5.6: Specific Energy

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In the dredging industry, the specific cutting energy is described as: The amount of energy, that has to be added to a volume unit of soil (e.g. sand, clay or rock) to excavate the soil. The dimension of the specific cutting energy is: kN/m² or kPa for sand and clay, while for rock often MN/m² or MPa is used.

For the case as described above, cutting with a straight blade, the specific cutting energy can be written as:

At low cutting velocities this gives for the specific cutting energy:

\[\ \mathrm{E}_{\mathrm{sp}}=\frac{\mathrm{P}_{\mathrm{c}}}{\mathrm{Q}_{\mathrm{c}}}=\frac{\mathrm{F}_{\mathrm{h}} \cdot \mathrm{v}_{\mathrm{c}}}{\mathrm{h}_{\mathrm{i}} \cdot \mathrm{w} \cdot \mathrm{v}_{\mathrm{c}}}=\frac{\rho_{\mathrm{s}} \cdot \mathrm{g} \cdot \mathrm{h}_{\mathrm{i}}^{2} \cdot \mathrm{w} \cdot \lambda_{\mathrm{HD}}}{\mathrm{h}_{\mathrm{i}} \cdot \mathrm{w}}=\rho_{\mathrm{s}} \cdot \mathrm{g} \cdot \mathrm{h}_{\mathrm{i}} \cdot \lambda_{\mathrm{H} \mathrm{D}}\tag{5-25}\]

At high cutting velocities this gives for the specific cutting energy:

\[\ \mathrm{E}_{\mathrm{sp}}=\frac{\mathrm{P}_{\mathrm{c}}}{\mathrm{Q}_{\mathrm{c}}}=\frac{\mathrm{F}_{\mathrm{h}} \cdot \mathrm{v}_{\mathrm{c}}}{\mathrm{h}_{\mathrm{i}} \cdot \mathrm{w} \cdot \mathrm{v}_{\mathrm{c}}}=\frac{\rho_{\mathrm{s}} \cdot \mathrm{v}_{\mathrm{c}}^{\mathrm{2}} \cdot \mathrm{h}_{\mathrm{i}} \cdot \mathrm{w} \cdot \lambda_{\mathrm{H I}}}{\mathrm{h}_{\mathrm{i}} \cdot \mathrm{w}}=\rho_{\mathrm{s}} \cdot \mathrm{v}_{\mathrm{c}}^{\mathrm{2}} \cdot \lambda_{\mathrm{HI}}\tag{5-26}\]

At medium cutting velocities a weighted average of both has to be used.