Dredgeability

Wear and tear of the pickpoints of the cutter, the teeth of the bucket or the ripper of the draghead depend on the strength and mineralogy of the rock. The need to regularly replace these parts may seriously affect the production rate of the dredger. Sihceous rock will result in a higher wear compared to calcareous rock. 

Sand and silt can be dredged by most types of plant. The plain suction dredger is usually only employed to dredge thick deposits of loose to moderately loose sand. 

Production rates of most types of dredgers are affected by the in-situ density, the friction angle, the grain size, the fines content and the permeabihty of the sand/ silt deposits. 

When dredging clay for reclamation purposes the work method is mostly aimed at limiting the disturbance of the material in order to preserve as much of the original strength as possible. The undrained shear strength, the Atterberg limits and the in-situ unit weight of the clay and/or silt deposits are the most important material properties. 

Preserving the strength can best be achieved by using a backhoe or grab dredger. 

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A number of tests are recommended to determine the dredgeability of soils and their behavior in placer mining or slurry mixing (Table 1-7). In nature, silts may be found in association with clays thus, the parameters for both silts and clays should be assessed. The following testing parameters are accepted by the industry. 

Tests done in the borrow area are mainly aimed at determining the quality, quantity and dredgeability of the soil. The soil properties (e.g. grading) might change during dredging and therefore not all tests are useful or at least need to be treated with care (e.g. Particle size distribution tests). 

Reference is made to section 9.1 for a more elaborate discussion on the suitability of clay as a fill material. Required data for judgement of the dredgeability of the various types of material see Table 3.1 in section 3.4.2.1. 

Data collection for Data collection for Data collection for dredgeability... 

The suitability of a potential borrow area depends on more aspects than the quality, quantity and dredgeability of the fill material. Other important boundary conditions include ... 

The major advantage of the seismic refraction method is that the derived sound velocity of an individual layer correlates very well to the compactness of the sediment or to the Unconfined Compressive Strength rock strength (UCS). However, whereas the UCS value is a rock strength property the seismic sound velocity is more a rock mass characteristic that may strongly be affected by joints, fractures and other discontinuities. These properties in combination with the UCS determine the dredgeability of rock. 

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