Hello, for compression test performed at atmospheric conditions i.e. unconfined compression test, whether Pref should be taken as 100 kPa?
During triaxial testing, we provide confining pressure of 100kpa then the values of E will be corresponding to 100kpa or 100kpa+atmospheric pressure i.e. 200 kpa?Also, if we want to create a model of the sample then should we apply a pressure of 100kpa from all directions to simulate the test at atmospheric conditions?
PLAXIS uses pref = 100 kPa by default which is 1 Bar (atmospheric pressure). This pressure has typically been used in the literature as 100 kPa. In the triaxial test pref = Sigma′3 . The actual stiffness depends on the minor principal stress Sigma′3 which is the confining stress. At this reference stress of 100 kPa, for oedometer conditions of stress and strain it can be assumed that Eoed-ref = E50-ref.
pref is the reference stress used to scale stiffness moduli Eoed-ref, Eur-ref, E50-ref to current values with respect to a current minor principal stress Sigma′3 . The reference pressure of 100 kPa is the same for all the three stiffness moduli.
Although 100 kPa is the default pref value that people are commonly using, however, it is a user defined input value, which allows you to specify a different value based on your experimental data. Typically, it can be the confining pressure in the triaxial test that best corresponds to in situ conditions.
Since HS and HSsmall models take the stress dependency for the stiffness into account, practically, using a different value for pref will only tell the software that the reference pressure at which the stiffness values are determined is different.
Hopefully an example can make this clearer:
Let’s assume that you are performing drained triaxial compression tests for a soil by unloading and reloading your samples at cell pressures of σ’3 = 250, 500, 1000 and 1400 kPa, respectively. Depending on the expected in-situ minor principal stresses, you can select one of the above cell pressures as your reference stress. For example, you could choose 250 kPa, but it could be any of the other three values. The secant modulus, E50 and the unloading-reloading modulus Eur determined for that particular reference value, will also correspond to the reference moduli values E50,ref and Eur,ref.
The equations to calculate a stiffness at a different pressure levels (σ'3) are given in the Material Models Manual, e.g.
The equation shows that for an unconfined test, if you input a zero (0) value for Pref as well as zero for sigma3, then the stiffness modulii E50 becomes E50-ref and Eur become Eur-ref which makes sense as it is unconfined. If you want to simulate a test at atmospheric conditions such as a triaxial test, then you still need a pressure that would be present from the insitu surrounding soil and that pressure is what you need to apply. On, the other hand, if you want to simulate a test at atmospheric conditions such as an unconfined test, in that case, there is no confining pressure so you will not apply any confining pressure on the soil in this case. This is because you cannot determine confining stress-dependent stiffness moduli in an unconfined test.
Thank you Faseel, it helped a lot.