Testing the Strength of Cohesive Soil - A Field Guestimate

It is impressive to me that Appendix A in Subpart P describes a method of classifying soil and rock stability with such simplicity that someone without a degree in geology can come up with a Type A, B, or C and can do so with confidence. What is even more amazing is that someone without a degree in geology can teach the subject to those needing to learn that skill. Once again, I find myself in awe at those who constructed Appendix A, and I applaud their success.

Testing the unconfined compressive strength of cohesive soils is just one example of this. Let’s look at a few takeaways on this from the standard. From Appendix A we find the following:

“Unconfined compressive strength?means the load per unit area at which a soil will fail in compression. It can be determined by laboratory testing, or estimated in the field using a pocket penetrometer, by thumb penetration tests, and other methods.”

The authors of Appendix A make the point that testing the strength of cohesive soils in the field is only an estimate. A lab test will be more accurate, but the field estimate is adequate when performed properly.

That last statement is key. Performing these tests properly gives us the best estimate, which is what we want. Appendix A also states:

“(iii)?Thumb penetration. The thumb penetration test can be used to estimate the unconfined compressive strength of cohesive soils.”

Here’s another statement:

“(iv)?Other strength tests. Estimates of unconfined compressive strength of soils can also be obtained by use of a pocket penetrometer or by using a hand-operated shearvane.”

In both statements, the word estimate is used, and rightfully so.

There are some helpful guidelines in Appendix A about how to conduct the thumb penetration test. The reader is encouraged to review those instructions. While there isn’t a lot said about using a pocket penetrometer in the standard, there are some key takeaways based on the totality of the discussion from terminology and phrases in Appendix A using inductive reasoning. Here are a few.

Only test the unconfined compressive strength of saturated cohesive soil. That means, only test the cohesive strength of soils that successfully pass the plasticity test.

Only test soils from the spoil pile that are freshly excavated and have not been exposed to either wetting or drying.

Take several samples and throw out the high and low readings. Choose average readings for final determination.

When using a pocket penetrometer, in order to get the most accurate reading, take an object and carefully cut a smooth side from the clay sample prior to inserting the penetrometer into the smooth side. This way we can more accurately see when the penetrometer has penetrated the sample the correct depth.

Do not stab the penetrometer into the soil sample. Simply use a smooth motion and push it into the sample the correct depth. Too fast or very slow will not provide the best estimate.

Follow the manufacturer's instructions completely when using a pocket penetrometer or a shearvane.

Never attempt to get a reading from the sides of the open excavation. We only test soil samples taken from the spoil pile.

Hopefully this has been helpful and has answered some questions. For more information on this and other excavation safety topics we refer you to www.trenchandexcavationsafety.com