Directionally dependent strength and dilatancy behavior of soil–structure interfaces

Soil–structure interfaces typically exhibit a shear behavior that is independent of the direction of relative displacement due to symmetry in the solid material\u27s surface profile. This experimental study investigates the interface shear behavior of surfaces with asymmetric profiles inspired by the scales of snake skin. The results of shear box interface tests on two sandy soils indicate that the peak and residual interface shear strengths and dilatancy are greater when the soil is displaced against the sharp edges of the asperities (cranial direction) than when the soil is displaced along the asperities (caudal direction). The experimental results indicate that the effect of asperity geometry on the interface shear response can be captured with the ratio of asperity length to asperity height (L/H). Analysis of the stress–dilatancy behavior indicates that interfaces with a relatively short asperity length follow a classical flow rule developed for soils. However, the relationship between the mobilized stress ratio and the dilatancy rate is shown to be a function of the shearing direction and asperity geometry. Implementation of snake skin-inspired profiles on the surface of geotechnical structures may provide benefits in performance and efficiency during installation and service life. In general, the results of this study indicate the behavior of the soil-structure interfaces sheared in the cranial direction is similar to that of interfaces between soil and fully rough surfaces. In contrast, the behavior of the soil-structure interface sheared in the caudal direction shares characteristics with that of interfaces with smooth surfaces, including the mobilization of smaller a interface strength and dilation

Hypoplastische Modelle für Boden-Bauwerkskontakte: Modellierung und Implementierung

The consideration of interfaces is an important issue when modelling the holistic global structural behaviour of geotechnical engineering structures. The most prominent example is the shaft friction of axial loaded piles and anchors. In this thesis, a stochastic assessment scheme was proposed and applied. This scheme was modified to take into account the special considerations for the assessment of interface models. Based on the assessment and a state-of-the-art review, theoretical considerations were used and a novel scheme was developed. This scheme uses reformulated mathematical operators as well as reduced stress and strain rate tensors, based on existing constitutive equations, to model interfaces. Shear stress mobilization and the volumetric behaviour are predicted more accurately, and the bearing behaviour of frictional contacts can be modelled in a better way. By using the novel scheme, an older hypoplastic interface model for granular interfaces was enhanced, and three different hypoplastic clay interface models were proposed. In addition to the theoretical constitutive model formulation, an implementation method was developed. This allowed a user-friendly implementation of advanced constitutive models as interface models using the capabilities of a commercial finite element software package. This concept was exemplary applied to various 3D boundary value problems and the benefits of such advanced hypoplastic interface model are discussed.Das Kontaktverhalten von geotechnischen Strukturen ist wichtig zur Berechnung des ganzheitlichen Strukturverhaltens bei der Berücksichtigung von Boden-Bauwerks-Interaktion. Bekannte Beispiele dafüur sind axial belastete Pfähle und Anker. In dieser Arbeit wurden verschiedene existierende Modelle zur Modellierung von Boden-Bauwerks-Kontakten mit Hilfe eines stochastischen Ansatzes untersucht und bewertet. Anhand dieser Bewertung und dem Stand der Forschung wurde basierend auf theoretischen Überlegungen ein Methode entwickelt. Diese erlaubt es, die Kontaktflächen mittels modifizierter mathematischen Operatoren und reduzierten Spannungs und Dehnungstensoren, basierend auf existierenden Kontinuums-Modelle zu berechnen. Dies führt zu verbesserten Modellierung der Scherspannungs-Mobilisierung und des volumentrischen Verhaltens der Kontaktzone. Basierend auf dem neuen Konzept wurde ein existierendes, hypoplastisches Modell für granulare Kontaktreibung verbessert und drei unterschiedliche hypoplastische Modelle für das Kontaktverhalten von feinkörnigen Böden entwickelt. Alle Modelle wurden verifiziert und mit experimentellen Daten validiert. Zusätzlich zur Formulierung der theoretischen Modelle wurde ein Konzept erarbeitet, mit dem die entwickelten Modelle in eine Finite-Elemente Software implementiert werden konnten. Hierzu werden existierende Boden-Kontinuumsmodelle benutzt. Die Implementierung dieser Modelle wurde mittels unterschiedlicher Randwertprobleme erfolgreich validiert und die Vorteile der Modelle sowie des Implementierungskonzeptes diskutiert. Die theoretischen Überlegungen und das benutzerfreundliche Implementierungsschema wird die Zugänglichkeit dieser zukunftsweisenden Modelle für Ingenieure verbessern. Hierdurch können die Ergebnisse der Modellierungen und die experimentelle Beobachtungen angeglichen werden, was die Aussagefähigkeit von Finite-Elemente Analysen weiter verbessern wird

Experimental study on monotonic to high-cyclic behaviour of sand-silt mixtures

The naturally deposited soil usually does not consist of pure coarse or fine-grained soil but of a mixture of both. The mechanical behaviour of a saturated fine sand mixed with varying amounts of low-plastic fines was evaluated by monotonic as well as high-cyclic triaxial tests. The test results were used to conclude on the effect of fines content on the critical state, phase transformation line, secant Young’s modulus, the residual strain accumulation as well as strain amplitude during drained cycles of the mixtures in relation to the global void ratio as well as to the equivalent void ratio. It was found that while the choice of void ratio definition is important for the uniqueness of the critical void ratio, both approaches can be used as state variables for the phase transformation line. However, some seemingly contradictive results are found from the drained high-cyclic tests. Eventhough, an increase of the residual strain accumulation with decreasing fines content compared at the same initial equivalent void ratio is rendered by the laboratory data, a unique and on fines content independent relationship between eacc could be established only with respect to the initial global void ratio

Bio-inspired geotechnical engineering: Principles, current work, opportunities and challenges

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Bio-inspired geotechnical engineering: principles, current work, opportunities and challenges

A broad diversity of biological organisms and systems interact with soil in ways that facilitate their growth and survival. These interactions are made possible by strategies that enable organisms to accomplish functions that can be analogous to those required in geotechnical engineering systems. Examples include anchorage in soft and weak ground, penetration into hard and stiff subsurface materials and movement in loose sand. Since the biological strategies have been ‘vetted’ by the process of natural selection, and the functions they accomplish are governed by the same physical laws in both the natural and engineered environments, they represent a unique source of principles and design ideas for addressing geotechnical challenges. Prior to implementation as engineering solutions, however, the differences in spatial and temporal scales and material properties between the biological environment and engineered system must be addressed. Current bio-inspired geotechnics research is addressing topics such as soil excavation and penetration, soil–structure interface shearing, load transfer between foundation and anchorage elements and soils, and mass and thermal transport, having gained inspiration from organisms such as worms, clams, ants, termites, fish, snakes and plant roots. This work highlights the potential benefits to both geotechnical engineering through new or improved solutions and biology through understanding of mechanisms as a result of cross-disciplinary interactions and collaborations