That would mean that the rock baseline would be 10 degrees below freezing, which seems improbable. I think it would be close to the yearly average temperature for Helsinki (6.5°C) that close to the surface. Maybe I'm missing your point?
6.5 sounds right. Average ground temperatures down where they are stable are under ten degrees C. So this project will definitely warm up the surrounding rock, which (I guess) will increase its effective heat capacity.
