4.6: Summary

Pore Pressure

Saturated sand

Atmospheric rock

Hyperbaric rock

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The cutting forces for sand, clay and rock can be described by a generic equation, where a number of terms dominate for each individual type of soil. Here dry sand, water saturated sand, clay, atmospheric rock and hyperbaric rock are distinguished. The influences of the different forces for each type of soil are summarized in Table 4-1. The general cutting mechanism is the Flow Type, which is mathematically the same as the Shear Type and the Crushed Type. If the forces on the blade depend on the length of the blade, such as the adhesive force A or the pore under pressure force W₂, the Curling Type may occur if the layer thickness is very small compared to the blade length. A mobilized blade length (height) is introduced. If the forces on the shear plane depend on the length of the shear plane, such as the cohesive force C and the pore under pressure force W₁, the Tear Type (or Chip Type) may occur if the layer thickness is large compared to the blade length. A mobilized shear strength is introduced.

However there may also be mixed soils like clay mixed with sand, resulting in a clay with internal friction. Or sand mixed with clay, resulting in a very low permeability. For clay the ratio of the adhesion to the cohesion is very important and little is known about this. Very weak clays may have an adhesion almost equal to the cohesion, but when the cohesion increases the ratio between adhesion and cohesion decreases. A 100 kPa clay may have an adhesion of just 5-10 kPa. For even harder clays the adhesion may drop to zero. The harder clays however seem to have some internal and external friction, increasing with the strength of the clay. A new topic is the cutting of permafrost, frozen clay. From preliminary research it appears that permafrost behaves more like rock, but how exactly is still a question. Future research will give an answer to these questions and hopefully the generic equations will also be applicable for these soils.