Jacques Locat

Distinguishing sediment waves from slope failure deposits: Field examples, including the 'humboldt slide', and modelling results

Abstract Migrating sediment waves have been reported in a variety of marine settings, including submarine levee–fan systems, floors of fjords, and other basin or continental slope environments. Examination of such wave fields reveals nine diagnostic characteristics. When these characteristics are applied to several features previously attributed to submarine landslide deformation, they suggest that the features should most likely be reinterpreted as migrating sediment-wave fields. Sites that have been reinterpreted include the ‘Humboldt slide’ on the Eel River margin in northern California, the continental slope in the Gulf of Cadiz, the continental shelf off the Malaspina Glacier in the Gulf of Alaska, and the Adriatic shelf. A reassessment of all four features strongly suggests that numerous turbidity currents, separated by intervals of ambient hemipelagic sedimentation, deposited the wave fields over thousands of years. A numerical model of hyperpycnal discharge from the Eel River, for example, shows that under certain alongshore-current conditions, such events can produce turbidity currents that flow across the ‘Humboldt slide’, serving as the mechanism for the development of migrating sediment waves. Numerical experiments also demonstrate that where a series of turbidity currents flows across a rough seafloor (i.e. numerical steps), sediment waves can form and migrate upslope. Hemipelagic sedimentation between turbidity current events further facilitates the upslope migration of the sediment waves. Physical modelling of turbidity currents also confirms the formation and migration of seafloor bedforms. The morphologies of sediment waves generated both numerically and physically in the laboratory bear a strong resemblance to those observed in the field, including those that were previously described as submarine landslides.

Instability conditions of marly hillslopes: towards landsliding or gullying? The case of the Barcelonnette Basin, South East France

Abstract Black marl hillslopes in the French Alps are strongly affected either by mass movements or by gully erosion due to their susceptibility to weathering processes. This paper presents experimental data (geomorphological, geotechnical, geomechanical and hydrological) on the parameters of the material of three well-known earthflows and of the two stratigraphic parts of the Callovo–Oxfordian black marl. The main objectives are to define the geomechanical behaviour of the various formations using different characterisations (grain size distribution, mineralogy, retention capacity, consolidation test, direct shear test and triaxial test) and to demonstrate how such soils can affect the stability of natural slopes. Particular care must be taken when interpreting the geomechanical tests on this evolving and very heterogeneous clay-rich material. There are strong relationships between the parent rock and the landslide materials. This paper presents a conceptual model of the development of strength over a period of time and investigates the parameters of the material for use in slope stability analysis and risk assessment.

Numerical analysis of the mobility of the Palos Verdes debris avalanche, California, and its implication for the generation of tsunamis

Abstract Analysis of morphology, failure and post-failure stages of the Palos Verdes debris avalanche reveals that it may have triggered a significant tsunami wave. Our analysis of the failure itself indicates that the slope is stable under aseismic conditions but that a major earthquake (with a magnitude around 7) could have triggered the slide. A post-failure analysis, considering the debris avalanche as a bi-linear flow, shows that peak velocities of up to 45 m/s could have been reached and that the initial movement involved a mass of rock less than 10 km wide, 1 km long and about 50–80 m thick. Initial wave height estimates vary from 10 to 50 m. Tsunami waves propagating to the local shoreline would be significantly smaller. Such a range demonstrates our lack of proper knowledge of the transition from failure to post-failure behavior related to mass movements. Further investigations and analyses of terrestrial and submarine evidence are required for a proper hazard assessment related to tsunami generation in the Los Angeles area.

Mineralogical and microtextural changes associated with lime stabilization of marine clays from eastern Canada

Abstract Four clayey soils displaying the range of properties of the marine clays from eastern Canada were selected for this study. They were treated with different amounts of quick, hydrated and agricultural lime, at different water contents. The mineralogical and the microtextural aspects involved in lime stabilization of these clays as well as the mechanisms responsible for gain of strength have been investigated. In soils stabilized with quick and hydrated lime at a water content near the liquid limit, (1) the clay minerals appear to be the main targets for chemical attack, (2) reaction products of CASH and CSH types are formed, affecting the pore size distribution, and (3) the gains in strength are related to the progressive formation of these new phases.

Instabilities along ocean margins: a geomorphological and geotechnical perspective ☆

Due to the growing development of offshore activities along margins, in the search for more natural resources, there has been a greater need for understanding the mechanics of slope formation in these areas. One aspect of this relates to the problems associated with slope stability issues. This article presents an perspective view of some geomorphological and geotechnical aspects related to instabilities along ocean margins. In order to properly address the issues of slope stability and other mass movements, we need to improve our technology in the field of sampling, high resolution seismic, and in situ measurements. Ultimately, the goal is to be able to carry out proper risk assessment analysis pertaining to submarine mass movement. This could be achieved by integrating the geotechnical characterisation of mass movements into a risk assessment methodology which can then be applied on a regional basis. With the initiation of a project on continental slope stability (COSTA) during the year 2000, major advances are expected on our understanding of the mechanics of submarine mass movements.

Thixotropic behavior during shear of a fine-grained mud from Eastern Canada

The quantitative determination of the hysteresis loop area developed during viscometer tests on shear rate-shear stress reverse paths, has been used to describe the thixotropic behavior of a clayey mud under flow conditions. Tests were conducted for liquidity indexes between 1.8 and 4.0, and for six pore water salinities ranging from 0.1 to 30 g/l. Admitting that a low hysteresis loop area denotes a well-developed thixotropic behavior for a given liquidity index, the thixotropy of the slurry is likely to be more strongly marked for salinities greater than the flocculation threshold. At a constant salinity, the results also indicate the existence of a simple linear relationship between the hysteresis loop area and the yield stress and the viscosity of the slurry. The restrictions of this approach and the possible implications of a strengthening/softening process on mudflow kinematics are also discussed.

Rheological properties of fine-grained sediment: the roles of texture and mineralogy

Rheological properties of fine-grained sediments depending on index properties and salinity were examined. To characterize flow behaviors as a function of soil type, groups were made for convenience: (i) low-activity clays (group 1), (ii) high-activity clays (group 2), and (iii) silt-rich soils (especially for iron tailings; group 3). Low-activity and high-activity clays have characteristics of pseudoplastic (shear thinning) fluids, and exhibit a decrease in viscosity with increasing shear rate. However, in terms of the change in soil structure due to particle–particle interactions, illitic and montmorillonitic clays have opposite responses to salinity. As most of our data were obtained on low-activity clays — mostly illitic mixtures — we implemented a test program to ascertain the influence of montmorillonite on flow behavior. Using the Bingham model, a simple relationship is presented in terms of the possible critical limits of rheological transitions from clay- to silt- to sand-rich soils.

The July 1996 flood deposit in the Saguenay Fjord, Quebec, Canada: implications for sources of spatial and temporal backscatter variations

In July 1996 a major rainstorm and flood took place in the Saguenay Fjord. Backscatter strength measurements made with a Simrad EM1000 multibeam echosounder in 1993, 1997 and 1999 have shown spatial and temporal variations, which are interpreted in relation to the occurrence of the flood. After empirical calibration of the different maps the data show an overall diminution of 5 dB in backscatter strength in 1997, 1 yr after the flood took place, while the data sets obtained in 1993 and 1999 show comparable levels of backscatter strength. The different data sets show a similar pattern of low and high backscatter patches, which represent variations of the backscatter strength of a few decibels. Several grain size and water content measurements were also carried out on sediment box core and grab samples from the Fjord bottom in 1997 and 1999. These have shown that the areas with higher acoustic backscatter correspond to the finer sediments, while the low backscatter patches correspond to the coarser material. The data show little relation between water content and backscatter strength, thus indicating a poor dependence between the impedance terms (bulk density and sound speed) and the backscatter. Having taken this into account, the major contribution to backscatter strength is assumed to result from differences in surface and volume roughness of the sediment. Since finer grain sizes offer higher backscatter the grain size of the sediment is not considered as being a major contributor to roughness and hence, backscatter strength. The major factor that seems to control roughness generation in the Saguenay Fjord is considered to be bioturbation. The areas where geological processes most physically disturb the bottom, which coincide with the areas where also most accumulation takes place and the coarser grain sizes are deposited, are more sparsely colonized by organisms. This results in a lower degree of bioturbation, lower roughness and thus, backscatter strength. This hypothesis helps explain the variation in backscatter strength observed between the different measurement years. The several million tons of sediment deposited in the Fjord bottom after the 1996 flood buried the benthic fauna, which was still recovering 1 yr later. The less bioturbated sediment at this time would present lower roughness with respect to the 1993 and 1999 data resulting in a lower backscatter strength.

Applicability of power law for describing the rheology of soils of different origins and characteristics

The rate-dependent rheological behaviour of soils of different origins and characteristics was studied and the ap- plicability of the power law model was examined. The studied soils were divided into three groups: (i) low-activity soils, (ii) high-activity soils, and (iii) silt-rich soils. The results show that the power law applies to all these soils and is represen- tative of soil behaviour in a strain rate range corresponding to debris flows, which is generally not the case with the Bing- ham model. For low-activity clays, the power law index, n, is typically equal to 0.12 and seems to increase with the plasticity index; it is larger (i.e., in the range of 0.2-0.6) for silt-rich soils. Comparison of n values for tests performed on intact and remoulded low-activity clay specimens indicates that the power law index is possibly strain-rate dependent.

Effects of lime on common soil and rock forming minerals

Abstract Common soil and rock forming minerals were tested in the presence of lime as slurries and pastes in order to assess their reactivity and the nature of the reaction products. Results include chemical analysis of the solutions, infrared spectroscopy, X-ray diffraction and SEM analysis of the solids involved. Accordingly, opal, chert, metabentonite, nepheline and dolomite were classified as highly reactive, oligoclase and quartz as reactive, all the fifteen other minerals tested were found poorly or not reactive. This order of reactivity is in good agreement with computer simulations performed to assure the qualitative results.