Micro-structural characterisation of weak layers of submarine landslides

Submarine landslides are common on all sediment bearing submarine slopes worldwide. They have the potential to damage expensive subsea infrastructure such as pipelines or telecommunication cables, and generate hazardous tsunamis. Numerous studies have shown that weak layers embedded within the slope stratigraphy play a crucial role in controlling the formation of submarine landslides; however, very little is known about their internal structure and composition. Although weak layers seem to be an essential pre-conditioning factor for slope failure, many questions remain unanswered, such as where with respect to weak layers do failure planes form: within the weak layer, above or below it? Previous studies usually relied on sedimentological and geotechnical sediment core and in-situ analyses to investigate weak layers. These analyses, however, do not provide insights into the internal structure of the sediments on a micro-scale level and hence, lack information needed to qualitatively and quantitatively investigate weak layers. Here, we present a new approach towards weak layer investigation that is based on high-resolution micro-Computed Tomography (µCT) imaging. µCT is used to visualise, and qualitatively and quantitatively investigate selected sediment samples taken from within weak layers and the background sediment of submarine landslides. Our results show clear compositional and structural differences between individual sub-units of the investigated weak layers, as well as the background sediment. These differences can be attributed partly to different sediment types, i.e. coarse- versus fine-grained sediments, but also reveal a dependency on the sedimentation regime. We find that pore space distribution is highly spatially variable and works on a sub-millimetre scale. Such high variability may be masked by standard bulk porosity measurements, which require larger (several centimetre) sediment samples and only provide information averaged over the entire sample. The identification of small-scale changes, however, appears to be crucial for the formation of weak layers. Our results therefore demonstrate the huge potential of µCT to investigate the internal structure of weak layers, obtaining information that is not resolved and lost in other analytical methods.