1015
A Simplified Contact Model for Sandy Grains Cemented with Methane Hydrate
Un modèle simplifié pour les contacts entre grains de sable cimentés par hydrates de méthane
Jiang M., Liu F., Zhu F., Xiao Y.
Department of Geotechnical Engineering, Tongji University, Shanghai 200092, China
Key Laboratory of Geotechnical and Underground Engineering (Tongji University), Ministry of Education, Shanghai
200092, China
ABSTRACT: Methane hydrates (MHs), regarded as one of the most promising future energies, extensively occupy the voids of soil
deposits in permafrost regions and deep seabed. The presence of MH largely changes the macro-mechanical properties of the host
deposits due to MH bonds among soils particles. This study introduces a new contact model for soil grains cemented by MH. This
model was experimentally calibrated from two rods cemented by different materials. The bond failure criterion was then related to the
strength properties of MH considering the effects of temperature, confining pressure, density and saturation of MH. The new model
was implemented into a discrete element code to simulate the mechanical response of a MH bearing soil specimen subjected to biaxial
loading conditions. The comparison between the simulation and experimental results shows that the new contact model can
qualitatively capture the effects of MH cementation. Strain softening and shear dilation of soils cemented with MH become
remarkable due to the presence of MH bonds. The cohesion of the MH bearing deposit substantially increases with the hydrate
saturation, while the internal friction angle is less affected.
RÉSUMÉ : Les hydrates de méthane (MH) sont considérés comme une source énergétique potentielle fondamentale pour le futur. Ils
abondent dans les régions avec permafrost et sur les fonds marins profondes. Les MH influencent grandement les propretés macro-
mécaniques des sols qui les contiennent en raison de liens qu’ils forment entre les particules des sols granulaires. Dans ce papier, on
introduit un nouveau modèle pour les liens entre particules de sol cémentes par MH. Le modèle a été calibré utilisant des données
expérimentales de pairs de barres métalliques (barres de Schneebeli) cimentées par diffèrent matériaux. Le critère de rupture pour les
liens (bonds) a été corrélé aux paramètres de résistance au cisaillement des MH incluant les effets de la température, la pression de
confinement, la densité et la saturation des MH. Le modèle a été implémenté dans un logiciel aux éléments discrets pour simuler la
réponse mécanique des échantillons de sols contenant des MH chargés en conditions biaxiales. La comparaison entre les résultats
expérimentaux et numériques montre que le nouveau modèle reproduit qualitativement les effets de la cimentation des liens MH. On a
observé que les liens MH causent un adoucissement et dilatation remarquables. La cohésion et l’angle de frottement des sols
contenant MH augmentent avec le dégrée de saturation des hydrates avec une augmentation plus significative pour la cohésion que
pour l’angle de frottement.
KEYWORDS: Methane hydrate bearing sands; bond; contact model; distinct element method; granular material
1 INTRODUCTION
As promissing resource of future energy, methane hydrates
(MHs) are extensively found in voids of sediments situating in
seabeds and permafrost regions at low temperatures and high
pressures (e.g., Kvenvolden, 1988). They greatly enhances the
strength of the host sediments. Moreover, MHs are prone to
dissociation due to change in envrionmental conditions and
human activities (e.g., installation offshore infrasturcutes).
Serious geohazards such as marine landslides are likely
triggered by instability of methane hydrate bearing sediments
(MHBS). Unfortunately, the mechanism of these geo-hazards is
poorly understood due to the lack of robust constitutive model
of MHBS in addition to limited experimental data.
A variety of influencing factors on mechanical properties of
MHBS were investigated using special triaxial compression
aparatus (Hyodo et al. 2005; Masui et al. 2005). In particular,
the hydrate habit (i.e., the distribution of MHs in the pore scale)
strongly affects properties of MHBS. For example, hydrates
acting as inter-particle cementation cause larger strength and
stiffness than pore-filling hydrates. It implies that hydrate habit
should be featured in a robust constitutive model. However
most models derived from laboratory tests at the macro scale are
unable to build the connection between macroscopic properties
of MHBS and micro structure of hydrate at the pore scale.
In the contrast, the distinct element model (DEM) proposed
by Cundall and Strack (1979) provides a solution for simulating
hydrate habits at the grain scale. Waite et al., (2009) identified
three habits: (1) pore filling with hydrates nucleating in the pore
without bridging two or more soil grains together; (2) load
bearing with hydrate bridging nearby soil grains and taking part
in the strong force chains of the granular assembly; and (3)
cementation with hydrates cementing at inter-particle contacts
as illustrated in Fig. 1(a). The first type naturally turns into the
second when the hydrate saturation exceeds 25-40%. Pore-
filling hydrate has been successfully modeled by Brugada et al.
(2010) and Jung et al. (2012) using DEM. However the model
of cementation-type hydrate remains unsolved partially due to
the difficulty in quantifying the strength of MH bonds. This
constitutes the strong motivation of this paper. The objective of
this study is to develop a suitable bond contact model for
MHBS, which is of critical importance to produce meaningful
macro-mechanical response of MHBS via DEM.
2 A BOND CONTACT MODEL FOR MHBS
2.1
A conceptual bond contact model and laboratory data
Fig. 1 shows a conceptual bond contact model (Jiang et al.
2006) of two disks with radii
R
1
and
R
2
bonded at a finite width
B
and a central thickness
t
. A dimensionless parameter
is
defined in terms of common radius
R
=2
R
1
R
2
/(
R
1
+
R
2
):
RB
(1)
