Actes du colloque - Volume 2 - page 144

1013
Technical Committee 105 /
Comité technique 105
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Figure 5 Variation of normalized deviator stress difference against
degree of MH saturation for T
a
, T
b
, T
c
a
a
b
c
d
e
j
k
l
60mm
80mm
160mm
Front
Side
a: Pressure gauge
g: Camera
b: Syringe pump(Upper)
h: LED light
c: Top cap
i: Pressure cell
d: Pedestal
j:Syringe pump(Lower)
e: Specimen
k:Thermocouple(60mm)
f: Confining plate l: Thermocouple(30mm)
Outside
Inside
a
a
i
b
j
e
d
c
h
g
f
f
Figure 6 Schematic overview of the high pressure plane strain
testing apparatus
Figure 7 Variation of principal stress difference and volumetric
s
train against axial strain for methane hydrate bearing sands
Figure 8 Volumetric strain and maximum shearing strain contours
at axial strain
ε
a
= 10% by PIV analysis
㻿
㻹㻴
㻩㻜㻑
㻿
㻹㻴
㻩㻠㻝㻚㻟㻑
㼀㼛㼥㼛㼡㼞㼍㻌㼟㼍㼚㼐㻌
㻿
㻹㻴
㻩㻜㻑
㼀㼛㼥㼛㼡㼞㼍㻌㼟㼍㼚㼐㻌
㻿
㻹㻴
㻩㻠㻤㻚㻜㻑
0
3
6
9
12
5
0
-5
0
3
6
9
12
Principal stress difference
(
'
1
-
'
3
)/2(MPa)
Volumetric strain
v
(%)
Axial strain
a
(%)
Temperature=5
c
=3.0MPa
T
c
S
MH
=0.0%
T
c
S
MH
=41.3%
Toyoura sand
S
MH
=48.0%
Toyoura sand
S
MH
=0.0%
0 10 20 30 40 50 60
0.0
0.5
1.0
1.5
2.0
Normalized deviator stress
difference (
q
MH
-
q
sand
)/
c
'
Degree of MH saturation
S
MH
(%)
T
a
T
b
T
c
n
=45%
T
a
'
c
=1MPa
'
c
=3MPa
T
b
'
c
=1MPa
'
c
=3MPa
'
c
=5MPa
T
c
'
c
=1MPa
'
c
=3MPa
be observed. Local deformation analysis was performed by
observing the cross-points of the mesh during shear tests and
using this data in PIV analysis. Thermocouples(k,l) were
installed at 60mm and 30mm from the bottom of the specimen
in order to measure the variation of the temperature during the
dissociation of MH. Tc and Toyoura sand were the materials
used for comparison. Specimens were prepared with water
contents equivalent to given degrees of MH saturation and
tamped in 12 layers to give a porosity of
n
=45%. Formation of
MH was performed using the same method as in the triaxial
compression tests. The specimens were saturated by filling the
pore water and consolidated at given effective confining stress
and then subjected to shear tests under drained conditions. The
speed of shear was 0.1%/min. Also, in order to understand the
behavior during MH production, MH was dissociated by
decreasing pore water pressure by 7MPa under constant cell
pressure and observing the behavior. After finishing MH
dissociation, the pore water pressure again rises and the
behavior was also investigated. The rate of depressurization
during dissociation of MH and repressurization was
0.5MPa/min. Figure 7 shows the shear test results for Tc and
Toyoura sand with and without MH. A marked increase in the
strength and stiffness due to the cementation of MH are
observed in both specimens. The increments of strength and
initial stiffness for Tc are lower than those of Toyoura sand. The
corresponding volumetric strain exhibits a dilative manner for
MH bearing Toyoura sand, whilst that of Tc shows a contractive
manner during shear. Figure 8 shows the results of the PIV
analysis during shear for the specimens. The upper part of the
figure shows volumetric strain and the bottom part shows shear
strain contour for specimens at 10% axial strain. On the left
hand side there is Tc without MH, then Tc with MH and then
Toyoura sand without and with MH. Clearer shear bands can be
observed in Toyoura sand than in Tc, so the effect of fines on
local deformation can be observed in this figure. The shear band
for Toyoura sand with MH is clearer than that of Toyoura sand
without MH, but there are no clear differences observed
between Tc with and without MH. Next, in order to simulate the
production of MH, pore water pressure was decreased whilst
keeping constant initial shear stress, MH was dissociated and
the deformation behavior of loss of cementation was
investigated. Specimens which had cell pressure 10MPa, pore
water pressure 7MPa and consolidated at effective stress 3MPa
had their pore water pressure decreased by 7MPa. Stress and
temperature conditions were set outside of the MH stability
state line and MH was dissociated. Figures 9 and 10 show
effective stress paths applied to the experiments. In these
figures, only the results for Toyoura sand are shown. Failure
envelopes for Toyoura sand with and without MH are shown by
broken and solid line, respectively. The dissociation tests were
performed in two cases. In Case 1, after the specimens were
isotropically consolidated, depressurization was conducted at a
rate of 0.5MPa/min. Then, repressurization was performed at
the same speed after finishing MH dissociation. These pore
water pressure histories correspond to real production of MH in
recovering pore water pressure after production. In Case 2, after
specimens were isotropically consolidated, initial shear stress
was applied at an amount greater than the host sand but less
than MH bearing sand. Depressurization is then conducted in
the same way as Case 1 and at the same speed. After finishing
MH dissociation, pore water pressure was increased at the same
rate. This test was done to simulate the element in the vicinity
of the production well, where the stress condition is close to
failure. In Figure 10, the specimen failed during repressurization
when the stress path reached the failure line of the host sand. In
Figure 11, the relationship between the temperature and pore
water pressure during the depressurization process is shown. As
can be seen, temperature decreased suddenly when the pore
pressure was decreased to the value of the state boundary curve.
This is due to the temperature absorption phenomena of MH
during dissociation. Also, at a pressure of 3MPa dissociation
and reformation repeats, and when dissociation is complete the
temperature of the specimen rises to room temperature. Figure
12 shows the relationship between the effective stress ratio and
active strain during dissociation tests of MH for Case 2. Point
(a) corresponds to the point before dissociation, where initial
shear stress has been applied. Point (b) corresponds to the point
where pore water pressure was decreased from 10MPa to 3MPa.
1...,134,135,136,137,138,139,140,141,142,143 145,146,147,148,149,150,151,152,153,154,...913