Challenges in slope stability assessment of contaminated fibrous sediments along the northern Baltic coast of Sweden

In several locations along Sweden's northern Baltic coast there are deposits of contaminated fibrous sediments that are known as fiberbanks. Historically, these anthropogenic fiberbanks and associated contaminants were formed outside pulp and paper factories because of unregulated waste discharges. Some of the fiberbanks are located on seafloor slopes that are unstable and may fail, which could result in large amounts of fiber and contaminant dispersion. Hydroacoustic surveys have revealed that some parts of the fiberbanks have been dispersed to a wider area by submarine landslides. However, the unusual nature of these cellulose-rich sediments makes it difficult to apply conventional techniques for assessing submarine slope stability. Therefore, a combination of investigation techniques and interpretation methods was tested to assess the stability and triggering mechanisms for submarine slope failure identified in fiberbanks from the Ångermanälven river estuary on the Baltic sea coast. The integration of bathymetric data, sediment lithology and geotechnical characteristics from in-situ and laboratory measurements, enabled the characterisation of underlying natural sediments and the fiberbanks at two sites. Despite low densities, which indicate that the fiberbank material is near the buoyancy level, the fiberbank deposits seem to be relatively stable. Our results indicate the underlying natural sediments are also stable and would require an external triggering mechanism to generate the observed slope failures. We suggest that pore water pressure, potentially related to groundwater table fluctuations, may be an important trigger for submarine landslides in the study area.