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Undrained triaxial tests on water-saturated methane hydrate–bearing clayey-silty sediments of the South China Sea

Canadian Geotechnical Journal

Volume 58 , Number 3
March 2021

Abstract
Résumé
1. Introduction
2. Testing device and sample preparation
3. Test results
4. Discussions
5. Conclusions
List of symbols
Acknowledgements
References

Abstract

Approximately 90% of gas hydrates are buried in fine-grained sediments, especially in the South China Sea. The potential instability of fine-grained sediments induced by hydrate dissociation requires investigation of the shear strength and pore pressure response of sediments during hydrate recovery. To date, most studies have focused on the undrained mechanical behavior of gas hydrate–bearing sand or gas hydrate–free clay — few studies have examined gas hydrate–bearing fine-grained sediments. Because of the low-permeability and water-saturated characteristics of the sediments in the South China Sea, a series of undrained triaxial shear tests were performed on water-saturated methane hydrate-bearing clayey–silty sediments in this area. The experimental results show that the failure strength of methane hydrate–bearing sediments (MHBSs) increases with the increase in hydrate saturation and initial effective mean stress. The excess pore-water pressure of MHBSs remains positive during shear. Cohesion in the Mohr–Coulomb model increases with the increase in hydrate saturation, while the internal friction angle in the Mohr–Coulomb model has little dependence on the hydrate saturation.

Résumé

Environ 90 % des hydrates de gaz sont enfouis dans des sédiments à grain fin, en particulier dans la mer de Chine méridionale. L'instabilité potentielle des sédiments à grain fin induite par la dissociation des hydrates nécessite une étude de la résistance au cisaillement et la réponse à la pression interstitielle des sédiments pendant la récupération des hydrates. Jusqu'à présent, la plupart des études se sont concentrées sur le comportement mécanique non drainé du sable contenant des hydrates de gaz ou de l'argile sans hydrates de gaz, et peu d'études ont examiné les sédiments à grain fin contenant des hydrates de gaz. En raison de la faible perméabilité et de la saturation en eau des sédiments de la mer de Chine méridionale, une série de tests de cisaillement triaxial non drainé a été effectué sur des sédiments argileux et limoneux contenant des hydrates de méthane saturés en eau dans cette région. Les résultats de l'expérience montrent que la résistance à la rupture des sédiments contenant des hydrates de méthane (MHBS) augmente avec l'augmentation de la saturation en hydrates et la contrainte moyenne effective initiale. L'excès de pression d'eau interstitielle des MHBS reste positif pendant le cisaillement. La cohésion dans le modèle de Mohr–Coulomb augmente avec l'augmentation de la saturation en hydrates, tandis que l'angle de frottement interne dans le modèle de Mohr–Coulomb dépend peu de la saturation en hydrates. [Traduit par la Rédaction]

1. Introduction

Natural gas hydrates (NGHs) are ice-like crystal compounds formed by natural gas and hydrogen-bonded water molecules under low-temperature and high-pressure conditions (Mahajan et al. 2007). They are widely found in marine and continental permafrost sediments below 300 m with the characteristics of vast reserves, wide distribution, and high energy density (Makogon et al. 2007). NGHs are considered the most promising energy resource that can satisfy environmental standards and economic viability in the 21st century (Yang et al. 2019). However, the exploitation of hydrates will trigger the decomposition of hydrates (Sun et al. 2019a; Wu et al. 2020a), which will increase the pore pressure and reduce the strength of the sediments (Li et al. 2016; Wu et al. 2019), possibly destroy the production well and drilling structure, or even trigger subsidence and submarine landslides (Kim et al. 2013; Sultan et al. 2004). Thus, to achieve safe and efficient hydrate exploitation and ensure the long-term stability of the gas hydrate–bearing sediments, it is important to understand the mechanical behavior of gas hydrate-bearing sediments in advance (Hyodo et al. 2017; Sun et al. 2019c; Li et al. 2019).

Understanding the shear strength and pore pressure response of sediments is critical for stability of the sediments (Mitchell and Soga 2005), and the in situ pore pressure always affects calculation of the bottom seismic reflector depth (Peltzer and Brewer 2000). To solve the mechanical stability problem, many studies have focused on the undrained mechanical behaviors of the gas hydrate–bearing sediments. Winters et al. (2007) studied the pore pressure effects on methane hydrate–bearing sandy and silty sediments during undrained shear and found that pore pressures decreased in sandy sediments, whereas the silty sediments had a positive pore pressure response. Ghiassian and Grozic (2013) and Grozic and Ghiassian (2010) conducted undrained triaxial shear tests on methane hydrate–bearing sand and the results showed that the occurrence of hydrates would increase the strength, stiffness, cohesion, and internal friction angle of the sediments. Sun et al. (2013) performed undrained triaxial experiments on methane hydrate–bearing sand and found that the failure strength increased with the increase in methane hydrate saturation and effective confining pressure. Yoneda et al. (2015) performed undrained triaxial tests for hydrate-free clayey-silty sediments and results showed a positive excess pore pressure, which indicates that the sediments are normally consolidated. Iwai et al. (2017, 2018) performed undrained triaxial tests on CO₂ hydrate–bearing sandy sediments, and they found that the strength and stiffness of the samples increased with the increase in hydrate saturation and negative excess pore-water pressure was observed. Priest et al. (2019) conducted undrained triaxial tests on the cores from offshore India and found that the coarse-grained hydrate–bearing sediments had greater undrained shear strength than the fine-grained hydrate-free sediments.

The host sediments used in previous studies are mainly sand. Only a few studies examined gas hydrate–bearing fine-grained sediments primarily because of the experimental difficulties in synthesizing NGHs in the fine-grained sediments and water-saturating the fine-grained sediments. However, 90% of NGHs are buried in fine-grained sediments (clays or silts) (Liu et al. 2019), especially in the South China Sea. Geological surveys show that the hydrate deposits in the South China Sea contain a large amount of NGHs (Yang et al. 2017). From May to July in 2017, the trial production of NGHs in the Shenhu Sea area of the South China Sea was completed with an average daily output of 5151 m³ and a maximum output of 35 000 m³/day. This successful production trial shows that the fine-grained hydrate deposits also have significant natural gas production potential (Chen et al. 2018; Li et al. 2018). As the NGHs in this area are buried in fine-grained and low-permeability deposits (Liu et al. 2020; Sun et al. 2019b; Wu et al. 2020), the NGH dissociation causes the increase in pore pressure and the overburden and underburden layers of the gas hydrate–bearing sediments act as low-permeability boundaries to fluid migration (Wang et al. 2020a). In addition, the depressurization method will also trigger volumetric compaction of the sediments. The compacted dense formation might have a low permeability leading to a reduction of gas production rate (Sun et al. 2019c). Thus, the undrained mechanical behavior of gas hydrate–bearing sediments in this area plays a vital role in the safe and efficient exploitation of NGHs (Li et al. 2019). However, few studies have conducted undrained triaxial experiments on gas hydrate–bearing sediments in this area. Luo et al. (2016) and Wang et al. (2017) conducted undrained triaxial experiments on methane hydrate–bearing sediments (MHBSs) of the South China Sea, and they both found that the decomposition of hydrates decreased the strength of MHBSs. However, they used the ice-seeding method to form hydrates, which is quite different from the hydrate accumulation processes in natural systems (Yun et al. 2007) and ignores the effect of the hydrate formation mechanism and nucleation site on the mechanical behavior of gas hydrate–bearing sediments (Priest et al. 2005). To further understand the in situ undrained mechanical behavior of gas hydrate-bearing sediments in this area, a series of undrained triaxial shear tests have been performed on water-saturated MHBSs of the South China Sea, also comparing the undrained triaxial test results with the drained one.

2. Testing device and sample preparation

2.1. Triaxial testing device

To reproduce the occurrence environment of methane hydrates in the sub-seabed, a temperature-controlled and high-pressure triaxial testing device has been developed. The triaxial shear testing device for MHBSs is schematically illustrated in Fig. 1, which is used for the in situ methane hydrate formation and also to exert the confinement and load. The in situ conditions are applied on the cylindrical sediment sample. The sample is 61.8 mm in diameter and 125 mm in height, and is larger than other triaxial testing samples (Hyodo et al. 2014; Li et al. 2019; Wang et al. 2019). The triaxial test device is equipped with a hydraulic oil source, ISCO pumps, and a thermostatic bath, which can provide an axial load capacity of 600 kN, a cell pressure capacity of 30 MPa, a pore-pressure capacity of 25 MPa, and a temperature range from –35 °C to room temperature. The temperature of the MHBSs can be adjusted within a range of ±0.1 °C by circulating antifreeze through a closed heat-exchange tube coiled in the cell of the thermostatic bath. A thermocouple placed near the sample in the triaxial cell is used to record the temperature of the sample with time. Data acquisition of the vertical displacements, loading, confining pressure, back pressure, and temperature is performed via the control and acquisition software on the computer.

Fig. 1.

2.2. Specimen preparation

The host sediments were drilled in the Shenhu area of the South China Sea, provided by China National Offshore Oil Corporation (CNOOC), whose burial depth is approximately 7.8–8.8 m. The dried marine sediments are shown in Fig. 2, and the particle-size distribution of the host sediments is shown in Fig. 3. The specific gravity of the host sediments is 2.73 g/cm³, and the grain size displays a wide spatial distribution of 0.188–120.3 μm. The major components of the sediments are clay (<4 μm, 32.29%) and silt (4–63 μm, 66.47%), while the sand (>63 μm, 1.24%) content is relatively low as shown in Fig. 3. In terms of the percentage of clay, silt, and sand, the sediments in this area belong to clayey silts (Shepard 1954). The median diameter d ₅₀ of the sediments is 7.27 μm with a coefficient of uniformity C _u = 7.12 and a coefficient of curvature C _c = 1.11. Therefore, the sediments are characterized as well-graded clayey silts according to the Unified Soil Classification System (ASTM 2017).

Fig. 2.

Fig. 3.

The specimen was prepared in accordance with the following procedures (Fig. 4). First, the dried sediments were well mixed with ultrapure water in a transparent sealed bag for an initial water saturation of 60%. Ultrapure water refers to the water that has few impurities and metal ions, which was chosen with the aim of reducing the influence of metal ions and other impurities on the hydrate formation. Then, the moist sediments were compacted into the mold (61.8 mm in diameter and 125 mm in height) in 15 layers, and each layer was compacted four times with a tamper. Then, the specimen was removed from the mold. To prevent the sample from being damaged when covering the rubber membrane, the unsaturated sample was placed in a freezer at 268 K (Fig. 4a). Then, the cylindrical sample was placed into a triaxial cell covered with a rubber membrane.

Fig. 4.

The pressurized methane gas was injected into the unsaturated specimen from the lower end of the pedestal. The temperature of the sediments was adjusted to 288 K to melt the ice inside the pore completely (Fig. 4b). The pore pressure was increased gradually to 9 MPa by gas injection (Fig. 4c). Simultaneously, the confining pressure was increased to 9.2 MPa at the same increasing rate, which maintained a pressure difference of 0.2 MPa with the pore pressure. After 24 h, the temperature inside the triaxial cell was adjust to 274 K to ensure that the pressure–temperature conditions were in the methane hydrate stability zone and form hydrates in the pores of the sample (Fig. 4d). When forming the hydrates, both the inlet and outlet were connected to the pump, which provided enough gas to maintain a constant pressure. The formation of hydrates will consume the gas in the pores. When the pressure decreases with consumption of gas in the pores, the pressure gradient will drive the gas from the pump to the pores. When the target hydrate saturation was reached, pure N₂ gas of 9.2 MPa was injected into the sample to replace the remaining CH₄ gas in the pores while the confining pressure was increased to 9.4 MPa. Then, the precooled water of 9.4 MPa (the confining pressure was increased to 9.6 MPa) was injected into the sample to replace the N₂ gas in the pores. When the amount of water inflow was equal to the amount of water outflow and exceeded the volume of the pore space, the sample was considered water-saturated. Afterwards, the pore pressure was increased to 12 MPa, while the confining pressure was increased to 12.2 MPa at the same increasing rate (Fig. 4e). Then, the temperature was adjusted to 281 K for 24 h to reproduce the in situ temperature and pressure conditions of the gas hydrate–bearing sediments of the South China Sea (Fig. 4f). Subsequently, the isotropic consolidation was performed with the initial mean effective stress of 1, 2, and 3 MPa. When consolidating the samples, both the inlet and outlet were connected to the pump. The pore pressure pump maintained a constant pressure of 12 MPa. The consolidation process reduces the pore volume, draining the excess water from the pores into the pump. After 10 h of consolidation, the pore volume and pump volume remained unchanged, and the consolidation process was considered complete (Wu et al. 2020b). Triaxial shear tests were performed successively at a constant strain rate of 0.04%/min in an undrained condition, which was sufficiently slow to ensure the equalization of pore pressure across the sample (Bishop and Henkel 1962), while monitoring the changes in deviator stress and excess pore-water pressure. When the axial strain reached 35%, the triaxial shear test was stopped.

2.3. Hydrate saturation calculation

To obtain an accurate relationship between methane hydrate saturation and mechanical properties such as failure strength and stiffness, it is important to determine the exact methane hydrate saturation in the sample (Ghiassian and Grozic 2013). The volumetric change of the pore volume ΔV _T induced by the decrease of temperature from 288 K to 274 K is as follows:

(1)

where V _p is the pore volume of the sample; Z ₁ is the compressibility factor of methane under the conditions of 9 MPa and 288 K (0.8412); Z ₂ is the compressibility factor of methane under the conditions of 9 MPa and 274 K (0.80425); T ₁ is the temperature of the sample under the condition of 288 K; T ₂ is the temperature of the sample under the condition of 274 K.

The volume of methane gas consumption induced by the formation of hydrates is

(2)

where V is the total methane gas volume consumed after the formation of the hydrates.

The volume of formed hydrates is

(3)

where P is the pore pressure of the sample under the condition of 9 MPa; n is the hydration number (5.75); and are the molar masses of methane and water, respectively; ρ is the density of the hydrate (0.916 g/cm³); R is the gas constant.

The hydrate saturation S _h is calculated as follows:

(4)

2.4. Test conditions

A series of tests on remolded MHBSs was performed under the following conditions: porosity approximately 45%; pore pressure 12 MPa; temperature 281 K; hydrate saturation approximately 0%, 20%, and 30%; initial effective mean stress 1, 2, and 3 MPa; and strain rate 0.04%/min. The specifics of the conditions are shown in Table 1. In Table 1, the case name is in the format "Case-mi-j" where "i" denotes that the triaxial shear test begins from i MPa of the initial effective mean stress and the "j" is the corresponding case number.

Table 1.

3. Test results

3.1. Stress–strain relationship

Figure 5 shows the relationship between deviator stress and axial strain ε _a at several methane hydrate saturations (approximately 0%, 20%, and 30%) when the initial effective mean stresses are 1, 2, and 3 MPa. The stress–strain curves of MHBSs are hyperbolic and exhibit a strain-hardening response to shear. The deviator stress q monotonically increases with the axial strain, and the slope of the q–ε _a curve drops until the quasi-elastic stage ends; then, the slope remains nearly constant in the plastic stage as shown in Fig. 5. The q–ε _a curves clearly depend on the methane hydrate saturation and initial effective mean stress, discussed separately in the next section. The critical state does not appear even for an axial strain of 35%. The strain-hardening is continuing during the entire period of shearing, which is different from the stress–strain relationship of some hydrate-bearing sand that showed strain-softening characteristic (Masui et al. 2005a; Miyazaki et al. 2011b). The difference in these responses is primarily due to the difference of the host sediments. The coefficient of uniformity C _u of some sandy sediments is less than 5 (Masui et al. 2005a; Miyazaki et al. 2011b), which is not considered well-graded (Mitchell and Soga 2005), and the particles are relatively large and uniform. When the sample is sheared exceeding a certain void ratio, some particles have to roll across others, resulting in the dilation and strain-softening of the sample (Hyodo et al. 2013a). While the clayey silts in this paper are well-graded, they contain a wide range of particle sizes and small particles fit into the voids between larger particles during shear, therefore the strain localization does not occur inside the sample (Wang et al. 2020b) and the sediments show strain-hardening behavior during the shearing process.

Fig. 5.

3.2. Excess pore-water pressure

The volumetric strain ε _v of the undrained triaxial tests is equal to 0, and it is important for undrained triaxial tests to measure the excess pore pressure development μ ^w during shear. The excess pore pressure development primarily depends on the collapse of the soil structure. Accordingly, strain is the dominant factor that controls the pore pressure generation (Terzaghi et al. 1996). Figure 6 shows the relationship between excess pore-water pressure μ ^w and axial strain ε _a at three different methane hydrate saturations and initial effective mean stresses. In all cases, the excess pore water pressure remains positive during shear, which indicates that all tests show a trend of overall net compressive volumetric behavior (Lade and Yamamuro 1996). The excess pore-water pressure increases initially and subsequently remains constant after reaching the maximum value. The sample presents a contractive behavior, which is consistent with the results of Winters et al. (2007) that the fine-grained clayey-silty samples have a positive pore pressure response. The pore pressure magnitudes that are developed in undrained loading depend on not only the initial effective mean stress, but also on methane hydrate saturation as shown in Fig. 6, which will be discussed separately in the next section.

Fig. 6.

3.3. Effective stress paths

The effective stress paths of the MHBSs are demonstrated on p′–q space in Fig. 7, where , . The effective stress paths of MHBSs are affected by both the methane hydrate saturation and initial effective mean stress and have a similar shape. For the same initial effective mean stress, the effective mean stress corresponding to higher methane hydrate saturation moves slightly to the right. With the stress path and excess pore pressure generation, it can be seen that the presence of hydrates makes the soil denser, but its effect is not as significant as can be observed for gas hydrate–bearing sand (Iwai et al. 2018). This might be due to the fact that remolded silts have lost their stress history, but sand samples tend to regain their strength. Based on the standard test method for consolidated undrained triaxial compression test for cohesive soils (ASTM D4767; ASTM 1995), the points at 15% strain were selected as the failure points and they were fitted with different initial effective mean stresses into straight lines on the p′–q plane shown in Fig. 7. It can be seen that the failure points of MHBSs are all near the inflection points of the effective stress paths and hydrate saturation has little effect on the failure lines.

Fig. 7.

4. Discussions

4.1. Effect of hydrate saturation

4.1.1. Strength and elastic properties

To further understand the effect of hydrate saturation on mechanical behavior, the relationship between normalized yield strength q _y and failure strength q _f and methane hydrate saturation S _h are plotted in Fig. 8 and Fig. 9, respectively. The yield strength q _y is defined as the strength at which the stress–strain response changes from quasi-elastic to plastic. The normalized yield strength q _n equals the yield strength with hydrates divided by the yield strength without hydrates at the same initial effective mean stress. The failure strength q _f is defined as the deviator stress of MHBSs under 15% strain based on the standard test method for consolidated undrained triaxial compression test for cohesive soils (ASTM 1995). The yield strength and failure strength of the sediments are shown in Table 2. The normalized yield stress and failure strength increase with the increase in methane hydrate saturation, because the hydrates forming at the grain contacts enhance the density of the sample, which increases with the increase in methane hydrate saturation (Hyodo et al. 2013a, 2013b; Miyazaki et al. 2011a). The failure strength is linearly related to the methane hydrate saturation, which is consistent with the results of Iwai et al. (2018). When the initial effective mean stress increases, the slope of the fitted line increases, i.e., a larger initial effective mean stress corresponds to a faster increase in failure strength with the methane hydrate saturation. The reason may be that the increase in initial effective mean stress provides the kinematic constraints and makes the sediments denser.

Fig. 8.

Fig. 9.

Table 2.

To reduce the influence of a setting–bedding error in the triaxial shear test (Miyazaki et al. 2011a), the secant Young's modulus E ₅₀ was used as an indicator of the elastic property. The secant Young's modulus E ₅₀ is the slope of a line that connects the starting point to the point corresponding to 50% of the failure strength on the q–ε _a curve. The normalized secant Young's modulus E _n50 equals the E ₅₀ with hydrates divided by the E ₅₀ without hydrates at the same initial effective mean stress. E _n50 increases with the increase of methane hydrate saturation as shown in Fig. 10. This is because the formation of hydrates increases the density and contact stiffness of the sediments, increasing the shear resistance and anti-deformation ability of the sediments (Uchida et al. 2012).

Fig. 10.

4.1.2. Cohesion and internal friction angle

Cohesion in the Mohr–Coulomb model is the main component of the shear strength of MHBSs (Lijith et al. 2019) and the internal friction angle in the Mohr–Coulomb model is an important engineering parameter to predict submarine landslides (Yoneda et al. 2015). Mohr's stress circles and the corresponding strength envelopes of different hydrate saturations are shown in Fig. 11. The Mohr's stress circles of MHBSs were drawn according to the failure strength and its corresponding effective confining pressure at failure according to the standard test method for consolidated undrained triaxial compression testing for cohesive soils (ASTM 1995). Based on the strength envelopes of different hydrate saturations, the cohesion and internal friction angle of different hydrate saturations can be obtained. The cohesion increases with the increase in methane hydrate saturation, while the internal friction angle does not change with the methane hydrate saturation in the undrained condition, Thus, the strengthening of hydrates to MHBSs is mainly due to cohesion instead of friction (Choi et al. 2018; Masui et al. 2005b). Cohesion c is linearly related to the methane hydrate saturation as shown in Fig. 12. The formula of the fitted line is as follows:

(5)

Fig. 11.

Fig. 12.

The internal friction angle ϕ remains constant at approximately 31°. Then, by bringing the relationships between cohesion c, internal friction angle ϕ, and methane hydrate saturation into the Mohr–Coulomb criterion, the criterion related to hydrate saturation of the South China Sea is obtained:

(6)

(7)

The nonlinear relationship between cohesion and hydrate saturation has been reported in other research (Miyazaki et al. 2011a); the difference may be due to the difference in their host sediments (sands and silts), which may lead to different hydrate occurrence and cementation (Lei et al. 2019; Liu et al. 2019). As well, different host sediments have different specific surfaces, leading to different cohesion (Wijeweera and Joshi 1990; Yun et al. 2007) and thus may lead to different mathematical relationships.

4.2. Effect of initial effective mean stress

4.2.1. Strength and elastic properties

To further understand the effect of the initial effective mean stress on the mechanical behavior of MHBSs, the relationship between failure strength q _f and initial effective mean stress is plotted in Fig. 13. The larger initial effective mean stress provides higher enhancement of the failure strength, as the higher initial effective mean stress corresponds to the higher interparticle coordination and frictional force among the particles (Hyodo et al. 2013a; Miyazaki et al. 2011b; Yun et al. 2007). The quasi-elastic stage increases with the increase in initial effective mean stress when the methane hydrate saturation remains constant, which is consistent with the results of Miyazaki et al. (2011a). Thus, the larger effective confining pressure corresponds to the larger recoverable deformation of the sediments. The relationship between the failure strength q _f and the effective confining pressure at failure , which is defined as the confining pressure minus the excess pore-water pressure at failure, was also obtained as shown in Fig. 14. The failure strength also increases with the increase in effective confining pressure at failure.

Fig. 13.

Fig. 14.

The secant Young's modulus E ₅₀ is higher, corresponding to the higher initial effective mean stress as shown in Fig. 15. The reason may be that in the initial elastic stage, the weak cementation between hydrates and soil particles or the soil particles themselves is the main migration in the sediments during shearing. The higher initial effective mean stress increases the hydrate–grain and grain–grain contact area, and forms a stronger binding force to restrain their migration and rearrangement, leading to a higher secant Young's modulus macroscopically (Wu et al. 2020b; Yan et al. 2017).

Fig. 15.

4.2.2. Maximum excess pore-water pressure

Understanding the pore pressure response of sediments is critical for the stability of the sediments (Mitchell and Soga 2005), and the in situ pore pressure always affects the calculation of the bottom seismic reflector depth (Peltzer and Brewer 2000). To further understand the effect of initial effective mean stress on the excess pore-water pressure, the relationship between maximum excess pore-water pressure and initial effective mean stress is plotted in Fig. 16. The maximum excess pore-water pressure increases with the increase in initial effective mean stress. The axial strain arriving at the maximum excess pore-water pressure increases with the initial effective mean stress.

Fig. 16.

There are two reasons for the change in pore pressure: a change in imposed total mean stress p and a change in in effective mean stress p′. The change in p results from the control of the test: dp = dq/3. The change in p′ is an indication of the soil response. To make the change in p′ clear, (du ^w – dq/3) is plotted versus axial strain ε _a in each case. As shown in Fig. 17, the increase of pore pressure caused by effective mean stress first increased and then decreased, the maximum value of which occurs at an axial strain of approximately 5%. The maximum excess pore pressure induced by effective mean stress also increases with the initial effective mean stress as shown in Fig. 18.

Fig. 17.

Fig. 18.

4.3. Comparison between drained and undrained triaxial tests

Figure 19 shows the comparison of stress–strain curves between drained and undrained triaxial tests of MHBSs at a hydrate saturation of approximately 30%. The difference between the drained and undrained triaxial tests is only the drainage form during the shearing process, and the other experimental steps are the same. Compared with the stress–strain curves of drained triaxial tests, the stress–strain curves of MHBSs subjected to undrained tests yield more easily than those subjected to drained tests, and the MHBSs subjected to undrained tests have lower strength. This is primarily because the effective mean stress of the MHBSs in the undrained experiment is lower than that in the drained tests due to the increase of pore pressure.

Fig. 19.

Figure 20 shows the comparison of effective stress paths of the MHBSs between drained and undrained triaxial tests at a hydrate saturation of approximately 30%. The slope of the total stress path for the drained triaxial tests is 3, and the magnitude of the aforementioned excess pore pressure is represented by the horizontal distance between the total stress path and the effective stress path in the drained tests. Based on the standard test method for consolidated drained and undrained triaxial compression testing for cohesive soils (ASTM 1995, 2001), the points under 15% strain were selected as the failure points and were fitted with different initial effective mean stresses into straight lines on the p′–q plane shown in Fig. 20. The mathematical formulas of the fitted failure lines of MHBSs under the drained and undrained tests are as follows:

(8)

(9)

Fig. 20.

As shown in the above formulas, the intercepts of the drained and undrained failure lines are the same, while the slope of the failure line in the undrained test is higher than that in the drained test. The constitutive equations may be helpful for the numerical simulation of gas hydrate–bearing sediments and assessment of the stability of hydrate deposits during hydrate exploitation of the South China Sea.

5. Conclusions

In this paper, undrained triaxial shear tests on water-saturated MHBSs were conducted to investigate the effects of the methane hydrate saturation (0%–30%) and initial effective mean stress (1, 2, and 3 MPa) on the strength and deformation characteristics of sediments in the South China Sea. The conclusions are as follows:

The stress–strain relationship of hydrate-bearing clayey silts presents a strain-hardening for different hydrate saturations (0%–30%) and different initial effective mean stresses (1–3 MPa). The failure strength corresponding to an axial strain of 15% increases with the increase in methane hydrate saturation (0%–30%) and initial effective mean stress (1–3 MPa).

The excess pore-water pressure of MHBSs remains positive during shear. The increase of pore pressure caused by effective mean stress first increased and then decreased, the maximum value of which occurs at an axial strain of approximately 5%.

Cohesion in the Mohr–Coulomb model increases linearly with the increase in methane hydrate saturation (0%–30%), while the internal friction angle in the Mohr–Coulomb model does not change with the methane hydrate saturation (0%–30%) in the undrained condition.

With the stress path and excess pore pressure generation, it can be seen that the presence of hydrate makes the soil denser, but its effect is not as significant as observed for gas hydrate–bearing sand. This might be due to the fact that remolded silts have lost their stress history, but sand samples tend to regain their strength.

The authors will be conducting similar tests on overconsolidated hydrate-bearing samples of the South China Sea in their next study to investigate how the presence of hydrates affects the sediments.

List of symbols

C _c: coefficient of curvature
C _u: coefficient of uniformity
c: cohesion (MPa)
d ₅₀: median diameter (μm)
E ₅₀: secant Young's modulus (MPa)
E _n50: normalized secant Young's modulus
n: hydration number
n ₀: initial porosity
P: pore pressure (MPa)
p: total mean stress (MPa)
p′: effective mean stress (MPa)
p ₀: initial mean stress (MPa)
: initial effective mean stress (MPa)
q: deviator stress (MPa)
q _f: failure strength (MPa)
q _n: normalized yield strength
q _y: yield strength (MPa)
R: gas constant
S _h: hydrate saturation (%)
T: temperature (K)
T ₁, T ₂: temperature of sample under condition of 288 and 274 K, respectively
u ^w: excess pore-water pressure
: maximum excess pore pressure induced by effective mean stress (MPa)
V: total methane gas volume consumed after formation of hydrates
V _g: volume of methane gas consumption induced by formation of hydrates
V _h: volume of formed hydrates
V _p: pore volume of sample
ΔV _T: volumetric change of pore volume induced by decrease of temperature
Z ₁, Z ₂: compressibility factor of methane under conditions of 9 MPa; 288 and 274 K, respectively
ε _a: axial strain
ε _v: volumetric strain (%)
ε _y: yield strain (%)
ε̇: strain rate (%/min)
μ ^w: excess pore water pressure (MPa)
: maximum excess pore water pressure (MPa)
ρ: density of hydrate
σ: normal stress
: maximum effective principal stress (MPa)
: minimum effective principal stress (MPa)
: effective confining pressure at failure (MPa)
τ: shear stress
: internal friction angle (°)

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant Nos. 51890911, U20B6005, and 51909025) and Fundamental Research Funds for the Central Universities (Grant No. DUT20LAB104).

References

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Canadian Geotechnical Journal cover image

Canadian Geotechnical Journal

Volume 58 • Number 3 • March 2021

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Received: 4 November 2019

Accepted: 2 May 2020

Published online: 8 May 2020

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Key Words

undrained triaxial tests
water-saturated
methane hydrate–bearing sediments
clayey silts
South China Sea

Mots-clés

tests triaxiaux non entraînés
saturés d'eau
les sédiments contenant des hydrates de méthane
limons argileux
la mer de Chine méridionale