Occluded Gas Research Articles

Experimental and Empirical Studies to Evaluate Liquefaction Resistance of Partially Saturated Sands

Partially saturated sands with occluded gas bubbles may occur naturally in the field or can be created artificially as a way of mitigating liquefaction effects. This study aimed to investigate the main parameters that influence the liquefaction resistance of partially saturated sands containing occluded gas bubbles. It also adopted a simplified approach and proposed empirical relationships that linked the liquefaction-resistance ratio to the bulk modulus, maximum volumetric strain, and excess pore-pressure ratio. Considering these objectives, a total of 60 stress-controlled dynamic triaxial tests were conducted. Nearly/fully saturated and partially saturated sand specimens prepared using sodium percarbonate were subjected to undrained cyclic loading with a loading frequency of 0.1 Hz. The loosely or densely packed sand specimens at different degrees of saturation (79–100%) were examined under different initial effective confining stresses (25–100 kPa) and back pressures (100–400 kPa). The analysis of the test results indicated that the liquefaction resistance of the partially saturated sands constantly increased as the degree of saturation decreased, and this effect was more prominent under higher initial effective confining stresses and lower back pressures. The adopted method satisfactorily captured this trend and showed reasonable matches between the empirical predictions and experimentally observed results from this study and previous research.

Evaluation of instability of quasi-saturated sand with entrapped gas due to decreasing total confining stress

The collapse of quasi-saturated sand with entrapped air due to decreasing total confining stress under undrained conditions has been widely reported in engineering practice. This material instability can fundamentally be related to dual effect of occluded gas bubbles: (1) altering undrained shear characteristics and (2) changing the relative compressibility between pore fluid and solid skeleton. Existing studies, however, have mainly treated these two effects independently. This work presents a theoretical framework to interpret the aforementioned instability that allows accounting for the interactions between the dual effect of gas. For this purpose, a state-dependent constitutive relation for gassy sand is employed that can well represent the gas effects of enhancing shear resistance and transferring unloading to soil skeleton. This soil model further enables evaluating the stability of quasi-saturated soils during constant deviatoric stress undrained shear with decreasing total mean stress, based on second-order work, the singularity of the symmetric part of the constitutive matrix, and test controllability. The experimentally observed collapse of loose gassy sand during the aforementioned stress path is analyzed by combining the constitutive model and stability criteria. This analysis finds that the reduction in effective stresses is initially driven by transferring unloading to soil skeleton, but becomes dominated by shear-induced contraction of soil matrix as failure is approached. Observed soil collapse can be well anticipated by probing the loss of controllability, yet lagging vanishing second-order work. The latter condition corresponds to the reversal in pore pressure and soil volume rate and represents a precursor for instability. Lastly, this work shows that the competition between the dual effect of the gas is key to understanding the non-monotonic dependence of instability potential on the properties of pore fluid and its compressibility.

A state-dependent constitutive model for coarse-grained gassy soil and its application in slope instability modelling

Free gas in sandy marine sediments is a common occurrence worldwide. A distinct feature of gassy sand is that, under undrained shearing, presence of occluded gas bubbles in the pore fluid can increase the undrained strength of sand at a relatively loose state, while reduce the strength of a relatively dense sand. Previous theoretical analyses have primarily focused on modelling the ‘beneficial’ effect of free gas on loose sand in migrating static liquefaction, with few attempts to describe the ‘detrimental’ effect of gas on dense sand under undrained loading. This study presents a state-dependent critical state model, which describes the distinct behavior of gassy sand with various states in a unified way. Comparison between the model predictions and test data of three gassy sands shows that the new model can capture the constitutive behavior of gassy marine sand over a wide range of initial states and degrees of saturation (typically between 85% and 100% for unsaturated marine sediments) using a single set of parameters. Parametric studies were performed to quantify the effects of gas (either ‘detrimental’ or ‘beneficial’) on sand with various initial states. The new model has been implemented in ABAQUS and used to simulate the stability of submarine slopes under undrained loading condition. It is found that free gas can improve and weaken the slope stability for loose and dense sand, respectively.

Liquefaction response of loose gassy marine sand sediments under cyclic loading

Gassy sediments have often been encountered in the marine seabed, and they have different features from common saturated and unsaturated soil. By developing and improving an effective methodology, triaxial gassy sand specimens with different initial gas content (saturation ≥85%) were prepared in the laboratory. Their state parameters can be controlled in real time. A series of undrained dynamic triaxial tests by a Global Digital System (GDS) dynamic testing apparatus were conducted to investigate the liquefaction characteristics of gassy sand sediments. The results show that the gassy sand can liquefy the same as the fully saturated sand, but gas existence monotonically increases the sand liquefaction resistance. The occluded gas bubbles have significant influences on sand liquefaction properties. The dynamic pore pressure of gassy sand shows obvious features of slower accumulation, greater amplitude fluctuation, and deeper groove shape in time history curves of pore water, resulting from the effects of gas compression/expansion, migration, and dissolution/exsolution. By introducing a parameter of saturation, a modified model was proposed to describe the evolution of dynamic pore pressure of gassy sands. It was found that the model parameter θ is linearly dependent on the initial gas content (or initial saturation degree Sr).

Migration of Volatile Organic Contaminations Through a Deformable Clay Liner: Effects of Occluded Gas Bubbles

In a solid waste containment facility, volatile organic contaminants (VOCs) can diffuse through the geomembrane and have a potential to pollute the underlying soil and groundwater. A compacted clay liner (CCL) is frequently adopted as a barrier to the pollution plume. In this study, a theoretical model is developed to describe the effect of trapped gas bubbles on the transport of dissolved VOCs in a saturated CCL. The present model couples the consolidation of the liner due to waste emplacement and the solute transport of the VOCs. The numerical results suggest that the gas partitioning between the aqueous phase and a trapped gas phase can greatly reduce the rates of transport of hydrophobic VOCs in the CCL. However, the mitigation effect due to un-saturation and vapor sorption for the hydrophilic VOCs is less significant, compared with the hydrophobic VOCs. The bubble wall is a promising barrier to the hydrophobic VOCs. For the hydrophilic VOCs, the effects of retardation is anticipated if the biotransformation process occurs due to presence of oxygen in occluded bubbles, which can be produced by certain agent admixture. Moreover, the influence of varying gas bubble volumes on the VOCs transit time can be negligible.

Evaluating the hydrogen isotope absorption/diffusion coefficient of CVD-SiC at high temperature

Deuterium diffusion coefficient measurements of CVD-SiC were carried out using a solubility and diffusivity measurement apparatus to investigate the permeation mechanism of the hydrogen isotope through CVD-SiC. Experiments were conducted with thin-sheet-type samples with thicknesses of 0.1mm, 1mm, and 2mm at 1073–1183K. Total amount of occluded gas into or released gas from different thickness but same weight sample were expected to be the same, but unexpectedly differed by more than 50%. As the release rates after sufficient time had passed were almost the same, and the 1-mm-thick sample had twice the surface area of the 2-mm-thick sample, the measurements were probably affected by adsorbed gas on the surface. The value of D/L2 (the diffusion coefficient divided by the square of the thickness), obtained by fitting to the theoretical formula but ignoring the early phase of discharge, was in good agreement for samples of different thickness at the same temperature, and was more than 5 orders of magnitude smaller than that obtained from the permeability measurement experiments. Therefore, we believe that the deuterium permeation through CVD-SiC is primarily dependent on the permeation rate through the grain boundaries.

Microstructural Observation of Ni-WC Surface Alloyed Carbon Steel

EPS (expanded polystyrene spheres) templates with thickness of a 10 mm coating on a surface that is mainly consisted of Ni-WC particles, are prepared. Carbon steels containing 0.45C%wt are then cast in the templates using V-EPC process (vacuum expandable pattern casting), forming the surface alloyed steels. The microstructures are observed and analyzed using optical microscope, SEM and EDS. Macrostructural observation showed that the surface alloyed zone is dense and there is no obvious defects, such as gas bubbles, occluded gas holes and delimitation. Microstructural investigations indicate that the samples from top to bottom are obviously consisted of three different zones, i.e., the top alloyed zone, the interim transitional zone and the bottom matrix zone. Ni-WC particles are totally decomposed during the molten steel infiltration. The microstructures in the surface alloyed zone are consisted of small amounts of fine WC+W2C carbides, large amounts of M3C+M7C3 carbides and dendritic matrix. There is a fine pearlite strip at the top of the transitional layer. Small amounts of carbide particles within the matrix grains and net carbide precipitation in the grain boundaries at the transitional zone can be observed.

Microstructural Observation of SiC Surface Alloyed Carbon Steel

EPS (expanded polystyrene spheres) template with a 5 mm coating on a surface that is mainly consisted of SiC particles are prepared. Carbon steels are then poured in the templates using V-EPC (Vacuum Expandable Pattern Casting) process. The microstructures are observed and analyzed using optical microscope, SEM and EDS. Macrostructural observation showed that the surface alloyed zone is dense and there are no obvious defects, such as gas bubbles, occluded gas holes and delimitation. Microstructural investigations indicated that the samples from top to bottom are obviously consisted of three different layers, i.e., the surface compound layer, the interim transitional layer and the bottom matrix layer. SiC particles are totally decomposed during infiltration. The microstructures in the surface compound layer are consisted of a large amount of graphite flakes, carbides, Si rich ferrites and dendritic pearlites. There is a fine pearlite strip at the top of the transitional layer and small amounts of carbide particles within the matrix grains and net carbides at the grain boundaries can be also observed in this layer.

Geomicrobiology and occluded O 2–CO 2–Ar gas analyses provide evidence of microbial respiration in ancient terrestrial ground ice

The study of ancient massive ground ice and sediment-rich ice bodies not only allows an understanding of the stability of permafrost, but also offers the possibility to understand the long-term preservation and viability of microbes within these ice bodies. Although microorganisms have been identified in ancient bodies of ground ice and permafrost, there still is a lack of convincing evidence that the microbes were metabolically active within the ice. In this study, we combine geomicrobiology methods with the measurements of occluded gasses in four ancient massive ground ice bodies to provide evidence of microbial respiration. In our samples, abundant cold-adapted heterotroph microorganisms were identified using culture-dependent and -independent methods. The occluded gas measurements, with low δ(O 2/Ar) (less than − 21.4%) and δ 13C CO 2 (as low as − 25.2‰) values and high δ 18O O 2 (up to + 16.3‰) values are suggestive of in situ consumption of O 2 by heterotrophic psychrophilic bacteria living within the ice matrix. Overall, the multi-disciplinary approach provides a strong set of tools from which to infer metabolic activity within bodies of ground ice. The analysis of δ(O 2/Ar), δ 18O O 2 and δ 13C CO 2 from occluded gasses in ice are strong isotope biosignatures that could allow detecting evidence of life on other planetary icy bodies such as Mars.

ADAGIO technique: From UO 2 fuels to MOX fuels

The amount of gas at the grain boundaries plays an important role in the fuel transient behaviour during accident conditions, such as a loss-of-coolant accident (LOCA) or a reactivity-initiated accident (RIA). Direct experimental determination of the grain boundary gas inventory has been performed for MOX fuel irradiated in an EDF pressurised water reactor (PWR) using the ADAGIO technique (ADAGIO is a French acronym meaning ‘Discriminatory Analysis of Accumulated Inter-granular and Occluded Gas’). The ADAGIO protocol applied to a MOX MIMAS fuel produced inter-granular gas fraction results that were consistent with those reached with other methods of evaluation i.e. electron probe microanalysis (EPMA). Furthermore, a new methodology for the numerical treatment of 85Kr release kinetics which was developed for UO 2 was applied to MOX fuels. The corresponding results evidenced two types of release kinetics. These kinetics were attributed to the inter-granular bubbles of the UO 2 matrix and the bubbles located in the restructured zones, i.e. Pu agglomerates.

Evidence of two gas release kinetics during the oxidation of an irradiated PWR UO 2 fuel

The ‘ADAGIO’ (French acronym for Discriminating Analysis of Accumulation of Inter-granular and Occluded Gas) experiments are dedicated to the determination of the gas inventory located at inter-granular position thanks to a partial oxidation of an irradiated UO 2 fuel. During these experiments the 85Kr gas release is measured as a function of time. The comparison of 85Kr release kinetic with the fuel oxidation kinetic brought into prominence two different gas release kinetics: the first one is proportional to the oxidation kinetic, the second one has a sigmoid shape and could be linked to the gas release of the so called ‘High Burn-up Structure’.

Gas hydrate measurements at Hydrate Ridge using Raman spectroscopy

Oceanic gas hydrates have been measured near the seafloor for the first time using a seagoing Raman spectrometer at Hydrate Ridge, Oregon, where extensive layers of hydrates have been found to occur near the seafloor. All of the hydrates analyzed were liberated from the upper meter of the sediment column near active gas venting sites in water depths of 770–780 m. Hydrate properties, such as structure and composition, were measured with significantly less disturbance to the sample than would be realized with core recovery. The natural hydrates measured were sI, with methane as the predominant guest component, and minor/trace amounts of hydrogen sulfide present in three of the twelve samples measured. Methane large-to-small cage occupancy ratios of the hydrates varied from 1.01 to 1.30, in good agreement with measurements of laboratory synthesized and recovered natural hydrates. Although the samples visually appeared to be solid, varying quantities of free methane gas were detected, indicating the possible presence of occluded gas in a hydrate bubble fabric.

Optimized method of producing washers of titanium hydride for plasma gun using occluded hydrogen gas

An optimized way of producing washers of titanium hydride for the application to a plasma gun using the occluded gas is presented. The amount of H2 gas (equivalently, gas pressure p) is entirely preadjusted in a gas reservoir of a simple instrument. The temperature T of a furnace is completely feedback controlled. Data show that when p is the order of 1atm, T needs to be higher than about 450°C in order to successfully produce washers of titanium hydride. Results on compressive strength of the loaded washers suggest that an appropriate ratio of atoms of hydrogen to titanium is less than H:Ti∼0.85:1.

Occluded Gas Content in Fast Breeder Reactor Fuels

A comprehensive analytical methodology based on hot vacuum extraction principle was developed for the determination of total/occluded gas content in ceramic fast reactor nuclear fuels such as (U, Pu)C and (U, Pu)O2 of different bulk compositions. In view of the stringent specifications imposed on these fuels with respect to total gas, detailed investigations were carried out to understand and interpret the implications of the definition of the occluded gas content. These investigations include selecting the temperature and duration of heating, method of gas collection (with and without gas refrigeration) as well as measuring the gas composition employing quadrupole mass spectrometer. Hydrogen was found to be the major constituent of the occluded gas (>95%) followed by minor quantities of N2, CO and CO2. These observations showed the necessity of introducing a vacuum degassing step after sintering and also helped in redefining the specification of total gas content. The total gas content is less than 0.1 cm3.g−1 excluding hydrogen. The measurement error is ±5%.

Modeling signal loss in surficial marine sediments containing occluded gas.

The presence of occluded gas in inland lakes, harbor muds, and surficial marine sediments is well documented. Surficial gassy sediments cause underlying beds to be acoustically impenetrable to seismic surveys; therefore, the modeling of signal loss arising from mudline reflection and transmission absorption is of particular interest. The Anderson and Hampton [J. Acoust. Soc. Am. 67, 1890-1903 (1980)] model for attenuation in gassy sediments was evaluated against the physical and acoustical properties of eight laboratory silty clay soils containing different amounts of occluded gas in bubbles of 0.2- to 1.8-mm diameter. The model was shown to give good agreement with measured data over the lower frequencies of bubble resonance and above resonance. It did not agree with measured data at frequencies below resonance, for which the model did not simulate the bulk properties of the gassy soils. The Mackenzie [J. Acoust. Soc. Am. 32, 221-231 (1960)] model for reflection loss was also examined for the gassy soils. The maximum reflection losses of 6 dB, at a grazing angle of 40 degrees, does not wholly support speculation by Levin [Geophysics 27, 35-47 (1962)] of highly reflective pressure-release boundaries arising from substantial reflection and absorption losses in gassy sediments. It was found that mudlines formed from sediments with significant occluded gas may be successfully penetrated, although the substantial absorption loss arising from signal transmission through the sediment prevents penetration of the surficial layers to much beyond a meter in depth.

The effect of nitrogen on analytical glow discharges studied by high resolution Fourier transform spectroscopy

The conditions in an analytical glow discharge may be considerably affected by a small amount of a foreign gas in the working gas (usually argon). Intensities of emission lines can change drastically, significantly influencing quantitative analysis by glow discharge optical emission spectrometry (GD-OES). However, gaseous elements are present as compounds or occluded gas in many samples; for example, nitrogen is a very important element in depth profiling nitrided steels or in the analysis of coatings containing nitrides. Analytically important nitrogen emission lines are in the vacuum ultraviolet (VUV) region, and the VUV Fourier transform (FT) spectrometer at Imperial College, London, was used to record the spectra emitted by Grimm-type sources using argon with various cathode materials, representing matrices of different reactivity with nitrogen. Controlled amounts of nitrogen were added to the working gas. Intensities and line profiles of emission lines—sample, argon (both atomic and ionic) and nitrogen (atomic and molecular band)—were recorded over wide spectral regions and the observed trends are presented. The self-reversal observed for the Ar I 811.5 nm and Ar I 763.5 nm resonance lines was reduced by the addition of nitrogen, implying a reduction in the population of argon metastable atoms.

An examination of the parameters that govern the acoustic behavior of sea bed sediments containing gas bubbles.

The acoustic properties of sea bed sediments containing occluded gas are dominated by the volume of gas contained in bubbles, the size of bubbles, and the elastic properties of the soil matrix. This study evaluated current theory developed by Anderson and Hampton to determine the sound speed and resonance frequency of gassy soils, and the models they used to determine the elastic properties of the soils. It compared calculated sound speeds, based on material properties simulated by the models, with measured sound speeds on "large bubble" laboratory soils produced in a similar manner to natural sea bed gassy soils. There was some evidence that the Anderson and Hampton equations accurately predicted sound speed at lower frequencies of bubbles resonance and below, but results were sensitive to inappropriate values for the elastic and damping properties of the soil. The bounds of sound speed based on the elastic properties of models that simulate "compressible fluid" or "suspension" behavior were grossly misleading when applied to large bubble soils. Conversely, sound speed based on models that correctly simulate the "bulk" or "matrix" properties of large bubble soils, at strain magnitudes and strain rates equivalent to acoustic signals, agreed well with measured data.

Partially saturated tailings sand below the phreatic surface

An opportunity to re–examine the liquefaction potential of hydraulically placed tailings sand has been provided by the Canadian Liquefaction Experiment (CANLEX). As part of this experiment, undisturbed samples of tailings sand were recovered after freezing the tailings in situ. Examination of undisturbed cores of frozen tailings sand clearly showed that the specimens were not fully saturated. This was confirmed by both physical measurements and laboratory tests in which gas was recovered from thawing specimens and analysed by gas chromatography. The gas was mainly air although a small amount of microbial gas was also present. Cryogenic scanning electron microscopy and confocal laser scanning microscopy were used to further confirm the existence of occluded gas bubbles. Triaxial undrained compression tests were carried out on undisturbed tailings specimens that were not back–saturated prior to shearing, in order to preserve the in–situ degree of saturation. Occluded air bubbles within the tailings sand, even if only in very small percentages by volume, are shown to have a marked effect on the response to undrained loading of the pore pressure within the tailings specimens. It is suggested that the liquefaction potential of tailings sand that could be expected to be contractive under undrained loading may be reduced by the occurrence of occluded gas bubbles within the voids. While it is not possible to quantify these effects accurately until further laboratory testing of both unsaturated and saturated loose tailings specimens has been conducted, this attribute could modify present engineering design. In this respect, obtaining undisturbed samples of granular soil is an important component of evaluating the liquefaction susceptibility of a specific deposit.

A Model for the Distant Activity of Comet Hale-Bopp

Numerical simulations of the evolving activity of comet Hale-Bopp are presented. A porous, spherical nucleus, 20 km in radius, made of ice and dust in equal mass fractions, is considered, assuming 5% of CO (by mass) to be occluded in the amorphous ice. The main effects included in the code are the following: crystallization of amorphous ice and release of occluded gas, condensation, sublimation and flow of gases through the pores, changing pore sizes, and flow of dust grains. A sharp rise in the activity of the nucleus occurs at a heliocentric distance of 7 AU preperihelion, marked by an increase in the CO flux and in the rate of dust emission by several orders of magnitude. This is due to runaway crystallization taking place a few meters below the surface, accompanied by the release of the trapped CO. There is a surge in the water vapor production rate as well, but this rate remains 2 orders of magnitude below the CO production rate. The runaway is eventually quenched, and a period of sustained, but variable, activity ensues. The emission of water molecules is found to surpass that of CO at a heliocentric distance of 3 AU. It remains almost constant through perihelion and starts declining beyond 1 AU postperihelion. The model predicts that the comet's activity (and brightness) will not increase dramatically as it approaches perihelion, as might be expected from its unusually high activity at 6-7 AU. It is found that a very porous dust mantle forms between 7 and 2 AU preperihelion, reaching a thickness of about 10 cm. This mantle leads to high surface temperatures. The onset of activity at 7 AU, the large CO flux, and the slowdown in the comet's activity rise as it nears perihelion are in very good agreement with the observations of comet Hale-Bopp.

Modelling Gas And Dust Release From Comet Hale–Bopp

Numerical simulations of the evolving activity of comet Hale-Bopp are presented, assuming a porous, spherical nucleus, 20 km in radius, made of dust and gas-laden amorphous ice. The main effects included are: crystallization of amorphous ice and release of occluded gas, condensation, sublimation and flow of gases through the pores, changing pore sizes, and flow of dust grains. The model parameters, such as initial pore size and porosity, emissivity, dust grain size, are varied in order to match the observed activity. In all cases, a sharp rise in the activity of the nucleus occurs at a large heliocentric distance pre-perihelion, marked by a few orders of magnitude increase in the CO and the CO2 fluxes and in the rate of dust emission. This is due to the onset of crystallization, advancing down to a few meters below the surface, accompanied by release of the trapped gases. A period of sustained, but variable, activity ensues. The emission of water molecules is found to surpass that of CO at a heliocentric distance of 3 AU. Thereafter the activity is largely determined by the behaviour of the dust. If a dust mantle is allowed to build up, the water production rate does not increase dramatically towards perihelion; if most of the dust is ejected, the surface activity increases rapidly, producing a very bright comet.