The first question that arises in any dredging project is: “what kind of soils need to be dredged?” It also goes without saying that the characteristics of the materials will largely determine the type and capacity of the dredger required, as well as the means by which it is transported.
Grain size and soil classification
Grain size or particle diameter is often used to classify loose soils. It is the standard basis for the classification of soils as composed by PIANC (Permanent International Association of Navigation Congresses).
Grain size classification defines soils as boulders and cobbles, gravels, sands, silts, clays and peats, and organic soils. Soils always consist of different grain sizes. Particle size distribution is used to quantify the relative composition of the soil and define the mean grain size – or mean particle diameter – which is a useful indication of the soil type. Sands, gravels and silts are the most commonly encountered soils in dredging projects.
Sand is an incoherent soil, which means there are no bindings or forces of attraction between the grains, nor are there any forces of attraction between the water and the grains. This makes sands relatively easy to dredge. Sand grains range from 0.06mm to 2mm and all particles are visible to the naked eye. The soil is not compressible (or hardly so) and, when dry, sand has very little cohesion.
There are three main categories of sand:
- fine (grains of 0.2-0.06mm)
- medium-fine (grains of 0.6-0.2mm)
- coarse (grains of 2-0.6 mm).
The finer sands are mostly found in calm waters, since any disturbance will bring the soil into suspension. Sand is used for capital dredging and coastal protection. A mixture of fine, medium-fine and coarse sand has proven to be the best material for landfill projects. Sands can also be encountered on maintenance dredging projects, and the coarser soils are used in the construction industry.
Gravel is easily identifiable by visual examination. It consists of particles ranging from 2mm to 60mm in diameter. Larger particles are regarded as cobbles and boulders. Gravel is an incoherent and non-cohesive soil, and it behaves in a similar manner as sand.
Since it is a loose soil, it can be dredged by means of suction. However, in comparison to sand, the particles are heavier and therefore more relative suction power is required.
Gravel is a raw material, that – together with coarse sand – is widely used in the construction industry. Aggregate dredging is done in rivers, where sand and gravel result from weathering and erosion of rock fragments. In Europe, offshore deposits can date back to the Ice Age.
Coarse sands and gravels
Gravel and coarse sands are usually found closely together and are often mixed. Gravel can, for instance, be buried under a layer of coarse or medium-fine sand in offshore burrows.
Silt consists of particles with a diameter between 0.002mm and 0.06mm. Most particles are invisible, with only grains of coarse silt just visible to the naked eye. A grain size distribution is indispensable to determine dredge ability. Coarse silt behaves like sand and is incoherent, where fine silt behaves like clay and exhibits strong cohesion. A mixture of a cohesive soil with a non-cohesive soil will most likely exhibit cohesive properties.
Siltation and sedimentation are a natural phenomena that occur in ports and estuaries, and lakes and reservoirs, which therefore require maintenance dredging. In order to dredge coherent, ductile soils, cutting action is needed to loosen the materials.
Clay is a soil consisting of particles smaller than 0.002mm. Clay exhibits strong cohesion and plasticity, making it nearly impossible to dredge just by means of suction. Cutting tools are needed to loosen the soil, after which it can be pumped to the hopper or to the disposal site via a pipeline.
In general, soils are composed of particles of different diameters, and they can be mixtures of different soil types. Moreover, in a single dredge site, different types can be encountered. To ensure the right dredging equipment is selected for a particular project, soil investigations are an important part of the preparations.
If you want to know more about which dredger is most suited to what soil type, leave a comment or go to the Equipment Selector.