Geotechnical site investigation challenges in extremely soft marine clays: A Baltic Sea case study

¹ BU Offshore Wind – Geoscience, Vattenfall, Amsterdam, the Netherlands
² BU Offshore Wind – Geoscience, Vattenfall, London, UK

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

Offshore geotechnical site investigations in the Baltic Sea are often challenged by complex depositional environments and the presence of soft Holocene sediments overlaying glacial deposits. This paper presents the experience from a site investigation campaign carried out in the Baltic Sea, with the objective of characterising an offshore wind farm export cable route through a combination of cone penetration testing and vibrocoring, supported by a preceding geophysical survey. The geophysical interpretation indicated the widespread presence of marine deposits overlying glacial till. The main operational challenge arose from the integrated seabed rig, which proved unstable under the extremely soft seabed conditions. Mitigation measures included the deployment of piston cores and an independent cone penetration testing unit, which allowed data collection but limited penetration capability into the underlying glacial till. Despite an extended campaign duration, the geotechnical investigation successfully achieved its objectives. In-situ and laboratory testing revealed extremely low-strength clays with high to very high plasticity at the mudline, with undrained shear strengths typically below 10 kPa within the upper four metres. The findings underline the importance of flexible site investigation strategies and adaptive equipment selection when dealing with soft marine clays in glaciated seabed environments.