@misc{NiemunisPradaSarmientoGrandasTavera, author = {Niemunis, Andrzej and Prada-Sarmiento, L. F. and Grandas Tavera, Carlos Eduardo}, title = {Paraelasticity}, series = {Acta Geotechnica}, volume = {6}, journal = {Acta Geotechnica}, number = {2}, issn = {1861-1125}, doi = {10.1007/s11440-011-0137-4}, pages = {67 -- 80}, abstract = {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.}, language = {en} } @misc{NiemunisGrandasTaveraPradaSarmiento, author = {Niemunis, Andrzej and Grandas Tavera, Carlos Eduardo and Prada-Sarmiento, L. F.}, title = {Anisotropic visco-hypoplasticity}, series = {Acta Geotechnica}, volume = {4}, journal = {Acta Geotechnica}, number = {4}, issn = {1861-1133}, doi = {10.1007/s11440-009-0106-3}, pages = {293 -- 314}, abstract = {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 Fr{\´e}chet 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.}, language = {en} } @incollection{GrandasTaveraTriantafyllidisKnittel, author = {Grandas Tavera, Carlos Eduardo and Triantafyllidis, Theodoros and Knittel, Lukas}, title = {A Constitutive Model with a Historiotropic Yield Surface for Sands}, series = {Recent developments of soil mechanics and geotechnics in theory and practice}, booktitle = {Recent developments of soil mechanics and geotechnics in theory and practice}, editor = {Triantafyllidis, Theodoros}, publisher = {Springer}, address = {Cham}, isbn = {978-3-030-28515-9}, pages = {13 -- 43}, abstract = {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.}, language = {en} } @incollection{MachačekGrandasTaveraTriantafyllidis, author = {Machaček, Jan and Grandas Tavera, Carlos Eduardo and Triantafyllidis, Theodoros}, title = {Coupled Numerical Simulations for Seismic Hazard Assessment of Large Opencast Slopes}, series = {Recent developments of soil mechanics and geotechnics in theory and practic}, booktitle = {Recent developments of soil mechanics and geotechnics in theory and practic}, editor = {Triantafyllidis, Theodoros}, publisher = {Springer}, address = {Cham}, isbn = {978-3-030-28515-9}, pages = {266}, abstract = {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.}, language = {en} } @incollection{NiemunisGrandasTavera, author = {Niemunis, Andrzej and Grandas Tavera, Carlos Eduardo}, title = {Essential Concepts of Neohypoplasticity}, series = {Desiderata Geotechnica}, booktitle = {Desiderata Geotechnica}, editor = {Wu, Wei}, publisher = {Springer}, address = {Cham}, isbn = {978-3-030-14986-4}, doi = {10.1007/978-3-030-14987-1}, pages = {132 -- 142}, abstract = {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.}, language = {en} } @incollection{NiemunisGrandasTavera, author = {Niemunis, Andrzej and Grandas Tavera, Carlos Eduardo}, title = {Computer Aided Calibration, Benchmarking and Check-Up of Constitutive Models for Soils. Some Con� clusions for Neohypoplasticity}, series = {Holistic Simulation of Geotechnical Installation Processes}, booktitle = {Holistic Simulation of Geotechnical Installation Processes}, publisher = {Springer International Publishing}, address = {Karlsruhe}, doi = {10.1007/978-3-319-52590-7_7}, pages = {168 -- 192}, abstract = {A computer aided calibration, benchmarking and testing of constitutive models is presented. A large collection of test data for Karlsruhe sand is used together with the neohypoplastic model as an example. The essential feature of the procedure is an automatic evaluation of the discrepancies between a test and its simulation. They can be minimized by modifications of material constants or used as a benchmark for different models. Apart from the curve-fitting part one may check whether a constitutive model violates the Second Law. For this purpose several specially designed stress loops are tried out. A shake down of all state variables is established. The neohypoplasticity and the barotropic hypoelasticity are used as examples in the thermodynamic tests. In conclusion several modification to neohypoplastic models are presented.}, language = {en} } @incollection{NiemunisGrandasTaveraWichtmann, author = {Niemunis, Andrzej and Grandas Tavera, Carlos Eduardo and Wichtmann, Torsten}, title = {Peak Stress Obliquity in Drained and Undrained Sands. Simulations with Neohypoplasticity}, series = {Holistic Simulation of Geotechnical Installation Processes}, booktitle = {Holistic Simulation of Geotechnical Installation Processes}, publisher = {Springer International Publishing}, address = {Karlsruhe}, doi = {10.1007/978-3-319-23159-4_5}, pages = {5 -- 114}, abstract = {The difference between undrained and drained peak friction angle is considerable (up tp 10^ ∘ 10∘), despite identical densities and pressures (at peak). This cannot be explained using the elastoplastic formalism. An attempt is made to describe this effect with neohypoplasticity. For this purpose two types of nonlinearity are used, the well-known term Y m_ ij ‖ ̇ ϵ ‖ Y mij‖ ϵ˙‖ and the novel skew-symmetric correction tensor which is added to the elastic stiffness.}, language = {en} } @incollection{OsinovChrisopoulosGrandasTavera, author = {Osinov, V. A. and Chrisopoulos, S. and Grandas Tavera, Carlos Eduardo}, title = {Vibration-induced stress changes in saturated soil: a high-cycle problem}, series = {Holistic Simulation of Geotechnical Installation Processes}, booktitle = {Holistic Simulation of Geotechnical Installation Processes}, publisher = {Springer International Publishing}, address = {Karlsruhe}, doi = {10.1007/978-3-319-23159-4_4}, pages = {69 -- 84}, abstract = {Numerical modelling of the cyclic deformation of soil can be performed with the use of two types of constitutive models: incremental plasticity models and so-called explicit cyclic models. The present study deals with the application of a model of the second type—the high-cycle accumulation model for sand—to the analysis of vibration-induced stress changes in saturated soil. The application of the model requires the concurrent solution of two coupled boundary value problems: a dynamic problem for the determination of strain amplitude and a quasi-static problem for the calculation of stress evolution. The coupling of the two problems has been implemented in a two-dimensional axisymmetric formulation with the finite-element program Abaqus. The model is applied to the analysis of stress changes near a concrete wall caused by a vibrating pile. The numerical results show that a large-amplitude vibration, e.g. during the installation of vibratory driven piles, may substantially reduce both the effective and the total stresses in front of the wall. The effective stress may be reduced to zero resulting in soil liquefaction.}, language = {en} } @incollection{OsinovGrandasTavera, author = {Osinov, V. A. and Grandas Tavera, Carlos Eduardo}, title = {A Numerical Approach to the Solution of Dynamic Boundary Value Problems for Fluid-Saturated Solids}, series = {Holistic Simulation of Geotechnical Installation Processes}, booktitle = {Holistic Simulation of Geotechnical Installation Processes}, publisher = {Springer International Publishing}, address = {Karlsruhe}, doi = {10.1007/978-3-319-18170-7_8}, pages = {149 -- 162}, abstract = {A dynamic boundary value problem for a fluid-saturated solid can be represented as two coupled boundary value problems for one-phase media. This allows us to solve the problem with a commercial computer program without a built-in procedure for the solution of dynamic problems with non-zero permeability, provided that the user is able to establish the required coupling between the two problems. This approach has been implemented in the present paper with the computer program Abaqus/Standard using the dynamic analysis for one-phase media as a built-in procedure without the need to construct a user-defined finite element.}, language = {en} } @techreport{GrandasTavera, author = {Grandas Tavera, Carlos Eduardo}, title = {Einfluss der Anisotropie von weichem Ton beim Grenzzustand von Baugrubenecken und der Gebrauchstauglichkeit von Verbauw{\"a}nden}, abstract = {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 k{\"o}nnen, wurde eine Erweiterung der Visko-Hypoplastizit{\"a}t entwickelt und in einer FE-kompatiblen Subroutine implizit implementiert. Die neue anisotrope Visko-Hypoplastizit{\"a}t ist in der Lage, die Ratenabh{\"a}ngigkeit, das Kriechen, die Relaxation, die Anisotropie, und die Akkumulation des Porenwasserdruckes infolge undrainierter Deformationszyklen wiederzugeben. Bez{\"u}glich der Standsicherheit wurde der unter Suspension gest{\"u}tzte Aushub des Eckschlitzes und das Verhalten des Schlitzes bis zum Bruchzustand simuliert. Der dadurch entstandene Bruchk{\"o}rper hat die Form eines vertikalen Viertelzylinders, der von seiner Unterseite her von einer geneigten Ebene begrenzt wird. Im Vergleich mit den in Deutschland {\"u}blichen verwendeten analytischen Verfahren zur Berechnung der Standsicherheit, die normalerweise von einem Erdkeilmodell ausgehen, ist der Mechanismus f{\"u}r den Eckschlitz mit dem Viertelzylinder kritischer. Dieses Ergebnis spricht f{\"u}r die Entwicklung eines analytischen Verfahrens, das auf diesem Mechanismus basiert. Die Simulationen der Gebrauchstauglichkeit von benachbarten Fundamenten infolge der Herstellung von Eckschlitzw{\"a}nden zeigen, dass die Setzungen stark von der Abwicklungsl{\"a}nge des Eckschlitzes abh{\"a}ngen und wenig vom Verh{\"a}ltnis der Schenkell{\"a}ngen L1 /L2 . Zus{\"a}tzlich nehmen die Setzungen mit der Gebrauchslast des Fundamentes zu. F{\"u}r 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 h{\"a}ngen stark von der Zeit ab. Ein schnellerer Aushub des Schlitzes und eine schnellere Betonierung der Wand f{\"u}hren zu gr{\"o}ßeren Standsicherheiten und kleineren Setzungen des Fundamentes, als eine langsamere Vorgehensweise.}, language = {de} } @misc{NitschMachačekWichtmannetal., author = {Nitsch, Antonia and Machaček, Jan and Wichtmann, Torsten and Grandas Tavera, Carlos Eduardo}, title = {Validierung und Kalibrierung eines hypoplastischen Stoffmodells f{\"u}r expansive B{\"o}den}, series = {Bautechnik}, volume = {100}, journal = {Bautechnik}, number = {11}, issn = {0932-8351}, doi = {10.1002/bate.202300062}, pages = {689 -- 698}, abstract = {Einige B{\"o}den neigen aufgrund ihrer tonmineralogischen Zusammensetzung bei Kontakt mit Wasser zur Volumen{\"a}nderung, welche sich ph{\"a}nomenologisch entweder als Expansion oder durch eine Quelldruckentwicklung {\"a}ußert. In der Baupraxis m{\"u}ssen sowohl die Quellph{\"a}nomene als auch das hydromechanisch gekoppelte Materialverhalten allgemein z.B. bei der Bemessung von Tunnelbauwerken oder Abdichtungen ber{\"u}cksichtigt werden. Existierende empirische Modelle sind f{\"u}r komplexe Randbedingungen oft unzureichend, doch die Anwendung h{\"o}herwertiger mechanischer Stoffmodelle stellt aufgrund komplexer Modellformulierungen und meist aufwendiger Bestimmung der ben{\"o}tigten Materialparameter eine große H{\"u}rde dar. In diesem Aufsatz wird ein von den Autor:innen k{\"u}rzlich vorgestelltes neuartiges hypoplastisches Stoffmodell f{\"u}r expansive B{\"o}den durch die Anwendung auf weitere experimentelle Studien validiert. Zur Erleichterung der Anwendung des Stoffmodells in der Praxis wird eine vereinfachte Parameterkalibrierung erl{\"a}utert, die keine aufwendige experimentelle Untersuchung erfordert.}, language = {de} }