disturbed.

However, there also exists an interesting exception for caliche and hardpan, and this is explained below by OSHA in the federal register. When subpart P was being revised, the question came up as to whether cemented soils could be considered “Stable Rock”. This discussion can be found in the Federal Register on page 45906 under issue 12. It reads as follows:

Issue 12 of the NPRM solicited comment on a suggestion that hardpan and caliche be moved from a “Type A” soil classification to a “stable rock” classification.

(For brevity, some of the discussion is being omitted here, but can be found on page 45906 of the Federal Register. Below is OSHA’s conclusion)

After careful consideration of the comments, OSHA has determined that upgrading the classification of hardpan and caliche from “Type A“ soil to stable rock, on a national basis, is not appropriate. The Agency notes that much of the support for this upgrade is tempered with recommended limitations. OSHA agrees with the comments that these terms have a wide range of meaning in different areas of the country. The Agency, however, does not preclude such a reclassification if done on a regional or site-specific basis, provided that the requirements of the standard are met. Specifically, this would require approval of a registered professional engineer, as required for the use of other tabulated data or site-specific designs. Appendix A does not permit this reclassification.

The key points to take away from the above are

(1) The last sentence in the above where it states that Appendix A names caliche and hardpan in the list of Type A soil types, refers to when the competent person is classifying the soil. OSHA does not permit the competent person to reclassify cemented soils such as caliche or hardpan as stable rock.

(2) An exception to this can be made only if a registered professional engineer will certify the cemented soil as stable rock. This is a practice that is currently done in some parts of the country.

Additional comment on this topic can be found in the federal register on page 45939 where it states:

OSHA recognizes that hardpan and caliche can be excavated with vertical sides under some site conditions. However, as pointed out even by the advocates of this practice, there are many restrictions (see discussion of Issue 12) which must apply. Therefore, the Agency declines to include hardpan and caliche in the stable rock classification in all circumstances for the purpose of this appendix. OSHA wishes to restate and reiterate that even though appendix A does not allow for it, the employer has the option of using this practice if approved by a registered professional engineer, or it is in accordance with tabulated data prepared by a register professional engineer.  

This is yet another example of how the federal register provides valuable insight into key aspects of excavation safety. That is also why a copy of the federal register has been included in the book Trench and Excavation Safety by the Book.

I hope you found this article interesting. For more information on the topic of excavation safety please visit www.trenchandexcavationsafety.com

Thank you