Abstract: Post-piled offshore jacket foundations usually temporarily rest on mudmats, which may have shear keys or skirts for increased sliding capacity. Scour protection may also be required. Therefore, the skirts or plates should fully penetrate into the scour protection. While current standards offer analytical solutions for penetration resistance in sandy or clayey soils, they may underestimate resistance when soil grain size is comparable to plate thickness. Here, small-scale laboratory tests examine this grain size effect during skirt penetration into granular soils, varying skirt thicknesses and using a sand and a gravel, with plate thickness to grain size ratios ranging from 0.4 to 63. A total of 30 penetration tests were conducted, along with triaxial and direct shear tests. Results show increased penetration load with thicker skirts and higher soil friction angles. Furthermore, the penetration load increased with the grain size to skirt thickness ratio. An alternative equivalent thickness using d99 to account for the soil grain size is proposed and validated against the conducted experimental tests. The new equivalent thickness enhances the theoretical interpretation because it reduces the range of back-fitted values for the soil friction angle to 4° in the studied cases, instead of 8° with the actual plate thickness.

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