Chalk–steel interface testing for marine energy foundations

To aid deployment and recovery of tidal stream generators, gravity-based foundations rather than fixed-foundation alternatives are being considered in areas where the foundation may be placed directly onto an exposed rock seabed. Horizontal loading is usually critical in such applications, therefore specific knowledge of the interface friction between the foundation (made of steel or concrete) and seabed 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 different 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 chalk's tensile strength (∼300 kPa). 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 additional degradation 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 unconfined compressive strength, which has been developed for other higher strength rock types.