Experimental observations of the penetration of spudcan footings in silt

Abstract

Mobile drilling platforms perform the majority of the world's drilling in water depths up to approximately 120 m. The installation of their large, inverted, conical spudcan footings into fine-grained silt material is considered problematic by an industry that has usually defined the conditions of installation as either undrained in ‘clays' or drained in ‘sands'. This is reflected in current SNAME industry guidelines. Little guidance is provided for calculating the penetration resistance of spudcans in partially drained conditions. The use of full flow-round penetrometers may provide the opportunity to directly correlate spudcan predictions with in situ measurements. Previous centrifuge testing of T-bars and spudcans has shown a similar bearing pressure for installation under undrained conditions in kaolin clay. However, to provide correlation between a penetrating spudcan and penetrometers in soils displaying partially drained conditions, relationships between size, shape and penetration rate are required. This technical note provides an experimental comparison of the ‘in-flight' installation of a T-bar penetrometer and a typically shaped spudcan footing in reconstituted calcareous silt. The experiments were conducted at an acceleration of 100 times that of Earth's gravity (known as 100g) in the drum centrifuge at the University of Western Australia. The primary variable was the rate of installation. The experimental aims were first to accurately measure the change in capacity with normalised velocity for both the T-bar and spudcan, second to compare these silt results to other published experimental data on kaolin clay soils, and third to investigate the relationship between bearing pressure on a T-bar and a spudcan footing.