# About the gravity and initial stress in OGS6

Dear all,
I’m working on a THM process (initialization is realized by TH process) to investigate the fault stability in 2D influenced by EGS stimulation. I have a question about the gravity and initial stress in OGS6, mainly in THM process:

part of prj code (y is from 0 to -7000m):
<initial_stress>sigma0</initial_stress>
<specific_body_force>0 -9.81</specific_body_force>
...
<parameter>
<mesh>THM_test_initialization</mesh>
<name>sigma0</name>
<type>Function</type>
<expression>26280*y*0.65</expression>
<expression>26280*y</expression>
<expression>0</expression>
<expression>0</expression>
</parameter>

Question in THM process:
What is the function of gravity in this sentence? <specific_body_force>0 -9.81</specific_body_force>
Would the gravity conflict with the initial stress I have set? Because the initial stress I set here is produced by gravity. Does the above prj code mean that I set gravity effect twice?

Dear Rui Feng,

as long as you don’t tell the programme otherwise, you have to specify an initial state that is in equilibrium. This means, yhat whatever you give for the initial stress is interpreted as the initial effective (!) stress and, in conjunction with your definition of bodyforce (gravity in your case) and your boundary conditions has to fullfill \partial_{x_j}(\sigma'_{ij} - p \delta_{ij}) + \varrho g_i = 0. So it’s not redundant but you have to make sure your initial effective stresses, initial pore pressures, initial body forces and initial boundary conditions are compatible. If you don’t want that, there are also options to specify non-equilibrium initial states and calculate from an arbitrary state (usually fine for elastic settings).

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