I live in northeast wisconsin and we have very little soil depth before hitting bedrock.  What kind of soil depth is required in my climate to make geothermal practical?

manifolds should be located at least 18" below frost, and the loops should, also. We have a few EarthLinked installations where the manifold is only 2' deep with vertical bore holes. The lines are insulated, and they are still efficient, well-working systems.

While it's always best to get the manifolds deeper, it still will work on an EarthLinked DX system to have them in the frost. All of the heat exchange is done typically in the bottom half of the boreholes.

I'm wondering if you don't have the required depth required if you can backfill the area? Is this cost effective? If for example you can only get it 4 feet down, and the system isnt working as well as expected, would 20 truck loads of dirt solve the problem?

 

Adding depth by adding dirt will certainly help the loop perform better. Hard to say how cost effective it would be. If you raise the loop temp by 10°, you gain about 1/2 of a point in COP.