Homa Lee

Weak Layers: Their Definition and Classification from a Geotechnical Perspective

Weak layers play a major role in the development of many large submarine landslides. A definition of a weak layer is proposed here using a geotechnical perspective: a layer (or band) consisting of sediment or rock that has strength potentially or actually sufficiently lower than that of adjacent units (strength contrast) to provide a potential focus for the development of a surface of rupture. Such a layer or a band can follow stratigraphic horizons, but this is not a requirement. From this it is proposed to define two types: inherited and induced weak layers. In addition, weak layers can develop in strain softening sediments where progressive failure can generate a surface of rupture without the need to invoke the role of excess pore pressures.

A geomechanical approach for the genesis of sediment undulations on the Adriatic shelf

[1]xa0This study is among the first to examine the genesis of the seafloor and subsurface undulations on the Adriatic continental shelf by integrating stratigraphic information and in situ and laboratory geotechnical measurements. Interpretation of sediment behavior is based on a 32-m-long borehole crossing (1) a possible shear plane and (2) a silty clay layer at about 20 m below seafloor (mbsf) on which sediment undulations are rooted and could be interpreted as a potential weak layer succession. Our main results in terms of triggering mechanism for the observed undulations show that under an earthquake, liquefaction and/or failure of the silty-clay sediments (weak layer) leading to deformation of the upper more cohesive sediments is possible only when such a layer is buried by less than 5 m. For greater burial thicknesses, this silty clay becomes stable under the confining lithostatic pressure exerted by the overlying sediment. This work shows that the seafloor and subsurface undulations observed in the study area are most probably the result of an early deformation process of the seafloor followed by a depositional process.

Submarine Mass Movements and Their Consequences: An Overview

Submarine mass movements pose a threat to coastal communities and infrastructures, both onshore and offshore. They can be found from the coastal zone down onto the abyssal plain and can take place on slope angles as low as 0.5°. They can move at velocities up to 50 km/h and reach distances over 1000 km. Their volume can be enormous, as illustrated by the 2.5 × 103 km3 Storegga slide. Similar to their sub-aerial counterparts, submarine mass movements can consist of soil or rock and can take the form of slides, spreads, flows, topples or falls, but in addition they can develop into turbidity currents. Their main consequences are linked either to the direct loss of material at the site where the mass movement is initiated or along its path and to the generation of tsunamis.

Detecting compaction disequilibrium with anisotropy of magnetic susceptibility

In clay-rich sediment, microstructures and macrostructures influence how sediments deform when under stress. When lithology is fairly constant, anisotropy of magnetic susceptibility (AMS) can be a simple technique for measuring the relative consolidation state of sediment, which reflects the sediment burial history. AMS can reveal areas of high water content and apparent overconsolidation associated with unconformities where sediment overburden has been removed. Many other methods for testing consolidation and water content are destructive and invasive, whereas AMS provides a nondestructive means to focus on areas for additional geotechnical study. In zones where the magnetic minerals are undergoing diagenesis, AMS should not be used for detecting compaction state. By utilizing AMS in the Santa Barbara Basin, we were able to identify one clear unconformity and eight zones of high water content in three cores. With the addition of susceptibility, anhysteretic remanent magnetization, and isothermal remanent magnetization rock magnetic techniques, we excluded 3 out of 11 zones from being compaction disequilibria. The AMS signals for these three zones are the result of diagenesis, coring deformation, and burrows. In addition, using AMS eigenvectors, we are able to accurately show the direction of maximum compression for the accumulation zone of the Gaviota Slide.

Towards an Approach for the Assessment of Risk Associated with Submarine Mass Movements

With the growing development of offshore natural resources, use of sea-floor transport and communication routes, considerations for the environment and the effects of global climate changes, and will for protecting populations and their infrastructures, the need for assessing risk associated with submarine mass movements is increasing. The present paper proposes an approach for the assessment of risk associated with submarine mass movements based on geotechnical characterisation.

Stability Analysis of the Hudson Apron Slope, Off New Jersey, U.S.A.

As part of the STRATAFROM project, the Hudson Apron area was selected for a detailed slope stability analysis. Results indicate that high pore pressure is necessary to trigger a failure. Under normal conditions, an excess pore pressure of more than 90% would be required for failure. On the other end, the actual strength profile would indicate a remaining marginal stability. Aggravating factors were the high sedimentation rate cyclicity and esulting layering inducing high excess pore pressures, and potentially gas pressures and earthquakes.