Effects of local scour on failure envelopes of offshore monopiles and caissons

•The effect of local scour on the ultimate bearing capacities and failure envelopes of the monopiles and caissons is investigated.•Parametric studies including scour depth, scour hole angle and soil strength heterogeneity ratio are performed.•The formulas are proposed to simulate the failure envelopes of the monopiles and caissons with scour.•A procedure of designing the monopiles and caissons to consider local scour effect is proposed. Local scour may induce bearing capacity loss of the foundation significantly by removing soil around the foundation and it has become one of the pivotal factors causing the failure of structures in offshore engineering. Therefore, it is indispensable to investigate the effect of local scour on bearing capacity behaviour of foundations. This paper focuses on the effects of dimensions of local scour hole on both ultimate bearing capacities and failure envelopes of the monopile and the caisson in undrained soil with different soil strength heterogeneity ratios using three-dimensional finite element method. The results show that the scour depth has a more significant effect on the ultimate bearing capacity than the scour hole angle. Meanwhile, the scour depth has a more significant effect on the lateral bearing capacity (i.e., horizontal bearing capacity and moment) than on the vertical bearing capacity. For given dimensions of local scour hole, the effect of local scour on the bearing capacity decreases as soil strength heterogeneity ratio increases. Moreover, local scour affects not only the size of the failure envelopes of the monopile and the caisson, but also their shape. Based on the above findings, a procedure of designing the monopile and the caisson to consider local scour effect is proposed, which could be helpful to the design of the foundations in practical engineering.