Forces acting on plane blade when soils are excavated by the plane blade keeping the depth of the blade constant are calculated by the theory of plasticity in this paper.
　According to both the result and many experiemtns, a new formula about excavation forces is develod. When a small mound is not made in front of the blade, horizontal force (F) acting on the plane blade is as follows
where, d:depth of excavation.
　φ:angle of internal friction. γ: specific weight.
　q:constant depending on both φand an angle of friction between soils and a blade.
　θ:angle of principle stress in three dimensional part.
　b:width of blade in two dimensional state.
　P1: Excavation force depending on cohesion of soils.
　P is as follows

where, C: cohesion of soils.
　δp:angle of friction between sils and a blade.
　α:constant depending on both φ and δ.
　When the small mound is made, the excavation force is calculated by assumption that weight of the mound is a force acting on soils. In the result, the excavation force increases in proportion to the small mound.