termite protection in earth sheltered psp

I'm considering building an earth sheltered barn/shed in central Illinois using local timber and pool liner. one big concern i have is termites. any ideas on how to keep them away? I'm wondering if a thin layer of charcoal/biochar would deter them.

That could work Michael but once the wood is in or touching the ground, you will never know for sure. A post base that holds the post off the foundation yet prevents uplift would be better. Research "post base" for all sorts of options.