Particle orientation and its influence on the mechanical behaviour of isotropically consolidated reconstituted clay

The behaviour of naturally occurring geological materials such as clay and sand depends on many factors. For example, stresses, strains, previous stress history, mineralogy and the depositional environment all contribute in some degree to a characteristic that all natural soils share, namely “structure”. The structure of clay, or more generally, the microstructure of microscopically sized clay mineral particles, is just as important as the many other parameters that are used to quantify the performance of clays. This paper examines the microstructure that results from the particle arrangement brought about during reconstitution in the laboratory and considers its relevance to the resulting stress–strain behaviour.

Samples of reconstituted kaolin clay were produced using two different procedures. In the first series of tests, kaolin slurry was simply isotropically compressed in one increment. In the second series, the slurry was first isotropically compressed to a low pressure and then completely remoulded. This was followed by isotropic compression to the same pressure as the other series. Specimens were taken from the two series of samples, reconsolidated at various isotropic pressures, and sheared under undrained conditions.

Scanning Electron Microscope (SEM) images indicated that the monotonically compressed samples (Series 1) exhibited an anisotropic microstructure that was distinct from the remoulded (Series 2) samples. Significant differences were also found in the consolidation and stress–strain characteristics of the samples produced in the two series.     

Predictions of a continuous hyperplasticity model for Bangkok clay

The performance of a new constitutive model called ‘kinematic hardening modified Cam clay’ (KHMCC) is presented. The model is described using the ‘continuous hyperplasticity’ framework. Essentially this involves an infinite number of yield surfaces, thus allowing a smooth transition between elasticity and plasticity. The framework allows soil models to be developed in a relatively succinct mathematical form, since the entire constitutive behaviour can be determined through the specification of two scalar potentials. An implementation of the continuous hyperplasticity model is also described. The model requires eight parameters plus a viscosity coefficient for rate-dependent analysis. The model is defined in terms of triaxial stress–strain variables for this study, and is used to model monotonic triaxial tests on Bangkok clay. Comparisons of the theoretical predictions with the results of cyclic undrained triaxial compression tests on Bangkok clay are also presented.     

Study of moisture migration in clay soils considering rate of freezing

The paper presents some methods of numerical modelling and a study of moisture changing in frozen clay soils considering rates of freezing in the foundation. An impact assessment of moisture changes in clay soils affecting strength characteristics during thawing was carried out in stability tests.     

Retention behaviour of natural clayey materials at different temperatures

The water retention capacity of geomaterials, and especially clayey soils, is sensitive to temperature changes as the physical mechanisms of retention, such as capillarity or adsorption, are affected by it. It is therefore a major issue to be able to define temperature-dependent behaviour of materials, especially for geo-energy and geo-environmental applications involving non-isothermal conditions. This paper presents results of experiments conducted on two representative materials: a hard clay (Opalinus clay) and a plastic clay (Boom clay), both of which have been considered as buffer materials for underground radioactive waste disposal, in Switzerland and Belgium, respectively. Two new devices were developed for this purpose to permit the analysis of water retention behaviour at different temperatures. The behaviour of these two materials at ambient (20 °C) and high temperature (80 °C) was observed and described through the evolution of the degree of saturation, the water content and the void ratio with respect to suction. It appears that the retention capability of the clays reduces significantly with an increase in temperature; on the other hand, the change in temperature had less of an effect on the total volume variation.     

Modelling the behaviour of an embankment on soft clay with different constitutive models

The paper investigates the effect of constitutive models on the predicted response of a simplified benchmark problem, an embankment on soft soil. The soft soil is assumed to have the properties of POKO clay from Finland and five different constitutive models are used to model the deposit. Two of the models are isotropic models, i.e. the Modified Cam Clay model and the Soft‐Soil model. The other models are recently proposed constitutive models that account for plastic anisotropy. The S‐CLAY1 and S‐CLAY1S models are embedded in a standard elasto‐plastic framework and account for anisotropy via a rotational hardening law. In addition, the S‐CLAY1S model accounts for bonding and destructuration. In contrast, the Multilaminate Model for Clay (MMC) accounts for plastic anisotropy by utilizing so‐called multilaminate framework. The results of numerical simulations show that accounting for anisotropy results in notable differences in the predicted settlements and horizontal movements compared to the predictions using the isotropic models. There are also significant differences in the K₀ predictions by the different constitutive models and this has a significant impact on the results. Copyright © 2006 John Wiley & Sons, Ltd.     

A theoretical and experimental study on the behaviour of lignosulfonate-treated sandy silt

The effectiveness of an environmentally friendly stabilising agent for soil, namely, lignosulfonate was examined through a series of laboratory tests. A simple bounding surface plasticity model was developed to capture the bonding effects induced by lignosulfonate. One of the appealing aspects of the model is that it can incorporate the mechanical behaviour of the bonded soil during shearing, including the brittle and ductile failure modes. Validity of the model was verified by experimental results of lignosulfonate-treated soils under different stress path conditions. The mechanical behaviour of chemically treated soil was adequately captured by the model.     

Improvement of a hypoplastic model to predict clay behaviour under undrained conditions

A shortcoming of the hypoplastic model for clays proposed by the first author is an incorrect prediction of the initial portion of the undrained stress path, particularly for tests on normally consolidated soils at isotropic stress states. A conceptually simple modification of this model, which overcomes this drawback, is proposed in the contribution. The modified model is applicable to both normally consolidated and overconsolidated soils and predicts the same swept-out-memory states (i.e., normal compression lines) as the original model. At anisotropic stress states and at higher overconsolidation ratios the modified model yields predictions similar to the original model.     

The stress-strain behaviour of rock material related to fracture under compression

Fracture is the main reason for the non-linear behaviour of hard rocks. The fracture mechanics of rock is studied in this article by analysis of the fracture process under compression. A constitutive model that describes the relationship between the macro deformation of rock and the micro fracture within rock is developed. The propagation of microcracks, the non-linearity of deformation, the loading-and-unloading hysteresis and the variation of the apparent Young's modulus and Poisson's ratio are studied using the developed model. The model simulations demonstrate that: (1) the fracture toughness, initial crack length, crack density, and Young's modulus are four crucially important parameters that affect the deformation behaviour of rock; (2) the elastic parameters (E and v) of the rock matrix should be measured in triaxial tests. If they are measured in uniaxial tests, the upper straight unloading portion of the stress-strain curve is suggested to be used for the purpose, unless the closure effect of open cracks will be included in the estimations. In addition (3), the slope of the reloading stress-strain curve is a measure of the damage in material.     

Thermo-plasticity of clays: An isotropic yield mechanism

Numerical modelling of the thermo-mechanical behaviour of soils is an important issue in the analysis of problems such as nuclear waste isolation or geothermal structures. The purpose of this paper is to present a new thermo-plastic mechanism for isotropic thermo-mechanical paths including thermal hardening. It is based on considerations of the thermal effect on void ratio. After a discussion of the experimental evidence, the formulation of the thermo-plastic yield mechanism is introduced. Typical features are analysed and the responses of the model discussed. The proposed model is validated on the basis of experimental results on two different clays.     

Capability of constitutive models to simulate soils with different OCR using a single set of parameters

Incorporation of void ratio as a state variable into constitutive models allows, in principle, to use a single set of parameters for soils with different OCRs. Two sets of experimental data on reconstituted clays are used for evaluation of three constitutive models of different complexity (Modified Cam clay model, 3SKH model, hypoplastic model for clays). Although all the models predict the influence of OCR correctly from the qualitative point of view, quantitative comparison using a suitable scalar error measure reveals merits and shortcomings of different models.     

Ultimate lateral bearing capacity of tetrapod jacket foundation in clay

The tetrapod jacket foundations that are always used to support offshore wind turbines have been investigated primarily in laboratory experiments. In this study, the ultimate lateral soil resistance on this type of foundation was investigated using the finite element method and the analytical upper bound plasticity method. The numerical results show good agreement with the theoretical upper bound solutions under the same pile spacings (S) and soil-pile adhesion factors (α). Three distinct failure mechanisms (mechanisms A, B and C) were established in terms of different pile spacings. The ultimate lateral pressure was subsequently determined using numerical analyses with consideration of the loading direction. The most critical loading direction angles (θ) vary with the soil-pile adhesion factors, and are θ = 0 for α = 1 and θ = π/4 for α = 0. Selected empirical equations were proposed to predict the ultimate lateral bearing capacity for engineering practice, considering the pile spacing, soil-pile adhesion and loading direction.     

An experimental study on static and dynamic bending behaviour of piles in soft clay

Static and dynamic lateral load tests were carried out on model aluminium single piles embedded in soft clay to study its bending behaviour. Model aluminium piles with length to diameter ratios of 10, 20, 30 and 40 were used. Static lateral load tests were conducted on piles by rope and pulley arrangement upto failure and load–deflection curves were obtained. Dynamic lateral load tests were carried out for different magnitudes of load ranging from 7 to 30 N at wide range of frequencies from 2 to 50 Hz. The load transferred to the pile, pile head displacement and the strain variation along the pile length were measured using a Data Acquisition System. Safe static lateral load capacity for all piles is interpreted from load–deflection curves. Dynamic characteristics of the soil–pile system were arrived from the acquired experimental data. The soil–pile system behaves predominantly in nonlinear fashion even at low frequency under dynamic load. The displacement amplitude under dynamic load is magnified by 4.5–6.5 times the static deflection for all piles embedded in soft clay. But, the peak magnification factor reduces with an increase in the magnitude of lateral load mainly because of increase of hysteretic damping at very soft consistency. The maximum BM occurs at the fundamental frequency of the soil–pile system. Even the lower part of the pile affects the pile head response to the inertial load applied at the pile head. The maximum dynamic BM is magnified by about 1.5 times the maximum static BM for model piles in tested consistency of clay. The maximum dynamic BM occurs at a depth of about 1.5 times the depth of maximum static BM for model piles, which indicates an increase of active pile length under dynamic load.     

Study on the swelling behaviour of blended clay–sand soils

Soils containing expansive clays undergo swelling that can be both detrimental and beneficial in various applications. In the Arabian Gulf coastal region, natural heterogeneous soils containing clay and sand (tills, shales, and clayey sands) support most of the civil infrastructure systems. Likewise, mixes of clay and sand are used for local earthwork construction such as roads and landfills. A clear understanding of the swelling behaviour of such soils is pivotal at the outset of all construction projects. The main objective of this paper was to understand the evolution of swelling with increasing clay content in local soils. A theoretical framework for clay–sand soils was developed using phase relationships. Laboratory investigations comprised of mineralogical composition and geotechnical index properties of the clay and sand and consistency limits, swelling potential, and morphology of clay–sand mixes. Results indicated that soil consistency of mixes of a local expansive clay and an engineered sand depends on the weighted average of the constituents. Mixes with 10% clay through 40% clay capture the transition from a sand-like behaviour to a clay-like behaviour. Influenced by the initial conditions and soil matrix, the swelling potential of the investigated mixes correlated well with soil plasticity (SP(%) = 0.16 (I _p)^1.188). The parameters sand void ratio and clay–water ratio were found to better explain the behaviour of blended clay–sand soils.     

Review of fundamental principles in modelling unsaturated soil behaviour

An unsaturated soil is a state of the soil. All soils can be partially saturated with water. Therefore, constitutive models for soils should ideally represent the soil behaviour over entire ranges of possible pore pressure and stress values and allow arbitrary stress and hydraulic paths within these ranges. The last two decades or so have seen significant advances in modelling unsaturated soil behaviour. This paper presents a review of constitutive models for unsaturated soils. In particular, it focuses on the fundamental principles that govern the volume change, shear strength, yield stress, water retention and hydro-mechanical coupling. Alternative forms of these principles are critically examined in terms of their predictive capacity for experimental data, the consistency between these principles and the continuity between saturated and unsaturated states.     

On the use of discrete element modelling to simulate the viscoelastic deformation behaviour of an idealized asphalt mixture

The use of discrete-element modelling (DEM) to simulate the behaviour of a highly idealized bituminous mixture under uniaxial and triaxial compressive creep tests is investigated in this paper. The idealized mixture comprises single-sized spherical particles (sand) mixed with bitumen and was chosen so that the packing characteristics are known and the behaviour of the mixture is dominated by the bitumen. The bitumen is represented as shear and normal (tensile and compressive) contact stiffnesses. Numerical sample preparation procedures for specimens containing spherical particles or clumps have been developed to ensure that the final specimen is isotropic and has the correct volumetric proportions. An elastic contact was used for the compressive normal contact stiffness and a viscoelastic contact was used for shear and tensile normal contact stiffness. Simulation results show that the idealized mixture is found to dilate when the ratio of compressive to tensile contact stiffness increases as a function of loading time. Uniaxial and triaxial viscoelastic simulations have been performed to investigate the effect of stress ratio on dilation and the numerical results have been verified with experimental data. The effects of introducing a proportion of frictional contacts and a more complex particle shape (clump) on dilation have been examined.     

Modelling the mechanical behaviour of unsaturated soils using a genetic algorithm-based neural network

Modelling the mechanical behaviour of unsaturated soils has been the subject of many research works in the past few decades. A number of constitutive models have been developed to describe the complex behaviour of unsaturated soils. Despite the significant advances in the constitutive theories for unsaturated soils, none of the existing models can completely describe the various aspects of the real behaviour of unsaturated soils. In this paper, a new unified approach is presented, based on the integration of a neural network and a genetic algorithm, for the modelling of unsaturated soils. In the proposed approach, a genetic algorithm was used to optimise the weights of the neural network. A three-layer sequential architecture was chosen for the neural network. The network had eight input neurons, five neurons in the hidden layer and three neurons in the output layer. The eight input neurons represented the initial gravimetric water content, initial dry density, degree of saturation, net mean stress with respect to pore-air pressure, axial strain, deviatoric stress, soil suction and volumetric strain, and the three neurons in the output layer represented the deviatoric stress, suction and volumetric strain at the end of each increment. The network was trained and tested using a database that included results from a comprehensive set of triaxial tests on unsaturated soils from the literature. The predictions of the proposed model were compared with the experimental results. The comparison of the results indicates that the proposed approach was accurate and robust in representing the mechanical behaviour of unsaturated soils.     

Numerical predictions of the behaviour of soft clay with two anisotropic elastoplastic models

The numerical predictions obtained with the Melanie and MIT-E3 models are compared. Firstly, the performance of the constitutive models is checked against undrained triaxial tests. The models are then used to replicate the behaviour of an embankment built on soft clay. The numerical results are compared with the field data in terms of settlements, lateral displacements and excess pore pressures. Additionally, the numerical predictions are also analysed in terms of yield area, contours of vertical effective stresses, horizontal effective stresses and shear stresses and in terms of effective stress paths.     

Thermo-hydro-mechanical simulation of ATLAS in situ large scale test in Boom Clay

Following the need for understanding and quantifying the effect of temperature on the response of a candidate host formation for radioactive waste disposal, finite element modelling of an in situ thermal experiment has been carried out. Based on a thermo-hydro-mechanical (THM) finite element approach including a consistent thermo-plastic constitutive model, it has been possible to reproduce the THM response of a clay formation submitted to in situ thermal loading. The simulated large-scale experiment, called ATLAS was designed in the underground research facility (HADES-URF) in Mol, Belgium. After an extensive literature analysis on the thermal, hydraulic and mechanical characteristics of Boom Clay, laboratory tests were simulated to calibrate model parameters. The results of the finite element modelling of the ATLAS experiment were compared with in situ measurements and revealed the necessity to account for flow diffusion in all three directions through a 2D axisymmetric analysis. Finally, those results were interpreted in the light of elasto-thermoplasticity, which emphasizes the significant role of thermo-plastic processes in the global THM response of the clay formation.     

煤层气解吸的温度效应

为了探索温度对煤层气解吸效果的影响,通过对沁水南部寺河煤矿3号煤的吸附/解吸实验,经过数学分析,获得该煤层气吸附/解吸的数学式与等量吸附热表达式。分析认为：随着储层温度升高,饱和吸附量减小;解吸过程滞后于吸附作用的原因主要是非物理吸附常数 c值作用,随着储层温度增高,c值增大;降压解吸伴随吸热(温降),同时阻碍了持续解吸过程。通过提高储层温度来促进解吸过程,在理论上可行,工程中应该受到重视。