1288
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
–
Since the apparatus can conduct a CBR test immediately
after the freeze-thaw process without moving the sample,
the effects of the freeze-thaw action on the bearing-
capacity characteristics of unbound granular base course
materials can be examined under clear boundary condition,
as well as the initial conditions.
2.2
Medium-size triaxial apparatus for unsaturated soils
A schematic diagram of a medium-size triaxial apparatus for
unsaturated soils is shown in Figure 2. One key feature of the
apparatus is the structural design of the cap and pedestal as
shown in Figure 2. Here, the versapor membrane filter is a kind
of microporous membrane filters made from hydrophilic acrylic
copolymer, and polyflon filter is a hydrophobic filter made from
polytetrafluoroethylene. The other key features are as follows:
–
Since the apparatus can use a medium-size cylindrical
specimen (
�
=150mm,
H
=300mm), a triaxial compression
test can be performed in accordance with the “Standard
Method of Test for Determining the Resilient Modulus of
Soils and Aggregate Materials (AASHTO Designation:
T307-99)” (AASHTO, 2003).
–
The apparatus can apply the matric suction from both ends
of the specimen (Figure 2). Besides, pore water is allowed
to drain from both cap and pedestal. Accordingly, the
apparatus can reduce the testing time by shortening the
length of drainage path to half of the specimen height, in
addition to the effect of versapor membrane filter.
–
The apparatus can apply axial load to a specimen with high
accuracy by both strain control method and stress control
method with only one hybrid actuator. Moreover, the
apparatus can perform both monotonic loading tests with
very slow loading rate, and cyclic loading tests in which
the maximum frequency of cyclic loading is up to about 10
Hz.
3 METHODOLOGY
3.1
Method of freeze-thaw CBR test
CBR tests on the specimens exposed to different patterns of
freeze-thaw history under three different water contents were
conducted by using the newly developed freeze-thawing CBR
test apparatus. As a test sample of the CBR test, a natural
crusher-run (C-40, Figure 3) made from angular, crush, hard
andesite, which is employed at the subbase course of pavement
structures in Japan, was used. The specimen was prepared by
compacting the air-dried samples (water content,
w
=1.8%) with
a vibrator at a degree of compaction (
D
c
) of 95% (“air-dried
condition”). Then, air-dried specimens were saturated with
permeating water for 1 hour (“saturation condition”), and after
the saturation process saturated specimens were allowed to
drain by gravity for 3 hours (“wet condition”). Therefore, there
were three types of specimens defined by the difference in
initial water content.
A freeze-thaw CBR test of C-40 was conducted as follows.
Freeze-thaw of the specimen was performed according to JGS
0172-2003, though this research adopted closed-system freezing
so that the initial water content of the specimen could be
maintained. The freeze-thaw process was repeated, and the
number of freeze-thaw process cycles (
N
f
) was given in three
patterns of
N
f
=0 (no freezing), 1, and 2 cycles. After subjection
to the freeze-thaw history, CBR test was carried out as per
Japanese Industrial Standards “Test Method for the California
Bearing Ratio (CBR) of Soils in Laboratory (JIS A 1211:
2009)”.
3.2
Resilient modulus test
Cyclic loading triaxial compression tests on C-40 were
performed under three different water contents in conformance
with the AASHTO Designation: T307-99 by using the newly
developed medium-size triaxial apparatus for unsaturated soils
as follows. In the air-dried condition (
w
=1.2%), an air-dried
specimen after compaction (
D
c
=95%) was isotropically
consolidated under an effective confining pressure (
σ
c
'
) of 49.0
150
Surcha
rge
Water supply
/ drainage
Coolant
circulating line
Insulation
Acrylic cell
Base cooling plate
Temperature
sensor (pt100)
Porous metal
with filter paper
O-ring
Water supply
/ drainage
Porous metal
with filter paper
Top cooling plate
Coolant
circulating line
Coolant
circulating line
Coolant
circulating line
Figure 1. Freeze-thawing CBR test apparatus.
Hybrid
Actuator
Triaxial Cell
Pedestal
Cap
Gap Sensors
LVDT
Dial Gauge
Specimen
150mm x 300mm
LLVDT
Load Cell
(a)
Water
Plumbing path
(
mm)
Screw
Air supply path
(
12mm)
Polyflon filter
Screw
Versapor membrane filter
Porous metal
O-ring
Figure 2. Medium-size triaxial apparatus for unsaturated soils.
(a) Schematic diagram of test apparatus.
(b) Structural design of cap and pedestal.
0.01
0.1
1
10
100
0
10
20
30
40
50
60
70
80
90
100
C-40
D
50
= 9.1 mm
U
c
= 37.1
F
c
= 5.20
PI = NP
s
=2.74 g/cm
3
dmax
=2.07 g/cm
3
w
opt
=8.2%
Soundness = 4.6%
Percentage passing (%)
Grain size (mm)
Figure 3. Physical properties of subbase course material.
(b)
