Gassy Soil Research Articles

A thermodynamic model to simulate the thermo-mechanical behavior of fine-grained gassy soil

Fine-grained gassy soil is a special kind of unsaturated soil, with the gas in the form of discrete big bubbles within the saturated matrix. These gas bubbles are sealed and considerably larger than the normal particle size. Based on the conceptual model of soil containing large gas bubbles proposed by Wheeler (Geotechnique 38(3):389–397, 1988a) and the granular solid hydrodynamics (GSH) theory, a thermodynamic model is presented to describe the mechanical properties and temperature effect of fine-grained gassy soil in this paper. The model assumes that the gas pressure is related to total stress and pore water pressure of soil, and the behavior of saturated matrix is controlled by “quasi-effective stress.” In addition, the effect of gas on the plastic deformation of soil skeleton is considered. Comparing with the experimental results, the ability of the model to describe the consolidation and undrained shear properties of fine-grained gassy soil is verified. What is more, the effect of temperature on fine-grained gassy soil considering the various responses for different drainage conditions and overconsolidation ratios is discussed and simulated by the proposed model. It is concluded that for fine-grained gassy soil with different overconsolidation ratios, the increase of temperature can increase the compressibility and thermal contraction under drained conditions, as well as the pore water pressure under undrained conditions, while the temperature effect on undrained shear properties depends on the initial conditions.

On evolving size and shape of gas bubble in marine clay under multi-stage loadings: microcomputed tomography (μCT) characterization and cavity contraction analysis

Discrete bio-gas bubbles commonly form in fine-grained marine sediments and have modified many aspects of the behavior of these sediments, including strength, stiffness, and permeability. Although the level of such modifications is known to govern by bubble shape and size, limited studies have been undertaken, mainly due to difficulty in nondestructively characterizing bubbles within a soil under in situ stresses. In this study, a mini-loading device was developed to perform one-dimensional loading tests on gassy marine clay and gassy silt in a microcomputed tomography (μCT). The evolving bubble shape, size, and pressure during loading were quantified, and the resulting stress fields around the bubble cavities were evaluated via elliptical cavity contraction analysis considering stress anisotropy. As the vertical load increased, bubble cavities were found to compress predominantly along the vertical loading direction, with little horizontal compression, because localized soil failure (LSF) and thus cavity collapse occurred mainly near the roof of the at-rest lateral earth pressure coefficient (K0)-stressed elliptical bubble cavities. The evolution of bubble shape and size under loading is significantly affected by stress anisotropy, which governs the extent and location of the LSF. A Gaussian mixture model is adopted to quantify the evolving distributions of bubble structure parameters, which are essential for developing more physically rigorous gassy soil models.

On the use of Fredlund gas–fluid compressibility relationship to model medium-dense gassy sand behavior

This paper presents the influence of gas bubbles trapped in a soil mass on the stress–strain–strength response of medium-dense sands. A hypoplastic constitutive sand model enhanced with the intergranular strain concept was coupled with the Fredlund gas–fluid compressibility relationship to capture gassy soil behavior. Boundary value element representations in a finite element platform of oedometer and saturated drained and undrained triaxial compression tests are performed for the calibration of soil parameters. For the numerical simulation of gassy soil behavior, pore fluid compressibility is modified to account for the presence of free gas in the pore fluid. The gassy soil mechanical response is studied by using only one set of parameters determined from the saturated soil response. The testing bed for this evaluation is a laboratory experimental program conducted on sands retrieved from the Oakridge Landfill, a sanitary landfill located in South Carolina, USA. The hypoplasticity sand model specialized with the Fredlund relationship reproduces reasonably well the stress–strain–strength response of these sands for a wide range of loading conditions and at a reasonable level of testing for the calibration of constitutive parameters. It is found that a slight reduction in the degree of saturation significantly decreases the undrained shear strength of the soil and causes changes in volume (i.e., drained-like behavior).

Stress-dilatancy behaviour of bubbled fine-grained sediments

Although the strength and stiffness of fine-grained gassy soil have been relatively well characterised, its dilatancy remains poorly understood. This study aimed to understand and formulate the stress-dilatancy behaviour of fine-grained gassy soil through a series of triaxial tests (19 in total) conducted on fine-grained gassy specimens where different initial gas volume fractions (ψ0) and initial pore water pressures (uw0) were considered. Compared to the stress-dilatancy relation of saturated fine-grained soil, the addition of gas bubbles was found to result in more contractive or dilative soil behaviour, depending on the combination of ψ0 and uw0. These distinct features cannot be captured by the existing stress-dilatancy functions for fine-grained saturated soils, which solely depend on the stress ratio. Therefore, a new stress-dilatancy function is proposed within the framework of critical state soil mechanics to quantify the distinct modification effects of bubbles (which either suppress or enhance dilatancy) on the dilatancy of gassy soils with different ψ0 and uw0 values in a unified manner. This new function has been validated and can be readily implemented in critical state based constitutive models for predicting the shear behaviour of fine-grained gassy soil and therefore, the response of various offshore structures (i.e., foundations and pipelines) founded on a gas-bearing seabed.

Three-dimensional analysis of unanticipated behavior of a deep excavation

A three-dimensional (3D) effective stress finite element analysis, modified to account for hydrofracturing and gassy soil behavior, is used to examine the potential for the venting of water and gas from a bedrock aquifer and through 13–14 m of low permeability clayey silt between the base of the excavation and the bedrock following excavation to about 24 m in an approximately 40 m thick clayey silt deposit. The clayey deposit contained sand lenses with dissolved gas. The analysis predicts that the exsolution of this dissolved gas, caused by a reduction in total stress due to the excavation, results in liquefaction of the sand in the lenses and consequent lateral deformations of the side slopes. The analysis predicts hydrofracturing through the remaining clayey silt when the excavation reaches its final depth and this explains the venting of water and gas from the underlying aquifer that was observed above a local bedrock high. The presence of gassy sand lenses created weak zones within the clayey deposit that influenced the path of the hydrofracturing. However, the analyses suggest that, for the depth of excavation and bedrock elevation examined, hydrofracturing and subsequent venting would have occurred even if there had been no sand lenses.

Effect of initial pore pressure on undrained shear behaviour of fine-grained gassy soil

Many of the world reserves of fossil fuels are located at various water depths in fine-grained sediment under the seabed. The fine-grained sediment contains relatively large biogas bubbles, which has been posing challenges to the stability of offshore foundations supporting oil and gas platforms. Although fine-grained gassy soil was found to exhibit different undrained shear strengths (cu) by altering the initial pore pressure, ui (relevant to water depth), systematic studies concerning the effect of ui on undrained shear behaviours of the soil are still lacking. This study reports a series of undrained triaxial tests aiming to compare and investigate the responses of reconstituted fine-grained gassy soil with the same consolidation pressure ([Formula: see text]), but at a wide range of varying ui (0–1000 kPa). The shearing-induced excess pore pressure (Δu) in the gassy specimens highly depends on ui. It can be either smaller than that of the saturated specimen with the same [Formula: see text] (due to partial dissipation of Δu into relatively large bubbles at low ui) or larger than that of the saturated specimen (related to collapse of relatively small bubbles at high ui). Consequently, the presence of bubbles had beneficially increased cu at relatively low ui (ui/[Formula: see text] < 0.6), and vice versa. The critical stress ratio of the reconstituted fine-grained gassy soil, however, did not appear to be altered by ui.

Experimental Validation of Terzaghi’s Effective Stress Principle for Gassy Sand

Gassy soils are a special case of unsaturated soils, for which the gas phase exists in the form of occluded bubbles of a size that allows the bubbles to fit within the void spaces without distorting the soil structure. The applicability of the original form of Terzaghi’s effective stress principle was tested for this class of soils. A series of globally undrained tests on gassy specimens, as well as drained effective stress path (ESP) tests on fully saturated specimens, was presented to validate this hypothesis. In a gassy test, the effective stresses were computed with the assumption that the Terzaghi’s effective stress principle was valid. In an ESP test, the effective stress path obtained from the experiment on an identical gassy specimen was applied to a saturated specimen with the drainage lines opened. Two groups of tests covering a degree of saturation (Sr) from 92 to 100% were performed. The application of the same effective stress path in the gassy and the ESP tests resulted in the same axial versus volumetric strain response, and therefore it was in accordance with the postulated validity of the Terzaghi’s effective stress principle under gassy states. Such results were found to be valid within the entire range of degrees of saturation explored in this study, and they provide new support for the use of the original definition of Terzaghi’s effective stress in geotechnical analyses for gassy deposits.

Mechanical behaviour of gas-charged fine sediments: model formulation and calibration

Multi-phase fluid conditions encountered in geotechnical and geo-environmental problems have led to the development of models that account for the influence of gas solubility and compressibility on the behaviour of soils of various grain sizes. Yet, no consideration has been given to damages related to the nucleation and growth of gas bubbles in fine-grained soils. The purpose of this paper is to present a Cam Clay based constitutive model extended to incorporate such detrimental effects on gassy soils. This is achieved by deriving an analytical expression relating the preconsolidation pressure to a damage parameter dependent on the gas content. That expression is coupled to a deviatoric yield surface accounting for inherent and stress-induced anisotropy. Comparisons of simulations and experimental results attest that introducing a single additional parameter to a conventional Cam Clay type model allows key behavioural features of gas-charged fine sediments to be reproduced.

Forensic numerical analysis of gas venting in Southwestern Ontario

A forensic modeling study examines the potential causes of gas and water venting observed during the excavation of a landfill in a 40m thick clayey till deposit in southwestern Ontario, Canada. The clayey till is known to be underlain by permeable, natural gas bearing, rock and gas has been diffusing through the clayey deposit over about the last 13,000–15,000years. Attention is focused on 2D modeling of hydrofracturing as well as gassy soil behavior. The model examines the predicted location of venting with and without the presence of gassy sand lenses and a discontinuous layer of basal till between the bedrock and the low permeability till. The modeling shows that hydrofracturing alone can explain the occurrence and location of the observed venting in one of the sub-cells. In the other sub-cell, hydrofracturing explains the occurrence but not the location of the observed venting or the presence of the sand in the venting water at this location. Modeling of appropriately placed gassy sand lenses is shown to explain the occurrence and location of the observed venting as well as the presence of both waters with different geochemistry from the bedrock and the till in the venting waters.

On the undrained unloading behaviour of gassy sands

Soils that contain large amounts of dissolved gas within the pore fluid are called gassy soils. Gassy soils are common in marine environments and it is important to further our understanding of the unloading behaviour of gassy soils because of their potential to initiate and propagate submarine slope failures. This paper focuses on the pore-pressure responses and volumetric strains of loose gassy sands under different undrained unloading stress paths in laboratory specimens. Special attention was given to the constant deviatoric stress (q-constant) undrained unloading stress path as it simulates the stress condition imposed by tidal drawdown — one of the potential triggers of landslides in gaseous marine sediments. Gas exsolution was observed when the pore pressure was reduced below the liquid gas saturation pressure. Upon further decreases in total stress, the resulting pore-pressure change was much less than the total stress change; hence, effective stress decreased rapidly and at a certain point the samples tested under the q-constant stress path collapsed. This paper has experimentally and theoretically shown that gas in free and (or) dissolved form is detrimental in undrained unloading stress paths.

On the undrained shear strength of gassy clays

The presence of methane gas will affect a soil’s strength, which will alter its resistance to submarine sliding and in turn impact offshore developments. This paper describes a constitutive model for determining the undrained shear strength of fine-grained gassy soils. The model results show that although the presence of gas has a beneficial effect on the soil strength in compressive loading, the peak strength is achieved at larger deformations. In addition, the model results showed that the undrained shear strength of gassy soils increases with increasing initial consolidation stress, increasing volumetric coefficient of solubility, and increasing initial void ratio.

Constitutive modeling of gassy sand behaviour

The behaviour of loose gassy sand was investigated with the use of a constitutive model. The constitutive model was modified from an existing model that focused on assessing the liquefaction of loose sands over a wide range of states and loading conditions. The modifications involved taking into account the compressibility and solubility of the pore gas and liquids. Hilf's equation, which calculates the pore-pressure change in a gassy or unsaturated soil subjected to an applied total stress, was incorporated into the model formulation. The initial degree of saturation is needed for model prediction, and the coefficient of volumetric solubility (Henry's constant) was introduced as a new model parameter. The modified model was used to predict the effect of gas on the undrained static behaviour of loose sand. The laboratory results of saturated specimens were modeled, and the predicted and observed behaviours were found to agree well. Results from gassy specimens were also predicted, and again the model predictions matched the test results. The model was used to confirm that gas has the effect of decreasing, but not eliminating, the susceptibility of loose sand to flow liquefaction. The major shortcoming of the modified model was its inability to predict the slight increase in effective normal stress that was observed in the initial stages of all the undrained triaxial tests. This shortcoming resulted in differences between the predicted and observed behaviour especially of strain-hardening specimens.Key words: gassy soil, liquefaction, constitutive modeling, triaxial testing.

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.

Excavations in gassy soils

Gassy soils are soils that contain gases under positive pressure. They can have a significant impact on the stability of excavations. This briefing note summarises two cases in southern Ontario, Canada, that highlight the importance of continued research into the behaviour of gassy soils.

Briefing: Excavations in gassy soils

Gassy soils are soils that contain gases under positive pressure. They can have a significant impact on the stability of excavations. This briefing note summarises two cases in southern Ontario, Canada, that highlight the importance of continued research into the behaviour of gassy soils.

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.

Consolidation of naturally gassy soft soil

Biogenic gas is produced and contained in fine-grained soils in environments such as river estuaries and landfill sites. Planning dredging operations or predicting the volume capacity of a landfill requires information about the amount of gas that can be held in the soil, and about the effect that the gas has on the soil behaviour. The purpose of this paper is to present some experimental results and interpretation relevant to these and related topics. A series of experiments has been carried out at Oxford University with soil of estuarine origin from the Slufter disposal site in the Netherlands. The soils were introduced in settling columns, and the different stages of consolidation were documented by measuring height, densities, excess pore pressures and amount of gas. At the end of primary consolidation the gassy soil was consolidated to higher stress levels by applying a hydraulic gradient. The growth rates of bacteria producing gas within the soil were accelerated or reduced by controlling the temperature in the range 10–30°C. The experimental results show a sequence of events during consolidation. In the first phase, gas was produced and accumulated within the soil. The overall density of the gassy soil decreased and pore pressures fluctuated unpredictably. At the end of this phase the amount of gas within the soil reached a critical threshold value and thereafter began to escape through cracks and fissures. This marked the start of a second phase of events. Although the gas production continued to be high, the total amount of gas within the soil slowly decreased. The soil began a new phase of consolidation, with the settlement accelerating, as the cracks and fissures provided a quick route for pore water dissipation. The self-weight stresses in these experiments do not exceed 1 kPa, so that higher stress levels were achieved by the application of a hydraulic gradient. During this stage, the gas within the soil became less influential as the soil gained in strength.

Consolidation of naturally gassy soft soil

Biogenic gas is produced and contained in fine-grained soils in environments such as river estuaries and landfill sites. Planning dredging operations or predicting the volume capacity of a landfill requires information about the amount of gas that can be held in the soil, and about the effect that the gas has on the soil behaviour. The purpose of this paper is to present some experimental results and interpretation relevant to these and related topics. A series of experiments has been carried out at Oxford University with soil of estuarine origin from the Slufter disposal site in the Netherlands. The soils were introduced in settling columns, and the different stages of consolidation were documented by measuring height, densities, excess pore pressures and amount of gas. At the end of primary consolidation the gassy soil was consolidated to higher stress levels by applying a hydraulic gradient. The growth rates of bacteria producing gas within the soil were accelerated or reduced by controlling the temperature in the range 10–30°C. The experimental results show a sequence of events during consolidation. In the first phase, gas was produced and accumulated within the soil. The overall density of the gassy soil decreased and pore pressures fluctuated unpredictably. At the end of this phase the amount of gas within the soil reached a critical threshold value and thereafter began to escape through cracks and fissures. This marked the start of a second phase of events. Although the gas production continued to be high, the total amount of gas within the soil slowly decreased. The soil began a new phase of consolidation, with the settlement accelerating, as the cracks and fissures provided a quick route for pore water dissipation. The self-weight stresses in these experiments do not exceed 1 kPa, so that higher stress levels were achieved by the application of a hydraulic gradient. During this stage, the gas within the soil became less influential as the soil gained in strength.

Cyclic liquefaction of loose gassy sand

Gas can be found in many soils, but none more common than in seabed soils. Gas-charged sediments are known to be widely distributed throughout the world's oceans. Numerous research programs have studied saturated soil response to cyclic loading; however, little is known about the behavior of gassy soils subjected to cyclic loading. The response of loose gassy specimens subjected to cyclic loading was studied in a laboratory program. Triaxial tests were performed on specimens of reconstituted Ottawa sand prepared to various gas contents and densities. The specimens were consolidated and then sheared under undrained cyclic loading. Gas content was measured at the beginning and end of each test using time domain reflectometry. The response of the gassy specimens has been shown to be influenced by the soil state and grain characteristics and the intensity, duration, and frequency of the cyclic loading. The propensity of a soil to liquefy is represented by a cyclic resistance ratio, defined as the cyclic stress ratio to cause liquefaction, or to cause a specified amount of strain. The results of this laboratory program show that the cyclic resistance ratio increases as gas content and density increase. The laboratory study demonstrates that the cyclic liquefaction potential of a soil is reduced by the addition of gas; however, soils containing some gas may still experience cyclic liquefaction provided they are sufficiently loose.Key words: triaxial testing, cyclic liquefaction, Ottawa sand, gassy, unsaturated.

An acoustic study of soils that model seabed sediments containing gas bubbles

The acoustic response of gassy seabed sediment is unique. It is a dispersive and extraordinarily attenuative natural material at frequencies which cause gas bubble resonance. It conceals the structure of the seabed from seismic profiling and it dampens acoustic signals that, for example, trigger acoustic mines. In the past, theoretical studies have formulated the probable cause of this response and crude experimental work has partially corroborated theory. This study measures compressional wave velocity and attenuation in a laboratory soil simulating natural gassy soil, and it investigates the structural properties that cause the unique acoustic response. It was confirmed that below the frequencies which cause resonance the soil behaves as a compressible material (containing gas), and above as a relatively incompressible material (containing no gas). Over the frequency range producing bubble resonance it is suggested that the soil should be modeled as a biphasic material of gas and a relatively incompressible saturated soil matrix (particles and fluid). Velocities for gassy soil were found to be as low as 220 m/s at frequencies below resonance and 1500 m/s above resonance; attenuations were found to be as high as 60 dB/cm for moderately gassy soil and as low as 1 dB/cm for soil with almost no gas.