This book which is online (partially)at google books illuminates the skilled slaves and former slaves who helped build the capitol city.

Black Men Built the Capitol: Discovering African-American History In and Around Washington, D.C.
By Jesse J. Holland
Published by Globe Pequot, 2007
ISBN 0762745363, 9780762745364
216 pages

http://books.google.com/books?id=eest66wNX2UC&printsec=frontcover&source=gbs_summary_r&cad=0