Surplus is the right word as shown for example [here on this Canadian government website][1].  

There they refer to surplus earthwork material which includes more than just soil.  I think the potential difficulty with 'surplus soil' is the alliteration but in context I think it will be fine. You may need to differentiate between 'fill' soil that you brought to site and surplus excavated soil that you removed.


  [1]: https://dot.ca.gov/programs/construction/construction-manual/section-4-19-earthwork#4-1903A(5)