Effect of overburden and confining stress on HSS model parameters.

Question

For the hardening soil and hardening soil with small strain models, PLAXIS material models manual states that the Pref = 100 kPa in PLAXIS (and it is better to leave it at that) and also Pref = confining stress in a drained triaxial test. 
 I am conducting analysis of box culverts wished in place at different depths using the hardening soil with small strain (HSS) material model. My overburdens range from 3 meters to 30 meters and I was thinking whether the parameters I am using will change or not for the different analyses as I go towards higher overburdens. Since reference pressure (Pref) is equal to the confining pressure in laboratory tests, do I need to change its value (and subsequently the other parameters) for different depths of soil cover? 
 I have no laboratory data but instead, I am using Brinkgreve's correlations for sandy soils from their 2011 paper. 
 I would appreciate help regarding this matter.

Faseel Khan · Answer

PLAXIS uses pref = 100 kPa by default which is 1 Bar (atmospheric pressure). And this pressure has been typically used in the literature as 100 kPa. In the triaxial test pref = Sigma&prime;3 . The actual stiffness depends on the minor principal stress Sigma&prime;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 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&prime;3 . The reference pressure of 100 kPa is the same for all the three stiffness moduli. In some cases it may be necessary to change the reference pressure for example in deep excavation situations where higher horizontal effective pressures are reached at deeper depths and it may be suitable to use a smaller value for reference pressure.

Effect of overburden and confining stress on HSS model parameters.

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