1018
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
4 SIMULATION RESULTS
The simulation was conducted on four specimens at
S
Hb
= 0%,
15%, 24% and 41% at a pore water pressure of 8 MPa and a
temperature of 5C in order to compare to the experimental data
published in Masui et al. (2005).
S
Hp
is assumed to be
approximately 26% according to the data in Masui et al. (2005).
Fig. 8 shows the simulated stress-strain response at an
effective confining pressure of 1 MPa and test results obtained
under the same conditions. Although the simulation can not
quantitatively reproduce the tests, it captures the essential
features such as strain softening at
S
H
> 26%. At higher
S
H
, the
peak strength is mobilized when the axial strain exceeds
approximately 3%, and the residual strength coincides at a large
strain regardless of hydrate saturation due to complete breakage
of hydrate bonds. This agrees well with experimental data.
However the peak deviator stress obtained from the simulation
is lower than the test results. Besides the difference between
biaxial and triaxial tests, one of the reasons is that the bond
tension and compression strength could be underestimated in
the model. The size of the specimen used in material strength
tests is much larger than inter-particle bonds in MHBS. The
strength measured from a large specimen is much lower than
that of a much smaller specimen.
Fig. 9(a) presents an example of the stress-strain behavior
under different confining pressures, which leads to a
relationship between the peak strength parameters and
S
H
as
depicted in Fig. 9(b). The presence of hydrate cause the increase
in cohesion, while no significant change in the internal friction
angle is found associated with increasing
S
H
. This agrees well
with the experimental observation (Masui et al. 2005). However
the friction angle obtained from the simulation (approximately
20) is lower than the test data (approximately 30). This could
be improved by introducing the inter-particle rolling resistance
in the model. The micro parameters associated with the rolling
resistance can be first calibrated from a simulation on a
specimen without MH bonds in order to reach high friction
angle. These parameters set are then brought into MHBS model.
Considering the inter-particle rolling resistance will result in a
higher peak deviator stress, which better matches the test data as
shown in Fig. 8(b).
0
4
8
12 16
0
1
2
3
S
=67%
0-26%
40%
50%
Deviator Stress
(
MPa
)
Axial Strain
(
%
)
(a)
0
4
8
12
16
0
3
6
9
26.4%
40.9%
50.1%
Deviator stress (MPa)
Axial strain (%)
S
H
=67.8%
(b)
Figure 8. Deviator stress vs. axial strain: (a) DEM simulation; and (b)
triaxial test results performed by Masui et al. (2005)
0 2 4 6 8 10 1
0.0
0.5
1.0
1.5
2.0
2.5
2
S
H
=50%
0.7MPa
0.52MPa
1.1MPa
Deviator stress
(
MPa
)
Axial strain (%)
1.5MPa
(a)
0 20 40 60 80
0.0
0.2
0.4
0.6
0.8
1.0
16
18
20
22
24
Cohesion
Friction angle
(
o
)
Cohesion
(
MPa
)
S
H
(
%
)
Friction angle
(b)
Figure 9. Simulation result (a) deviator stress vs. axial strain at different
confining pressure for a specimen with SH=50%; and (b) peak strength
parameters at different SH.
5 CONCLUSIONS
This paper proposed a two-dimensional bond contact model of
MHBS for considering the bonding effect of MH. The bond
strength envelope was partially derived from laboratory data.
The model parameters are related to the hydrate saturation,
confining pressure, temperature and density of MH. Using this
model, the DEM simulation of the biaxial test is capable of
capturing the major mechanical response of MHBS specimen
such as strain softening and shear dilation at high hydrate
saturation. This study can help to better understand the
connection of the microscopic formation habit of MH to
macroscopic mechanical behaviors of MHBS.
Though the DEM simulation produced results qualitatively
comparable to available test data, quantitative agreement
remains still a challenge. The current model ignores the bond
thickness, which however affects the hydrate saturation and the
bond strength parameters. Consideration of inter-particle rolling
resistance in the model will improve the model performance.
Moreover, the size effect on the bond strength remains unclear
and deserves more caution. Further investigation on these issues
is definitely needed in the future work.
ACKNOWLEDGEMENTS
This work is funded by China National Funds for Distinguished Young
Scientists (No. 51025932), and the EU FP7 IRSES grant (No. 294976).
REFERENCES
Brugada J. et al. 2010. Discrete element modelling of geomechanical
behaviour of methane hydrate soils with pore-filling hydrate
distribution.
Granular Matter
12(5, SI): 517-525.
Cundall P.A. and Strack O.D.L. 1979. A discrete numerical model for
granular assemblies.
Géotechnique
29(1), 47–65.
Ellyin F. and Xia Z.H. 2006. Nonlinear viscoelastic constitutive model
for thermoset polymers.
Journal of Engineering Materials and
Technology
128:579-585.
Hussein A. and Marzouk H. 2000. Behavior of high-strength concrete
under biaxial stresses.
ACI Materials Journal
97(1):27-36.
Hyodo M. et al. 2005. Basic research on the mechanical bahavior of
methane hydrate-sediments mixture.
Japanese Geotechnical Society
45(1):75-85.
Jiang M.J. et al. 2003. An efficient technique for generating
homogeneous specimens for DEM studies.
Computers and
Geotechnics
30(7): 579-597.
Jiang M.J. et al. 2012a. Contact behavior of idealized granules bonded
in two different interparticle distances: An experimental
investigation.
Mechanics of Materials
55: 1-15.
Jiang M.J. et al. 2012b. An experimental investigation on the
mechanical behavior between cemented granule,
Geotechnical
Testing Journal
(
ASTM
) 35(5): 678-690.
Jiang M.J. et al. 2006. Bond rolling resistance and its effect on yielding
of bonded granulates by DEM analyses.
International Journal for
Numerical and Analytical Methods in Geomechanics
30(8): 723-
761.
Jung J. et al. 2012. Stress-strain response of hydrate-bearing sands:
Numerical study using discrete element method simulations.
Journal of Geophysical Research-Solid Earth, 117, B04202,
doi:10.1029/2011JB009040.
Kupfer H. et al. 1969. Behavior of concrete under biaxial repeated
loading.
ACI Journal Proceedings
66(52): 656-666.
Kvenvolden K.A. 1988. Methane hydrate-a major reservoir of carbon in
the shallow geosphere?
Chemical Geology
71(1-3):41-51.
Masui A. et al. 2005. Effects of methane hydrate formation on shear
strength of synthetic methane hydrate sediments.
Proceedings of
the 5th International Offshore and Polar Engineering Conference
,
Seoul, Korea: 364-369.
Nadreau J.P. and Michel B. 1986. Yield and failure envelope for ice
under multiaxial compressive stresses.
Cold Regions Science and
Technology
13:75-82.
Waite W.F. et al. 2009. Physical properties of hydrate-bearing soils.
Reviews of Geophysics
47, RG4003:1-38.
