Constitutive Modeling of Geomaterials: Advances and New Applications
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The results demonstrate that the angularity of grains is considerably affecting the behavior of soil.
Computational Modeling of Multiphase Geomaterials
This paper describes the results of numerical simulation of 2D assemblies of elliptical-shaped particles in direct shear test using discrete element method DEM. The effects of vertical stress, eccentricity and aggregation on the macro behavior of granular materials in direct shear test are investigated. Assemblies of approximately elliptical-shaped particles in random order represent soil specimens. The results indicate that by increasing the vertical stress, volumetric strain and pick friction angle decrease. In addition, simulation results show that by increasing the eccentricity of particles in an assembly, vertical strain, mobilized angle at pick state and packing friction increase due to reaching their maximum value and then decrease.
Furthermore, for well-graded assemblies with larger particles, maximum value of friction angle and strength were earned. Two-dimensional numerical simulations were conducted by employing the discrete element method DEM to study the micromechanical behavior of elliptical-shaped assembly in the direct shear test. In this study, the trend of anisotropy changes and the difference between their principal directions during shear deformation is investigated and the relations between the micromechanics parameters and average stress tensor are argued. The results demonstrate that anisotropy coefficients of soil fabric, contact normal force and contact shear force increase and rotate considerably after shearing starts and reach to their maximum value at peak state.
After peak, the shear strength decreases rapidly as the anisotropies of contact normal force and contact shear force. Furthermore, mobilized internal friction angle, which is obtained by assuming non-coaxiality between anisotropy directions, shows better fit with mobilized internal friction angle obtained from average stress tensor. For granular materials, their principal directions of stress and inelastic strain rate may become non-coaxial non-coincident when subjected to non-proportional loading, e. Non-coaxiality, as an important aspect of anisotropy of granular materials, may also produce significant effects on other plastic behaviors, such as dilatancy of materials.
A micromechanics-based framework is developed to model the proportional behaviors. The back stress defined in the classical plasticity is interpreted as contribution of fabric anisotropy and its evolution can be quantified with the deviation of principal direction of stress rate from that of stress.
As an assessment for validity of the proposed non-coaxial model, this study uses this model to examine the non-coaxial behaviors of the simple shear deformation. It has been found that fabric anisotropy plays significant roles on non-coaxiality as well as dilatancy of granular materials. The degree of non-coaxiality strongly depends on fabric anisotropy under simple shear. Due to fabric anisotropy, both dilatancy also becomes less. All predictions are of agreement with the measured from a series of simple shear tests.
In this paper a new formulation is presented to encompass the induced anisotropy during shear deformation in granular mass. The fabric anisotropy is affected by a number of parameters such as the mobilized stress ratio, internal friction angle, and the state of fabric. Non-coaxiality between stress and fabric tensor is included in this equation. This proposed formulation is derived by considering the interaction of particles across a potential sliding plane at micro-level.
It is further developed to incorporate the initial fabric anisotropy. The changes of fabric in the presence of inherent anisotropy will be predicted by these equations. Verifying of the formulation is presented with numerical simulations and experimental tests. Alkali activation of a flyash causes dissolution of its mineral constituents and also, significant transition in the characteristics of the residue.
As a result, the residue attains the crystallinity, which is different from the ash. In order to quantify the final crystallinity, peak intensity of a mineral present in the X-ray diffractograms XRD of these samples has been demonstrated to be a useful parameter. In this context this paper presents a methodology to determine crystallinity of the residue by employing XRD and the Fourier transform infrared FTIR analysis of these samples.
Also, this study establishes that such activation causes significant changes in the structural framework, which confirms formation of flyash zeolites viz. Various experimental results show that small strain stiffness and its decay with shear strain depend on confining pressure amongst other factors. In this paper, a selection of experimental evidence is briefly discussed. The Intergranular Strain formulation by Niemunis. Herle for the hypoplastic theory is extended for the same effect and evaluated.
Furthermore, application of the concept to elastoplastic models is discussed. Mathematical formalisms of shear-volume coupling for geomaterials, also known as stress-dilatancy relationships, are central components to a majority of constitutive models for soils. This paper investigates various stress-dilatancy formulations comparatively such that the consequence of adopting them in constitutive modeling can be easily understood.
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The objective of this paper is to examine the influences of moisture content, loading speed, and degree of compaction on the shear behavior of compacted sandy soil under unsaturated conditions. A series of triaxial compression tests with various experimental conditions were carried out by using triaxial apparatus for unsaturated soil. The test results show that the shear strength of unsaturated soil decreases with the increase in moisture content and increases due to increment of loading speed.
The effect of degree of compaction is also discussed. The results reveal that moisture content, loading speed, and degree of compaction have strong influences on the shear behavior of unsaturated sandy soil. The behavior of saturated coal under compressive loads is very important for evaluating the coal mass stability and mining hazards control. This paper presents a numerical study of localized deformation process in saturated coal sample. The three-dimensional images of a coal sample before compression were acquired by microCT scanner.
Then the images were used to create finite difference model. The material properties of different groups in saturated coal were assumed based on previous experiments results. Finally, the FLAC. Results indicate that the nature of internal material distribution in coal sample affect failure features greatly. It also shows that a diffuse, wide shear strain localization band progressively thinning during the failure process.
This paper presents a modeling procedure for simulating the monotonic undrained torsional shear behavior of sands, including stress-strain relationship, and excess pore water pressure generation, while considering the void ratio and stress level dependence of stress-strain-dilatancy behavior of sand. A unique set of soil parameters is required by the model to satisfactorily predict the undrained behavior of loose and dense Toyoura sand over a wide range of initial void ratios and confining pressures, as proven by simulating experimental data produced by the authors and found in the literature.
Cemented Granular Materials CGMs consist of a particle skeleton and a solid matrix partially filling the interstitial space.
In this broad class we encounter a number of typical geotechnical materials such as sedimentary rocks Sandstones, Conglomerates and Breccia as well as naturally and artificially cemented sands. These materials, while showing a brittle behavior under shearing at low confining pressures, are ductile at high confinements. The micro mechanisms involved, that are cement disaggregation, grain crushing and fragment rearrangement, are known to be different in these two cases. Several constitutive models based on the elasto-plastic framework have been developed to describe the behavior of CGMs.
However lack of correlations between the underlying failure mechanisms and the only internal variable plastic strain , results in the use of parameters that are hard to physically identify, let alone to calibrate. In this paper, we tackle the constitutive modeling of CGMs from a more physical angle, which starts from a micro mechanical observation of grain and cement failure, to a statistical homogenization of grain scale quantities for the constitution of their continuum counterparts. In particular, while the established Continuum Breakage Mechanics approach lends itself to the description of grain crushing process, a novel definition of damage for the cement phase is introduced and shown to be measurable.
The whole formulation of the new constitutive model is confined in a thermo-mechanical framework, with explicit links between the internal variables breakage, damage and plastic strain and the evolving microstructure of the material. As a consequence, the model possesses only a few physically identifiable and experimentally measurable parameters. The behavior of the model is assessed against experimental observations and its novel features are highlighted.
This paper presents a highly-coupled thermo-plasticity model for unsaturated soils. The effect of temperature on the mechanics of unsaturated soils is briefly addressed. Finally, the model is validated by the means of comparisons with experiments. This constitutive model constitutes an effectivetoolfor modelling the thermo-hydro-mechanical THM behaviour of geomaterials involved in the confinement of nuclear waste disposal. The dilatancy of clay has long been considered as a function of the current stress state independent of the loading history. Experimental evidence, however, indicates the dilatancy behavior of over-consolidated OC clay bears close correlation with the overconsolidation ratio OCR of the soil.
The dilatancy relation is integrated into a bounding surface model to predict the behavior of OC clays. With only three extra parameters added to the MCC model which can be easily calibrated by triaxial compression tests, the new model is shown to offer good predictions for the experimental data. The yield condition, flow rule and hardening law are determined independently and contradict each other sometimes, which results the probability of violating thermodynanics laws.
Thermodynamic-based constitutive models for soils are newly developed constitutive models. This approach has the advantage that the models are guaranteed to obey the laws of thermodynamics by use of dissipative incremental function and free energy function, while retrospective criteria need not be applied. The development history of constitutive models based on thermodynamic approach is outlined, the recent study situations are analysed, and the classification of the models is discussed which indicate a broad field of applications to modeling constitutive behavior for soils.
In present research, a constitutive model is developed for prediction of the mechanical behavior of saturated cemented sands according to generalized plasticity and critical state soil mechanics concepts. The model is based on differences between normal consolidation curves of cemented and uncemented soil in. In order to consider the effects of cementation, six new parameters are added to the generalized plasticity model proposed by Manzanal et al.
Deviatoric stress-axial strain besides volumetric strain-axial strain curves are predicted and compared with experimental data for verification. Results show fairly good performance of presented model. Based on researching the features of the modified Cam-clay model and UH model, the mechanism of models modeling the behaviors of the over-consolidated OC clays can be revealed.
For the OC clays, the curves predicted by the modified Cam-clay model have abrupt changing points, so the model cannot model continue and gradual changing process well. UH model can rationally model many characteristics of the OC clays well, including shear dilatancy, strain-hardening and softening, and stress path dependence behavior. This paper focuses on the study on the deformation behavior and strength character of natural Shanghai soft clay undergoing principal stress axes rotation.
Undrained hollow cylinder shear tests were performed on natural Shanghai soft clay under isotropically and anisotropically consolidated conditions to investigate the effects of principal stress rotation. Effects of coefficient of intermediate principal stress and rotational angle of principal stress axes on the stress-strain behavior and strength characteristics of soft clay were studied. Test results show that the stress-strain behavior and undrained strength characteristics of soft clay vary significantly with the coefficient of intermediate principal stress and rotational angle of principal stress axes.
The three-dimensional anisotropic bounding surface model for natural soft clay proposed by the authors is adopted to simulate the behaviors of Shanghai soft clay under principal stress rotation. Comparisons between simulations and experiments are given to further demonstrate the validity of the model. How to incorporate the impact of the internal structure into the constitutive modeling has attracted much research interest. Towards the grand challenge of developing a constitutive models embedded with particle-scale mechanism, this paper mainly concerns about the internal structure and its role in constitutive modeling.
The investigations to be reported address issues including 1 why the internal structure is important; 2 how the internal structure should be quantified; 3 how to reflect the impact of internal structure in constitutive modeling. State transformation is common in soil shearing problem and causes much difficulty in soil description in numerical simulation. For characterizing the state transformation of sand, dilatancy angle and friction angle are linked with soil state parameter, i. Further, state dependent dilatancy angle and friction angle are introduced into classical MC model.
Based on the theory of critical state, the state parameter considering the direction of principal stress during shear is introduced into dilatancy equation and the plastic hardening modulus expression to develop a new constitutive model for sand which can display the influence of the shearing direction of principal stress. By comparison with the related experimental data and analyzing, the method on simulating the stress-strain relationship under the different shearing direction of principal stress is proved to be correct and accurate.
A constitutive model of unsaturated soils coupling skeletal deformation and capillary hysteresis is developed based on the modified Cam-clay model. Theoretical simulations are compared to the experimental data available in the literature, showing that the new model captures very well the main features of unsaturated soil behavior. Unlike other unsaturated soil models, the new model treats the degree of saturation as an internal variable, whose evolution equation explicitly accounts for the capillary hysteresis. The new model can smoothly transit to the modified cam-clay model of saturated soils when the degree of saturation becomes one.
The new constitutive model is implemented into a finite element code, U-DYSAC2, in which the displacement of solid skeleton, pore pressure and air pressure are primary nodal unknowns. An implicit algorithm is developed to integrate the constitutive relation at the Gaussian-point level.
The proposed numerical procedure is used to simulate a typical two-dimensional unsaturated soil problem, illustrating the applicability and capabilities of the new numerical procedure. A rearrangement of the hypoplastic constitutive equation is proposed that enables the incorporation of an asymptotic state boundary surface of an arbitrary pre-defined shape into the model, with any corresponding asymptotic strain rate direction.
This opens the way for further development of hypoplastic models.
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Predictions of a new clay hypoplastic model based on the propsoed approach are shown for demonstration. A simple model to describe time-dependent behavior for various kinds of soils is presented. The present model can describe various time-dependent behaviors not only of normally consolidated soil but also of over consolidated and naturally deposited soils in the same manner without violating the objectivities. To consider the influence of over consolidation, the subloading surface concept defines a state variable.
To describe the behavior of naturally deposited soil, another state variable. The validity of the present models is verified using various kinds of simulations of time-dependent behavior in one-dimensional and three-dimensional conditions. It is necessary to guarantee the uniqueness of the solution by DEM.
At first, this paper aims to make sure the uniqueness for biaxial compression test with oval particles using parametric study. It was found that the strain rate has an important role comparing with other affecting factors. Secondly, the simulations were conducted until large strain to study the initiation of shear band formation and the existence of steady state. It was clarified that the initiation of the shear band formation occurs around the peak strength for dense material and the steady sate does not necessarily exist.
In order to know about the accelerated creep properties of volcanic breccias, triaxial compression rheological experiments are carried out on rock servo-controlled triaxial rheology equipment. Based on the experimental results, the accelerated creep phase of creep curve of volcanic breccias is analyzed. It is shown that the strain rate increases sharply, the rock damage rapidly increases; the acoustic emission dramatic increases with the increase of time during the stage of accelerated creep, and the accelerated growth of rock damage leads to the sharp increase of creep rate.
Then based on the accelerated creep deformation, the relation of damage variable versus time is obtained, and and the creep damage constitutive equation of volcanic breccias is established. It is shown that the established nonlinear creep damage models based on time function is correct and reasonable. For materials of granular characteristics, structural problem is one key and difficult point of the present study.
In the paper, the full state function theory for material structure is proposed based on investigation on the state of material structure as the starting point. The full state function of material structure is a function reflecting the material structure properties, so it also can reflect the variation rules of material mechanics and geometric characteristics when materials are affected by external effect, which is the constitutive relation of materials.
The constitutive relation set up based on the full state function is shortened as full constitutive relation, because for constitutive relation the structural characteristics of materials are taken into consideration. Besides the constitutive relation that takes material structure into account that can be deduced on the basis of the full state function theory, the constitutive relation of which the form is identical to that of classical theory can also be deduced when materials deteriorate as classical ideal elastic-plastic materials.
The proposed theoretical method enables the change of dynamic process analysis of structural change into state analysis and the establishment of bridge and method to analyze the relationship between particle structure scale and macro scale. In addition, it also lays a foundation for the solution of material structural problems macroscopically. Methane gas hydrate, usually found beneath permafrost and in marine continental margin sediments worldwide, has attracted interest as a possible energy resource and as a potential agent in climate change and seafloor instability.
Here, in order to describe the mechanical behavior of gas hydrate-bearing sediments GHBS , an elastoplastic model is proposed based on the framework of a critical state model. The presence of gas hydrate can increase yield stress, enhancing the cohesion, peak strength and stiffness of the sediments. Therefore, GHBS is considered as the bonded material in the proposed model. A new bonding strength parameter is introduced into the yield function to evaluate the effect of gas hydrate on the yield behavior of the earth materials.
Dilatancy is assumed to be a function of the bonding strength and the stress ratio, instead of the single parameter stress ratio, which can reflect the direction of plastic strain increment more realistically. The proposed model can transform into the modified Cam Clay model when hydrate saturation reduces to zero. Finally, the proposed model has been used to predict the stress-strain behaviors of GHBS in triaxial tests, and it is demonstrated that the proposed model has the capability to describe the behavior of GHBS. Inter-particle bonds exist in both naturally and artificially cemented geomaterials.
The bonds affect the mechanical response of the materials and make it differ from the remolded counterparts. In this paper, some recently proposed constitutive models for bonded soils are reviewed from the perspective of yield surface, flow rule and hardening law. The approach of phenomenological and thermomechanical modeling is discussed. The use of discrete element method to shed light on the modeling issue is explored.
In this paper, the mechanical behavior of sand, was systematically described and modeled. Without losing the generality of the sand, a specific sand called as Toyoura sand, a typical clean sand found in Japan, has been discussed in detail. In the model, the results of conventional triaxial tests of the sand under different loading and drainage conditions were simulated with a fixed set of material parameters. The model only employs eight parameters among which five parameters are the same as those used in Cam-clay model.
Once the parameters are determined with the conventional drained triaxial compression tests and undrained triaxial cyclic loading tests, and then they are fixed to uniquely describe the overall mechanical behaviors of the Toyoura sand, without changing the values of the eight parameters irrespective of what kind of the loadings or the drainage conditions may be. The capability of the model is discussed in a theoretical way. Based on the UH model, a structured UH model is formulated to express the constitutive relation of structured clay.
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The keynote of the work is presenting a moving normal compression line MNCL in void ratio-mean effect stress plan. By adopting a simple law of the structure potential evolution, void collapse due to the structure decay can be described. In the structured UH model, only 2 more parameters than that of the Cam-clay model are introduced, which are respectively structure potential and its decay rate factor. Comparisons of the model predictions and tests results indicate that the structured UH model is qualified in capturing the main behaviors of the structured clay.
A procedure is developed where stresses from a finite element analysis are incorporated into a limit equilibrium framework to evaluate the stability of unsaturated soil slopes. Based on a known seepage field and pore pressure distribution in the slope, an independent stress-deformation analysis is performed using the Barcelona Basic Model Alonso et al,  to calculate the internal stresses in the slope. Subsequently, slip surfaces from the limit equilibrium analysis are introduced using a series of rectangle planes.
The maximum and minimum principal stresses at the centre of each plane can be calculated from the internal stresses and they are introduced into the slope stability analysis depending on the stress level relative to the strength of the unsaturated soils. Finally, a parametric study is carried out to show the effect of suction on the calculated factor of safety. Static and dynamic numerical calculations were performed to study the dynamic response of a high core rockfill dam. Then based on the initial static stress filed, dynamic analyses of the dam subjected to earthquakes were conducted with the program EB3D in traditional way of visco-elastic model and Equivalent Linear Method.
The dynamic behavior of the high core rockfill dam during the earthquake was analyzed, and the seismic characteristics of response were investigated. Compared with traditional Equivalent Linear Method, it is shown that the Nonlinear Method is more reasonable in describing the dynamic response of high core rockfill dam subjected to the earthquake motion.
When a conventional method such as linear regression is insufficient to identify the constants and parameters of an advanced constitutive model, numerical optimization techniques are often used. In employing numerical optimization to constitutive model calibration, an objective function essentially needs to be formulated in the form of nonlinear least squares where the sum of the differences between the measured and computed data points are quantified.
When a minimum value of the objection function is obtained, the corresponding variables are the optimized model constants and parameters. Due to the complexity in the format of objective functions in constitutive model calibration, gradient-based methods are seldom used.
In this paper, non-gradient based methods namely Direct algorithm was implemented to calibrate a modified Cam-Clay model against laboratory data. While crack propagation is considered as the predominant mechanism of damage in quasi brittle rocks, recent observation on cracking evolution on porous rocks have shown that nucleation of new cracks and back-sliding mechanism of cracks are perhaps as important as the crack propagation. The model proposed here tends to explain the laboratory results that could not be explained within framework of crack-sliding model.
A crack nucleation mechanism is then considered to generate an increasing number of cracks based upon a statistical distribution of strain energy cumulated on grain to grain contacts. The increasing role of crack growth and crack coalescence is modeled by an avalanche like mechanism that leads finally to the failure of a porous rock. Predictions of the model are compared with experimentally results in terms of strains and acoustic emission. Inverted conical spudcan footings are the most popular foundation for jack-up platforms in the offshore industry.
Before a jack-up can be located at a site, the penetration resistance during spudcan installation must be accurately predicted. This is usually performed using bearing capacity formulae from industry standards, with the use of site-specific numerical modelling not yet common place. It is used to simulate the installation process of spudcans in sand overlying clay. The robustness of this large-deformation analysis method is validated against published results of numerical analyses of spudcan penetration into clay.
This comparison is performed to show that the numerical simulations are suited to capture the behaviour of spudcan penetration into seabeds. A series of parametric studies are then conducted to investigate the potential for punch-through during spudcan penetration in sand overlying clay. Spara som favorit. Skickas inom vardagar. JH Yin in Constitutive modeling of geomaterials has been an active research area for a long period of time.
Different approaches have been used in the development of various constitutive models. A number of models have been implemented in the numerical analyses of geotechnical structures. Spara som favorit. Skickas inom vardagar. JH Yin in Constitutive modeling of geomaterials has been an active research area for a long period of time. Different approaches have been used in the development of various constitutive models.
A number of models have been implemented in the numerical analyses of geotechnical structures.