Experimental investigation on the mechanical properties of methane hydrate-bearing sand formed with rounded particles

Journal of Natural Gas Science and Engineering 45 (2017) 96e107

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Experimental investigation on the mechanical properties of methane hydrate-bearing sand formed with rounded particles Shintaro Kajiyama, Yang Wu*, Masayuki Hyodo, Yukio Nakata, Koji Nakashima, Norimasa Yoshimoto Graduate School of Science and Technology for Innovation, Yamaguchi University, Ube, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 December 2016 Received in revised form 24 March 2017 Accepted 7 May 2017 Available online 25 May 2017

A series of triaxial compression tests were performed on the methane hydrate-bearing sands formed with rounded glass beads and natural sands to examine their mechanical properties. The effect of particle characteristics on the mechanical response of methane hydrate-bearing sands with regard to the exertion of bonding force and de-bond mechanism between grains is explained from the grain-scale viewpoint. In comparison with the natural sand, methane hydrate-bearing glass beads owns a similar initial stiffness and rapidly attains the peak shear strength at a smaller axial strain. The obvious post-peak strain-softening behaviour of methane hydrate-bearing glass beads which differs from the tender post-peak strain-softening tendency of methane hydrate-bearing natural sand is observed. It is attributed to almost simultaneous exfoliation of hydrate mass from the glass beads with smooth surface within the straightly sheared layers. The substantial resource of shear strength enhancement for methane hydrate-bearing glass beads is the cohesion but the shear strength of methane hydrate-bearing natural sand was jointly governed by the cohesion and angle of internal friction. In contrast to the increasing tendency of difference of shear strength with the level of effective confining pressure for methane hydrate-bearing natural sand, the methane hydrate-bearing glass beads exerts a stronger effect of bonding force on the difference of shear strength at relatively lower pressures but this effect decreases at higher pressures. © 2017 Elsevier B.V. All rights reserved.

Keywords: Shear strength Methane hydrate Particle shape Stick-slip Bonding Glass beads

1. Introduction Methane gas hydrates are clathrate solid constituted of methane gas tripped within the hydrogen-bonded of water molecules. They are founded at the deep seabed and sub permafrost area and formed under specific temperature and pressure conditions (e.g., 3e10 MPa and 275e285 K for methane hydrate). Natural gas hydrates are expected to be the potential energy resource for the further (Chong et al., 2015b; Englezos, 1993; MacDonald, 1990; Makogon, 2010; Sloan and Koh, 2007; Waite et al., 2009). During the process of gas production, the excessive or unsteady deformation of marine ground induced by the dissociation of hydrate would influence the stability of production well and even lead to the marine subsea landslide (Jin et al., 2016; Kleinberg et al., 2003; Nixon and Grozic, 2007; Pauli et al., 2003; Vedachalam et al.,

* Corresponding author. Graduate School of Science and Technology for Innovation, Yamaguchi University, Tokiwadai 2-16-1, Ube 755-8611, Japan. E-mail address: [email protected] (Y. Wu). http://dx.doi.org/10.1016/j.jngse.2017.05.008 1875-5100/© 2017 Elsevier B.V. All rights reserved.

2015; Xu and Germanovich, 2006). Thus, well understanding the mechanical properties of methane hydrate-bearing sediment is essential to safely exploit the methane gas from marine ground. The experimental investigations on the mechanical behaviour of methane hydrate-bearing sediment attracted the interests of mangy researchers (Ebinuma et al., 2005; Grozic and Ghiassian, 2010; Hyodo et al., 2013a, 2013b, 2005; Li et al., 2016; Masui et al., 2005; Priest et al., 2009, 2005; Winters et al., 2007; Yoneda et al., 2015; Yun et al., 2007; Zhang et al., 2012). It was indicated that the presence of methane hydrate increased the stiffness and strength and promoted the dilation behaviour (Priest et al., 2009, 2005). investigated the influences of hydrate formation and hydrate morphology on the variation in the seismic velocity of samples and pointed out that the hydrate cemented grain contact and dramatically increased the seismic velocity (Yun et al., 2007). examined the mechanical behaviour of THF hydrate formed with sand, silt and clay and indicated that the shear characteristics of hydrate-bearing sediment were jointly affected by the particle size, confining pressure and hydrate concentration (Miyazaki et al.,

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2011). examined the shear strength of methane hydrate-bearing sediments formed with different host sands at different methane hydrate saturations and effective confining pressures (Hyodo et al., 2013a, 2013b). conducted an extensive experiment study on the effects of methane hydrate saturation, effective confining pressure, density, pore water pressure and temperature on the shear behaviour of methane hydrate-bearing sediment formed with silica sand in triaxial test (Yoneda et al., 2015). investigated the mechanical response of pressure core sample from the seabed in the eastern Nankai Trough using an advanced pressure core nondestructive analysis technology. These experimental studies made a great contribution to the understanding of the influential factors governing the mechanical response of methane hydrate-bearing sediment. However, the particle characteristics had been removed from the relevant examinations. The past experimental and numerical studies revealed that the mechanical and physical properties of granular materials were altered by the particle characteristics (Cho et al., 2006; ~ a et al., 2007). The particle shape Nouguier-Lehon et al., 2003; Pen and size reflected the material composition and grain formation (Anthony and Marone, 2005). experimentally verified that the rise in particle angularity and surface roughness increased the frictional strength and distribution of stress within sheared layers. Additionally, the arrangement of packed grains, distribution of the pore space and hydrate morphology are also inevitably dependent on the particle shape of host material for formatting the methane hydrate-bearing sediment. In spite of this, the particle characteristics on the mechanical behaviour of methane hydrate-bearing sediment is often neglected without further explanation. Only a limited number of relevant study had been conducted (Kingston et al., 2008). formed methane hydrate in four different granular materials and explained that the grain size and shape affected the way hydrate bonded a sediment (Kajiyama et al., 2015, 2017). performed the plane strain test on the methane hydrate-bearing sand formed with glass beads and noticed the earlier appearance of shear band in comparison with natural sands without examining the grain-scale mechanism of different particle characteristics (Yu et al., 2016). conducted the discrete element simulation using spherical or elongated particles to represent different shapes of host sands and pointed out the different effects of particle shape on the shear response of pore-filling and cementation hydrate types. However, the effect of particle characteristics on the shear and deformation behaviour of methane hydrate-bearing sediment is still unclear and requires further investigation. This study presents the experimental study on the mechanical properties of methane hydrate-bearing sands formed with different host materials to investigate the influence of particle characteristics in triaxial loading condition. To represent the limit condition of particle shape, glass beads with spherical shape is deliberately selected as the host material. Although the glass beads were used as the porous medium to examine the formation and dissociation behaviour of methane hydrate in past laboratory tests (Chong et al., 2015a; Hachikubo et al., 2011; Spangenberg et al., 2005; Yang et al., 2015). The mechanical properties of methane hydrate-bearing hydrate had not been examined. These results can be employed as a benchmark for further discrete element method analysis. Another reconstituted sample with a similar grading curve to glass beads is also tested. A comparison study on the shear behaviour of methane hydrate-bearing sands formed with glass beads and natural sands under similar test conditions is also made. The grain-scale structure is hypothesized to interpret the different exertion of bonding force and de-bond failure mechanisms induced by the variation in particle characteristics.

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2. Tested materials and test procedure 2.1. Test apparatus The triaxial compression tests were performed using an innovative temperature-controlled and high-pressure triaxial apparatus as shown in Fig. 1. This apparatus was capable of reproducing the in situ environment of the deep marine ground. The cell pressure and pore pressure could be independently controlled at high pressure and low temperature level. The maximum axial load of test apparatus is up to 200 kN. The test apparatus can provide a maximum cell pressure capacity of 30 MPa with an accuracy of 0.1 MPa and a maximum pore pressure capacity of 20 MPa with an accuracy of 0.05 MPa, respectively. The allowable temperature range in the triaxial cell space is from
35  C to 30  C. A big tank governing the temperature control system of specimen installed outside is controlled by circulating the cell liquid into the triaxial cell. The cell pressure is supplied by the oil pressure and the pore pressure is controlled by the upper and lower syringe pumps. The bytyl rubber membrane with depth of 3 mm is intentionally used to resist the high pressure. The methane hydrate is completely dissociated after the completion of shearing process. A gas flow meter is designed to measure the methane hydrate saturation of specimen. The measurement of gas amount is accompanied with the adjustment the valve of pipe equipped with the gas flow meter. The axial displacement, cell pressure, volumetric change and pore pressure during the entire shearing process were automatically recorded by computer. The description of the test apparatus was comprehensively given in the previous work (Hyodo et al., 2013b). 2.2. Tested materials Glass beads is an assembly of rounded particles and one kind of solid granular materials. Its specific gravity Gs is 2.500, a maximum void ratio emax and a minimum void ratio emin are 0.584 and 0.448. Tb is a reconstituted sand prepared with the grading curve and composition in consistence of those of the fine-grained sediment in methane hydrate concentration zone in Nankai Trough (Hyodo et al., 2017; Suzuki et al., 2009). “T” stands for the “Turbidite”. The Japan Nankai Trough Exploratory Test Wells (Tokai-oki to kumano-nada) was conducted in 2004 and the main mineralogy composition was measured by the X-Ray diffraction method (Fujii et al., 2005; Minagawa et al., 2008; Suzuki et al., 2009). Tb is constituted of silica and, silica fines, kaolinite fines and mica fines and its mineralogy composition is similar to the marine sediment obtained from Nankai Trough. The specific gravity Gs of Tb is 2.661. The fines content of Tb is 8.9% quantified by weight. Table 1 lists the physical parameters of the two granular materials. Toyoura sand is a fine sand and consists of sub-rounded to subangular grains. Toyoura sand owns a specific gravity of 2.650, a maximum void ratio of 0.977, a minimum void ratio of 0.597 (Hyodo et al., 2013b). conducted an intensive study on the mechanical behaviour of methane hydrate-bearing sand using Toyoura sand as host material. Thus, Toyoura sand was selected as another natural sand for a comparison study. Fig. 2 shows the grain sizedistribution curves of glass beads, Tb and Toyoura sand. It is seen that majority of grain sizes for three granular materials are from 0.05 mm to 0.3 mm. 2.3. Test procedure Fig. 3 shows the preparation procedure of the methane hydratebearing sand specimens for triaxial compression test in accordance of a temperature and pore pressure path specifically designated

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(a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) (m) (n) (o) (p) (q) (r) (s) (t) (u)

Specimen 30(diameter)×60 mm(height) Pedestal Cell Confining pressure amplifier Inner cell Syringe pump for Inner cell Upper syringe pump Lower syringe pump Mass flow meter Confining fluid thermal control tank Thermometer Methane gas Load cell Pipe line Membrane Water tank Displacement transducer Confining pressure gage Confining pressure gage Pore pressure gage Pore pressure gage

(n)

(i)

(p)

(t) (q) (g) (m) (c) (l) (o)

(k) (a) (e) (s) (j)

(d)

(b) (u)

(f) (r)

(h)

Fig. 1. The schematic diagram of the innovative temperature-controlled and high-pressure triaxial apparatus.

Table 1 Physical properties of tested host granular materials and other natural sand. Tested material

Gs

emax

emin

d50 (mm)

Ar

Rc

Glass beads Tb

2.500 2.661

0.584 1.090

0.448 0.561

0.115 0.147

1.04 1.31

1.01 1.33

Ar: aspect ratio; Rc: roundness coefficient; emax: maximum void ratio; emin: minimum void ratio.

(Makogon and Sloan, 1994). The tested granular material was mixed with specific initial water content strictly controlled for the target methane hydrate saturation SMH. The target methane hydrate saturation SMH was prepared as 50% for glass beads, 30% and 50% for Tb. SMH is defined as the ratio the methane hydrate volume to the summary of methane hydrate volume and remained void volume. The cylindrical specimens 30 mm in diameter 60 mm in height were prepared using tamping method, with the target porosity n as 40%. The granular material was poured into the mold by 15 layers, densely tempted 40 times for each layer. The same tamping method and tamping times were employed to prepare the different specimens. The specific test procedures were simply reviewed here. ① The specimen was frozen in the freezer and this intended to make the specimen stand by itself. Then, the specimen covered by the bytyl rubber membrane was installed on the pedestal in the triaxial cell. ② The cell pressure and pore (back) pressure were simultaneously increased to 4.2 MPa and 4 MPa at a constant effective confining pressure, extraction of the pore pressure from cell pressure, as 0.2 MPa. The methane gas was injected into the specimens by the upper and lower syringe pumps. ③ To smoothly form the methane hydrate in specimen, the temperature was increased to the outside of methane hydrate phase boundary and terminally reduced to 1.0 MPa ④ The capillary water and injected methane gas were synthetized to methane hydrate when the temperature was

reduced into the stability field at a constant pore pressure. Subsequently, the pore pressure and temperature were kept constant in the entire hydrate formation process and this procedure last 24 he36 h according to the property of host sand. The conversion into hydrate mass from the pore water and methane gas was judged by the observation of the amount of gas flowing. The steady in the syringe pump indicated that the reaction between the methane gas and capillary water completed. ⑤ and ⑥ After the generation of methane hydrate, the specimen was saturated with circulated water. An amount of pure water equal to approximately three times that of the specimens was filtrated through them in order to fully saturate the specimens. The saturation procedure last almost from 24 h and became even longer once the amount of fines particles included in samples was increased. It was believed that the specimen was nearly-full saturation after the above test procedure. The back pressure and temperature were adjusted in compliance of the requirement of test conditions. The pore water pressure was raised from 4.0 MPa to 10.0 MPa at an incremental value of 0.2 MPa. The shear stress was applied on the specimen at a constant speed of 0.1%/min after consolidation. Table 2 lists the triaxial shear test conditions of glass beads, Tb and Toyoura sand (Hyodo et al., 2013b). To validate the formation pattern of methane hydrate between grains, the conversion process of methane hydrate from the capillary water and methane gas between grain contacts is also observed using a testing apparatus equipped with an electron scanning microscope, as shown in Fig. 4. The temperature and pore pressure can be controlled to reproduce the seabed conditions which is also simulated in triaxial compression test. It is clearly seen that the previously added water is mainly distributed between the grain contacts with red color and displays meniscus shape. The methane hydrate was formed after a span of time and firmly cemented the neighboring grains under the conditions that the pore pressure was 5.0 MPa and temperature was 3  C.

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Passing percent (%)

100 80

3. Test results

(a)

Glass beads Toyoura sand Tb

3.1. Stress-strain relationship in triaxial test

60 40 20 0 0.001

0.01

0.1 Grain size (mm)

10

1

50

Density (%)

40

(b)

Glass beads Toyoura sand Tb

30 20 10 0 0.001

99

0.01

0.1 Grain size (mm)

1

10

Fig. 2. Grain-size distribution curves of glass beads and natural sands (a) Passing percent (b) Density.

Fig. 5 (a) and (b) show the deviatoric stress-axial strain and volumetric-axial strain relationships of pure glass beads at three effective confining pressures of 1.0 MPa, 3.0 MPa and 5.0 MPa. The initial stiffness and peak shear strength of glass beads specimens increased with the level of effective confining pressure. The stickslip behaviour of pure glass beads at effective confining pressures of 3.0 MPa and 5.0 MPa was clearly seen in Fig. 5 (a). It was resulted from the continual rotation and slip of rounded grains subjected to increasing shear stress. The quick rotation and movement of rounded grains led to the drastic change in internal skeleton of tested specimen. This distinctive load oscillations of glass beads had also been reported in the past studies (Anthony and Marone, 2005; Cain et al., 2001; Johnson et al., 2008). Fig. 5 (b) presented that the pure glass beads initially exhibited contractive behaviour and slightly tended to dilate as the shear stress continued at all effective confining pressures. But the maximum variation in volumetric strain was within 1%. Fig. 6 (a) and (b) demonstrated the stress-strain relationship of methane hydrate-bearing glass beads at different effective confining pressures. The experimental results indicated that the initial stiffness and peak shear strength of methane hydratebearing glass beads were larger than those of pure glass beads specimens at effective confining pressures of 1.0 MPa, 3.0 MPa and 5.0 MPa. The hydrate formed in the pore space among glass beads grains strengthened the skeleton and sustained a higher shear stress. This was in accordance with the previous investigations (Hyodo et al., 2013b; Masui et al., 2005; Miyazaki et al., 2011). The presence of methane hydrate largely enhanced their initial stiffness and the methane hydrate-bearing glass beads owned similar initial stiffness at different effective confining pressures. The peak shear strength appeared at a small level of axial strain in the entire

Fig. 3. Preparation procedure of methane hydrate-bearing sands for triaxial test.

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Table 2 Test conditions of triaxial shear tests on methane hydrate-bearing sands. 0

Tested material

Effective confining pressure sc (MPa)

Temperature T (oC)

Pore pressure P. P (MPa)

Porosity n (%)

Methane hydrate saturation SMH (%)

Glass beads

1.0 1.0 3.0 3.0 5.0 5.0 3.0 3.0 3.0 1.0 1.0 3.0 3.0 3.0 5.0 5.0 5.0 5.0 5.0

20 5 20 5 20 5 20 5 5 5 5 5 5 5 5 5 5 5 5

10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

38.3 37.4 35.0 31.3 35.9 36.8 40.7 40.5 40.6 39.3 39.4 39.4 38.9 39.3 39.4 39.3 39.2 39.1 39.4

0.0 37.3 0.0 37.8 0 47.4 0.0 38.9 73.8 0.0 54.3 0.0 26.3 38.7 0.0 24.2 35.1 41.9 51.3

Tb

Toyoura Sand (Hyodo et al. (2013b))

Fig. 4. Formation of methane hydrate between grain contacts under electron scanning microscope.

shearing process. The stick-slip behaviour was largely suppressed with the formation of methane hydrate among glass beads grains. The hydrate impeded the rotation of grains and provided the additional shear resistance strength. The presence of methane hydrate significantly intensified the dilation behaviour of methane hydrate-bearing glass beads in Fig. 6(b). The dilation behaviour was suppressed as the bond breakage induced by the increasing effective confining pressure. It was attributed to that the particles experienced a remarkable vertical movement than that in horizontal direction (Miyazaki et al., 2011). also commented that the freedom of movement of grains was reduced by the effective confining pressure. A marked post-peak strain-softening behaviour of methane hydrate-bearing glass beads at effective confining pressures of 1.0 MPa and 3.0 MPa was seen. It is noticed that the strain-softening behaviour attenuated as the confining pressure increased. Such tendency could be attributed to the relatively easier exfoliation of hydrate mass from glass beads with smooth surface at higher pressures. Moreover, the occurrence of de-bond breakage also reduced the peak shear strength and attenuated the post-peak strain-softening tendency. The past study on the weakly bonded artificial soil revealed that the rise in the effective confining pressure increased the possibility of de-bond breakage (Malandraki and Toll, 2000). The strain-softening attenuation tendency with the rise in the effective confining pressure had also been observed for cemented sand with cement content of 10% and 15% at high pressures varying from 1 MPa to 12 MPa by (Marri, 2010). Besides (Kajiyama et al., 2015), performed the plane strain compression tests at high pressures on the methane hydrate-bearing glass beads and also obtained the similar results. A more remarkable strain-

softening behaviour of methane hydrate-bearing glass beads was seen due to the plane strain loading condition. Fig. 7 (a) and (b) show the deviatoric stress and volumetric strain plotted against the axial strain of Tb at an effective confining pressure of 3.0 MPa with three levels of methane hydrate saturation. The formation of hydrate increased the initial stiffness and peak shear strength and promoted the dilation tendency. The test data presented in Figs. 5 and 6 had been combined and displayed in Fig. 8 (a) to show the effect of presence of methane hydrate on the stress-strain relationship of glass beads. The solid and broken lines represent the results of methane hydrate-bearing glass beads and hydrate-free glass beads. Fig. 8 (b) shows the relationship between the difference of deviatoric stress of hydratebearing glass beads and hydrate-free glass beads against the axial strain at various effective confining pressures. The difference of deviatoric stress of methane hydrate-bearing glass beads qMH and pure glass beads qhost at the same level of axial strain has been calculated to express the shear strength enhancement due to the formation of methane hydrate. This evaluation method was adopted in previous experimental study (Hyodo et al., 2013b; Yoneda et al., 2015). These curves experienced their peak values at a small axial strain less than 2% and suddenly dropped down to a stable value as axial strain increased in Fig. 8(b). The exertion of bonding force at an earlier shearing stage was attributed to the different contact states between glass beads due to the variation in particle characteristics. The methane hydrate-bearing glass beads could quickly obtain a higher shear resistance strength. The results also demonstrated that the difference in deviatoric stress decreased with the increasing confining pressure. The bond contact in sheared

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Fig. 5. Test results on pure glass beads (a) Stress-strain relationship (b) Volumetricaxial strain relationship.

layer was gradually damaged as the specimen passed the peak shear strength and bond contact in sheared layer was almost destructed after the specimen entered into the critical state at high pressures.

4. Comparison of shear strength for methane hydratebearing sands with different host materials 4.1. Comparison of the stress-strain relation Fig. 9 (a) displays the deviatoric stress plotted against the axial strain of pure glass beads, Toyoura sand and Tb at an effective confining pressure of 3.0 MPa. Tb and glass beads exhibited the strain-hardening behaviour but Toyoura sand displayed a slight post-peak strain-softening behaviour. The deviatoric stress of glass beads was smaller than those of other two natural sands at higher axial strain. This difference was originated from the inherent characteristics of granular material. The contraction behaviour of Toyoura sand and Tb and a slight dilation behaviour of glass beads after shearing were noticed in Fig. 9 (b). Fig. 10 (a) and (b) show the deviatoric stress-axial strain and volumetric-axial strain relationships of methane hydrate-bearing sands formed with glass beads, Toyoura sand and Tb. It was distinctive that the peak shear strength of methane hydratebearing glass beads exceeded those of methane hydrate-bearing natural sands. Methane hydrate-bearing glass beads exhibited a

101

Fig. 6. Test results on methane hydrate-bearing glass beads (a) Stress-strain relationship (b) Volumetric-axial strain relationship.

remarkable post-peak strain-softening behaviour. The deviatoric stress of methane hydrate-bearing glass beads was lower than those of methane hydrate-bearing natural sands after the destruction of hydrate in sheared layer when the shearing was terminated. The methane hydrate-bearing glass beads exhibited stronger dilation behaviour in comparison with other natural sands. Additionally, no remarkable variation in the volumetric strain of methane hydrate-bearing Toyoura sand and Tb was observed. Fig. 11 presents the difference in deviatoric stress of methane hydrate-bearing sands and its corresponding host sands plotted against the axial strain for three different host materials. The maximum difference in deviatoric stress originated from the bonding force for glass beads and Tb appeared at a small level of axial strain. It was clear that the maximum difference in deviatoric stress for glass beads was larger than others. It was attributed to that the hydrate in pore space connected the rounded particles and this made the movement of grains a difficult task. It reversely resulted in the rapid increment of shear strength for methane hydrate-bearing glass beads. Another reason was that the shear strength of pure glass beads was smaller than those of two natural sands. All three curves became stable as the axial strain increased but some difference in deviatoric stress could still be seen for three granular materials when the shearing was terminated.

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Fig. 7. Test results on Tb at an effective confining pressure of 3.0 MPa and variable methane hydrate saturations (a) Stress-strain relationship (b) Volumetric-axial strain relationship.

4.2. Grain-scale mechanism for methane hydrate-bearing sands with different grain shapes Fig. 12 describes the hypothetical image of the exertion and breakage pattern of bond between the rounded and irregular-shape grains in grain-scale viewpoint. The two-grain model in the left portion intends to explain the quick exertion of bonding force among rounded grains. The assembly of grains in the right portion is to interpret the remarkable post-peak strain-softening behaviour of methane hydrate-bearing glass beads with smooth surface. The grain contact orientation and configuration due to the variation in particle shape affect the bonding conditions and even the exertion of bonding force among grains. The rounded glass beads grains are in contact with the neighboring grains by the faceface contact. The irregular natural sand grains are also connected by the face-corner contact type in exception of the face-face contact. These particle-to-particle contact types had been observed in granular material specimens from three-dimensional X-ray computed tomography (Druckrey et al., 2016). Additionally, for the natural sand grain with concaves, the hydrate formed from the pure water retained in the concave of grain made a minor contribution to the rise in shear strength in comparison with the meniscus hydrate bridge among grains (Otsuka et al., 1988). investigated the adhesion force between the particles and glass plate using impact separation method. They found that the adhesion force increased as the

Fig. 8. The relationship between difference in deviatoric stress and axial strain of methane hydrate-bearing glass beads at different effective confining pressures.

particle shape approached sphere and attributed it to the different effective area of contacts and contacting states. In term of the pattern of de-bond breakage, the failure of methane hydrate-bearing glass beads is characterized with the rapid exfoliation of hydrate mass from the glass bead when the increasing shearing stress exceeds the interface shear resistance between the glass beads and hydrate mass. The almost simultaneous exfoliation of hydrate mass from the glass beads in the straightly sheared layer was shown in the up-right portion in Fig. 12, which corresponded to the sudden strain-softening behaviour of methane hydrate-bearing glass beads. Conversely, the relatively larger roughness of natural sand increased the shear resistance between the grains and hydrate mass and reduced the possibility of exfoliation or altered the exfoliation pattern. Consequently, the tender strain-softening behaviour was seen for the Toyoura sand with the presence of methane hydrate. (Gregory et al., 2001) performed the triaxial tests on the frozen glass beads and frozen natural sands under various test conditions. They also observed the obvious post-peak strain-softening behaviour of frozen glass beads with rounded shape and smooth surface in comparison with the frozen natural sand with irregular shape and larger roughness. The marked strain-softening behaviour of glass beads was attributed to that the soils skeleton offered minimal frictional resistance after the peak strength.

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Fig. 9. Comparison of different host sands under similar experimental conditions (a) Stress-strain relationship (b) Volumetric-axial strain relationship.

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Fig. 10. Comparison of methane hydrate-bearing sands formed with different host materials under similar experimental conditions (a) Stress-strain relationship (b) Volumetric-axial strain relationship.

4.3. Mohr-Coulomb failure envelope of methane hydrate-bearing sands The Mohr-Coulomb failure criterion is the most well-known and empirical failure strength criterion for soils and rocks. Fig. 15 (a) and (b) show the Mohr's circles and the Mohr-Coulomb failure envelope of pure Toyoura sand and methane hydrate-bearing Toyoura sand with methane hydrate saturation around 40% for a comparison. The stress-strain relationship of hydrate-free Toyoura sand and the methane hydrate-bearing Toyoura sand are presented in Figs. 13 and 14. The corresponding Mohr's circles and Mohrcoulomb failure envelope of hydrate-free glass beads and methane hydrate-bearing glass beads are presented in Fig. 16 (a) and (b). Herein, the Mohr-Coulomb failure envelope refers to the line tangential to the Mohr's circles. It is well accepted that the shear strength of soil is jointly governed by the cohesion cd and angle of internal friction fd . The cohesion is the intercept of the failure envelope with the shear stress axis and the angle of internal friction is the slope of failure envelope. When the failure envelope is plotted for pure Toyoura sand, a value of cohesion is obtained due to appearance of particle breakage at high confining pressures. The angle of internal friction refers to the inter-particle friction for movement, rolling and rearrangement of sand grains. For methanehydrate bearing sands, the cohesion of methane hydrate-bearing sands is the bonding force at the inter-particle level by cementing agents. The angle of internal friction refers to the inter-particle

Fig. 11. The relationship between the difference in deviatoric stress and axial strain of methane hydrate-bearing sand formed with Toyoura sand, Tb and Glass beads at a confining pressure of 3.0 MPa.

friction for movement, rolling and rearrangement of sand grains bonded by hydrate. A rise in cohesion from 0.40 MPa to 0.45 MPa and angle of internal friction from 28.7 to 32.9 could be seen for

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Fig. 12. Exertion and breakage of bonding strength of methane hydrate-bearing materials formed with glass beads and natural sand.

Fig. 13. Stress-strain relationship of pure Toyoura sand at three levels of effective confining pressure.

Fig. 14. Stress-strain relationship of methane hydrate bearing Toyoura sand at three levels of effective confining pressure.

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Fig. 15. The Mohr's circles and Mohr-Coulomb failure envelop (a) Pure Toyoura sand (b) Methane hydrate-bearing sand. Fig. 16. The Mohr's circles and Mohr-Coulomb failure envelop (a) Pure glass beads (b) Methane hydrate-bearing glass beads.

Toyoura sand with methane hydrate saturation raising from 0% to around 40% in Fig. 15. The clusters of grains heterogeneously cemented by hydrate was created during shearing and it raised the angle of internal friction for Toyoura sand once methane hydrate was formed. It was demonstrated that the shear strength increment of methane hydrate-bearing natural sand was jointly governed by the cohesion and angle of internal friction. It was noted that the cohesion significantly increased from 0.12 MPa to 2.81 MPa for glass beads as the methane hydrate saturation varied from 0% to around 40% in Fig. 16. The cohesion of pure glass beads was lower due to a smaller amount of particle crushing in comparison of pure Toyoura sand subjected to the same level of shearing stress (Wu et al., 2014, 2013). The cohesion was believed to be the substantial resource of shear strength enhancement. In contrast, the angle of internal friction reduced due to the lager increment of cohesion. The test results indicated that the increment of cohesion for methane hydrate-bearing glass beads was larger than that of methane hydrate-bearing natural sand. The rounder particle of host granular material become, a higher cohesion for methane hydrate-bearing granular material is. The methane hydrate-bearing glass beads exhibited a higher stiffness behaviour and attained its higher peak strength in comparison with the methane hydrate-bearing natural sands from Fig. 10. Accordingly, it is ascertained that the cohesion makes a dominant contribution to the shear strength enhancement for methane

hydrate-bearing glass beads. 4.4. Difference of shear strength due to the formation of hydrate Fig. 17 presents the difference of shear strength between the methane hydrate-bearing sand and hydrate-free sand (qMH e qhost) plotted against the methane hydrate saturation for different methane hydrate-bearing sands. The results indicated that the difference of shear strength increased with the increasing methane hydrate saturation for the Toyoura sand and Tb samples at a given effective confining pressure. It was noted that the points representing the difference of shear strength of methane hydrate-bearing glass beads at effective confining pressures of 1.0 MPa and 3.0 MPa lied above and outside the region of natural sands. This value of methane hydrate-bearing glass beads became lower than those of natural sands once the effective confining pressure was increases to 5.0 MPa. At relatively low effective confining pressure of 1.0 MPa, the hydrate among rounded glass beads grains firmly connected the grains and the methane hydrate-bearing glass beads rapidly gained shear strength at a small level of axial strain. Therefore, it suggested that the methane hydrate formed between glass beads due to the grain contact states exerted the shear strength at an earlier stage than

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variation in the particle characteristics. The main findings of this study are summarized as follow.

Fig. 17. The relationship between the difference of shear strength and methane hydrate saturation of different methane hydrate-bearing sands.

other natural sands (Tselishchev and Val'tsifer, 2003). studied the effect of contact type on the capillary force by a liquid bridge between two grains with different shapes and showed that the capillary force between two rounded particles connected by the face-face contact was higher than that for face-corner type. Similarly (Torskaya et al., 2014), also noticed that the capillary pressure among the partially-saturated granular materials with rounded shape is higher than that among natural sands from the pore-scale modelling. At higher effective confining pressures, the exfoliation of hydrate mass from glass beads with smooth surface was liable to occur and almost no development of mechanical interlocking restricted the further shear strength increment. It was indicated that the effect of cementation due to the formation of hydrate on the difference of shear strength between the methane hydratebearing glass beads and hydrate-free glass beads was greater at effective confining pressures of 1.0 MPa and 3.0 MPa and this effect diminished with increasing effective confining pressure. In the contrast, the difference of shear strength for methane hydratebearing Toyoura sand increased with the level of effective confining pressure. It was resulted from the easier creation of cluster of grains cemented by the hydrate at higher effective confining pressures and methane hydrate saturations. In addition, the increasing tendency of the curve of Tb with the level of methane hydrate saturation is higher than that of Toyoura sand at the same effective confining pressure. It was believed that a larger cluster of grains was liable to be created for Tb with fines particles. In comparison of the methane hydrate-bearing sand formed with only coarse grains at a larger methane hydrate saturation, the methane hydrate-bearing sand containing fines gained a higher shear strength (Hyodo et al., 2015).

1. The presence of methane hydrate in both of the glass beads and natural sand specimens increased the initial stiffness, peak shear strength and intensified the dilation behaviour. The methane hydrate-bearing glass beads owned similar initial stiffness at different effective confining pressures and their peak shear strength appeared at a small level of axial strain in comparison of natural sand. 2. Pure glass beads exhibited stick-slip behaviour and this phenomenon became remarkable with the rise in the effective confining pressure. The load oscillation due to the stick-slip of glass beads was suppressed once the methane hydrate was formed among grains. The rounder particle of host granular material become, a higher cohesion for methane hydratebearing granular material is. The cohesion made a dominant contribution to the shear strength enhancement of methane hydrate-bearing glass beads. However, the shear strength enhancement of methane hydrate-bearing natural sands was jointly governed by the cohesion and angle of internal friction. The substantial resource of the shear strength enhancement of methane hydrate-bearing granular material differed with the variation in the particle characteristics. 3. The tender strain-softening behaviour of methane hydratebearing natural sand was seen at three levels of effective confining pressure. The methane hydrate-bearing glass beads exhibited remarkable post-peak strain-softening behaviour at effective confining pressures of 1.0 MPa and 3.0 MPa. Furthermore, the strain-softening behaviour attenuated with the increasing effective confining pressure. The failure of bonding is liable to occur at the interface between the rounded grain and hydrate mass due to the smooth surface of glass beads. 4. These points representing the difference of shear strength of methane hydrate-bearing glass beads at effective confining pressures of 1.0 MPa and 3.0 MPa lied above and outside the region of natural sands. It was attributed to the rapid shear strength increment at an earlier stage and the relatively lower shear strength of pure glass beads. The difference of shear strength of methane hydrate-bearing glass beads became lower than those of natural sands once the effective confining pressure was increased to 5.0 MPa. Conversely, the difference of shear strength of methane hydrate-bearing Toyoura sand exhibited an increasing tendency with the level of effective confining pressure. Acknowledgement: A part of the work was supported by JSPS KAKENHI Grant-in-Aid Fundamental Research (A) of Grant No.25249065 and Grants-inAid for JSPS Fellows 15F15368. The authors wish to express their sincere thanks to the people concerned.

5. Conclusions References This study presented an experimental study on the shear response of methane hydrate-bearing sand formed with rounded glass beads at different effective confining pressures. The mechanical behaviour of methane hydrate-bearing granular materials formed with rounded glass beads and natural sands were compared to investigate the influence of particle characteristics on the strength and deformation behaviour in triaxial compression test. The effects of particle shape and surface roughness on the macroscopically-observed shear behaviour were examined. The hypothetical grain-scale structure was proposed to compare the exertion of bonding force and de-bond breakage due to the

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