Building addition over clay soils

[masterdesign]

I am looking at adding an addition onto the short end of an existing pole barn. The addition would be the same width as the pole barn and have a length of about 25% of the length of the pole barn. Initial research suggests the site is entirely clay which is subject to shrink/swell behavior. Existing drawings for the pole barn are not available.

After I receive a geotech report, designing a properly proportioned foundation for the the new addition is simple enough. The challenge I am trying to wrap my head around is reconciling the movement of the existing structure with the new structure. Regardless of how well the new and existing foundations are designed, there could easily be differential foundation movements between the two due to over/under sizing of foundation components.

I am not seeing a way around this short of designing a building expansion joint between the existing and new structures. Am I missing something or have others tied structures together in this scenario and found acceptable results?

Thank you in advance for any replies.

 

[dik]

Maybe construct a joint to allow movement...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[TigerGuy]

First, design the addition as a free-standing structure, so differential movement between existing and new does not cause problems with each other. Flexible joints, as dik says, are the way to go.

Second, how expansive is the clay? The geotechnical report should provide guidance on how deep into the soil the issues are. Typically, your surficial few feet (1-2 meters) of soil will experience seasonal changes in moisture. This is where the shrinkage and swelling will be the biggest issue.

Third, how was the existing structure built? How is it performing? Does it move up and down through the year?

If the existing structure is stable and not moving, your best option may be to remove the "active" zone of clay and replace it with non-expansive fill.

 

[EireChch]

Do you care about differential settlement in a barn??

 

[masterdesign]

Thanks for all the replies. Sounds like I definitely will go with a building joint at the new/existing.

Tiger: Waiting on geotech to get definite answer on how active the clay is. But you make a good point, I will tell the geotech to include a remove and replace option (if applicable) in their report. Checking on how the existing building has performed is on my list as well. If the existing building has been moving, then I will need to take additional precautions.

Eire: the use of the structure will be changed to commercial so movement is a concern.