Numerical modeling of progressive failure and its application to spreads in sensitive clays

Les étalements sont de grands glissements de terrain survenant dans les argiles sensibles. Les méthodes d’analyse de la stabilité utilisant la méthode à l’équilibre limite donnent des coefficients de sécurité élevés et ne peuvent s’appliquer à ces glissements. Le mécanisme de rupture progressive expliquerait l’initiation et la propagation de la surface de rupture et la dislocation du sol en horst et en grabens, typiques aux étalements. Une méthode numérique est présentée afin d’identifier les paramètres influençant la rupture progressive et de valider son application aux étalements. Cette méthode évalue les contraintes présentes initialement dans le talus et modélise l’initiation et la propagation de la rupture progressive. Il est démontré que les hautes pentes, fortement inclinées ayant un coefficient de pression des terres au repos élevé sont susceptibles à la rupture progressive et que la surface de rupture se propage sur une grande distance. La rupture est favorisée par un sol ayant une grande fragilité lors du cisaillement. Une faible résistance à grande-déformation du sol favorise une grande distance de propagation. Les argiles de l’est du Canada, pouvant présenter une forte sensibilité et une grande fragilité lors du cisaillement, sont donc susceptibles à la rupture progressive et celle-ci explique l’occurrence d’étalements dans ces sols.Spreads are a type of large landslide occurring in sensitive clays. Stability analyses using the limit equilibrium method give factors of safety that are too large and are therefore not applicable to this type of landslide. The progressive failure mechanism is believed to explain the initiation and propagation of the failure surface and the dislocation of the soil mass in horsts and grabens, typical of spreads. A numerical method is presented to identify the parameters influencing progressive failure and to validate the application of this mechanism to spreads. The method evaluates the stresses acting in the slope before failure and models the initiation and propagation of the progressive failure. It is demonstrated that high, steep slopes, with a large earth pressure ratio at rest, are more susceptible to progressive failure and the failure surface propagates over a large distance. Failure is more likely to occur when soil with high brittleness is involved. Soil with low strength at large deformation induces failure propagation over a larger distance. Eastern Canadian clays can exhibit high sensitivity and large brittleness during shear and are susceptible to progressive failure, which explains the occurrence of spreads in these soils

Finite Element Modeling of the Las Colinas Landslide Under Earthquake Shaking

The Las Colinas landslide that occurred at Santa Tecla (El Salvador, Central America) due to the earthquake of 13 January 2001 is considered as one of the most destructive landslides ever known. This paper studies the ability of Hill’s sufficient condition of stability (1958), which is based on the sign of second-order work, for predicting and describing this catastrophic massive landslide. The general expressions of both local and global second-order work criteria and its implementation into finite element codes are given. By using the non-associated elasto-plastic Hardening Soil Model and the local second-order work criterion, it is demonstrated that potentially unstable stress-strain states can occur strictly inside the Mohr-Coulomb failure surface in axisymmetric conditions. The Las Colinas landslide under earthquake shaking is simulated in plane strain conditions, using the pseudo-static method as loading variable with the non-associated Hardening Soil Model available in the finite element code PLAXIS. The location of the zone of negative values of the local second-order work makes it possible to successfully exhibit the landslides mechanism observed on the Las Colinas slope. Moreover, the comparison with the safety factors calculated by using the methods of slices and the shear strength reduction technique confirms that the global second-order work is a more pertinent indicator for predicting the global stability of the Las Colinas slope

Accounting for effects of cyclic loading in design of offshore wind turbine foundations

In the design of foundations for offshore structures, it is generally important and required to consider the effects of cyclic loading caused by waves and wind. Therefore, NGI has developed a framework since the early 1970s that has been utilized in the design of various offshore structures. However, there are currently no accepted guidelines on how to account for this effect in design of monopile foundations. With the introduction of large-diameter monopile foundations for offshore wind turbines, it has become necessary to revisit and modify our existing procedure. The primary difference between monopile foundations for offshore wind turbines and those for the oil and gas industry is that the design of the former is typically not governed by a global failure mechanism during extreme storm loading because of their rather ductile response. Additionally, offshore wind parks often consist of over one hundred turbines, which necessitates more efficient design methods to optimise each individual foundation within a huge field with varying soil stratigraphy. This paper presents an efficient procedure using the finite element method to account for the effects of cyclic loading in design of monopile foundations. Furthermore, it provides recommendations for further improvements.publishedVersio

Effects of fines content in numerical simulation of CPTu in silty sands

The Cone Penetration Test (CPTu) does not measure soil properties directly; rather, it measures cone resistance, sleeve friction and pore water pressure, that correlated with laboratory test data, allow estimation of soil properties. These correlations work reasonably well in clean sand; however, they have been observed to provide poor estimation when fines are present. The aim of this work is to improve the estimation of fine contents in silty sands from CPTu measurements. Large deformation finite element analyses in ABAQUS implicit scheme with the "zipper technique" is used to simulate CPTu. The SANISAND model is used to capture the soil behaviour and account for fines content by altering the critical state line. Coupled consolidation analyses are performed to account for development of pore pressures during cone penetration. The influence of fines and drainage on the tip resistance and developed pore pressure is analysed. Existing field CPTu measurements would be back calculated using laboratory test data, to ascertain the accuracy of the developed framework. The results are analysed in tandem to improve the understanding of effect of fines content on CPTu response.publishedVersio

Cyclic Capacity of Monopiles in Sand under Partially Drained Conditions: A Numerical Approach

Cyclic loading of saturated sand under partially drained conditions may lead to accumulated strains, pore pressure buildup, and consequently reduced effective stress, stiffness, and shear strength. This will affect the ultimate limit state capacity of monopile foundations in sand for offshore wind turbines. This paper calculates the performance of large-diameter monopile foundations, which are installed in uniform dense sand, subjected to storm loading using the partially drained cyclic accumulation model (PDCAM). The simultaneous pore pressure accumulation and dissipation is accounted for by fully coupled pore water flow and stress equilibrium (consolidation) finite element analyses. Drainage and cyclic load effects on monopile behavior are studied by comparing the PDCAM simulation results with simulation results using a hardening soil model with small strain stiffness. At the end, a simplified procedure of PDCAM, named PDCAM-S, is proposed, and the results using this approach together with PLAXIS 3D and the NGI-ADP soil model are compared with the PDCAM results.Cyclic Capacity of Monopiles in Sand under Partially Drained Conditions: A Numerical ApproachacceptedVersio

On Common Research Needs for the Next Generation of Floating Support Structures

The world is facing several industrial and societal challenges, such as providing enough renewable energy and enough safe and healthy food as formulated in the United Nations sustainable development goals. Using floating stationary structures, the ocean can contribute to solving several of the challenges. New applications need new types of structures, with which we have limited experience. These support structures will be diverse, but also have essential research needs in common. Design of novel floating structures need reliable descriptions of the marine environment. This is particularly challenging for semi-sheltered coastal regions, with complex topography and bathymetry. Novel structures are likely to be compliant, modular and/or multi-body, requiring increased understanding and rational models for wave-structure interaction. Structures with sustainable, safe, and cost-efficient use of materials, including untraditional ones, must be developed. Smart, affordable, and reliable mooring systems and anchors for novel applications are necessary for station keeping. Digital solutions connecting the various stages of design and operation, as well as various design disciplines, researchers, and innovators, will be necessary. Sustainability will be an integral part of any new design. To unlock the potential of novel floating structures, we need to understand the requirements of the applications, as well as the associated technology gaps and knowledge and research needs. This paper highlights research needs for innovation within floating offshore wind, floating solar power plants, novel aquaculture structures, and coastal infrastructure.acceptedVersio