Unconfined Compression Test vs. Cohesion
Unconfined Compression Test vs. Cohesion
(OP)
If a soils testing Laboratory gives Unconfined Compressive Strengths (qu) of a rock sample can that be correlated to the Rock Cohesion by qu/2?
Thank you.
Thank you.





RE: Unconfined Compression Test vs. Cohesion
RE: Unconfined Compression Test vs. Cohesion
On the more basic geometric question, the answer is, no but close. If you perform an unconfined compressive strength test you will get a maximum deviator stress for some failure criterion. That maximum deviator stress is the diameter of a Mohr's circle. So half of that value is some measure of shear strength for some measure of normal loading.
Cohesion is the shear strength for a normal load of zero. You can't determine the shear strength for a normal load of zero when you are performing a test that includes a normal load reaction. Just because sigma3 is zero doesn't mean that the normal load is zero.
If you consider the P-Q plot of a unconfined compressive strength test (or the stress path to failure), you will have a point that falls on the line "alpha" not a point that falls on the line "phi". These are geometrically related, but not equal.
Your question specifically related to rock. In soil, we have a parameter called "undrained shear strength". The undrained shear strength of a soil can be described by one-half of the unconfined compressive strength, but that's a separate topic.
Not to confuse that is. . . .
f-d
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