Failure Mechanisms in Cemented Hydrate-Bearing Sands - American

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Failure Mechanisms in Cemented Hydrate-Bearing Sands Shmulik Pinkert*,† and Jocelyn L. H. Grozic Department of Civil Engineering, University of Calgary, Calgary, AB T2N1N4, Canada ABSTRACT: A significant portion of our knowledge on gas hydratebearing sands comes from experimental results on laboratory-synthesized specimens. The failure mechanics are often interpreted using components of the stress−strain curves, which capture the specimen’s global (largescale) response to shear. In this paper, we postulate on the microscale mechanics, which lead to a variety of interesting global behaviors. Two mechanisms of failure during shear are postulated: one involves debonding of the hydrate particle from the soil solid, and the other involves crushing or breakage through the hydrate itself. Both modes of failure lead to similar peak strengths, which arise from both friction and apparent cohesion induced by the hydrate bonding; however, the differences observed in postpeak softening may be attributed to the different failure mechanisms. Global specimen responses such as sudden strength loss, occurrence of double shear banding, and differences in postpeak behavior are manifestations of the microscale hydrate sand interactions.

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INTRODUCTION Determining the mechanical properties of gas hydrate-bearing sediments is an essential prerequisite to evaluate a hydrate accumulation for potential gas production, to predict slope instability, or even methane release. Currently, mechanical properties are evaluated on the basis of laboratory experiments, which can be performed on undisturbed samples or on artificial laboratory formed specimens. Extraction of an undisturbed natural hydrate-bearing sample for experimental use requires special techniques such as pressure coring and pressure transfer systems; because of the considerable expense associated with these methods, the majority of experimental knowledge has been acquired using laboratory-formed hydrate-bearing specimens. Artificial specimens facilitate the creation of known assemblages in which the composition, grain characteristics, and phase distribution within the specimen are well characterized and repeatable. Laboratory-formed sediments can be constructed using different techniques, which are generally divided into three groups according to the hydrate−soil interaction they represent; pore-filling, load-bearing, and cementation habits, as shown qualitatively in Figure 1. The mechanical response under loading is different for each of these models. For pore-filling hydrate morphology, the small-strain deformation behavior is less affected by the presence of hydrate because

the soil particles are free to move and rotate. With further deformation, however, re-engagement of soil particles with the pore-filled hydrate may convert the system into a load-bearing hydrate−soil interaction.1 For a load-bearing morphology, the hydrate provides additional support to the soil structure but without entirely preventing particle displacement and rotation. In a cemented hydrate-soil structure, the hydrate bonds with the soil particles and thus creates a significantly stiffer and stronger sediment. This paper focuses on a hydrate cementing morphology in sand specimens. The cementation properties are strongly related to the hydrate saturation, which affects the stiffness,2,3 strength,4−10 and dilation11−13 of the sediment. The correlation between hydrate saturation and the sediment’s mechanical properties has been investigated by examining the bond width and thickness and a bond failure envelope, which includes compression, shearing, and rotation effects within the bond.14−16 These cementation bonds affect shear failure in two distinct ways: (1) the hydrate can break, either within the hydrate or between the hydrate and sand particle leading to a loss of stiffness and cohesion; or (2) the hydrate can act like additional small particles in the sand structure, which will force the hydrate and/or sand particles to ride over other particles, leading to an increase in dilation. Although a number of experimental tests have been performed on hydrate-bearing sands,7,17,18 the microstructure interaction during shearing has not been extensively investigated and many questions remain about the mechanical behavior of hydrate-bearing sands at the microlevel. This paper presents a simple theory of hydrate− Special Issue: In Honor of E. Dendy Sloan on the Occasion of His 70th Birthday Received: July 9, 2014 Accepted: November 12, 2014

Figure 1. Hydrate−soil interaction models for the three main hydrate growth morphologies. © XXXX American Chemical Society

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dx.doi.org/10.1021/je500638c | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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sand interaction during shear failure by qualitatively describing how the soil characteristics and hydrate growth morphology govern the observed response.

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DEBONDING AND CRUSHING OF HYDRATE IN SANDS Tension Failure. The mechanical interaction between hydrate and different soil solids has been experimentally examined through tensile tests. Jung and Santamarina19 examined adhesive versus tensile failure of CH4 and CO2 hydrate attached to a flat surface of calcite or mica, in which either a debonding between the hydrate and the soil solid or a hydrate−hydrate tensile failure was observed. Figure 2

Figure 3. Illustration of three possible failure modes in cemented hydrate-bearing sand.

overall sand/hydrate structure, and hydrate breakage during shearing changes the overall porosity and may increase overall mobilized friction. Thus, the sediment’s strength properties, namely cohesion and friction, will be modified during the shearing process.

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HYDRATE BREAKAGE AND DOUBLE SHEAR BAND The concepts of hydrate breakage discussed above have been observed through manifestations presented in laboratory data and results. Hydrate breakage has been inferred through the presentation of a double shear band. Experimental Procedure. A series of drained triaxial tests were conducted on CO2 hydrate-bearing Ottawa sand. Figure 4

Figure 2. An illustration of adhesion and tensile failure modes.

qualitatively illustrates the two failure modes observed. The experimental results showed that whether a tensile or debonding failure occurred depended on the mineral surface properties; for CO2 and CH4 saturated samples, mica experienced an adhesion failure with debonding between the mineral surface and the hydrate, while calcite failed in tension with breakage occurring within the hydrate. Aman et al.20 presented direct micromechanical experimental measurements of the adhesion force between cyclopentane hydrate and calcite or quartz solids. These experiments also indicate that the strength of the hydrate/solid bond depends on the solid mineralogy, specifically the soil surface composition. Shear Failure Mechanisms in Hydrate-Bearing Sand. In the tensile test, the failed hydrate does not have any further interaction with the solid particles. However, in shear failure, the crushed/debonded material will continue to interact with other particles in its new (postpeak) morphology. In hydratebearing sand, this postpeak interaction has been observed in triaxial tests, where a clear yield point is observed4,21 and the sediment response changes after initial peak strength (i.e., failure). This well-defined yield point has also been noted in other cemented soil (e.g., the works by Leroueil and Vaughan22 and by Airey23). Analogous to the two modes of tensile failure discussed previously, during shearing of hydrate-bearing sands, hydrate can be debonded from the sand particles or break through the hydrate as shown in Figure 3. Generally, hydrate breakage indicates that the hydrate shear strength is lower than the hydrate−sand interface bond. A mixed failure mechanism, as illustrated schematically in Figure 3, could also occur. In this mechanism, the hydrate is debonded from the sand grains, but then some hydrate breakage occurs during shearing (thus less volumetric change would be observed relative to debonding with no breakage). Hydrate debonding and hydrate breakage affect the sediment shearing; debonded hydrate particles remain as small solid particles which occupy the pore spaces in the

Figure 4. Particle size distribution curve of 20/30 Ottawa sand. Percent passing, PP, versus particle size, d.

shows the particle size distribution curve of Ottawa sand, a poorly graded rounded to subrounded sand. A moist specimen was prepared to 5/10 cm diameter/height and placed in a highpressure temperature controlled triaxial apparatus. A nominal cell pressure of 0.7 MPa was applied, and then the pore air was replaced by CO2 gas by flushing at room temperature. Cell and back (CO2 gas) pressures were increased to 3.7 MPa and 3 MPa, respectively, and the cell temperature decreased to 3 °C. The specimen was allowed to equilibrate for approximately 15 h, during which time an increase in P-wave velocity was observed, indicating hydrate formation. Cell pressure was then increased to 4 MPa, thus consolidating the specimen to a 1 MPa effective confining stress. Shearing was induced by applying axial displacement at a rate of 0.1 %·min−1. Hydrate saturation was calculated on the basis of void volume evaluation, initial water content, and mass balance calculation B



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for CO2 hydrate, considering a CO2 hydrate density of 1.107 g· cm−3.24 Results and Analysis. Figure 5 shows the stress−strain curve obtained from one of these series of tests. The response

Figure 7. Failure envelopes for the first and the second failure points in the hydrate-bearing experiments. Shear stress, τ, versus normal stress, σ. The dashed and the solid lines represent the Mohr−Coulomb failure envelopes for the hydrate-bearing sand at the first and the second peaks, respectively.

(due to lack of additional water after a sufficiently long formation time prior to shear), and therefore the cohesion cannot be higher than chydrate, which indicates that the friction must increase to reach the second peak. An increase in friction can be attributed to the hydrate, which is being crushed with additional shearing. Crushing will have the effect of decreasing the porosity such that the specimen is behaving as a denser sand; the modified friction angle, ϕs+hyd, is used to describe the specimen in this state. When initial failure occurs, the apparent cohesion from hydrate bonding will be lost and thus no longer relevant to the failure behavior. Therefore, a failure envelope for the second peak is proposed with no cohesion and a modified friction angle ϕs+hyd, as illustrated in Figure 7. The modified friction angle, with no cohesion, is used to describe the response of the sediment after the initial peak stress, that is, after the first failure. According to Mohr−Coulomb theory, different friction angles refer to different failure planes, when applying the same loading conditions. Figure 8a illustrates the two failure plane directions associated with the two peaks (with the pole located according to triaxial compression test conventions). After testing, two shear bands, with different failure directions (Figure 8b), were observed on the specimen. A general

Figure 5. Drained triaxial test result, at 42% hydrate saturation (mass fraction). Deviatoric stress, q, versus axial strain, εa.

shows an unusual behavior; after reaching a peak stress of approximately 2.7 MPa, the stress shows a slight decrease during the strain increment of (1.5−3.5) %. Then there is a sudden and significant drop (in less than 1 s) in deviatoric stress, followed by a second increase to a higher deviatoric stress of approximately 3 MPa. All gauges were monitored at that moment and no technical error was found. Such a phenomenon of a “recovery” after a sharp drop in deviatoric stress has been observed in hydrate-bearing sand,18 and cemented sandstone.25 A Mohr−Coulomb representation of the peak strength of both the hydrate-bearing test and a similar drained test on pure sand are illustrated in Figure 6. The failure envelope for pure

Figure 6. Mohr−Coulomb representation of the failure points in the pure sand test and in the hydrate saturated sample (Figure 5). Shear stress, τ, versus normal stress, σ. The dashed and the solid lines represent the Mohr−Coulomb failure envelopes for pure sand and for the hydrate-bearing sand at the first peak, respectively.

sand is denoted by the dotted line, which is characterized by a friction angle, ϕsand, and no cohesion, as would be expected. The failure state of the hydrate saturated sample at the first peak stress is also plotted. It is assumed that the intact hydrate within the sediment does not affect the friction properties26 but only the bonding between hydrate and soil, which is expressed by an apparent cohesion in Mohr−Coulomb terms. Therefore, a failure envelope to describe the first peak strength is illustrated (Figure 6) with cohesion value offset, chydrate, and the friction angle, ϕsand. This friction angle and cohesion offset are used to describe the hydrate-bearing sediment prior to and at peak stress. Figure 7 shows the Mohr−Coulomb circles of the first peak and second (recovered) stresses of the hydrate-bearing specimen. Additional hydrate will not form during the test

Figure 8. (a) Failure directions, marked in red, according to the two observed peak strengths (according to Figures 6 and 7), (b) specimen after testing with a double shear band, and (c) the agreement between the two, shown upon a linear illustration of the specimen. C



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Figure 9. Stress−strain and ultrasonic P-wave responses (signal, S, versus time, t), of two samples with the same initial conditions but different hydrate formation times. The solid black and gray lines represent the samples with 15 h and 24 h hydrate formation, respectively.

Figure 10. Hydrate formation progress. (a) Experimentally measured hydrate formation progress (subfigures a to f) in a water film, after Beltran and Servio,29 and accordingly, (b) an illustration of the hydrate growth in the capillary water at the sand grain contacts.

angularity or the particle distribution, both scenarios provide indirect support to the hypothesis that a cohesion intercept governs the behavior up to the initial peak stress, while postpeak behavior is governed by no cohesion and an increased friction angle.

agreement of the failure directions was found between the Mohr−Coulomb prediction and the observed shear bands (Figure 8c). The first shear band, corresponding to peak failure, matches well a prediction using a cohesion offset and the friction angle for unhydrated sand. The second shear band, formed in the already partially failed material, developed a friction angle which can be described by using a modified angle that takes into account the presence of the hydrate solid particles within the soil skeleton matrix. The occurrence of double shear bands is in general agreement with Jiang et al.27 who noted a difference in the internal friction angle inclination of the developed shear bands of very dense granular materials at peak and residual stresses in the order of 3 to 4 degrees using Distinct Element Modeling. Although this experimental outcome cannot be explicitly related to the hydrate-crushing theory, it provides an explanation to the odd stress−strain response observed (Figure 5). In a hydrate cementation morphology, after hydrate breakage occurs, the relatively rounded sand particles (Ottawa sand 20/ 30) may also become more angular due to stiff crushed hydrate pieces which are still cemented at the particle contacts. These evolved angularities can directly affect the mechanical behavior by altering the shearing and rolling resistance15,16,28 or indirectly influence the friction angle. For hydrate particles that entirely separate from the hydrate-sand structure, it is reasonable to assume a more well-distributed particle grading relative to pure sand. Whether the crushed hydrate affects the

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EFFECT OF FORMATION TIME ON HYDRATE STRUCTURE To examine the influence of formation time on the resulting hydrate structure, two different specimens with the same moisture content (i.e., designed for same hydrate saturation) were tested in drained shear under the same stress conditions. After cell and back pressures were applied, the temperature was reduced and the specimens were allowed to equilibrate and form hydrate for different durations; 15 h and 24 h. Upon shearing, the specimens reached the same peak stress but exhibited different postpeak reductions (Figure 9a), where the specimen with the longer formation time had less strength reduction. Figure 9b shows ultrasonic P-wave signals, which were taken after hydrate formation and just prior to shearing. The obtained P-wave traces show virtually the same arrival time, but different wave amplitudes. A fundamental hydrate-formation mechanism can be considered, for which the similarity and diversity in the aforementioned aspects in the tests can be explained. As mentioned, during the current experiments, the hydrate was formed using the excess-gas method. In this method, it can be D



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assumed that the water is located within the grain contacts due to capillary forces. Figure 10 presents experimental based results of hydrate formation progression in pore water. As can be seen from Figure 10a,29 the hydrate begins to form at the gas water interface and grows, with time, inward. On the basis of this concept, within the sediments (Figure 10b), the water in direct contact with CO2 gas will form hydrate first. Hydrate formation in the next water layer requires CO2 gas to pass through the already formed hydrate shell. Accordingly, the hydrate formation rate exponentially decreases with time. It should be noted that the hydrate shell presented in Figure 10b may experience cracking and spreading during the formation process due to volumetric change in the already formed hydrate, but regardless, the water at the grain-contact will be the last to be converted into hydrate because of surface tension effects. P-waves were propagated through the specimen using piezocrystals, from the specimen bottom to the receiver at the top. P-wave velocity provides an indication of the elastic response of the sediment. Generally as hydrate grows at the particle contacts, the P-wave arrival time reduces and the wave amplitude increases due to an increase in stiffness. As seen in Figure 9b, no significant differences in P-wave arrival time were observed; although it would be expected the 24 h specimen would have a quicker arrival time, minor variations in height between the two specimens may account for slight increase in arrival time. However, the two specimens show large variations in wave amplitude; Priest et al.30 and Kneafsey and Nakagawa31 have shown that the presence of water during the formation process will increase the attenuation and thus reduce the wave amplitude. Therefore, it can be inferred that the 15 h specimen contains more water and thus less hydrate. This concept is described qualitatively in Figure 11, where the P-wave travels through the stiff hydrate bridge,

Figure 12. Two options for failure mechanisms of a partly hydrateformed model.

formation times can be translated into different hydrate saturation scenarios, which affect the postpeak behavior in the sediment strength. Therefore, the differences in the measured deviatoric stress and postpeak response between the two specimens (Figure 9a) can be attributed to the breakage/crushing mechanisms; if the failure mechanism was pure debonding, we would expect to see the same postpeak behavior. This is in agreement with the conclusion from the “double shear band” test, which was discussed previously, whereby the same failure mechanism (breakage/crushing) was found to be dominant and thus altering the postpeak frictional characteristics.

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DISCUSSION Wang and Leung32 presented a series of triaxial tests on Portland cemented sand. The results showed a strong relation between cement content and residual strength, in which higher initial cementation led to an increase in residual deviatoric stress (even after the contacts were broken). Considering Mohr−Coulomb parameters, it can be assumed that there is no cohesion after large shearing, but only an increase in friction angle. The alteration of strength properties during shearing is similar to our postulation on the behavior of hydrate-bearing sands. After breakage, the cementation material (in this case hydrate) can be considered as a new grain shape, with an increased angularity (this is particularly influential if the original sand is rounded, such as Ottawa sand). Hydrate bond resistance to compression, shearing and particle rotation has been investigated analytically and numerically.14−16 Bond length and width were correlated with hydrate saturation and bond strength, and programed in a Discrete Element Method (DEM) for the evaluation of the physical macroscale response. However, after large shearing the DEM predicted a return to the uncemented properties, in which the friction angle did not modify. This is in contradiction with the experimental results by Wang and Leung;32 although it is possible that the differences are due to the cementing material properties. The current work adds another dimension to our understanding of hydrate−sand interaction in which the microscale behavior after failure was investigated. The two concepts discussed here deal with one of the least complex scenarios, where uniformly graded sand is used as the hydrate host sediment. Nonetheless, the uniformity (and also the roundness) of the sand enabled more direct interpretations about the microscale behavior to be made. The two presented case studies show different and interesting postpeak strength behavior, in which the first clearly shows an increase in strength after a postpeak stress reduction, and the second shows different postpeak degradations for differing hydrate structures. The analysis for the two case studies shows a relation between the hydrate failure mechanism and a potential

Figure 11. Qualitative description of the observed P-wave arrival time and wave amplitude differences when hydrate is not yet fully formed.

hence arriving at a similar time for both specimens regardless of whether the hydrate is fully formed or not. However, the amplitude is significantly reduced in the 15 h specimen, indicating the free water remains. On the basis of the hydrate distribution model suggested in Figure 10, a failure mechanism is proposed as illustrated in Figure 12. If a debonding failure mechanism governs, then the hydrate particles will still cause the same amount of dilation, even though not all the hydrate is fully formed, since the hydrate-bridging structure is not damaged. Therefore, for the debonding scenario, the failure response will not be so affected by differences in hydrate formation. However, if the stiff hydrate structure breaks, the quantity of solid particle (and not the free water) will affect the modified friction angle of the crushed sediment. In this case (and only after failure), different E



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Notes

increase in the specimen friction. This increase in friction is associated with the presence of smaller solid hydrate particles. Therefore, for this particular Ottawa sand, a crushing/breakage hydrate behavior was determined to be more likely to occur than a debonding behavior, in which the hydrate structure does not break. This sort of failure mechanism may also be expected in other sand types for which low capillary forces exist, such as uniform, rounded to subrounded sands. Although not included in the scope of this paper, the fundamental theory of breakage/debonding within the hydrate suggested here can be expanded for more complex soil structures, such as well-graded sands with varying fines contents, and sands of different angularity and/or sphericity. In addition, the effects of microscale very localized dissociation within the shear zone (e.g., the work by Durham et al.33) should also be considered.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Jeffrey Priest for his discussions leading to the development of the scientific knowledge.

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(1) Brugada, J.; Cheng, Y. P.; Soga, K.; Santamarina, J. C. Discrete Element Modelling of Geomechanical Behaviour of Methane Hydrate Soils with Pore-Filling Hydrate Distribution. Granular Matter 2010, 12, 517−525. (2) Priest, J. A.; Best, A. I.; Clayton, C. R. I. A Laboratory Investigation into the Seismic Velocities of Methane Gas HydrateBearing Sand. J. Geophys. Res.: Solid Earth 2005, 110, B0410. (3) Priest, J. A.; Rees, E. V. L; Clayton, C. R. I. Influence of Gas Hydrate Morphology on the Seismic Velocities of Sands. J. Geophys. Res.: Solid Earth 2009, 114, B11205. (4) Yun, T. S.; Santamarina, J. C.; Ruppel, C. Mechanical Properties of Sand, Silt, and Clay Containing Tetrahydrofuran Hydrate. J. Geophys. Res.: Solid Earth 2007, 112, B04106. (5) Waite, W. F.; Santamarina, J. C.; Cortes, D. D.; Dugan, B.; Espinoza, D. N.; Germaine, J.; Jang, J.; Jung, J. W.; Kneafsey, T. J.; Shin, H.; Soga, K.; Winters, W. J.; Yun, T. S. Physical Properties of Hydrate Bearing Sediments. Rev. Geophys. 2009, 47, RG4003. (6) Jiang, M.; Yan, H. B.; Zhu, H. H.; Utili, S. Modeling Shear Behavior and Strain Localization in Cemented Sands by Two Dimensional Distinct Element Method Analyses. Comput. Geotech. 2011, 38, 14−29. (7) Miyazaki, K.; Tenma, N.; Aoki, K.; Yamaguchi, T. A Nonlinear Elastic Model for Triaxial Compressive Properties of Artificial Methane-Hydrate-Bearing Sediment Samples. Energies 2012, 5, 4057−4075. (8) Zhang, X. H.; Lu, X. B.; Zhang, L. M.; Wang, S. Y.; Li, Q. P. Experimental Study on Mechanical Properties of Methane-HydrateBearing Sediments. Acta Mech. Sin. 2012, 28, 1356−1366. (9) Hyodo, M.; Nakata, Y.; Yoshimoto, N.; Yoneda, J. Mechanical and Dissociation Properties of Methane Hydrate-Bearing Sand in Deep Seabed. Soils Found. 2013, 53, 299−314. (10) Pinkert, S.; Grozic, J. L.H. An Analytical-Experimental Investigation of Gas Hydrate Bearing Sediment Properties. Canadian Geotechnical Conference and the 11th Joint CGS/IAHCNC Ground Water Conference, Montreal, Canadea 2013; Paper 645. (11) Pinkert, S.; Grozic, J. L. H. Prediction of the Mechanical Response of Hydrate Bearing Sands. J. Geophys. Res.: Solid Earth 2014, 119, 4695−4707. (12) Masui, A.; Miyazaki, K.; Haneda, H.; Ogata, Y.; Aoki, K. Mechanical Characteristics of Natural and Artificial Gas Hydrate Bearing Sediments. In Proceedings of the 6th International Conference on Gas Hydrates, Vancouver, Canada, 2008. (13) Sultan, N.; Garziglia, S. Geomechanical Constitutive Modelling of Gas-Hydrate-Bearing Sediments. The 7th International Conference on Gas Hydrates, Edinburgh, Scotland, July 17−21, 2011. (14) Jiang, M. J.; Yu, H. S.; Harris, D. Bond Rolling Resistance and its Effect on Yielding of Bonded Granulates by DEM Analyses. Int. J. Numer. Anal. Meth. Geomech. 2006, 30, 723−761. (15) Jiang, M.; Liu, F.; Zhu, F.; Xiao, Y. A Simplified Contact Model for Sandy Grains Cemented with Methane Hydrate. The 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris, France, 2013; pp 1015−1018. (16) Jiang, M.; Zhu, F.; Liu, F.; Utili, S. A Bond Contact Model for Methane Hydrate-Bearing Sediments with Interparticle Cementation. Int. J. Numer. Anal. Meth. Geomech 2014, DOI: 10.1002/nag.2283. (17) Ghiassian, H.; Grozic, J. L. H. Methane Hydrate Formation Under Controlled Pressure in Conventional Triaxial Apparatus. In Proceedings of the 63rd Canadian Geotechnical Conference, Calgary, Alberta, 2010; pp 12−16.

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CONCLUSIONS Two fundamental shearing failure models of hydrate-bearing sand were presented; one in which the hydrate within the pore space is either debonded from the sand particles or crushed into smaller particles. Using laboratory test results, which represent a whole sediment response (i.e., a mesoscale response), these two microscale failure mechanisms were examined. Two case studies were presented. In the first, a significantly sudden drop in stress was observed during the test, followed by an increase in stress after the drop. A Mohr− Coulomb analysis indicated that crushing rather than debonding occurred, and that the friction angle following initial crushing increased due to a better sediment grading. The apparent cohesion initially present within the cemented hydrate−sand structure was lost at failure and was not recoverable, and thus no longer relevant to the failure behavior analysis. Verification of this theory was provided by the observation of a double shear band in the specimen at the end of the test, indicating two distinct shearing modalities. In the second case study, two hydrate-bearing sand specimens with different hydrate formation durations were examined. Ultrasonic P-wave measurements showed the same arrival time for each, but different amplitudes of the P-wave signal were observed. This phenomenon was interpreted by considering a mechanism of progressive hydrate formation at the grain contacts, which is time dependent. On the basis of this suggested mechanism, the triaxial test results, which showed the same peak stress but different postpeak behaviors, were analyzed and a breakage/crushing mechanism was found to better describe the differing postpeak results. Thus, hydrate formation time is suggested to have the greatest effect on the postpeak response, in which the hydrate breakage occurs. It was concluded that an alternative failure mechanism may exist for different sand types, when different grading, angularitym, and sphericity exists.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Tel.: +972-4-829-5889. Present Address

† S.P.: Civil and Environmental Engineering Faculty, Technion−Israel Institute of Technology, Haifa 32000, Israel.

Funding

The author’s gratefully acknowledge financial contributions from MITACS and Golder Associates. F



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(18) Jayasinghe, A. G. Undrained Triaxial Compression Strength of Methane Hydrate Bearing Sediments. Ph.D. Dissertation, University of Calgary, Calgary, Alberta, Canada, 2013. (19) Jung, J. W.; Santamarina, J. C. Hydrate Adhesive and Tensile Strengths, Geochem. Geophys. Geosyst. 2011, 12. (20) Aman, Z. M.; Leith, W. J.; Grasso, G. A.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Adhesion Force between Cyclopentane Hydrate and Mineral Surfaces. Langmuir 2013, 29, 15551−15557. (21) Rees, E. V. L; Kneafsey, T., J.; Nakagawa, Geomechanical, S.; Properties of Synthetic Hydrate Bearing Sediments. The 7th International Conference on Gas Hydrates, Edinburgh, Scotland, July 17−21, 2011. (22) Leroueil, S.; Vaughan, P. R. The General and Congruent Effects of Structure in Natural Soils and Weak Rocks. Géotechnique 1990, 40, 467−488. (23) Airey, D. W. Triaxial Testing of Naturally Cemented Carbonate soil. J. Geotechn. Eng. 1993, 119, 1379−1398. (24) Keenan, J. H.; Keyes, F. G.; Hill, P. G.; Moore, J. G. Steam Tables: Thermodynamic Properties of Water Including Vapor. Liquid and Solid Phases; Wiley, New York, 1978; pp 156. (25) Alvarado, G.; Coop, M. R.; Willson, S. On the Role of Bond Breakage Due to Unloading in the Behavior of Weak Sandstones. Geotecnique 2012, 62, 303−316. (26) Soga, K.; Lee, S. L.; Ng, M. Y. A.; Klar. A. Characterisation and Engineering Properties of Methane Hydrate Soils. In Characterization and Engineering Properties of Natural Soils; T. S. Tan, et al., Eds.; A. A. Balkema: Lisse, Netherlands, 2006; Vol. 4, pp 2591−2642. (27) Jiang, M.; Zhu, H.; Li, X. Strain localization analyses of idealized sands in biaxial tests by distinct element method. Front. Arch. Civil Eng. China 2010, 4, 208−222. (28) Hyodo, M.; Nakata, Y.; Yoshimoto, N.; Orense, R.; Yoneda, J. Bonding Strength by Methane Hydrate Formed among Sand Particles. AIP Conf. Proc. 2009, 1145, 79−82. (29) Beltrán, J. G.; Servio, P. Morphological Investigations of Methane−Hydrate Films Formed on a Glass Surface. Cryst. Growth Des. 2010, 10, 4339−4347. (30) Priest, J. A.; Best, A. I.; Clayton, C. R. I. Attenuation of seismic waves in methane gas hydrate-bearing sand. Geophys. J. Int. 2006, 164, 149−159. (31) Kneafsey, T. J.; Nakagawa, S.: Repeated methane hydrate formation and dissociation in a partially water saturated sand: Impact on hydrate heterogeneity and sonic-frequency seismic properties. The 7th International Conference on Gas Hydrates, Edinburgh, Scotland, July 17−21, 2011. (32) Wang, Y. H.; Leung, S. C. A Particulate-Scale Investigation of Cemented Sand Behavior. Can. Geotech. J. 2008, 45, 29−44. (33) Durham, W. B.; Kirby, S. H.; Stern, L. A.; Zhang, W. The Strength and Rheology of Methane Clathrate Hydrate. J. Geophys. Res. 2003, 108.

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