Source for the ko,oc value for the advanced model

Question

Hi all, 
 Based on Maayne and Kulhawy, the ko,oc relationship is shown below. 
 
 For Plaxis, the following relationship is used 
 
 Can I know the source for this one? 
 
 Thanks.

Vasileios Basas · Answer

Hi Nitharshan, 
 Chapter 2.9 (On the initial stresses) of the PLAXIS Material Models manual explains how this equation is derived. Contrary to other empirical formulas, the equation used in PLAXIS follows the theory of a one-dimensional compression test.