Interpretation of UCS (Unconfined Compressive Strength Test)

[sybie99]

I am a structural engineer working on a project where we need to confirm soils used to back fill behind a wall are okay for building a hardstand on.

The material is sandstone mixed with some clayey material. We have had the soils tested by a laboratory, who stabilised the soild with 4% cement. The results we have includes UCS results. Values are average 1.4MPa at 100% compaction. My question is how to interpret these results. As a structural engineer I need a certain bearing capacity from my soil. How do I use the UCS results?

Thanks in advance

 

[fattdad]

the mathimatical answer is different than the engineering answer.

Math answer: UCS represents the vertical stress to failure on an unconfined sample (i.e., where the horizontal stress is zero). So, half of that value (i.e., 0.7 Mpa) would represent the shear strength. For your sample that'd be 14.6 ksf. (Sorry, I just have to convert to make sure this makes sense to me.)

If you JUST base your bearing strength on that value alone, you'd then go to the bearing capacity equation and use 14.6 ksf as your value for "c."

Qallow=cNc/fs

So if you use a safety factor of 3 and Nc (for the case of no friction angle) is 5.14 (refer to Terzhagi and Peck), you'd get Qallow=25 ksf.

Engineering answer:

Get your geotechnical engineer to answer the question. I'd consider it against generally accepted engineering practice to take one UCS test without any geologic context and divine an allowable bearing pressure. I mean you are not addressing discontinuities in the overall rock mass, you are not addressing the character of the infill in the discontinuities and you are not addressing ground water conditions. Also, is this rock mass uniform throughout the entire bulb of stress influence?

We don't have much to go on, but that's my take on the OP.

f-d



¡papá gordo ain’t no madre flaca!

 

[Ron]

By the term "hardstand" I will assume you mean "pavement". Given that assumption, bearing capacity is not a parameter that is useful for you. The UCS; however, is useful. Your loading is transient. You are concerned about the capability to resist heavy loads for a short period of time and to be done so repetitively.

There are numerous correlations that can be used to determine your pavement capacity, using the UCS. You need to know the thickness of this material and what will be on top of and under it. Given these few pieces of data, you can determine the load capacity using either flexible pavement or rigid pavement methods of analysis. For your application, I would probably use elastic layer analysis, so you will only need the elastic modulus of each layer of material and its Poisson's ratio. So as to not confuse myself, I'll use US units for this....

Your 1.4 MPa material is equivalent to about a 200 psi soil-cement. Considering that, for elastic layer analysis I would use an elastic modulus of about 600 ksi, and a Poisson's ratio of about 0.25 to 0.30.