Clay is one of the three most commonly encountered soil types in riverbed and seabed, other than sand and silt. A major mechanical property of clay is the adhesion factor that reflects the ratio between its cohesive and adhesive strength under different water content. Currently, this adhesion factor is not exactly known. It is important however, to get a better understanding of the relationship between cohesive and adhesive forces, since large surfaces on dredging tools can generate a lot of resistance, therefore slowing production. In this study, experiments were undertaken to determine the actual relation between adhesion and cohesion. The results can help the implementation of analytical cutting models, in turn, allowing the optimal cutting angle in dredging practice to be calculated.
In the last two decades, the global economy and population have been growing steadily. The already densely populated coastal areas are becoming even more crowded, calling for new artificial islands to provide additional space for housing and recreational areas, etc. As global trade increases, there is greater demand for dredging activities to keep ports and waterways navigable. Due to these global trends, the demand for dredging, trenching and deep-sea mining activities have grown dramatically.
To be able to optimise these activities and increase overall production, understanding the physics and the analytical or numerical build-up of these cutting processes becomes key. In these engineering practices, underwater excavation is one of the major procedures, which involves complicated physics. In dredging, the cutting process is required to obtain the soil from the river/ seabed. In offshore wind farm installation, the vulnerable power cables transporting the generated energy must be buried a few metres under the seabed for their protection, so trenching becomes indispensable. In deep-sea mining, underwater excavation is one of the major ways in which to retrieve the highly concentrated minerals from the seabed.
To reasonably estimate the cutting force and cutting energy needed for underwater excavation on cohesive soil, Miedema (2014, 2017) has developed a model, ‘The Delft Sand, Clay and Rock Cutting Model’, in which several sets of equations are derived for each type of seabed soil. In these equations, the cutting depth, the cutting angle, the cutting speed and the soil properties are used as input parameters, while the cutting forces and the specific energy are the output results. In the case of clay, the adhesion factor, defined as the ratio between the cohesive strength and the adhesive strength of the cohesive soil, should be known as one of the input parameters. However, this factor is not well studied or recorded. In this research project, efforts were made to determine the adhesion factor by investigating literature research and conducting laboratory experiments. Details on this research can be found further in this article.
Adhesion factor and its significance
Clay is the collective noun for a fine-grained natural soil material consisting of clay minerals combined with metal oxides. The soil material properties change with the amount of water the clay contains, thus it is difficult to give definitive values of the properties of clay. In dredging practice, Miedema, (2014, 2017) summarised the clay cutting into the following three types: the flow type, the tear type and the curling type, in which the flow type is the most commonly seen failure mechanism in clay (as shown in Figure 1).