A simple hypoplastic model with loading surface accounting for viscous and fabric effects of clays

This paper presents a simple hypoplastic model capturing mostly all salient features of clays: rate dependency, time dependency and inherent and induced anisotropy without being restricted to only viscoplastic clays. Therefore, due to the strain rate decomposition into three parts, nonviscous clays, that is, rate‐independent clays, can also be simulated. The incorporation of a loading surface allows to capture the behaviour of normal and overconsolidated clays. The model requires eight material parameters, which are simple to calibrate from standard laboratory tests. In total, 77 simulations of five different clayey‐like soils are compared with experimental data. The simulations contain one oedometer test with loading–unloading–reloading cycles, creep and relaxation stages, both undrained and drained triaxial tests in compression and extension, as well as eight incremental response envelopes capturing also the directional response of Beaucaire Marl clay. Some limitations of the model such as the description of temperature effects on the behaviour of clays are also pointed out

AVISA: anisotropic visco-ISA model and its performance at cyclic loading

In this work, a constitutive model able to capture the strain rate dependency, small strain effects and the inherent anisotropy is proposed considering the influence of the overconsolidation ratio (OCR). Small strain effects are captured by using an extended ISA plasticity formulation (Fuentes and Triantafyllidis in Int J Numer Anal Methods Geomech 39(11):1235–1254, 2015). The strain rate dependency is reproduced by incorporating a third strain rate mechanism (in addition to the elastic and hypoplastic strain rate). A loading surface has been incorporated to define a three-dimensional (3D) overconsolidation ratio and to account for its effects on the simulations. Experimental investigations using Kaolin Clay and Lower Rhine Clay with horizontal bedding plane have shown that under undrained cycles of small strain amplitudes (<10$^{-4}$), the effective stress path in the p–q space is significantly inclined towards the left upper corner of the p - q plane. Consequently, a transversely (hypo)elastic stiffness has been successfully formulated to capture this behaviour. The performance of the model has been inspected by simulating the database of approximately 50 cyclic undrained triaxial (CUT) tests on low-plasticity Kaolin Clay (Wichtmann and Triantafyllidis) considering different deviatoric stress amplitudes, initial stress ratios, displacement rate, overconsolidation ratio and cutting direction. Furthermore, 4 CUT tests conducted on high-plasticity Lower Rhine Clay were simulated, whereby the influence of the displacement rate, as well as the deviatoric stress amplitude, has been analysed. The simulations showed a good congruence with the experimental observations

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

Performance of different constitutive soil models: from element tests to the simulation of vibratory pile driving tests

The present study deals with the investigation of the applicability (by means of parameter calibration), robustness and prediction quality of advanced constitutive soil models for the numerical investigation of complex geotechnical problems. The range of available constitutive soil models extends from simple linear to time-dependent and hydromechanically coupled nonlinear modelling approaches. It is the user's task to select a constitutive model suitable for the problem at hand. This requires in-depth knowledge of the soil behaviour as well as the strengths and weaknesses of the available constitutive models, most of which have only been validated using element test simulations. The procedure from parameter calibration using laboratory tests under well-defined boundary conditions (element tests) to the simulation of boundary value problems is complex in many respects and is often not followed with advanced constitutive models due to the large number of parameters required and the necessary laboratory tests. In this paper, the prediction quality of three models, namely Hypoplasticity with Intergranular Strain, Sanisand and Hypoplasticity with Intergranular Strain Anisotropy is inspected. The investigation is carried out based on back-calculations of laboratory tests and a well-documented model test to evaluate their suitability in representing complex soil mechanical aspects, such as the material behaviour under cyclic loading, particularly pore pressure accumulation. The parameter calibration is performed both "manually" as well as with a specially developed automatic calibration software. Subsequently, model tests of vibratory pile driving in water-saturated sand are simulated using the previously calibrated parameters

AVISA: anisotropic visco-ISA model and its performance at cyclic loading

In this work, a constitutive model able to capture the strain rate dependency, small strain effects and the inherent anisotropy is proposed considering the influence of the overconsolidation ratio (OCR). Small strain effects are captured by using an extended ISA plasticity formulation (Fuentes and Triantafyllidis in Int J Numer Anal Methods Geomech 39(11):1235–1254, 2015). The strain rate dependency is reproduced by incorporating a third strain rate mechanism (in addition to the elastic and hypoplastic strain rate). A loading surface has been incorporated to define a three-dimensional (3D) overconsolidation ratio and to account for its effects on the simulations. Experimental investigations using Kaolin Clay and Lower Rhine Clay with horizontal bedding plane have shown that under undrained cycles of small strain amplitudes (\10-4 ), the effective stress path in the p–q space is significantly inclined towards the left upper corner of the p - q plane. Consequently, a transversely (hypo)elastic stiffness has been successfully formulated to capture this behaviour. The performance of the model has been inspected by simulating the database of approximately 50 cyclic undrained triaxial (CUT) tests on low-plasticity Kaolin Clay (Wichtmann and Triantafyllidis) considering different deviatoric stress amplitudes, initial stress ratios, displacement rate, overconsolidation ratio and cutting direction. Furthermore, 4 CUT tests conducted on high-plasticity Lower Rhine Clay were simulated, whereby the influence of the displacement rate, as well as the deviatoric stress amplitude, has been analysed. The simulations showed a good congruence with the experimental observations

Experimental investigation and constitutive modeling of the behaviour of highly plastic Lower Rhine Clay under monotonic and cyclic loading

A new experimental series on the highly plastic (I_P = 34 %) Lower Rhine Clay (LRC) is presented. The study comprises tests on normally as well as over consolidated samples under monotonic and cyclic loading. The loading velocity has been varied in order to evaluate the strain rate dependency of the LRC behaviour testifying i.a. the well-known reduction of undrained shear strength with decreasing displacement rate. Isotropic consolidation followed by a cyclic loading with constant deviatoric stress amplitude leads to a failure due to large strain amplitudes with eight-shaped effective stress paths in the final phase of the tests. The inherent anisotropy has been additionally evaluated using samples cut out in either the vertical or the horizontal direction. Furthermore, the behaviour of LRC is compared with the behaviour of low plastic Kaolin silt (I_P = 12:2 %). A new visco-hypoplastic-type constitutive model with a historiotropic yield surface has been used to simulate some of the experiments with cyclic loading. Even the eight-shaped stress loops at cyclic mobility are reproduced well with this model. The data of this paper can be also used by other researchers for the examination, calibration, improvement or development of constitutive models dedicated to fine-grained soils under monotonic and cyclic loading.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Coupled hydro-mechanical behaviour of a Kaolin Clay in the context of the geothermal use of geotechnical structures

An experimental study regarding the hydro-mechanical behaviour of a Malaysian Kaolin is presented. Strain rate-controlled oedometer tests have been conducted on compacted samples. The influence of initial water content on the pore-size distribution (PSD) of compacted samples was investigated by Mercury Intrusion Porosimetry (MIP) tests. The drying path of the soil-water characteristic curve was experimentally determined for initially compacted samples and slurry samples. The preconsolidation stress was found to increase with increase in initial dry density and with decrease in initial water content. The compression curves merge into a single line, as soon as they have reached full saturation during loading. Samples at the dry side of Proctor water content showed a bi-modal PSD, whereas a mono-modal PSD was found for Proctor water content and at the wet side. The different initial compaction states of the samples were getting reflected in the drying SWCC until a suction of about 2 MPa, beyond which the drying paths were found to be identical