I live on a steep sloped lot with many oaks (not sure which one but I hear them referred to as "scrub" oaks) The previous owner added approximately 2 ft. of soil around some large oaks & used railroad ties up against trees to as a "retaining" wall. It's been this way for at least 2 years. The large oak is dying on one side. The ones with the ties are losing bark next to where the ties are. Is there anything I can do to salvage these trees? (Removing all of the old fill is not a viable option) I need to do some more work in some other areas. --How can I add fill without killing the trees?

I highly recommend you read the Sick Tree Treatment topic.

http://www.dirtdoctor.com/view_question.php?id=130

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Keeping it clean and green here, Boss.
Reduce, Reuse, Recycle

You have to remove the dirt from around the trees as soon as possible. Also look under "tree planting epidemic" on dirtdoctor.com

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Gary

Sounds to me like the trees probably aren't getting enough water and nutrients due to the all the extra soil that has been heaped on top of their roots.