Geological and geotechnical characterisation of soft Holocene marine sediments: A case study from the north Irish Sea

The seabed of the Western Irish Sea Mud Belt (north Irish Sea) comprises a thick sequence of fine-grained Holocene marine sediments that overlay complex glacial deposits. The Holocene sequence preserves a record of environmental change in the area. Part of this area is earmarked for a number of offshore wind farm developments. The area poses a number of geotechnical challenges to such developments, including soft marine silts and clays as well as shallow gas. This paper characterises the soil conditions in the area in the context of offshore renewable energy development using various geophysical techniques, geotechnical data and shallow (3 m) sediment samples obtained across multiple research surveys between 2009 and 2020. The purpose is to assess the use of such techniques, including multichannel sparker seismic and underwater multichannel analysis of surface waves (UMASW), in characterising these kinds of sediments. It is also intended to build on previous research and provide a reference dataset and case study for further work in developing offshore wind in the area. UMASW and in-situ cone penetration testing with pore pressure measurement (CPTu) results showed good agreement in characterising these Holocene sediments, with multichannel sparker delineating shallow gas from surrounding sediments. Core profiles show distinct lithological variation in the upper 3 m, which can be related to CPTu parameters, but is not resolvable by UMASW profiles. Paleoenvironmental reconstruction for this upper 3 m suggests changes in depositional setting over the past ca. 7000 years, which could influence sedimentary structure. These parameters have implications for cable installation design.