Then we need effective stress principle which says that the effective stress is given by the difference between total stress and average pressure on the solid grains. And here the average pressure is given where we take already into account that absorbed water does not produce any stresses. Then for the effective stress we write a constitutive relationship. Here we have the total strain, we have the thermal strain, which is given here, here we have the thermal expansion coefficient of the solid, and these are strain terms which are not connected with stress changes.

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