The role of sediments physical properties in preconditioning submarine landslides initiation: a sedimentological and a numerical investigation

Abstract

Submarine mass movements, in particular submarine landslides, are the result of slope sediment destabilization and downslope transport of the failed sediments. This downslope movement can damage offshore infrastructure and generate damaging tsunamis, thus submarine landslides pose a risk to populated coastal environments. In the past decades, mapping the size and distribution of submarine landslides revealed differences between submarine landslides on seismically active and passive continental margins. In addition, significant variations in sediments’ shear strength were also found between these environments. Although laboratory measurements and numerical modelling have tested the link between sediment shear strength and failure plane formation, key questions still remain regarding the relationship between the conditions in the slope sediments prior to failure and submarine landslides’ size and distribution. The present PhD thesis aims to improve the understanding regarding the influence of sediments’ physical properties and shear strength, on the initiation and development of submarine landslides, particularly large-scale, voluminous slides. To address this aim, in the first part of the thesis, post-slide sediments from a large-scale submarine landslide, the Currituck landslide complex, were analysed by means of geotechnical and sedimentological analyses. This characterization suggested that with similar lithological composition, the consolidation process plays an important role in the sediments’ shear strength development. Variations in shear strength control the development of strength interfaces and appear to have a major influence on the factor of safety. In the second part of the thesis, four sediment endmembers were tested by means of 3D numerical modelling in order to investigate how the size and volume of slope failure changed according to variations in sediment type and shear strength. The analysis suggested that shear strength plays a major role in controlling the development of the slope area that is susceptible to failure, and that the sediment type is important for the lateral development of shear localization. The results indicated that the following conditions facilitate large-scale slope failure: dense, sandy slope sediments and under- to normally-consolidated clayey slope sediments. The results of this thesis provide both dependent and independent evidence of the influence of the sediments’ physical properties on slope failure, by linking shear strength variations to the size and distribution of slope failure. Thus, bridging between many natural slope-scale observations and laboratory-scale measurements. This thesis emphasises how the Currituck landslide complex can be used as a key site to understand slope failure preconditioning factors in non-glaciated margins. In addition, this work lays the foundation for future numerical simulations exploring the influence of sediment composition and strength on slope stability.