# FISH function for elastic modulus increment with confining pressure

**URL:** <https://forum.itascainternational.com/t/fish-function-for-elastic-modulus-increment-with-confining-pressure/1996>\
**Category:** FLAC\
**Created:** [March 18, 2025, 11:43am UTC](https://forum.itascainternational.com/t/fish-function-for-elastic-modulus-increment-with-confining-pressure/1996 "2025-03-18T11:43:25Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![agapanto](https://avatars.discourse-cdn.com/v4/letter/a/f4b2a3/32.png) [@agapanto](https://forum.itascainternational.com/u/agapanto)\
**Post date:** [March 18, 2025, 11:43am UTC](https://forum.itascainternational.com/t/fish-function-for-elastic-modulus-increment-with-confining-pressure/1996/1 "2025-03-18T11:43:25Z")

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Hi all,

I’d like to initialize a non linear distribution of elastic moduli in my FLAC slope model. In particular, I would like to vary the moduli according to the confining pressure (sig2).  
I’m therefore trying to write a fish function called “linear\_mod\_z” but I cannot understand why it’s not working.  
It seems that the script does read the _sig2(i, j)_ value.

Here is the script of the function:

```auto

def linear_mod_z
	loop i (1,izones)
	  	loop j (1,jzones)
			if z_group(i,j) = 'clays' then
				bulk_mod(i,j) = (_Km * sig2(i, j) + _Kq) ; Bulk_modulus
				shear_mod(i,j) = (_Gm * sig2(i, j) + _Gq) ; Shear_modulus
			endif
		endloop
	endloop
end

set _Km = 38.517 _Kq = -3.3236 _Gm = 23.11 _Gq = -1.9941

linear_mod_z

```

I’m using FLAC v.8.

Can anyone tell me where I am wrong?

Thanks a lot in advance.

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<div class="post-metadata">

**Author:** ![jwang](https://avatars.discourse-cdn.com/v4/letter/j/b782af/32.png) [@jwang](https://forum.itascainternational.com/u/jwang)\
**Post date:** [March 18, 2025, 2:09pm UTC](https://forum.itascainternational.com/t/fish-function-for-elastic-modulus-increment-with-confining-pressure/1996/2 "2025-03-18T14:09:57Z")

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There is no such intrinsic fish function sig2(), you have to calculate the intermediate principal stress based on the stress components, see ps3d.fis from the FLAC installation folder: Itasca\FLAC810\Datafiles\Fish\3-Library\Ps3d

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<div class="post-metadata">

**Author:** ![agapanto](https://avatars.discourse-cdn.com/v4/letter/a/f4b2a3/32.png) [@agapanto](https://forum.itascainternational.com/u/agapanto)\
**Post date:** [March 18, 2025, 2:37pm UTC](https://forum.itascainternational.com/t/fish-function-for-elastic-modulus-increment-with-confining-pressure/1996/3 "2025-03-18T14:37:45Z")

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Thanks a lot jwang! I will have a look at the suggested documentation.