This paper extends available solutions of combined vertical, horizontal and moment (VHM) capacity envelopes to a novel foundation concept that combines a skirted mat with a suction caisson to enable selfinstallation and preloading of the footing. This research notably explores the effect of the circular plan geometry of the footing, and importantly the effect of the central caisson, on the failure mechanisms and the resulting VHM capacity through finite element analysis. Results demonstrated that the central caisson more than doubles the horizontal capacity, with moderate increases in the vertical and moment loading directions, respectively.
Foundation load combinations of vertical (V), horizontal (H) and moment (M) are typical in the offshore environment, due to platform self-weight as well as wind, waves and current. The industry increasingly embraces the use of VHM interaction surfaces to describe foundation capacity, rather than (semi-) empirical modifications to the classical bearing capacity theory that assumes predominantly vertical loading characteristic of onshore applications. These failure envelopes are affected by footing shape and embedment as well as the soil shear strength profile. Skirted foundations are often used in shallow waters. The skirt, which may extend up to 0.5 diameters below the mudline, is used predominantly to increase horizontal capacity of the foundation. Recent research established the combined vertical, horizontal and moment (VHM) capacity of skirted foundations, though this excludes the combined load capacity of circular skirted mats. Note that only in the most recent study on strip footings were the skirts explicitly modeled. Earlier investigations assumed the soil confined within the skirt to act as a rigid plug, enabling simplification of the problem to a solid footing encompassing the soil plug. This simplification is only valid, however, where the failure mechanism does not extend inside the skirt, as shown by Bransby and Yun (2009).
