Coating for underground pipes in our RMH

We are building a 6" diameter RMH in our greenhouse at a cooperative home in Marquette Michigan. Our plan is to essentially build an RMH but instead of building a cob bench as the mass we are using a garden bed in the greenhouse as the mass. We are nearly finished but the city inspector is concerned about how we are going to make sure that the pipes underground don't degrade from exposure to water and soil. We have been doing some research and have come up with two possibilities: cover the entirety of the underground pipes with concrete, or, cover the underground pipes with some sort of heat and corrosion resistant resin. We are leaning towards the resin approach due to the ease of access for future members of the co-op if maintenance is necessary. However, the resins we have found will melt at 300 degrees F.

ebay

Our biggest question is: what temperatures do the heat exchange pipes in the mass section of the RMH typically reach?

We would also like to know if anyone else who has built an underground heat exchanger has any experience or pro-tips for us.

Thank you

I'd go with the concrete. So cheap, pretty easy.
I'd use the high strength stuff that includes fibers.
Even after the metal fails,the channel s will persist.

Concrete would be easy and effective. If you don't want to use that, you could skip the ducts and just build the channels from masonry - block or brick would do the job and last. If you can get recycled material, it will be cheap and the looks will be irrelevant.