FG Bodenmechanik, Grundbau, Geotechnik
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- Boden (1)
- Constitutive modelling (1)
- Druckluft (1)
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The successful simulation of the behaviour of expansive soils using FEM strongly depends on the acting hydromechanical processes and their proper consideration. During swelling processes, part of the pore water is bound to the solid.
However, the associated change of volume fractions in the porous medium and its contribution to the overall material behaviour is not accounted for in most numerical frameworks. A numerical framework based on the theory of porous media and a novel
constitutive model is proposed. The theory of porous media includes a distinct swelling water phase that does not participate in the flow through the pore space. A bidirectional mass transport enables the transition from free to swelling water during change
in water content. The reduction of the effective pore space is considered at the finite element level. By consistent balancing in the governing equations, the swelling induced reduction of hydraulic conductivity is directly accounted for. The performance of
the proposed framework is demonstrated by back calculation of laboratory tests on different materials. Its capabilities to simulate complex loading paths in (un)saturated conditions with a single set of parameters are shown.
Advanced constitutive models are required for realistic prediction of the seismic behavior of geotechnical structures, such as dams and embankments, under undrained alternating loading conditions. The constitutive models should be able to capture accurately two important aspects related to the boundary value problem: the influence of a non-zero initial static shear stress and the multi-directional nature of the earthquake loading. Experimental studies show that the deformation characteristics of granular materials under undrained cyclic loading conditions depend on the initial static shear stress, initial void ratio and loading direction. In this paper, two advanced constitutive models from the hypoplasticity family are compared with each other and with experiments regarding to their ability to account for the influence of the initial static shear stress on liquefaction.
Infrastructure in permafrost landscapes affects the ground thermal regime and may enhance permafrost degradation. Stability and usability of the infrastructure depend likewise on the thermal conditions of the subsurface. Permafrost in the study area in Ilulissat (West Greenland) is typically warm and often ice-rich, so infrastructure is likely to accelerate thawing of the ground. Numerical models can be used to estimate the influence of infrastructure on permafrost development, subsidence, and ponding near roads. In this study, we investigated the influence of roads in Ilulissat using the one-dimensional, laterally coupled heat-transfer model CryoGrid. We used ERA5 reanalysis data (1980-2020) and projections according to the SSP5-8.5 scenario (2020-2100) as climate forcing and validated them with site data from the local weather station (1991-2021). Borehole data from Ilulissat and the north-eastern surroundings provide information on soil stratigraphy properties to first built the model for the natural, undisturbed tundra. In the second model set up, we added the elevated gravel embankment as linear infrastructure to the model with several tiles for the roadway and the shoulder. In a third model run, we quantified the effect of snow by changing the snow distribution near the road to imitate plowing. We utilized measured data from the weather station and iButtons for validation of the model runs. Our simulation show that the construction of a road had a direct influence on the thermal regime by warming the soil. A general deepening of the active layer is projected within the current century, which is supported by borehole observations.
Likewise, the associated measures of maintenance such as accumulation of cleared snow affect the response of the soil, as an insulator of the embankment, preventing stored heat from escaping. As the current state permafrost is vulnerable to climate change, it is important to rely on models to provide realistic projections.
Die Standsicherheit von nicht-ebenen Schlitzwand-Ecklamellen in weichem, normalkonsolidiertem bindigen Boden und die Setzungen von benachbarten Fundamenten infolge des Herstellungsvorgangs einer Schlitzwand wurden mit Hilfe der FE-Methode numerisch untersucht. Um das K0−anisotrope Verhalten des Bodens besser simulieren zu können, wurde eine Erweiterung der Visko-Hypoplastizität entwickelt und in einer FE-kompatiblen Subroutine implizit implementiert. Die neue anisotrope Visko-Hypoplastizität ist in der Lage, die Ratenabhängigkeit, das Kriechen, die Relaxation, die Anisotropie, und die Akkumulation des Porenwasserdruckes infolge undrainierter Deformationszyklen wiederzugeben. Bezüglich der Standsicherheit wurde der unter Suspension gestützte Aushub des Eckschlitzes und das Verhalten des Schlitzes bis zum Bruchzustand simuliert. Der dadurch entstandene Bruchkörper hat die Form eines vertikalen Viertelzylinders, der von seiner Unterseite her von einer geneigten Ebene begrenzt wird. Im Vergleich mit den in Deutschland üblichen verwendeten analytischen Verfahren zur Berechnung der Standsicherheit, die normalerweise von einem Erdkeilmodell ausgehen, ist der Mechanismus für den Eckschlitz mit dem Viertelzylinder kritischer. Dieses Ergebnis spricht für die Entwicklung eines analytischen Verfahrens, das auf diesem Mechanismus basiert. Die Simulationen der Gebrauchstauglichkeit von benachbarten Fundamenten infolge der Herstellung von Eckschlitzwänden zeigen, dass die Setzungen stark von der Abwicklungslänge des Eckschlitzes abhängen und wenig vom Verhältnis der Schenkellängen L1 /L2 . Zusätzlich nehmen die Setzungen mit der Gebrauchslast des Fundamentes zu. Für die gleiche Gebrauchslast nimmt die Setzung mit dem Abstand des Fundamentes zum Schlitz stark ab. Sowohl die Standsicherheit des offenen Schlitzes als auch die Setzungen des benachbarten Fundamentes hängen stark von der Zeit ab. Ein schnellerer Aushub des Schlitzes und eine schnellere Betonierung der Wand führen zu größeren Standsicherheiten und kleineren Setzungen des Fundamentes, als eine langsamere Vorgehensweise.
The neohypoplastic model [12] eliminates several shortcomings of the hypoplastic vW-HP model [17]. The most important improvements are presented here. The general form of the constitutive equation is slightly modified by two additional terms in brackets. The functions and are completely reformulated in order to deal with numerous problems of the old model: perpetuum mobile of the second kind, too small dilatancy during triaxial tension paths, poor predictions of peak stress and inconsistent behaviour of the model at the upper density limit. A new state variable similar to the one from the Sanisand model [3] is introduced. The hitherto used state variables: the stress and the void ratio are preserved, of course. Moreover, a new kind of nonlinearity is proposed: the rotation of the deviatoric stress response. The problems connected to the extension intergranular-strain extension are not discussed here.
Einige Böden neigen aufgrund ihrer tonmineralogischen Zusammensetzung bei Kontakt mit Wasser zur Volumenänderung, welche sich phänomenologisch entweder als Expansion oder durch eine Quelldruckentwicklung äußert. In der Baupraxis müssen sowohl die Quellphänomene als auch das hydromechanisch gekoppelte Materialverhalten allgemein z.B. bei der Bemessung von Tunnelbauwerken oder Abdichtungen berücksichtigt werden. Existierende empirische Modelle sind für komplexe Randbedingungen oft unzureichend, doch die Anwendung höherwertiger mechanischer Stoffmodelle stellt aufgrund komplexer Modellformulierungen und meist aufwendiger Bestimmung der benötigten Materialparameter eine große Hürde dar. In diesem Aufsatz wird ein von den Autor:innen kürzlich vorgestelltes neuartiges hypoplastisches Stoffmodell für expansive Böden durch die Anwendung auf weitere experimentelle Studien validiert. Zur Erleichterung der Anwendung des Stoffmodells in der Praxis wird eine vereinfachte Parameterkalibrierung erläutert, die keine aufwendige experimentelle Untersuchung erfordert.
Earthquake stability assessments of large opencast mine slopes (dimensions exceeding several hundred meters) are complex and non-linear problems, often addressed using pseudo-static approaches that neglect material-induced failures and the role of pore-fluids. In this study, a numerical approach is used to understand the dynamic response of saturated and unsaturated soils. Since the requirements for such simulations are often not yet met by commercial software packages, user-defined finite elements and user-defined material models have been implemented in Abaqus/Standard. To account for the large depth of the finite element model, a scaling procedure of the system of equations is proposed to purge the influence of initial stress. The constitutive model parameters are calibrated based on laboratory tests and by back calculation of downhole measurements. Large-scale fully coupled finite element simulations are performed to study the response of a flooded opencast mine under earthquake loading. The present work illustrates the importance of the pore-fluids treated as independent phases in the context of seismic analysis of slopes. The simulations show strong wave diffraction effects for inhomogeneous dump structures, resulting in smaller displacements in near-surface areas of the slope. Furthermore, it was found that large areas of the dump show a (significant) temporary decrease of effective mean pressure.
A simple, yet versatile yield surface in the stress space is
combined with a hypoplastic equation to simulate the influence of recent deformation history on the mechanical behaviour of sand. This yield surface is used to describe the intensity of anelastic flow. In the model, the state is fully described by the current stress, the void ratio and a novel back stress-like tensor. This new state variable determines the shape and size of the yield surface and accounts for recent deformation/stress history. The direction of the anelastic flow upon shearing is obtained from a generalization of the Taylor’s dilatancy rule. As a distinctive feature of the model, this dilatancy is able to reproduce the strong contractancy upon reversals observed in experiments without the need of additional state variables. The model corrects some known shortcomings of previous
hypoplastic models like overshooting and the excessive accumulation of
stress/strain upon strain/stress cycles of small amplitude (ratcheting). Laboratory tests are simulated to show the capabilities of the model to reproduce the soil behaviour under monotonic and cyclic loading conditions after different deformation histories.
Apart from time-driven creep or relaxation, most viscoplastic models (without plastic and viscous strain separation) generate no or a very limited accumulation of strain or stress due to cyclic loading. Such pseudo-relaxation (or pseudo-creep) is either absent or dwindles too fast with increasing OCR. For example, the accumulation of the pore water pressure and eventual liquefaction due to cyclic loading cannot be adequately reproduced. The proposed combination of a viscous model and a hypoplastic model can circumvent this problem. The novel visco-hypoplasticity model presented in the paper is based on an anisotropic preconsolidation surface. It can distinguish between the undrained strength upon triaxial vertical loading and horizontal loading. The strain-induced anisotropy is described using a second-order structure tensor. The implicit time integration with the consistent Jacobian matrix is presented. For the tensorial manipulation including numerous Fréchet derivatives, a special package has been developed within the algebra program MATHEMATICA (registered trade mark of Wolfram Research Inc.). The results can be conveniently coded using a special FORTRAN 90 module for tensorial operations. Simulations of element tests from biaxial apparatus and FE calculations are also shown.
Paraelasticity
(2011)
A 3D hysteretic, fully reversible constitutive model with rate-independent damping and with variable secant stiffness is proposed. A reversible dilatancy–contractancy effect is an optional feature as described in the companion paper (Niemunis et al. in Acta Geotech, 2011). The present paper describes the basic aspects of the model only. A strain path reversal is defined, and a treatment of the past reversals using a stack structure is proposed.