From literature on this subject it is known that the process of cutting clay or rock is different from that of water saturated sand. Sand is often modeled as a continuum with an internal friction angle and a sand/steel friction angle (external friction angle), but without cohesion and adhesion. Clay is considered to be a continuum with cohesion (internal shear strength) and adhesion (external shear strength), but with an internal friction angle and a clay/steel friction angle (external friction angle) equal to zero. Rock is considered as a continuum with cohesion (internal shear strength), but without external shear strength, so without adhesion. Rock is also considered to have internal and external friction. Based on the equilibrium of forces on the chip of soil cut, as derived by Miedema (1987) for soil in general, criteria are formulated to predict the failure mechanism when cutting clay or rock. There are 4 failure mechanisms that can be distinguished depending on the type of material to be cut. The shear type for sand, the curling type, flow type and tear type for clay and the flow type and tear type for rock. The flow type is also known as ductile failure and the tear type as brittle failure. The cutting equations derived allow the prediction of the failure mechanism and the cutting forces involved. In this paper simplifications have been applied to allow a clear description of the phenomena involved. The theory developed can be applied to trenching, deep sea mining, drilling and dredging.

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