Not a good idea to do it. Can you use a sledge hammer and knock it apart first, then use the chain saw after you have it apart? Can you post a picture of it, I am not sure I understand the structure

If the chain saw hits the soil the blade will be become dull within seconds of hitting the soil, and possible ruined. If your not familiar enough with the use of the chain saw and what can damage the blade, you really should not be using such a dangerous tool. Use an electrical Sawzall that you can rent if you do not own one. They sell blades to cut tree branches and larger wood frame members. Much safer then chain saw when working near the ground or near metal.

Woodbridge nailed it...sawzall with disposable "demo" blades..these can cut through wood and an occasional nail.

the "chain" for a chainsaw runs 25 bucks or so...a 9" demo blade about 5 bucks. Crow bars and sledge hammers can also be used.