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Abstract Pipelines in Arctic regions are at risk of being damaged by gouging ice masses(gouge depths of 5m-8.5m). As protection, pipelines can be buried in trenches. When trenching is performed in Arctic soil, subsea permafrost may be encountered. Subsea permafrost is found in the continental shelves of the Arctic seas at water depths up to 100 m. The upper units (the first 1m to 100m)generally comprise thawed layers, which act as an insulating layer to the permafrost below. Layers of thawed clayey soil have better insulating properties than those of coarser material. Consequently, more shallow subsea permafrost, which is located in the top 10 m of the seabed, is most likely to be encountered in clayey soils. Ground deformation caused by seabed ice gouges will demand that pipeline trenches be excavated up to 10 m deep. This leads to the conclusion that permafrost in arctic areas can be encountered while trenching, mainly while excavating clayey soil. For proper design and construction of efficient subsea excavation equipment (subsea trenchers) thorough knowledge of physical characteristics of permafrost is necessary, and failure mechanisms, cutting forces and other important parameters should be assessed. A literature review identified that knowledge of excavation in permafrost is limited. Therefore Delft University of Technology and IHC Merwede joined forces to fill the knowledge gap. The main goal of this research is to identify the governing failure mechanism of frozen clay. Knowledge of the failure mechanism of a material under certain cutting conditions leads to the ability to calculate cutting forces in similar conditions and to determine the required cutting energy per cubic meter of material, which is an essential parameters during design and operation of arctic trenchers. This study found that frozen clay has a tensile and compressive strength comparable to soft rock. During the process of freezing, ice lenses are formed in clay, making the material inhomogeneous and anisotropic. The failure mechanism of frozen clay in an arctic trenching process is studied experimentally by means of a linear cutting test setup. Samples of undisturbed Dutch river clay were excavated, frozen and cutin submerged conditions at near atmospheric pressure. The observed failure mechanism of submerged frozen clay appeared predominately tensile brittle. Further research has been planned to obtain more accurate characteristics by using clay from Arctic areas, and conducting experiments carried under higher hydrostatic pressures. |
