Confined or Unconfined?

[matest]

When specifying compressive strengths of cemented soils how do I know if it is appropriate to specify confining pressure or not?

 

[EireChch]

You could do either, for unconfined you will obviously get a Unconfined compressive strength.

For confined you would place in a triaxial and measure phi and c'.

I would say for nearly all man made cemented soils it’s unconfined strength you want

 

[matest]

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

 

[EireChch]

I have done 4 projects where we used cement stabilization and all had at least 5% cement which gave a UCS of 0.6MPa+.

Clays are tested in unconfined test too. You can get Su as UCS/2.

If a soil has a UCS of greater than 0.6MPa (or Su greater than 0.3MPa) it is considered a rock based on BS 5930.

 

[BigH]

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).

 

[fattdad]

it'd be all about the stress path, eh? And, amount of confinement.

In the near surface, UCS. Then too as BigH points out?

What's the design objective?

f-d

ípapß gordo ainÆt no madre flaca!