Is there a way for the user to modify the mass balance equation that Plaxis solves in the fully coupled solution of unsaturated flow and stress-strain?

Question

Hi, I was wondering if it is possible to implement in the coupled solution of unsaturated flow and stress-strain a term that relates the change in saturation to the volumetric change due to alterations in effective stresses. 
 In the Plaxis manual, equation 33 indicates that the variation in saturation is solely represented by the variation in pore pressure. However, I would like to include an additional term that relates the variation in saturation to the variation in volumetric deformation, as shown in the equation below. 
 
 With this implementation, it would be possible to simulate the variation in saturation under a constant suction condition as proposed by Zhou et al (2012). 
 
 ZHOU, A. N.; SHENG, D.; SLOAN, S. W.; GENS, A. Interpretation of unsaturated soil behaviour in the stress - Saturation space, I: Volume change and water retention behaviour. Computers and Geotechnics, v. 43, p. 178&ndash;187, 2012b.

Dennis Waterman · Answer

Dear Uilian, 
 Have you tried to simulate this already? Because I think it already works ... when partially saturated soil is compressed in PLAXIS we can observe that the suction is reducing, which can only be explained by the fact that the degree of saturation increases due to the reduction of void ratio. Hence, this is more or less the effect you're seeking for. In SoilTest we have the possibility to model a constant suction oedometer test so it should be rather fast to check if the effect you're looking for works in PLAXIS. 
 With kind regards, 
 Dennis Waterman