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I'm trying to get the compressive strength (and Young's modulus) of a clay sample.

The method I found is called 'Unconfined Compressive Strength of Cohesive Soil'. According to ASTM D2166M, I have to set the loading rate to 0.5-2%/min (axial strain).

The machine I have though, is stress-controlled. It measures the load in Pa/s.

Since concrete samples can be tested through a similar method; is there a method that I can use to measure the elastic modulus and/or compressive strength of the clay using something with a rate of Pa/s?

For example, if I set the loading to something very low (e.g 1Pa/s) will I get acceptable results? (Note: for concrete it is acceptable to use 0.15MPa/s)

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Unconfined compressive strength test is generally a undrained soils test. The issue with a constant load machine is that you will likely load it too quickly as well close to the failure point, which will produce inaccurate readings and measurements at failure (i.e. you will end up missing the point of failure, and needing a new sample again). Otherwise, the machines are technically doing the same thing in both tests.

With that said, your description of the unconfined compressive strength as "young's modulus" is inaccurate in the sense that the young's modulus is generally used only to describe the elastic behavior of materials and how it behaves within the elastic region (or behavior that closely mimics the elastic region), which is not really true in soils. If you are unfamiliar with soil testing, you should not perform testing on soils samples as that can cause major errors regarding designing proper foundations and prompt people to make wrong decisions.

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  • $\begingroup$ I'm sorry if the description is not clear enough. I meant that I could calculate the young's modulus (slope) using the same test. If my only concern is to calculate the young's modulus, would that test suffice? $\endgroup$ – Mhmd Nov 27 '18 at 7:42
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    $\begingroup$ To be honest, you should just use your machines and manually load the sample slowly, that may produce adequate results based on your scenario. There are people who o this with dial and gages - it's not like you need to use the machine in its intended constant load way as long as it produce accurate load readings and strain readings. $\endgroup$ – Isa Nov 27 '18 at 15:04
  • $\begingroup$ Why are you collecting the stress-strain curve slope? $\endgroup$ – Isa Nov 27 '18 at 15:11
  • $\begingroup$ I need to calculate the young's modulus for a research I'm working on. Basically, my aim is to find several relations between the dynamic and static properties of soil. It's just that the machine I planned to use malfunctioned and now I'm searching for work arounds. $\endgroup$ – Mhmd Nov 27 '18 at 18:40
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    $\begingroup$ If you are actually doing research, I would suggest finding relationships for triaxial test parameters. Those would provide more consistent figures for your purpose. I also want to note that I got the test wrong - it is supposed to be an undrained test. I have modified my answers to reflect that. I am not a geotech by trade so forgive me for that major mistake. $\endgroup$ – Isa Nov 27 '18 at 22:21
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I would use a Shear Box test for this. Stress testing is for maximum load-bearing capacity...

Shearing is what gives you the modulated graphic results you seek. Have a look at Smith's Soil Mechanics 9th ed for more details.

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