Sigma_zz not established by Gravity when using initial effective stress (THM Process)

1

Dear OGS Community,

I am running a large 3D coupled THM simulation and am facing a critical issue with the initialization of the vertical stress.

My goal is to use the gravitational body force to establish the correct lithostatic vertical stressfield, while maintaining user-defined functional definitions for the horizontal stresses sigma_xx and sigma_yy for tectonic testing.

I defined the solid density in all my mediums.

relevant parts of my .prj file:

<processes>
    <process>
        <specific_body_force>0 0 -9.81</specific_body_force>
        <initial_stress type="effective">sigma0</initial_stress>
    </process>
</processes>

<parameter>
    <name>sigma0</name>
    <type>Function</type>
    <expression>10000 * z</expression>
    <expression>13000 * z</expression>
    <expression>0</expression> 
    <expression>0</expression>
    <expression>0</expression>
    <expression>0</expression>
</parameter>

The Problem:
Despite the correct density and gravity setting, the vertical stress remains numerically zero (~ 1e-38 Pa) throughout the simulation.

When I define sigma_zz in the effective stress field, the results look much better and realistic.

My question: How can I ensure sigma_zz is correctly established solely by the gravitational body force while retaining the functional sigma_xx and sigma_yy definitions? Is there a necessary flag or parameter that is causing the body force to be ignored for the vertical stress calculation? Or can I simply define g as well, or do they interfere with each other?

Thank you,
2ophie

2

Hi, Sophie. I guess you have to define the vertical initial stress on the third line, instead of putting it as zero, maybe rho * 9.81 * z.
The six lines of expression means stress_xx, stress_yy, stress_zz, stress_xy, stress_yz, stress_xz, this way you can get the vertical stress.