Can OGS apply initial stress to a multilayer soil model?

Usability
1

Hello everyone!
I’m simulating a model consisting of multiple layers of soil and energy piles. By learning the prj file of the benchmark in the OGS source code /test/data, I know how to define the material parameters of different soils. But when I was ready to put initial stress on the model, I ran into difficulties. It seems that the simulated objects in the existing reference files are all homogeneous soils, so an expression of a first-order function can be used in to express the increase of initial stress with depth. When I want to apply initial stress to multi-layer soil, I need to use a piecewise function to express the change of initial stress due to the different material properties of different soils. I tried to write several prj files, and found that <initial_stress> can only be defined once, so I could not define the initial stress of different soils by defining multiple <initial_stress>.
Is there any way to solve the above problem? I am also curious if there are other ways to perform initial geostatic equilibrium in OGS.I look forward to your response. Thanks.
截图 2025-01-06 21-20-56

2

Hi PHLi,

probably the easiest solution for your problem is to use advanced functional expressions for the initial stress.

In this XML snippet* this has been used to model a bimaterial domain:

        <parameter>
            <name>E</name>
            <type>Function</type>
            <expression>if (x &lt; 2) 19; else if (x &lt; 2.1) 0.95; else 9.5;</expression>
        </parameter>

x, y, z are the Cartesian coordinates of the location at which the expression is evaluated.
Note that < and > must be escaped (&lt;, &gt;) since the expression is embedded in an XML file.

So, in your case the parameter would be something like

        <parameter>
            <name>sigma_ic</name>
            <type>Function</type>
            <!-- xx component -->
            <expression>if (x &lt; 2) 19; else if (x &lt; 2.1) 0.95; else 9.5;</expression>
            <!-- yy component -->
            <expression>if (x &lt; 2) 19; else if (x &lt; 2.1) 0.95; else 9.5;</expression>
            <!-- zz component -->
            <expression>if (x &lt; 2) 19; else if (x &lt; 2.1) 0.95; else 9.5;</expression>
            <!-- xy component -->
            <expression>if (x &lt; 2) 19; else if (x &lt; 2.1) 0.95; else 9.5;</expression>
            <!-- yz component, in 3D only -->
            <expression>if (x &lt; 2) 19; else if (x &lt; 2.1) 0.95; else 9.5;</expression>
            <!-- xz component, in 3D only -->
            <expression>if (x &lt; 2) 19; else if (x &lt; 2.1) 0.95; else 9.5;</expression>
        </parameter>

ExprTk is used to parse and evaluate the expressions.
It allows great flexibility for the expressions.
Take a look at the examples on the ExprTk website.

You can specify effective and total initial stresses. You can switch between them with the type attribute of the <initial_stress> setting.

Best regards,
Christoph

* Some info about OGS’s XML patch files can be found here.

3

Hi Christoph,
Thank you very much for your detailed response. It has resolved a problem that has been troubling me for quite some time.
I will try to follow your method.
best regards，
PHLi