Assessment of seabed trench formation based on marine sediment properties and chain-bar penetration tests

Seabed trenches, as a geological hazard, adversely influence the safety of adjacent anchors. Seabed trenches predominantly appear in the Gulf of Guinea, where the seabed soils exhibit some features, e.g., high plasticity, high water content and low shear strength. However, the marine engineering geology where seabed trenches appear is not well understood, as well as the trenching process related to soil erosion. In this paper, the information about seabed trenches was summarized, and marine engineering geology where seabed trenches appear was analyzed in detail. It was found that the marine clay with high plasticity index, high organic content, higher sensitivity, low effective unit weight and shear strength is easier to form seabed trenches. Then, penetration tests of a chain-bar penetrometer were conducted to investigate the soil deformation and erosion near the surface. Experimental observations reveal that the normalized soil resistances from both penetrometers exhibited similar trends, despite different soil deformation mechanisms. In the cyclic tests, the soil resistance was degraded significantly in the first 20 cycles, and water flow induced by chain motion eroded the soil particles near the chain links. This study provides insights into the marine engineering geology and development process of seabed trenches.