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.

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The paper discusses the mechanisms governing the shear strength along the inside of the skirt wall of suction anchors with and without stiffeners during and after installation by underpressure. Methods to calculate the shear strength along the skirt wall are proposed and used to calculate the shear strength after installation for a range of clays. The results are used to propose a simplified method to estimate the shear strength along the inside of the skirt wall after installation.

The design of OWTs relies on integrated load analyses tools that simulate the response of the entire OWT (including the rotor-nacelle assembly, support structure and foundation) under combined aerodynamic and hydrodynamic loading. Despite all efforts to develop accurate integrated models, these often fail to reproduce the measured natural frequencies, partly due to the current foundation modelling. This paper presents a new foundation model for integrated analyses of monopile-based OWTs. The model follows the macro-element approach, where the response of a pile and the surrounding soil is condensed to a force-displacement relation at seabed. The model formulation uses multi-surface plasticity and it reproduces key characteristics in monopile foundation behaviour that are not accounted for in current industry practice. The basic features of the model are described and its limitations are discussed. The performance of the macro-element model is compared against field test measurements and results from FEA. The comparison indicates that the macro-element model can reproduce accurately the non-linear load-displacement response and hysteretic behaviour measured in field tests and computed in FEA. This confirms that the model can simulate the pile and soil behaviour with the same level of accuracy as FEA, but with a considerable reduction in computational effort.

The paper describes the principles of skirted foundations and anchors in clay and their applicability for various types of offshore platforms and different types of loading. Procedures for penetration analyses and calculation of capacity are presented. It is argued that skirted anchors can carry both wave loads and large permanent pull-out loads. Introduction Skirted foundations and anchors have proven to be competitive alternatives to more traditional foundation solutions like piles and drag anchors in various types of soils and for a wide range of fixed and floating offshore platforms. Skirted foundations have been used for gravity platform jackets, jack-ups, subsea systems and seabed protection structures, and skirted anchors have been used for floaters (process barges, production vessels, loading buoys, storage buoys) and tension leg platforms. Skirted structures also have the potential of being used for several other purposes. Examples of a skirted anchor and a skirted jacket are shown in Figs. 1 and 2. Some main reasons for the success of skirted foundations and anchors are that they give potential for:significant cost savings compared to more traditional foundations and anchors. Skirted foundations and anchorsmay be cheaper to fabricate, need less expensive installation equipment, can be installed by controlled and simple marine operations, and require shorter offshore installation time.shorter anchor lines and accurate positioning of anchor. Skirted anchors have significant uplift load capacity, high positioning accuracy, and require no drag-in operation or proof loading. This reduces interference with mooring systems of other structures and with other platforms. It also makes them well suited for fibre rope applications.removal and reuse. Relocatable structures can be used atmore than one site and may make marginal fields profitable. Removal of the structure also provides a clean site after exploitation and accommodates environmental concerns. Principle of skirted foundation and anchor concept Geometry. Skirted foundations and anchors are normally cylindrical units made by steel or concrete. The cylinders are open at the bottom and closed by a cap at the top (Fig. 3). The term "skirted foundation" is used herein when the concept is applied for fixed platforms, and "skirted anchor" when it is pplied for floaters. Each foundation or anchor may consist of a single cylinder or several attached cylinders (Fig. 4). Installation. Installation of skirted foundations and anchors are based on the principle that they penetrate partly into the soil under weight. Further penetration is achieved by pumping water out from the top of the cylinder, creating an underpressure inside the cylinder. The difference between the hydrostatic water pressure outside the cylinder and the reduced inside water pressure gives a differential pressure that acts as a penetration force in addition to the weight (Fig. 3). Short term capacity. After penetration, the water outlet is normally closed, and skirted foundations and anchors may achieve substantial capacity, both for vertical downward loads, horizontal loads, vertical uplift loads, moments, and combinations of these loads. The capacity to carry wave loads is in clays governed by an undrained shear failure in the soil, and the capacity depends on depth of skirt penetration, cylinder diameter, soil strength and the combination of horizontal, vertical and moment loads.

Many geotechnical problems involve undrained behavior of clay and the capacity in undrained loading. Most constitutive models used today are effective stress based and only indirectly obtain values for the undrained shear strength. To match the design profiles of undrained shear strengths, in active (A), direct simple shear (D) and passive (P) modes of loading are complicated. This paper presents the elastoplastic constitutive model NGI-ADP which is based on the undrained shear strength approach with direct input of shear strengths. Consequently, exact match with design undrained shear strengths profiles is obtained and the well-known anisotropy of undrained shear strength and stiffness is accounted for in the constitutive model. A non-linear stress path-dependent hardening relationship is used, defined from direct input of failure strains in the three directions of shearing represented by triaxial compression, direct simple shear and triaxial extension. With its clear input parameters the model has significant advantages for design analysis of undrained problems. The constitutive model is implemented, into finite element codes, with an implicit integration scheme. Its performance is demonstrated by a finite element analysis of a bearing capacity problem.

Abstract:In 1994, a landslide occurred in the municipality of Sainte-Monique, Quebec. The debris of the landslide had graben and host shapes, typical of spreads in sensitive clays. The geotechnical investigation shows that the soil involved is a firm to stiff, sensitive, nearly normally consolidated grey silty clay of high plasticity. This soil exhibits a high sensitivity and a high brittleness during shear and is therefore susceptible to progressive failure. Traditional stability analysis cannot explain this landslide. This gives the opportunity to examine the applicability of progressive failure analysis to this spread. Using the finite elements method, it is demonstrated that the initiation and observed extent of the failure surface are explained by a soil having high brittleness during shear and a large-deformation shear strength close to the remoulded shear strength of the soil. The dislocation of the soil mass can also be explained by the active failure occurring in the soil mass above the failure surface during or shortly after failure propagation. It is therefore numerically demonstrated that progressive failure explains the initiation and the extent of the failure surface of this spread.Key words: progressive failure, spread, sensitive clay, large-deformation shear strength, brittleness.Résumé : Un glissement de terrain est survenu en 1994 dans la municipalité de Sainte-Monique, Québec. Les débris présentaient des formes de horsts et grabens typiques des étalements dans les argiles sensibles. L'investigation géotechnique démontre que le sol impliqué dans ce glissement est une argile silteuse grise normalement consolidée, sensible, de forte plasticité et ayant une consistance ferme à raide. Ce sol présente une sensibilité et une fragilité lors du cisaillement élevées et peut donc être susceptible à la rupture progressive. Les analyses de stabilité traditionnelles n'arrivent pas à expliquer ce glissement. Ceci offre donc l'opportunité d'examiner l'application du concept de la rupture progressive sur cet étalement. À l'aide de programmes d'éléments finis, il est démontré que l'initiation et l'étendue de la surface de rupture observée peuvent être expliquées par un sol ayant une grande fragilité lors du cisaillement et une résistance à grandes déformations près de la résistance du sol remanié. La dislocation de la masse de sol en horsts et grabens est expliquée par la rupture active survenant dans la masse de sol au-dessus de la surface de rupture pendant ou peu après la propagation de la rupture. Il est donc démontré numériquement que la rupture progressive explique l'initiation et l'étendue de la surface de rupture d'un étalement.

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