The safe and economic use of mobile jack-up structures is still hindered by limited understanding of the installation of their large conical 'spudcan' footings in layered sand over clay sites. This paper addresses this by detailing experimentally observed bearing failure modes induced during the penetration of a spudcan through a layer of sand into underlying normally consolidated clay. Digital images were captured continuously by installing a half-spudcan against a transparent window, and analysed using a particle image velocimetry technique coupled with close-range photogrammetry correction. As the experiments were performed in a geotechnical centrifuge the observed mechanisms occurred in stress conditions of similar magnitude to the offshore case. The experimental evidence provides: (a) failure modes at different spudcan penetration depths; (b) the transitional failure mechanism at the event of peak bearing resistance; and (c) the changes in overall failure mechanism due to varying geometric and strength conditions of the layered soil. The results provide useful references for the development and validation of analytical or numerical solutions for this problem.