Constant Volume Conditions Research Articles

Understanding Relaxation in the Kob-Andersen Liquid Based on Entropy, String, Shoving, Localization, and Parabolic Models.

We assess the validity of a range of models of glass formation based on molecular dynamics simulation results of the Kob-Andersen (KA) model system under a wide range of constant volume and constant pressure conditions. These models include the Adam-Gibbs model emphasizing configurational entropy, the string model emphasizing collective particle exchange motion, the shoving model emphasizing material elasticity, the localization model emphasizing dynamical free volume, and parabolic models based on the ideas of dynamic facilitation and, alternatively, the hypothesis that glass formation involves an avoided critical point. We demonstrate that these seemingly disparate models all provide a reasonable description of structural relaxation and diffusion data for the KA model system under all simulation conditions considered. Hence, the present study points to some unity in our understanding of the relationship between leading models of glass formation, supporting inferences drawn from previous studies of polymeric glass-forming liquids.

Effective approach to predict soil-water retention curve of bentonites considering adsorption and capillarity

Understanding soil–water retention behavior is a longstanding topic. Water retained in soils can be decomposed into adsorptive and capillary components, controlled by different physicochemical mechanisms. The capillary water retention was frequently discussed in literatures, but the adsorption role was rarely considered, especially for high active clays (e.g., bentonite). In this study, two novel equations for quantifying adsorptive and capillary water retentions are proposed, generating a twofold model to continuously simulate soil–water retention behavior of bentonites. Only 5 parameters, i.e., the maximum adsorption capacity, characteristic adsorptive and capillary suctions, and uniformities of adsorptive and capillary pores, are defined with clear physical meanings. A graphical method was suggested to firstly determine the maximum adsorption capacity, and the remaining parameters are efficiently estimated by non-linear curve fitting. The water retention data for various bentonites, representing a variety of hydration conditions, initial compactness, montmorillonite content and suction range, are used to assess the model performance. The predictions agree well with the measured total, adsorptive and capillary water contents of a Wyoming bentonite, and the fitting curves also match well with test data for other bentonites over the full suction range. Adsorption parameters are distributed within a narrow range, while the characteristic capillary suction is also distributed within a limited range under constant volume condition. The proposed model allows reasonable predictions about the capillary onset and the transition from adsorption to capillarity, revealing obvious superiority by comparison with other hybrid models. This work offers a new pathway to quantitively assess the soil–water retention curve of high active clays.

Temperature effects on adsorption and capillarity water retention mechanisms in constrained unsaturated soils

AbstractThis paper focuses on the impact of elevated temperatures on the adsorptive and capillarity water retention mechanisms of unsaturated soils under constrained (constant volume) conditions. This topic is critical for simulating the thermo-hydraulic behavior of soils in hydrogeological or geotechnical applications, including climate change effects on near surface soils, energy piles or soil borehole thermal energy storage systems in unsaturated soil layers, and buffers for geological nuclear waste repositories. A nonisothermal soil water retention curve (SWRC) that separately considers the temperature-dependency of the key parameters governing adsorptive and capillarity water retention mechanisms and soil physical parameters (e.g., surface tension, contact angle, adsorption capacity, cation exchange capacity, mean cavitation suction, air entry value and equilibrium film thickness) was developed to provide insights into the impact of temperature on water retention over the full suction range. The nonisothermal SWRC was validated using experimental data on high plasticity clays, with a good prediction of temperature effects on adsorption and capillarity water retention mechanisms in constrained unsaturated soils.

Granular behaviour under bi-directional shear with constant vertical stress and constant volume

This paper aims to investigate the role of bi-directional shear in the mechanical behaviour of granular materials and macro-micro relations by conducting experiments and discrete element method (DEM) modelling. The bi-directional shear consists of a static shear consolidation and subsequent shear under constant vertical stress and constant volume conditions. A side wall node loading method is used to exert bi-directional shear of various angles. The results show that bi-directional shear can significantly influence the mechanical behaviour of granular materials. However, the relationship between bi-directional shear and mechanical responses relies on loading conditions, i.e. constant vertical stress or constant volume conditions. The stress states induced by static shear consolidation are affected by loading angles, which are enlarged by subsequent shear, consistent with the relationship between bi-directional shear and principal stresses. It provides evidence for the dissipation of stresses accompanying static liquefaction of granular materials. The presence of bi-directional principal stress rotation (PSR) is demonstrated, which evidences why the bi-directional shear of loading angles with components in two directions results in faster dissipations of stresses with static liquefaction. Contant volume shearing leads to cross-anisotropic stress and fabric at micro-contacts, but constant vertical stress shearing leads to complete anisotropic stress and fabric at micro-contacts. It explains the differentiating relationship between stress-strain responses and fabric anisotropy under these two conditions. Micromechanical signatures such as the slip state of micro-contacts and coordination number are also examined, providing further insights into understanding granular behaviour under bi-directional shear.

The effect of shear rate and overconsolidation ratios on undrained clay–suction caisson interface behaviors

The interface resistance during installation is crucial for the stability and safety of suction caisson in offshore geotechnical engineering, which is strongly affected by the penetration rate and soil–structure interface mechanical properties. This research conducts a series of clay–structure interface shear tests using modified direct simple shear device to fully study the mechanical behavior of clay–suction caisson interface. The effect of shear rate, over consolidation ratios (OCRs), interface boundary conditions, stress levels, and interface roughness were considered. Results show that as the OCR increases, the strength of both the clay and interface increase but show distinct patterns under constant volume (CV) and constant normal load (CNL) boundary condition. It was found that the interface strength is positively related to interface roughness and shear rate impact both the clay and corresponding interface strength. Under CNL conditions, the strength of normally consolidated (NC) clay decreases with rising shear rate, while the over consolidated (OC) clay demonstrate a opposite trend. In contrast, the effect of shear rate on interface behavior gets complicated owing to the combination of roughness, stress levels, and OCRs. Under CV conditions, the shear strength of clay and interface exhibits a logarithmic growth relationship with shear rates. The result of this work can provide a basis for interface resistance evaluation for suction caisson installation in clay.

Effects of microbially induced calcite precipitation on static liquefaction behavior of a gold tailings sand

Loose tailings are susceptible to static liquefaction during which they lose a substantial amount of their strength. This study examines a sustainable technique known as Microbially-Induced Calcite Precipitation (MICP) to improve the static liquefaction resistance of gold mine silty sand tailings. These materials were enriched with Sporosarcina pasteurii, consolidated in a direct simple shearing apparatus, and subjected to several injections of a cementation solution. Calcified tailings were then sheared under constant-volume and constant vertical stress conditions to evaluate their undrained and drained shearing behaviors. Results showed that bio-mineralization can prevent the occurrence of static liquefaction in tailings by reducing their contraction tendency. This is demonstrated by the strong strain-hardening behaviors of the treated tailings specimens compared to the strain-softening and undrained strength loss in specimens of the untreated tailings. Substantial increases in the tailings undrained and drained shear strengths (by up to 30 - 50 kPa), improvements (by up to 5 MPa) in their tangent moduli, and more than 5° rise in their friction angles are observed in the direct simple shear tests following MICP-treatment. The critical state line of tailings is also found to be steeper and shifted to denser void ratios following MICP treatment. These changes reduce liquefaction susceptibility of tailings and enhance their resistance against static liquefaction. Post-treatment acid dissolution further indicates that CaCO3 contents of about 4% to 11% precipitated in the treated specimens. This amount decreases with increasing specimens void ratio. Changes in the microstructural fabric of the cemented tailings particles are also characterized using scanning electron microscopic (SEM) images and X-ray diffraction (XRD) analyses.

Numerical investigation of gas migration behaviour in saturated bentonite with consideration of temperature

AbstractGas migration behaviour in saturated, compacted bentonite, especially under rigid-boundary conditions, is controversial. Gas breakthrough phenomena, observed under higher pressure gradient conditions in laboratory experiments, are described in literatures by adopting visco-capillary or dilatancy-controlled flow concept. Since, under rigid-boundary conditions, volumetric expansion is restricted and/or water dissipation is not detected, these concepts cannot be implemented satisfactorily. Instead, a diffusion and solubility-controlled (DSC) flow concept was previously found to be adequate for describing the behaviours at lower temperatures (20 °C). The DSC concept describes gas breakthrough as a function of gas solubility. Breakthrough occurs when concentration of dissolved gas reaches or surpasses the solubility limit in the entire specimen. In this work, the DSC flow concept is applied to validate gas migration and breakthrough experiments conducted at higher temperatures, e.g. 40 and 60 °C. Good agreements are observed between the experimental and predicted results, suggesting that the DSC flow concept can be applied to describe gas migration behaviour satisfactorily in rigidly confined saturated bentonites (under constant volume conditions) for various temperature regimes. Results also show that helium dissolution and diffusion processes in saturated bentonite are sensitive to test temperature and pressure conditions. The processes become more stable with increasing gas injection pressure and ambient temperature.

Gas transport model and numerical solution in roof rock based on the theory of free gas diffusion

Accurately identifying the source of coalbed methane is of great practical significance in preventing gas concentration overruns and improving extraction efficiency. At present, it is widely believed that underground gas mainly comes from coal desorption, and the mechanism of gas flow inside the coal seam is explored in depth, while the gas released inside the rock is often neglected, and there is insufficient understanding of its internal transportation law. In this study, the density gradient theory (FGDGD) in the coal matrix was used as a guide to characterize the adsorption and transport properties of gases within the rock. The anthracite and mudstone from the same roadway in Yuxi mine were selected for the experiments, and coal rock samples with particle sizes of 0.425 mm–0.25 mm were ground. The adsorption constant test and adsorption process experiments were carried out under the constant volume condition, and the transport behavior of gas within the rock was simulated and numerically solved, and the content and flow behavior of gas in the rock were discussed in detail. The results are as follows: (i) The simulation curves based on the density gradient model fit well with the experimental data for both coal and rock downholes, indicating that the gas transport law inside the rock downhole is also in accordance with the density gradient theory, which broadens the scope of application of the theory. (ii) The free gas diffusion coefficients Dks (rock) are smaller than Dks (coal) for the gases in the rock matrix and coal matrix, which may be due to the difference in the pore structure of the rock matrix, resulting in lower adsorption sites than those of the coal. (iii) Although the gas content inside the rock is relatively low compared with that of coal, it is significant for reducing the prediction error of gas influx in engineering practice. Therefore, it is necessary to consider the storage of gas in rocks and its migration behavior when studying the sources of gas emissions in engineering fields.

Comparative Analysis of Gas Adsorption and Diffusion Characteristics of the Coal Matrix under Constant Pressure and Constant Volume Conditions

Comparative Analysis of Gas Adsorption and Diffusion Characteristics of the Coal Matrix under Constant Pressure and Constant Volume Conditions

Shear rate effect on clay-structure interface strength properties in various interface boundary conditions

Based on the modified simple direct shear device which can directly measure the interface pore pressure and interface shear displacement, a series of interface shear tests and corresponding pure clay shear tests were conducted at an undrained state in constant normal load (CNL) boundary conditions or equivalent undrained state in constant volume (CV) boundary conditions. The clay-structure interfaces, consisting of seabed clay and Speswhite kaolin clay with overconsolidation ratios (OCR) of 1 and 3, were tested at three shear rates, respectively V1 = 0.0002 mm/s, V2 = 0.001 mm/s, and V3 = 0.01 mm/s. The results demonstrated that the shear strength of the clay-structure interface is lower than that of pure clay, and this difference is more pronounced under CV boundary conditions. In CNL condition, though the pure clay strength decreases with increasing shear rate at OCR = 1 and increases with increasing shear rate at OCR = 3, the shear rate effect on clay-structure interface strength is not obvious. In CV condition, the strength of the interface with the normally consolidated (NC) and over consolidated (OC) clay increases approximately linearly with the shear rate on the semi-logarithmic scale. the shear rate parameter ρ is used to describe the growth rate of pure clay or clay-structure interface shear strength with a tenfold increase in shear rate. As for normally consolidated clay, in CV condition, the corresponding shear rate parameter satisfies that ρ (with R1 roughness)> ρ (pure clay)> ρ (with R2 roughness). The rate parameter corresponding to NC seabed clay is significantly higher than the rate parameter corresponding to NC Speswhite kaolin clay. For OC clay, the shear rate parameter for interface strength is higher than that for pure clay, meeting with the relationship that ρ (with R1 roughness)> ρ (with R2 roughness)> ρ (pure clay).

Advances in Coupling Computational Fluid Dynamics and Discrete Element Method in Geotechnical Problems

In some cases, the water content in granular soil increases to the extent that it becomes saturated, which noticeably alters its responses. For example, the pore water pressure within saturated granular soil would increase rapidly under sudden external loading, which is equivalent to undrained or constant volume conditions. This reduces the effective stress in soil dramatically and may result in catastrophic failure. There have been different numerical approaches to analyse such a failure mechanism of soil to provide a deeper understanding of soil behaviour at the microscopic level. One of the most common numerical tools for such analysis is the discrete element method (DEM) due to its advantage in obtaining microscopic properties (e.g., statistics on particle contacts and fabric), reproducibility and simple feedback control. However, most DEM studies ignore the fluid phase and merely consider the solid particles while the fluid pressure is indirectly calculated by mimicking undrained condition to a constant volume condition. Note that fluid’s influence does not limit to the change of pore water pressure. For example, the external loading would induce the movement of fluid, and the fluid-solid interaction could subsequently drag the solid particles to shift within the system. In addition, the state of soil could change from solid to suspension under an excess hydraulic gradient. Therefore, the study of the fluid-solid mixture is essential as it is a typical scenario in geotechnical practice, and the simulations of saturated sand should be conducted in numerical forms in which both the solid and fluid phases can be modelled.

Hydro-mechanical modelling of swell behaviour of bentonite buffer

Hydro-mechanical modelling of swell behaviour of bentonite buffer

Heterogeneous Swelling of an Isotropically Compacted Bentonite-Based Material: Experimental Observations and Modelling

This paper presents a comprehensive investigation of the swelling behaviour of a compacted bentonite–sand mixture subjected to hydration under constant volume conditions. Contrary to previous studies, the tested sample was isotropically compacted before being hydrated under constant volume conditions until full saturation was reached. The total axial pressure, total radial pressures at four different heights of the sample, and injected water volume were recorded over time. The experimental data reveal a complex and non-uniform evolution of the axial and radial stresses over time, as well as anisotropy of the total stresses, which persist at the saturated equilibrated state. To gain further insights, a numerical analysis was performed using an advanced hydromechanical framework for partially saturated porous media, accounting for the evolving microstructure of the material. The complex evolution of the total axial and radial pressures with time is attributed to the advancing hydration and swelling front in the sample, along with the development of irreversible strains. The good agreement between the numerical results and the experimental data enables validation of the developed framework. Implications for engineered barriers in deep geological disposal of radioactive waste are discussed.

Combustion comparative analysis of pyrolysis oil and diesel fuel under constant-volume conditions

The article discusses the research results on the combustion of pyrolysis oil derived from the pyrolysis of HDPE plastics after its distillation. The tests were carried out in a constant-volume combustion chamber in conditions similar to those in a compression-ignition engine with a compression ratio of 17.5:1. The phases of premixed and diffusion combustion and the ignition lag were determined. Diesel fuel combustion tests were performed under similar pressure-temperature conditions. Comparative analysis was used to draw conclusions as follows: the duration of the premixed combustion phase for pyrolysis oil is approximately 5% shorter than that for diesel oil; with an increase in the initial pressure in the chamber, the premixed combustion time in both cases was shortened, while the diffusion combustion time slightly increased, practically without any significant change; the maximum combustion rate for the premixed combustion phase for pyrolysis oil was nearly 20% higher than that for diesel fuel; the heat released from the premixed combustion phase for pyrolysis oil was found to be 20% higher compared to that one for diesel fuel; the ignition lag for pyrolysis oil was slightly longer compared to that for diesel fuel. The presented parameters significantly impact both the development of combustion and the thermal efficiency of the internal combustion engine. Summing up, pyrolysis oil can be applied as a substitute for diesel fuel as a single or blended component with it.

The homogenisation behaviour of BCV bentonite – A laboratory and numerical study

Bentonite is considered as a suitable sealing material for radioactive waste geological repositories because of its high swelling capacity and low permeability. Regardless of the construction procedure and initial form of the bentonite, i.e. blocks, pellets, powder or combinations and mixtures thereof, the placement of the bentonite barrier results in initial heterogeneities which could potentially form preferential pathways for the migration of radionuclides. This study focused on the laboratory investigation of the homogenisation of Czech BCV bentonite. The analysis comprised the performance of four sets of tests on dual-density samples composed of two layers of bentonite compacted to various dry densities. Each set of tests considered two identical samples saturated from different sides under constant volume conditions. Homogenisation was investigated through the measurement of swelling pressure on both sides of the samples and via the determination of the final density distribution in a vertical profile from the post-mortem analysis. The results confirmed a substantial degree of homogenisation, with only a small density gradient revealed by the post-mortem analysis. The density distribution profiles indicated a gradual change in the vertical direction with no distinct border between the original layers. A higher degree of homogenisation was found in the samples with a higher overall density and, consequently, a higher swelling potential. No clear effect of the direction of saturation on homogenisation was identified from the density profiles. The final swelling pressures corresponded well to swelling pressures obtained on single-density samples with density equal to the global density of the dual-density samples. The numerical modelling represented well the final swelling pressures and the swelling pressure evolution curves for both the single- and dual-density samples (including the distinct experimentally-observed swelling pressure peak when the dual-density sample was saturated from the high-density end). More accurate results were obtained while considering the porosity and the degree of saturation-dependent permeability. However, the model did not fully capture the homogenisation of the dual-density samples in terms of the dry density profiles. While the tendency towards homogenisation was predicted correctly, two distinct density zones were evident in the simulations, while the experiment revealed a gradual density distribution.

Modeling of granular soil-structure interface under monotonic and cyclic loading with the nonlinear approach

A constitutive model that can accurately predict the behavior of the soil-structure interface is beneficial to the analysis and design for foundation engineering performance. This paper develops a new model with the nonlinear approach for simulating the monotonic and cyclic behaviors of the granular soil-structure interface. Based on the hyperbolic function, the concept of normalized shear displacement is proposed, and the basic model for the effects of the normal stress, the nonlinear shear modulus, and the friction angle on shear strength during shearing is deduced. On this basis, the Pyke criterion is modified to describe the cyclic loading responses. The relation between the friction angle and the state parameter of the interface in different interface states is established under the framework of critical state soil mechanics (CSSM) theory to further reflect the interface behaviors corresponding to the changes of void. Taking the phase transformation state as a reference state of volume increment, the incremental equation of stress-dilatancy induced by shearing is developed. In addition, the basic model is extended to be able to describe the interface mechanical behaviors under constant normal load (CNL), constant stiffness (CNS), and constant volume (CV) conditions by considering the characteristics of three typical stress loading paths. Only 10 calibrated parameters are required to reproduce the complex properties in the monotonic and cyclic behaviors. The predictive performance of the proposed interface model is examined by the previous experimental results.

Preparation of Si@PVDF composite microspheres by electrostatic spraying for enhanced energetic characteristics

In this study, Si@PVDF composite microspheres are prepared by the electrostatic spraying method. The spheres are with average diameter of around 2 μm (when at ideal stoichiometry), within which Si nanopowder is uniformly distributed. The exothermic properties of samples are studied through simultaneous thermogravimetry and differential scanning calorimetry. Benefiting from the improved interfacial contact quality, Si@PVDF shows increased chemical reactivity in terms of reduced reaction onset temperature, peak temperature and apparent activation energy than those of physically mixed Si/PVDF counterpart. Si/PVDF cannot be ignited through Joule heating, either in the powder of pellet form, while self-sustained combustion propagation is observed for Si@PVDF pellets. The combustion behaviors of Si@PVDF pellets are systematically studied in open-air (where light signal is analyzed) and constant-volume (where pressure signal is analyzed) conditions. The results show that the combustion process is influenced by the equivalence ratio, porosity of the pellet and the accessibility of O2 gas in the environment. Overall, the encapsulation of Si nanopowder into PVDF microspheres using electrostatic spraying shortens the mass and heat transfer distance and thus realizes enhanced energetic characteristics.

Hydraulic and volume change behaviors of compacted highly expansive soil under cyclic wetting and drying

The wide engineered application of compacted expansive soils necessitates understanding their behavior under field conditions. The results of this study demonstrate how seasonal climatic variation and stress and boundary conditions individually or collectively influence the hydraulic and volume change behavior of compacted highly expansive soils. The cyclic wetting and drying (CWD) process was applied for two boundary conditions, i.e. constant stress (CS) and constant volume (CV), and for a wide range of axial stress states. The adopted CWD process affected the hydraulic and volume change behaviors of expansive soils, with the first cycle of wetting and drying being the most effective. The CWD process under CS conditions resulted in shrinkage accumulation and reduction in saturated hydraulic conductivity ( k sat ). On the other hand, CWD under CV conditions caused a reduction of swell pressure while has almost no impact on k sat . An elastic response to CWD was achieved after the third cycle for saturated hydraulic conductivity ( k sat ), the third to fourth cycle for the volume change potential under the CV conditions, and the fourth to fifth cycle for the volume change potential under the CS conditions. Finally, both swell pressure ( σ s ) and saturated hydraulic conductivity ( k sat ) are not fundamental parameters of the expansive soil but rather depend on stress, boundary and wetting conditions.

Shear behaviors of recycled aggregate sand in constant volume simple shear tests interrelated with particle shape

Compared with quartz sand, recycled aggregate sand is characterized by the high liquefaction and shear resistance because of its irregular particle shape. However, the interrelationship between its particle shape and shear behaviors is seldom investigated. This paper investigates the role of the particle shape on the shear behaviors of recycled aggregate sand by conducting a series of monotonic and cyclic simple shear tests under undrained constant volume conditions. The particles’ morphology parameters are assessed in terms of sphericity, roundness and regularity with microscopic observation and image processing. According to the experimental results, the recycled aggregate sand with irregular shapes mobilizes greater gradients of the flow liquefaction lines under monotonic shear conditions. The irregularity of the particle shape also leads to greater peak shear stress and dynamic shear modulus, indicating stronger shear and liquefaction resistances under the cyclic shearing. Furthermore, the interrelationships between the peak shear stress, dynamic shear modulus and particle shape of the samples are revealed and established by well fitted functions, which could serve as references for the selection of recycled aggregate sand and corresponding designs of the foundation.

Исследование особенностей зажигания и горения высокоплотных зарядов в условиях постоянного объема

The study of combustion and ignition of charges with promising propellants or powder compositions in terms of initial conditions is an urgent problem of modern ballistics. Generally, combustion and ignition of charges are tested in a manometric closed vessel. The charge ignition conditions may affect the subsequent reactions. When designing a shot with a new charge, it is necessary to know what determines the moment of ignition (gas pressure, time, pressure rise rate, type of igniter, etc.) and which factor has a greater effect on the ignition. This paper presents a study of some features of high-density charge ignition under constant volume conditions. A brief description of the manometric setup is given. The effect of mass and type of igniter and combustion surface area on the high-density charge ignition and subsequent combustion is studied. The pressure-time dependences corresponding to high-density charge combustion in a textolite container with different diameters are presented and analyzed. The ignition of the paste high-density charge is revealed to be affected by the geometric dimensions of the container. The parametric experimental study results show that the ignition conditions of a model high-density propellant (MHDP) strongly depend on the method of initiation, namely the number of burning particles and ambient pressure; however, these factors do not affect the dependence of the MHDP combustion rate on pressure. The ignition momentum is obtained as a function of the ratio of the container burning surface circumference to the burning surface area.