Soil test - Triaxial (HSM)

Unfortunately, I have the drained triaxial test data (without unloading and reloading) it's a loading increment until a 14% of axial deformation, so what I can get is the data "E50" and I am unable to calculate the Eur, on the other hand I have data from the oedometric tests contain unloading and reloading, can I obtain a Eur rigidity from the oedometer data and use it as Eur in the HSM parameters?
Best regards.
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  • Dear Adnan,

    It is possible to use the unloading/reloading stiffness from the oedometer test instead.
    We know that based on Hooke's law you can correlate the Eur,oed to the Eur,triax and for a low Poisson's ratio (vur) these are Eur,oed ~ 1.1 * Eur,triax
    In any case, you will probably need to do a calibration with SoilTest for the Eoed,ref, therefore, you can also directly use the Eur,oed and see if that matches the laboratory data you have.

    Regarding the different reference pressure, you can use the equations in our Material Models manual that define the E50, Eur and Eoed based on the reference pressure you have (σ'3 for triaxial test or σ'1 for oedometer test) and the power m parameter. 
    In our latest manuals, i.e. CONNECT Edition V20 Update 2, these are equations 6.3, 6.5 and 6.16, respectively

    Typically for sand m=0.5, so, for example, you can calculate E50,ref = E50 * sqrt(pref/σ'3) in which pref = 100kPa and σ'3 is the reference pressure you have.

  • Dear Stefanos,

    Thank you very much for your detailed response, it is so helpful.

    In this context, I just have a small question, why always on Plaxis we take the Poisson's ratio "0.2" for the soil? (I guess that's why we have 1.1 between Eur and Etriax).

    Thank you in advance,

    Adnan.

  • Hi Adnan

    A Poisson's ratio of 0.2 is a typical pure elastic unloading/reloading Poisson's ratio. This works well for e.g. the Hardening Soil model as a default value. In Mohr-Coulomb model it cannot just be this unloading/reloading Poisson's ratio, as the Mohr-Coulomb model just has one stiffness. Typical ranges is 0.3-0.35. To be sure, please run a SoilTest simulation to see if this is a valid assumption.

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