WP2 – Site investigation methods

Work Package 2 focuses on developing a practical and reliable site investigation strategy for offshore wind sites with shallow weak rock, where conventional soil and rock methods each have their limitations. The work package will assess how in-situ tools such as CPTu, borehole geophysical logging, high-pressure dilatometers, and lab test results can be best utilized to characterize weak rock strength and stiffness.

Current practice relies on reconnaissance seismic surveys, followed by seabed CPT and boreholes with sampling and downhole testing. While seismic data is valuable for identifying and mapping the depth to bedrock, it is often insufficient for estimating strength, especially where rock is weathered. Seabed CPTus are also limited in that they may not reach the top of the bedrock or may refuse within a few metres. Borehole based in-situ testing, offshore index tests, and advanced laboratory testing therefore remain essential. 

A central challenge in WP2 is sampling of weak rock. Push sampling works well in residual soil and extremely weak rock, while rotary coring is better for stronger rock; the transition zone between them is the most difficult to sample without disturbance. WP2 will therefore highlight the potential of high capacity CPTu systems for collecting in-situ data of extremely to very weak rock that is challenging to sample, providing data that can be used directly for design lines.

The work package will examine how incomplete CPTu profiles, rock descriptions and laboratory test results in highly variable rock can be interpreted together to build robust design profiles. Where high-quality sampling is not possible, greater reliance must be placed on in-situ testing, index data and developed correlations. Overall, WP2 will provide a streamlined framework for weak rock site characterization, with uncertainty management and stronger links between field data, laboratory testing and foundation design.