Understanding the shear behaviour of the rock-grout interfaces is a critical aspect in the design of piles socketed into rock, particularly for axial load transfer assessments. For a planned wind farm development offshore Ireland, an experimental campaign of Constant Normal Stiffness (CNS) tests was conducted to characterise the shear behaviour of the rock-grout interface. This study utilised an ultra-high-strength grout, typical for offshore rock socket applications, considering three rock types: mudstone, sandstone and siltstone described from site investigation.

The CNS tests were performed to accurately replicate in-situ conditions, the rock samples were prepared with a surface roughness mimicking that of typical post-drilling traces as closely as possible. The testing program included both monotonic and cyclic loading conditions to assess the shear resistance and degradation of the interface under different displacement conditions. The constant normal stiffness values were derived from site-specific geotechnical data and applying various correlations.

The CNS test results provide valuable insights into the rock-grout interface behaviour directly relevant to the design of monopiles socketed into rock. A detailed methodology for sample preparation is outlined, including the geometry of the surface roughness, offering a practical framework for preparing and executing future CNS testing campaigns. The testing performed aims to contribute to more robust and informed preliminary design considerations for design of offshore monopile foundations in rock.