Thanks, as I thought. I was just wondering is there a general rule when one is chosen over the other, such as when an soil lies within a certain strength range. Obviously soils tend to be tested in confined conditions while concrete/rock are tested in the unconfined condition. The results we achieve are deemed relative rather than absolute values on different mixes hence the lack of clarity on test procedure
In my experience, it is always unconfine compression - for that is what you can measure in the laboratory . . . An interesting possibility is for a concrete pad of RCC - which is a cemented aggregate (typically taken as about 12 to 15 MPa unconfined) - but what or how do you consider the strength when the pad is overlain by earth fill confining pressures of, say, 5 to 6 MPa vertical (and side pressures say 0.5 to 0.7x the vertical overburden pressures)? Is unconfined realistic? Could one really in the normal course of work determine the sconfined compressive strength in a laboratory setting (yes, with highly specialized equipment perhaps).