pref value from in situ testing

Question

Could someone help me how to determine the pref value for the HS (or HSS) material model when the stiffness parameters (E50, Eoed, Eur) are determined based on in situ borehole pressuremeter test results? Should the pref value then be the value of applied pressure for a certain loading cycle of the test? 
 Many thanks in advance!

Vasileios Basas · Answer

Hi Jovan, 
 E 50,ref , E oed,ref and E ur,ref are all material stiffness moduli identified at p ref , which is the reference minor stress. 
 The corresponding &ldquo;operational&rdquo; stiffnesses E 50 , E oed and E ur are are defined to be minor stress-dependent using the frequently adopted power laws (Eq. 138, 140 and 152 of the Material Models manual), and they increase with pressure. They become equal to their reference values (E 50,ref , E oed,ref and E ur,ref are) only when &sigma;&rsquo; 3 = p ref . For any other minor stress value, they are not equal. 
 In the triaxial compression test, p ref corresponds to the confining stress &sigma;' 3 . However, the same principle applies to the pressuremeter test, where you typically do a data reduction procedure by adjusting the measurements to a reference stress level, such as the effective in-situ lateral stress, &sigma;&prime; h . In that case, the reference stiffness values correspond to the reference p ref = &sigma;&prime; h . 
 Cross-check your in-situ measurements with the information you get from the PLAXIS SoilTest module to ensure that you have your model's optimised parameters.

pref value from in situ testing

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