358
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
method for frost susceptibility of soils, which uses specimens in
a disk form 10 cm in diameter and 5 cm in thickness and is
capable of falling head permeability tests using a burette before
and after a freeze-thaw cycle (designated hereinafter as the frost
heave test apparatus capable of permeability tests). The second
is a frost heave test apparatus equipped with bender elements
(BEs) for measuring the velocity of shear waves propagating in
the specimen before and after a freeze-thaw cycle (designated
hereinafter as the frost heave test apparatus equipped with BEs).
As shown schematically in Figure 2a, two pairs of BEs are
provided, in the upper and lower plates and in the mold,
respectively. The BE pair in the cooling plates measures the
velocity (
V
s
)
vh
of the shear wave that oscillates horizontally and
propagates vertically, while the BE pair in the mold measures
the velocity (
V
s
)
hh
of the share wave that oscillates and
propagates horizontally (Kawaguchi et al. 2001, Yamashita and
Suzuki 2001). The BE pair in the mold was removed in freeze-
thaw tests to avoid any damage by freezing, and mounted again
after thawing for BE tests. The third apparatus is a direct box
shear test apparatus that permits freeze-thaw test in the shear
box (designated hereinafter as the temperature-controllable
direct box shear apparatus). The shear box is schematically
represented in Figure 2b. The specimen is a disk 6 cm in
diameter and 4 cm in thickness. A coolant is circulated in the
piston and pedestal for temperature control. The shear box, or
the circumferential surface of the specimen, is thermally
insulated by a two-centimeter-thick acrylic resin layer. A rubber
sheet, 0.3 mm in thickness, is placed between the upper and
lower halves of the shear box during freeze-thaw tests to
prevent water leakage, and is removed for shear tests to leave a
0.2mm clearance between the box halves.
Figure 3 shows grain size distribution curves for the two
frost-susceptible fine-grain soils used in this study. One is
weathered volcanic ash obtained at Kitami City, Hokkaido
(sample V) which was used in the tests using the two frost
heave test apparatus. The test specimens were prepared by
compacting the volcanic ash sample conditioned to be slightly
drier than with the optimum water content. The other, used in
the temperature-controllable direct box shear tester, is a mixture
of clay commercially available in dry powder and silt at a ratio
of 1:1 by weight, which was made to a slurry at twice the liquid
limit and then consolidated one-dimensionally to a vertical
stress
v
= 100 kPa (sample CL,
w
L
= 38%,
I
P
= 19).
3 RESULTS AND DISCUSSION
3.1
Void ratio and coefficient of permeability
Falling head permeability tests were performed before and after
the freeze-thaw tests using the frost heave test apparatus in
order to study effects of freeze-thaw cycles on the void ratio
e
and the coefficient of permeability
k
. Sixteen specimens were
prepared using a rammer and a mold at three levels of
compaction energy: 126, 284, and 550 kJ/m
3
(Nakamura et al.
2011). The vertical stress was
v
=10 kPa for all the tests. Six of
the specimens underwent three freeze-thaw cycles and
k
was
measured before and after each cycle. The frost heave test
method specified in the JGS 0172-2009 standard was used. In
the thawing tests, the specimen was dewatered through the top
and bottom surfaces held at 5
C, and then saturated again.
Figure 4 shows the rate of frost heave
U
h
(mm/h) and frost
heave ratio
ξ
(%) of the specimen as functions of void ratio
e
at
the beginning of freezing. A greater
e
means more pore water
and lower tensile strength, which might suggest easier ice lens
formation. Actually, however, both
U
h
and
ξ
are lower at higher
e
at the beginning of freezing, presumably because of inhibition
of continuity of the unfreezable water needed for frost heave
(Nakamura et al. 2011). The results of the three consecutive
freeze-thaw cycles, indicated by the points connected with lines,
show that the repeated cycles lead to convergence to fairly
constant values of
U
h
and
ξ
.
Figure 5 represents changes in the void ratio
e
and
coefficient of permeability
k
of the specimen through the freeze-
thaw cycles. The data for each specimen are connected by lines.
As observed in Figure 4, the freeze-thaw cycles decrease greater
initial
e
values and increase smaller initial
e
values, eventually
leading to convergence to a relatively limited range of
e
values
between 1.1 and 1.3. Ono et al.(2003) reported that freeze-thaw
history decreases
e
of clay at a normally consolidated state and
increases
e
of clay with a larger over consolidation ratio. This
observation is in agreement with the present results assuming
that higher compaction energies on the specimen result in over
consolidation in terms of the specimen’s mechanical
0.001
0.01
0.1
1
10
0
20
40
60
80
100
Percent passing (%)
Grain size
Clay
Silt
Sand
Gravel
: Sample V
: Sample CL
Screw
Resistance
temperature
detector
Acrylic
resin
Coolant
Specimen
(mm)
Figure 2. Test apparatus diagrams (a): frost heave apparatus equipped
with BEs, b): temperature-controllable direct box shear apparatus)
Figure 3. Grain size distribution of samples used in the tests
Figure 4.
U
h
and
ξ
as functions of
e
at the beginning of freezing
Water
Vertical stress
Water
BE
(
V
s
)
vh
(
V
s
)
hh
BE
Upper plate
Lower plate
Piston
Shear
Vertical stress
Coolant
Coolant
Coolant
Water
Resistance
temperature
detector
a)
b)
0.2
0.4
0.6
0.8
1.0
Rate of frost heave
: Cycle 1
: Cycle 2
: Cycle 3
Sample V
U
h
(mm/h)
0.8
1.0
1.2
1.4
1.6
0
40
80
120
Frost heave ratio
Void ratio at the beginning of freezing,
e
(%)
