Question
How much,(if any) fill can you place around the base of pine trees without killing them ?

Answer
Very very little! A tree has a flare at the base of the trunk if the fill is over this flare the tree will start to die back. The fill soil pushes the air out of the soil and the roots will die from lack of air and compaction of the weight of the soil. So I would not recommend that any fill dirt be added around the tree.