The Mechanics of Soil Cutting Blades
Journal of Agricultural Engineering Research 1964, 9 (No. 4)
M.S Osman
Soil Dynamics in Tillage and Traction
Agricultural Handbook No. 316
Agricultural Research Service United States Department of Agriculture
Gill W.R. & Vanden Berg G.E. 1968
Theory of Bulldozer Action in Friable Soil
P.E.R Cook & A.R. Reece
Proceedings of the Second International Conference of the International Society for Terrain Vehicle Systems August 29 – September 2, 1966
Quebec City, Canada
University of Toronto Press
Several Articles In:
Proceedings of the 9th European Conference of the ISTVS,
Harper Adams University College
UK, Sept 2003
The Fundamental Equation of Earth Moving Mechanics
A.R. Reece
Proceedings of the Instn Mech Engrs 1964-65
Also look up work by Daniel Hettiaratchi, and P.C.J Payne. All of the references above do have their own references which does prove useful. One that I think will be particularly useful for discs is the second one down. As well check out these websites.
Hope this helps