Formation and Implications of Glacially-derived Gravel Lag Deposits; Mapping a Geo-constraint to Shallow Offshore Infrastructure

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4. Formation and Implications of Glacially-derived Gravel Lag Deposits; Mapping a Geo-constraint to Shallow Offshore Infrastructure

ABSTRACT: Soils deposited across previously glaciated margins can often contain, by the nature of their depositional environment, a high content of gravels and cobbles. These coarser-grained soils can either be deposited and remain in situ (e.g., glaciofluvial channel fill) or they can accumulate into denser lag deposits, as Holocene (and preceding) marine inundation of the previously terrestrial surface rework the glacigenic soils, winnowing out the fine-grained material. These coarse-grained soils can be classified as an engineering geological constraint (geo-constraint) to offshore wind projects in several ways, including during site investigation campaigns (e.g., early refusal of Cone Penetration Tests), subsea cable trenching works (e.g., unfavourable for jetting, can cause plough deviation) and during Wind Turbine Generator (WTG) foundation installation works (e.g., refusal of driven piles or suction caissons). By using an integrated suite of geoscience data comprising shallow, high-resolution seismic (e.g., pinger), multibeam backscatter, seabed grab samples, and vibrocores, an assessment can be made of how prevalent these deposits are across a lease area or a cable route, and maps can be generated to reveal where lag deposits are likely to occur within cable burial depths (e.g., 2 m below seafloor). Here, we demonstrate the benefits of using an integrated approach on a wind farm development in the North Sea, identifying seismostratigraphic units containing diffraction hyperbolae indicative of coarse sediments, ground-truthed against geotechnical locations. The results demonstrate excellent correlation between the integrated datasets and provide valuable insights to de-risk future cable trenching operations.

5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG2025)

4 - Ground models