I am trying to understand where the terms for the bulldozing equation come from and cannot find mention of it in my searches. The equation is mostly sourced from the book, "Theory of land locomotion - the mechanics of vehicle mobility" by M.G. Bekker (1962) The first term in the equation is derived from the passive earth pressure equations and only this term is used for tracked vehicles, the additional terms only apply to wheels. These additional terms are what I am unsure of and do not know how they have been derived. It seems as if they have something to do with the geometry of the wheel but beyond that I am unsure. where bw(m) is the width of the contact area, α(◦) is the angle of approach of the wheel, z_max(m) is the depth of the rut left by the wheel, γ(gcm−3) is the soil density, D(m) is the diameter of the wheel, and lr(m) is the distance of rupture. Kγ is the modulus of density of soil deformation, c(Pa) is the soil cohesion, Kc is the modulus of cohesion of soil deformation, φ(◦) is the angle of internal friction; all of these are experimental soil parameters. The diagram below might give some additional context. Xc = 45◦ - φ /2, z_max = h Any help would be much appreciated.