Characterising chalk-concrete interfaces for offshore renewable energy foundations

Deployment of renewable energy foundations, be it piles or gravity based structures, may come into contact with chalk in southern UK waters and in other parts of European offshore and nearshore deployment. To aid more appropriate design it is useful to understand the interface shear strength between the foundation and the underlying rock where this is exposed at surface or where the foundation penetrates. In this paper, the interface shear strength between chalk and unbonded concrete is investigated for constant normal stress conditions over a range of normal stresses using tilt table and specialised interface shear testing apparatus. The results show that the interface strength of chalk is significantly influenced by the normal stress used during testing where at lower stresses the interface strength exceeds the basic friction angle determined for a chalk-chalk interface and degradation of the interface strength below the basic friction angle occurs when normal stresses exceed 73% of the tensile strength of the chalk material. This degradation is more severe at small displacements than previously observed for chalk-steel interfaces. At low normal stresses and displacements, the shear strength of the chalkconcrete interface can be represented by an alpha type approach related to the chalk unconfined compressive strength as previously developed for higher strength rocks.