TY - GEN A1 - Nitsch, Antonia A1 - Leuthold, Jan A1 - Machaček, Jan A1 - Wichtmann, Torsten A1 - Grandas Tavera, Carlos Eduardo T1 - Experimental investigations of the stress-dependent swelling behavior of reconstituted claystone T2 - Fachsektionstage Geotechnik 2021: 24. Symposium Felsmechanik und Tunnelbau N2 - An experimental campaign giving a holistic overview on the hydro-mechanically coupled behavior of Opalinus Clay shale is presented. Oedometric swelling tests show a pronounced stress pathdependent swelling behavior that is governed by the local permeability and the mechanical preloading stress. A linear coupling between deformation and volumetric water content can be established during initial saturation that causes a swelling-induced reduction of the hydraulic permeability. Swelling deformation and pressure result in the mechanical degradation of the material with a reduction of preloading stress and elastic stiffness. Based on the experimental results, fundamental characteristics of the material behavior are derived and form the basis for the future development of a new constitutive model that is able to predict the hydro-mechanically coupled material response Y1 - 2021 UR - https://www.researchgate.net/profile/Antonia-Nitsch/publication/353039348_Experimental_investigations_of_the_stress-dependent_swelling_behavior_of_reconstituted_claystone/links/60e583454585156c95e89477/Experimental-investigations-of-the-stress-dependent-swelling-behavior-of-reconstituted-claystone.pdf ER - TY - GEN A1 - Grandas Tavera, Carlos Eduardo A1 - Wichtmann, Torsten A1 - Machaček, Jan A1 - Nitsch, Antonia ED - Zdravkovic, Lidija ED - Kontoe, Stavroula ED - Tsiampousi, Aikaterini ED - Taborda, David T1 - An extended theory of porous media for expansive soils T2 - 10th European Conference on Numerical Methods in Geotechnical Engineering (NUMGE2023) N2 - 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. KW - Swelling; Expansive soils; Partial saturation; Hydro-mechanical coupling; Porous Media Y1 - 2023 U6 - https://doi.org/10.53243/NUMGE2023-379 ER - TY - GEN A1 - Nitsch, Antonia A1 - Leuthold, Julia A1 - Machaček, Jan A1 - Grandas Tavera, Carlos Eduardo T1 - Experimental investigations on hydro‑mechanical processes in reconstituted clay shale and their significance for constitutive modelling T2 - Rock Mechanics and Rock Engineering N2 - The hydro-mechanical processes of a reconstituted clay shale (Opalinus Clay) were studied and implications for constitutive modelling of swellable clay shales derived. The study aims to provide further insights into these coupled processes. For that, oedometric swelling tests with varying boundary conditions and loading paths, oedometric compression tests under dry and saturated conditions, and permeability tests were performed. The results showed a unique preloading line for the dry state when plotting the void ratio against the vertical stress. For saturated states, this line was shifted to the left (to smaller stresses) which revealed a severe reduction of preloading stress due to saturation. All loading paths converged in a normal compression line (NCL) regardless of the mechanical boundary conditions during saturation. A trend line affine to the NCL was established for swelling pressures at different initial void ratios. The influence of saturation on the mechanical properties became further evident, as the un-/reloading stiffness was significantly reduced. A linear relationship between volumetric deformation and saturation was found. A severe swelling induced reduction of permeability in the saturated state was demonstrated and confirmed by the analysis of consolidation processes during oedometric loading. The experimental findings were used to derive fundamental assumptions for constitutive modelling and were discussed within a numerical framework for expansive soils. KW - Hydro-mechanical behaviour KW - Swelling KW - Oedometric compression KW - Permeability KW - Constitutive modelling Y1 - 2023 U6 - https://doi.org/10.1007/s00603-022-03202-1 SN - 1434-453X SN - 0723-2632 VL - 56 IS - 4 SP - 2457 EP - 2478 ER - TY - GEN A1 - Grandas Tavera, Carlos Eduardo A1 - Nitsch, Antonia A1 - Machaček, Jan T1 - A 1D Hypoplastic constitutive model for expansive soils T2 - Acta Geotechnica N2 - This paper presents a simple hypoplastic constitutive model that describes the essential features of the material behaviour of partially saturated clayey soils observed in oedometric compression tests. The model is formulated in terms of net stress and degree of saturation. The total strain rate is decomposed into a portion related to the changes in saturation and a portion for the evolution of net stress. However, no distinction is made between plastic and elastic strains. With this strain rate decomposition, the maximum swelling strain/stress are obtained by simulating wetting processes under constant stress/strain conditions. In addition to the void ratio, the model includes two scalar variables to track the loading history (preloading). The calibration of the model constants using common laboratory tests is discussed. Confined and unconfined swelling tests under oedometric conditions with subsequent loading and unloading phases carried out on three different materials were satisfactorily simulated by the model. Its promising results call for an extension to a 3D formulation. Y1 - 2022 U6 - https://doi.org/10.1007/s11440-022-01548-y SN - 1861-1133 SN - 1861-1125 VL - 17 SP - 4313 EP - 4331 ER - TY - GEN A1 - Machaček, Jan A1 - Staubach, Patrick A1 - Grandas Tavera, Carlos Eduardo A1 - Wichtmann, Torsten A1 - Zachert, Hauke T1 - On the automatic parameter calibration of a hypoplastic soil model T2 - Acta Geotechnica N2 - This paper presents an approach for the automatic parameter calibration (AC) of a hypoplastic constitutive soil model. The calibration software developed in this work simplifies the parameter calibration, reduces the subjective “human” factor on the calibration result and lowers the entry hurdle for the use of the hypoplastic constitutive model. The performance of the software was demonstrated by comparing automatically calibrated parameter sets for two sands and their related simulations of the underlying experimental data with simulations using two reference parameter sets. The first reference parameter set was calibrated the classical way, "by hand", and the second was calibrated using the AC tool ExCalibre. Two different optimization methods were used, namely the Differential Evolution (DE) and the Particle Swarm Optimization (PSO). The simulations performed with the parameters obtained from the AC agree well with the experimental data and show improvements over the reference parameter sets. With respect to the optimization method, the performance of the DE proved superior to that of the PSO. Various measures of comparison were examined to quantify the discrepancy between experiment and simulation. By repeating 500 calibration runs, the dispersion of parameters was determined and correlations between different parameters of the hypoplastic model were found. Y1 - 2022 U6 - https://doi.org/10.1007/s11440-022-01669-4 SN - 1861-1125 VL - 17 IS - 11 SP - 5253 EP - 5273 ER - TY - CHAP A1 - Machaček, Jan A1 - Grandas Tavera, Carlos Eduardo A1 - Triantafyllidis, Theodoros ED - Triantafyllidis, Theodoros T1 - Coupled Numerical Simulations for Seismic Hazard Assessment of Large Opencast Slopes T2 - Recent developments of soil mechanics and geotechnics in theory and practic N2 - 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. Y1 - 2020 SN - 978-3-030-28515-9 SN - 978-3-030-28516-6 SP - 266 PB - Springer CY - Cham ER - TY - GEN A1 - Nitsch, Antonia A1 - Machaček, Jan A1 - Wichtmann, Torsten A1 - Grandas Tavera, Carlos Eduardo T1 - Validierung und Kalibrierung eines hypoplastischen Stoffmodells für expansive Böden T2 - Bautechnik N2 - 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. Y1 - 2023 U6 - https://doi.org/10.1002/bate.202300062 SN - 0932-8351 VL - 100 IS - 11 SP - 689 EP - 698 ER -