Remblais hydrauliques en dédiant une attention particulière à la liquéfaction

Hydraulic fills are often deposited in a loose state making them susceptible to liquefaction. The paper focuses on flow/static liquefaction, a phenomenon that has led to a number of catastrophic failures. The current understanding of flow/static liquefaction, based on the concepts of critical/steady state and state parameter, is reviewed. Theoretical concepts such as that of controllability lead to a more rigorous definition of the undrained instability phenomena associated with flow liquefaction. Hydraulic fills are often characterised by in situ tests. Advanced numerical analyses of the piezocone penetration test (CPTu) on soils exhibiting undrained softening are presented and discussed. Two case histories involving the liquefaction of hydraulic fills are described. The first one concerns a tailings dam where hydraulic fill liquefaction was a consequence of an independent foundation failure. Tailings liquefaction, however, was the major contributor to the devastating consequences of the failure. The second case is the failure of a harbour quay where backfill liquefaction was the immediate cause of the failure and its consequences. Some general considerations on the liquefaction of hydraulic fills close the paper.Postprint (published version

Numerical analysis of suction embedded plate anchors in structured clay

As offshore energy developments move towards deeper water, moored floating production facilities are increasingly preferred to fixed structures. Anchoring systems are therefore of great interest to engineers working on deep water developments. Suction embedded plate anchors (SEPLAs) are rapidly becoming a popular solution, possessing a more accurate and predictable installation process compared to traditional alternatives. In this paper, finite element analysis has been conducted to evaluate the ultimate pullout capacity of SEPLAs in a range of post-keying configurations. Previous numerical studies of anchor pullout capacity have generally treated the soil as an elastic-perfectly plastic medium. However, the mechanical behaviour of natural clays is affected by inter-particle bonding, or structure, which cannot be accounted for using simple elasto-plastic models. Here, an advanced constitutive model formulated within the kinematic hardening framework is used to accurately predict the degradation of structure as an anchor embedded in a natural soft clay deposit is loaded to its pullout capacity. In comparison with an idealised, non-softening clay, the degradation of clay structure due to plastic strains in the soil mass results in a lower pullout capacity factor, a quantity commonly used in design, and a more complex load–displacement relationship. It can be concluded that clay structure has an important effect on the pullout behaviour of plate anchors.Peer ReviewedPostprint (author's final draft

Ice formation in unsaturated frozen soils

This paper presents a procedure for determining unfrozen water saturation in a partially saturated frozen soil (clayey silt) using bulk electrical conductivity (EC) measurements. A modification of Archie’s law is proposed to describe the relationship between soil bulk EC, temperature, porosity and degree of unfrozen water saturation. Compacted samples have been prepared at a dry density around 1.90 Mg/m3 and at dif-ferent degrees of saturation. Samples have been then subjected to freezing paths up to -15 °C. Measurements of bulk EC along the temperature decrease and freezing paths have been used to calibrate parameters associ-ated with the proposed model. These calibrated models allow determining the amount of ice content for a given state of the partially saturated soil (porosity, initial degree of water saturation and temperature). The soil freezing retention curve has been also estimated by combining the Clausius-Clapeyron equation with water retention data on drying. A good agreement has been observed between the estimation based on EC measurements and results from water retention data, which validates the proposed procedure.Postprint (published version

Anisotropy effects on the undrained stability of cuts in clays

It has long been recognised that soils are generally anisotropic with some of their properties varying depending on the direction of measurement. In particular, strength anisotropy should be a main concern in relation with the stability assessment of geotechnical cases. Nevertheless, the incorporation of this feature is rarely considered in routine slope stability analyses in spite of the fact that its absence may lead, in some cases, to an overestimation of the factor of safety. In the paper, the effects of strength anisotropy on the undrained stability of cuts in clays are examined. A literature review reveals that clays have different patterns of undrained strength anisotropy depending on their overconsolidation ratio. A constitutive model is introduced, able to account for the different observed variations of undrained strength with loading direction. A series of numerical stability analyses have been performed to explore the effects of the type of undrained strength anisotropy on the stability of cuts. The effects of slope inclination, bedding orientation, and pattern of undrained strength anisotropy on factors of safety and failure mechanisms are examined and discussed. In addition, a real case study of the failure of an underwater cut is analysed showing that undrained strength anisotropy is able to explain satisfactorily the collapse observed.Postprint (published version

Modelos elasto-plásticos de estado crítico para análisis numéricos de problemas geotécnicos. I- Formulación básica y principales modificaciones

Los modelos de estado crítico han tenido una importancia capital en la aplicación de la elastoplasticidad a la caracterización del comportamiento mecánico de materiales geotécnicos. En esta serie de dos artículos se revisa el estado actual de desarrollo de estos modelos y su aplicación a análisis numéricos de problemas geotécnicos. El primer artículo se inicia con una breve nota sobre el desarrollo histórico de los modelos pasando a continuación a describir, de forma resumida, la formulación básica y sus principales consecuencias. El artículo se completa con una descripción y breve discusión de las principales modificaciones propuestas a la formulación básica, en especial aquellas más frecuentemente utilizadas en análisis numéricos. Los temas seleccionados para ser objeto de una atención especial son: superficie de fluencia en el lado supercrítico, superficie de fluencia para suelos consolidados sin deformación lateral, componente elástica del modelo, comportamiento plástico en el interior de la superficie de fluencia principal y formulación en el plano desviador. El segundo artículo de la serie tratará de la implementación numérica de estos modelos y ofrecerá ejemplos de aplicación.Peer Reviewe

Numerical implementation of an elastoplastic model for unsaturated soils

This paper describes some issues related to the numerical implementation of a constitutive model for unsaturated soils based on the BBM [1]. The focus of the paper is on the stress variables used and on the numerical algorithms adopted. Conventional stress variable approach (net stress and suction) as well as the approach that takes into account the degree of saturation (Bishop’s stress and suction) are examined. To solve the constitutive stress–strain equations, two stress integration procedures have been implemented, an explicit stress integration scheme with automatic substepping and error control techniques [2] and a fully implicit stress integration scheme based on the Backward-Euler algorithm with substepping [3]. Their performances during the integration of the constitutive laws are compared

Triaxial tests on frozen ground: formulation and modelling

Artificial Ground Freezing (AGF) is a controllable process that can be used by engineers to stabilise temporarily the ground, provide structural support and/or exclude groundwater from an excavation until construction of the final lining provides permanent stability and water tightness. In this work, the process of ground freezing is studied using a constitutive model that encompasses frozen and unfrozen behaviour within a unified effective-stress-based framework and employs a combination of ice pressure, liquid water pressure and total stress as state variables. The parameters of the constitutive model are calibrated against experimental data obtained from samples retrieved during construction of Napoli underground, in which AGF was extensively used to excavate in granular soils and weak fractured rock below the ground water table

Elastoplastic modelling of a foundation on an unsaturated soil

This paper presents a coupled flow-deformation finite element analysis of a shallow foundation on an unsaturated loosely compacted silt subjected to variations in the water level. The behaviour of the silt foundation was simulated using the Barcelona Basic Model (BBM) which was implemented into the PLAXIS finite element code. Material parameters were calibrated from laboratory tests reported in the literature. The influence of partial soil saturation and of fluctuations of the groundwater level on the behaviour of footing is investigated

Report on THM/THMC model improvement

Deliverable 10Preprin

Capabilities and requirements of numerical models

Deliverable 11Preprin