# Deformation at the free field boundary

**URL:** <https://forum.itascainternational.com/t/deformation-at-the-free-field-boundary/2205>\
**Category:** FLAC\
**Created:** [August 15, 2025, 12:51am UTC](https://forum.itascainternational.com/t/deformation-at-the-free-field-boundary/2205 "2025-08-15T00:51:42Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![song](https://avatars.discourse-cdn.com/v4/letter/s/ecd19e/32.png) [@song](https://forum.itascainternational.com/u/song)\
**Post date:** [August 15, 2025, 12:51am UTC](https://forum.itascainternational.com/t/deformation-at-the-free-field-boundary/2205/1 "2025-08-15T00:51:42Z")

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I have a dynamic/seismic FLAC model with liquefied material at the free boundary. I have used free boundary and this is large lateral deformation at the free boundary (liquefied here), what is the cause and how I can prevent this?

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**Author:** ![jwang](https://avatars.discourse-cdn.com/v4/letter/j/b782af/32.png) [@jwang](https://forum.itascainternational.com/u/jwang)\
**Post date:** [August 15, 2025, 1:30am UTC](https://forum.itascainternational.com/t/deformation-at-the-free-field-boundary/2205/2 "2025-08-15T01:30:50Z")

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Using liquefied material at the zones near the free field boundary is not recommended. The free field boundary performs a small-strain calculation even though the main grid is executing in large strain mode. In order to reduce the mismatch between large-strain and small-strain calculations, two or three columns of zones near the lateral boundaries are usually assigned with high strength properties to minimize the large deformation, check the earthdam example here: [Earth Dam | US Minneapolis - Itasca Consulting Group, Inc.](https://www.itascacg.com/software/support/examples/earth-dam)

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**Author:** ![song](https://avatars.discourse-cdn.com/v4/letter/s/ecd19e/32.png) [@song](https://forum.itascainternational.com/u/song)\
**Post date:** [August 15, 2025, 1:42am UTC](https://forum.itascainternational.com/t/deformation-at-the-free-field-boundary/2205/3 "2025-08-15T01:42:21Z")

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Thanks. This is what I did for other projects. However, I didn’t do for this one where we got large deformation. Just to confirm this is the case I can use high strength materials.