A series of triaxial tests has been carried out on the mechanical properties and dissociation characteristics of sands containing methane hydrate using an innovative high pressure apparatus which has ...been developed to reproduce the in-situ conditions expected during proposed methane extraction methods. It was found that the strength of MH sand increased with MH saturation due to particle bonding. Dissociation by heating caused large axial strains for samples with an initial shear stress and total collapse for samples consolidated in the metastable zone. In the case of dissociation by de-pressurization, axial strains were generated by increasing effective stress until a stable equilibrium was reached. However, re-pressurization led to the collapse in the metastable zone.
A series of undrained monotonic and cyclic triaxial tests were performed on silica sand at two initial densities and different confining pressures from 0.1 to 5 MPa to investigate their shear ...response and crushing behaviour. The influence of particle crushing on the undrained shear strength and pore-water pressure was examined. To clarify the evolution of particle crushing, undrained monotonic and cyclic tests were terminated at several distinctive stages and sieving analysis tests were subsequently performed on the tested specimens. In the undrained monotonic test, specimens exhibited remarkable dilation behaviour and experienced no apparent particle crushing at low confining pressures. An increase in the mean stress suppressed the dilatancy due to a faster increase of the pore-water pressure, giving rise to the occurrence of particle crushing. In the undrained cyclic test, a higher confining pressure and cyclic stress ratio resulted in a much higher relative breakage. At a specific cyclic stress ratio, the relative breakage increased as the cyclic loading progressed. The confining pressure and shear strain amplitude played a significant role in controlling the evolution of particle breakage. The correlation between the relative breakage and plastic work for specimens under isotropic consolidation, undrained monotonic, and cyclic loadings has been validated experimentally.
A series of tests were conducted in order to investigate the shear strength and deformation behavior of methane hydrate-bearing sediments during dissociation using the thermal recovery method or ...depressurization method. An innovative temperature-controlled high pressure triaxial apparatus which can reproduce the in situ conditions of hydrate reservoirs was used. The results indicate that: (1) the failure strength of isotropically consolidated methane hydrate-bearing sediments which dissociated completely using the thermal recovery method is less than that of pure Toyoura sand. However, the initial stiffness and volumetric strain are higher than that of pure Toyoura sand. (2) The thermal recovery method will cause the failure of methane hydrate-bearing sediments when the axial load is higher than the strength of methane hydrate-bearing sediments after dissociation. (3) The depressurization method will not cause collapse of methane hydrate-bearing sediments during depressurization. However, water pressure recovery will lead to failure when the axial load is larger than the strength of the methane hydrate-bearing sediments after dissociation. (4) The depressurization rate shows little effect on the ultimate deformation of methane hydrate-bearing sediments, while the initial deformation rate increases with increasing depressurization rate. (5) The larger the reduction of pore pressure, the larger axial strain and volumetric strain.
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•Both thermal recovery and depressurization method were used to dissociate methane hydrate.•The mechanical behavior during dissociation was obtained.•Thermal recovery method will lead to failure of specimen in certain conditions.•Depressurization will not lead to collapse, while water pressure recovery will lead to failure in certain conditions.•The effects of depressurization rate and reduction of pore pressure on the deformation behavior were obtained.
AbstractA significant increase in effective stress can be induced in hydrate-bearing reservoirs when the depressurization method is applied. A series of drained triaxial shear tests were performed on ...hydrate-bearing sediments with various hydrate saturations to investigate their mechanical characteristics under effective confining pressures of up to 20 MPa. The results show that significant particle crushing of the host sand occurs during shearing under high pressures, and there is no remarkable effect of hydrate saturation on the degree of particle breakage. As the effective confining pressure increases, the stress–strain curves of the hydrate-bearing specimen transformed from strain-softening to strain-hardening. The peak stress ratio and internal friction angle of the sediments gradually decrease and tend to be constant with the increased confining stress, whereas the cohesion in hydrate-bearing sediments exhibits an increasing tendency. The critical state line (CSL) of hydrate-bearing sediments in the e-lnp′ space under low-to-high effective confining pressures intersects with the normal consolidation line under the same hydrate saturation. Furthermore, the CSL moves upward and rotates clockwise as the hydrate saturation increases.
This investigation examines the effect of clamshell ash (CSA) and lime additives on the physico-mechanical characteristics of kaolinite clay soil stabilized at the optimum silica fume content. ...Laboratory tests were performed to assess plasticity, shrink-swell characteristics, compaction characteristics, unconfined compressive strength (UCS), shear strength characteristics, mineralogical and morphological microstructure characteristics of stabilized soil specimens. The kaolinite clay soil was stabilized at its optimum silica fume content (6%) to produce the highest strength and was altered with three non-identical proportions of clamshell ash and lime (3%-9%). Cylindrical soil specimens, 76 mm in height and 38 mm in diameter, were moulded and treated for curing periods of 1, 7, 14, and 30 days to examine the strength of the altered soil. The findings revealed that, adding clamshell ash and lime significantly alters the plasticity, shrink-swell, maximum dry unit weights, and optimum moisture contents of the silica fume-stabilized soil. In terms of strength, the beneficial effects of CSA and lime additives were found to be more significant with more extended curing periods. Incremental increases in curing periods resulted in further enhancements in UCS, cohesion, and internal friction angle, indicating continued strength development over time. Microstructural analysis using field emission scanning electron microscopy and X-ray diffraction provided insights into the interparticle bonding mechanisms and microstructural changes induced by the addition of CSA and lime. The emergence of cementitious phases and pozzolanic responses between soil particles and stabilizers contributed to the densification and strengthening of the stabilized soil matrix. The findings of this study provide valuable insights into the potential of clamshell ash and lime additives to enhance the engineering properties of kaolinite clay soil stabilized with silica fume. These results have implications for sustainable soil stabilization practices, offering a promising approach to improve the performance of soils for various engineering applications, including construction and geotechnical projects.
Exploitation of gas from deep-sea methane hydrate-bearing layers might lead to some geological disasters, including marine landslides and excessive settlement of marine ground. The first offshore gas ...production tests for methane hydrate-bearing sediments were carried out in eastern Nankai Trough. However, knowledge on mechanical behavior of gas hydrate reservoirs with similar gradation and minerology component to the marine sediment is still insufficient. Consequently, proper modeling of geomechanical properties of methane hydrate-bearing sediments is crucial for reservoir simulation and deep ocean ground stability analysis for long-term gas production in the future. This study conducted a series of triaxial shear tests to examine the shear response of methane hydrate-bearing sediments with a similar grading curve and minerology components to the hydrate-rich sediments in Nankai Trough. The test results demonstrated that the presence of hydrate mass between sand grains altered the stress-strain pattern from strain-hardening to postpeak strain-softening. A simple constitutive model based on several empirical relationships of granular materials is proposed to describe the stress-strain relationship of methane hydrate-bearing sediments under triaxial stress condition. This model can reproduce the enhancement of shear strength, initial stiffness, and dilation behavior of methane hydrate-bearing sediments containing different amounts of fines content with a rise in the methane hydrate saturation at a wide range of effective confining pressures. The numerical results indicate that the parameter A associated with initial stiffness of stress-strain curve and the parameter α related with dilation properties are jointly governed by the confining pressure, fines content, and hydrate saturation.
•A simple constitutive model is proposed to capture the mechanical properties of methane hydrate-bearing sediment.•The influence of hydrate saturation on the stress-dilatancy relationship is included.•Mean stress at the critical state is an influential parameter governing stress-strain pattern.•The effect of the presence of hydrate on the peak shear strength, initial stiffness and dilation behavior is simulated.
The Acheulean technological tradition, characterized by a large (>10 cm) flake-based component, represents a significant technological advance over the Oldowan. Although stone tool assemblages ...attributed to the Acheulean have been reported from as early as circa 1.6–1.75 Ma, the characteristics of these earliest occurrences and comparisons with later assemblages have not been reported in detail. Here, we provide a newly established chronometric calibration for the Acheulean assemblages of the Konso Formation, southern Ethiopia, which span the time period ∼1.75 to <1.0 Ma. The earliest Konso Acheulean is chronologically indistinguishable from the assemblage recently published as the world’s earliest with an age of ∼1.75 Ma at Kokiselei, west of Lake Turkana, Kenya. This Konso assemblage is characterized by a combination of large picks and crude bifaces/unifaces made predominantly on large flake blanks. An increase in the number of flake scars was observed within the Konso Formation handaxe assemblages through time, but this was less so with picks. The Konso evidence suggests that both picks and handaxes were essential components of the Acheulean from its initial stages and that the two probably differed in function. The temporal refinement seen, especially in the handaxe forms at Konso, implies enhanced function through time, perhaps in processing carcasses with long and stable cutting edges. The documentation of the earliest Acheulean at ∼1.75 Ma in both northern Kenya and southern Ethiopia suggests that behavioral novelties were being established in a regional scale at that time, paralleling the emergence of Homo erectus- like hominid morphology.
Methane hydrate‐bearing sediments with different amounts of fines content and at three densities were artificially prepared under controlled temperature and pressure conditions. The void ratios of ...specimens after isotropic consolidation tend to decrease with a rise in fines content. The fines particles enter into the pore space between sand grains and densify the specimens. A series of triaxial compression tests were performed to systematically investigate the influences of fines content and density on the shear properties of hydrate‐free sediments and methane hydrate‐bearing sediments. The test results demonstrate that a rise in fines content within methane hydrate‐bearing sediments significantly enhances peak shear strength and promotes dilation behavior. These influences are particularly prominent for specimens at loose packing state. A decrease in void ratio increases the shear strength and stiffness of hydrate‐free sediments and methane hydrate‐bearing sediments containing fines content of 0% and 8.9%. It is noted that the formation of methane hydrate in samples with varying amounts of fines content increases the stress ratios at the critical state. The addition of fines particles into coarse‐grained sand grains alters the internal microstructure of sand matrix and the hydrate formation pattern in the pore space between sand grains and fines particles.
Key Points
A series of triaxial compression tests were performed on methane hydrate‐bearing sediments and hydrate‐free sediments with different amounts of fines content and at three levels of density
The rise in fines content and decrease in void ratio enhanced the peak shear strength and promoted dilation behavior of methane hydrate‐bearing sediments
The presence of methane hydrate increased the stress ratios at the critical state of methane hydrate‐bearing sediments with various amounts of fines content
Depressurization is an effective method to produce methane gas from methane hydrate reservoirs. However, during gas production, sediments consolidate due to increasing effective stress. Revealing the ...compressive characteristics of methane hydrate-bearing sands during consolidation is essential for an accurate understanding of sediment properties and for the development of a constitutive model. Therefore, a series of isotropic consolidation tests was performed on sand in which methane hydrate was artificially generated, and its compressibility characteristics were evaluated. Furthermore, to assess prolonged production, creep compressive behavior was investigated. The experimental results showed volumetric strain due to increasing confining stress decreased with increasing initial methane hydrate saturation. Particle crushing during consolidation was inhibited by the presence of methane hydrate. It was confirmed that the increase in the effects of methane hydrate on soil compressibility followed a power function with the increase in methane hydrate saturation. Creep deformation was observed during the stress holding period regardless of the presence of methane hydrate. Creep behavior during the stress holding period was related to the extent to which the creep component had dissipated before the stress holding period in the past. A theoretical concept for creep strain was proposed based on the experimental results.
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