# Can we use "fast-flow" for structure-soil interaction problems?

**URL:** <https://forum.itascainternational.com/t/can-we-use-fast-flow-for-structure-soil-interaction-problems/490>\
**Category:** FLAC3D\
**Created:** [May 2, 2022, 3:19am UTC](https://forum.itascainternational.com/t/can-we-use-fast-flow-for-structure-soil-interaction-problems/490 "2022-05-02T03:19:56Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![FLAC3D\_GEO](https://avatars.discourse-cdn.com/v4/letter/f/f1d935/32.png) [@FLAC3D\_GEO](https://forum.itascainternational.com/u/FLAC3D_GEO)\
**Post date:** [May 2, 2022, 3:19am UTC](https://forum.itascainternational.com/t/can-we-use-fast-flow-for-structure-soil-interaction-problems/490/1 "2022-05-02T03:19:57Z")

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According to the manual, “fast flow” cannot be used for the “models with interfaces”. I am just wondering whether the “interfaces” include those “links” generated automatically when the structural elements are created? In another words, Can I use “fast flow” for the structure-soil interaction modelling, where the structure is modelled with liners or shells?

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**Author:** ![cheng](https://avatars.discourse-cdn.com/v4/letter/c/ecccb3/32.png) [@cheng](https://forum.itascainternational.com/u/cheng)\
**Post date:** [May 3, 2022, 9:38pm UTC](https://forum.itascainternational.com/t/can-we-use-fast-flow-for-structure-soil-interaction-problems/490/2 "2022-05-03T21:38:03Z")

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This has not yet been officially tested. For 1-D structures, shells and geogrids (if considered permeable) should be ok while liners (since zones be separated) may not. Anyway, this is just a very preliminary answer.

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**Author:** ![FLAC3D\_GEO](https://avatars.discourse-cdn.com/v4/letter/f/f1d935/32.png) [@FLAC3D\_GEO](https://forum.itascainternational.com/u/FLAC3D_GEO)\
**Post date:** [May 4, 2022, 5:00am UTC](https://forum.itascainternational.com/t/can-we-use-fast-flow-for-structure-soil-interaction-problems/490/3 "2022-05-04T05:00:37Z")

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Thank you, Cheng! Side question: Presumably shells and liners are all impermeable?

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**Author:** ![cheng](https://avatars.discourse-cdn.com/v4/letter/c/ecccb3/32.png) [@cheng](https://forum.itascainternational.com/u/cheng)\
**Post date:** [May 4, 2022, 5:41pm UTC](https://forum.itascainternational.com/t/can-we-use-fast-flow-for-structure-soil-interaction-problems/490/4 "2022-05-04T17:41:59Z")

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Pore-pressures are linked with gridpoints which link with zones. By default in FLAC3D, a boundary of the grid is considered impermeable. Shells do not separate zones, so no new boundary is created. Liners, on the contrary, do separate zones and create new boundaries at the separation, so that place is considered impermeable. But physically, shells and liners may both be considered impermeable.