Serge Leroueil

The Varnes classification of landslide types, an update

The goal of this article is to revise several aspects of the well-known classification of landslides, developed by Varnes (1978). The primary recommendation is to modify the definition of landslide-forming materials, to provide compatibility with accepted geotechnical and geological terminology of rocks and soils. Other, less important modifications of the classification system are suggested, resulting from recent developments of the landslide science. The modified Varnes classification of landslides has 32 landslide types, each of which is backed by a formal definition. The definitions should facilitate backward compatibility of the system as well as possible translation to other languages. Complex landslides are not included as a separate category type, but composite types can be constructed by the user of the classification by combining two or more type names, if advantageous.

An efficient technique for generating homogeneous specimens for DEM studies

Abstract A new technique, designated as the Multi-layer with Undercompaction Method (UCM), capable of generating homogeneous specimens for DEM studies is presented herein. The specimen are compacted in several layers using undercompaction criteria based on average planar void ratio. The proposed approach was compared to other available methods and was proven to generate very homogeneous specimens provided a nonlinear average undercompaction criterion was used. Furthermore, the UCM method was efficient for a variety of density conditions ranging from very loose to dense states.

Observational Approach to Membrane and Area Corrections in Triaxial Tests

The corrections to account for the changes in the cross-sectional area and for the restraint of the membrane during a triaxial test may vary considerably depending on the actual behavior of the soil specimen. This paper shows how observations and measures can lead to a choice of corrections which are in good agreement with the true behavior of the soil-membrane system both for bulging failures and for failures along a shear plane. The data obtained by compression tests on rubber and rigid dummies offer a valuable basis for quantifying the required area and membrane corrections. An example illustrates the merits of the proposed corrections and shows that different interpretations of the membrane correction may have an important implication on the effective stress cohesion intercept.

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.

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.

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.

Consolidation of Natural Clays and Laboratory Testing

A numerical model has been developed, making use of a strain-rate-dependent compressibility law and a permeability-void ratio law, both experimentally established. The numerical program is used to simulate the behavior ofclay specimens submitted to oedometric tests (MSL, CGT, creep, etc.). The following conclusions are drawn: 1. The effective stress-strain clay behavior depends on the type of test carried out. In particular, the preconsolidation pressure value varies from one test to another. 2. Each element throughout the specimen follows a specific stress-strain relation depending on its strain rate history. Moreover, the oedometric curve, as usually interpreted from a multiple-stage loading (MSL) test, does not agree with any of these relations, not even with the average curve followed by the whole specimen. 3. The pore pressure isochrones maintain the same shape during controlled-gradient (CGT) or creep tests, in particular when the effective stresses correspond to the preconsolidation pressure.

Submarine Mass Movements in the Upper Saguenay Fjord, (Québec, Canada), Triggered by the 1663 Earthquake

Extensive submarine mass movements have been mapped in the upper reaches of the Saguenay Fjord, Quebec, Canada. Our analysis indicates that they were most likely triggered by the 1663 earthquake which shook a large area in Eastern Canada. Signs of instabilities have been observed at various places and although the triggering mechanism is the same, the types of mass movements differ largely including slides, spreads and flow failures in the soft Holocene sediments. Seismic and morphological investigations have revealed the presence of a fault -like deformation possibly linked to terrestrial fractures suggesting that the epicentre of the 1663 earthquake may be located in the area.

Modelling the combined effect of strain rate and temperature on one-dimensional compression of soils

Strain rate and temperature have important effects on the behaviour of soils. The present paper enhances the thermo-viscoplastic modelling of soils by taking advantage of the most recent understanding of the effects of temperature and strain rate on soils. In particular, modelling of the evolution of the vertical yield stress at any void ratio is made possible with the use of an advanced model for the dependence of vertical yield stress on temperature, as well as the use of the unique effective stress-strain-strain rate concept. The first part of the paper presents an overview of the experimental and constitutive theoretical works performed on the effects of strain rate and temperature. The second part describes a strain-rate and temperature coupled model, and the third part deals with the numerical validation of the proposed model.

Effect of soil microstructure on the compressibility of natural Singapore marine clay

The effect of soil microstructure on the compressibility of natural Singapore marine clay is studied in this paper. It was found that natural Singapore marine clay can sustain higher void ratio than reconstituted Singapore marine clay at the same effective stress in one-dimensional compression. This difference is most noticeable between yield stress and two times the yield stress. This void ratio difference is commonly attributed to soil microstructure, and the decrease in this difference at higher stresses is attributed to progressive destructuration. As a consequence of progressive destructuration, the virgin compression curve is nonlinear and the compression index along the virgin compression range is variable, which is only noticeable during the constant rate of strain consolidation test on high-quality samples. In the absence of high-quality samples and constant rate of strain consolidation tests, an empirical method is proposed to obtain a first-order estimation of a nonlinear virgin compression cur...