Chalk-steel Interface testing for marine energy foundations

To aid ease of deployment and recovery of tidal stream generators gravity based foundations rather than fixed foundations are being considered in areas where the foundation may be place directly onto an exposed rock seabed. Horizontal loading is usually critical in such applications, therefore specific knowledge of the interface friction between the foundation and seabed surface is important for design. This paper presents results of an interface testing programme of chalk-steel interfaces carried out utilising a computer controlled interface shear tester under constant normal stress conditions against steel of varying roughness. Results indicate that interface strength is significantly affected by the normal stress applied as interface strength degrades for normal stress levels in excess of 30% of the chalks tensile strength. Large displacement tests revealed a tendency of the ultimate interface frictional resistance to drop to values very similar to that of the basic chalk-chalk interface at normal stresses up to 300 kPa, whereas substantial degradation additional was noticed for normal stresses above 700 kPa. At low normal stresses and displacements the behaviour of the chalk steel interface was captured by an alpha type approach related to the rock UCS which has been developed for other higher strength rock types.