1056
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
to check the uniformity of temperature in the triaxial cell.
Compared to other suction and temperature controlled triaxial
systems reported in the literature, this new system, perhaps, is
the first one which can test cyclic behaviour of soil under
different suction and temperatures. In addition, this system is
equipped with three pairs of Hall-effect transducers, measuring
local axial and radial strains at center portion of each soil
specimen. After calibration by a micrometer, the resolution and
accuracy of each Hall-effect transducer is about 1 and 3 μm,
respectively. For a specimen height of 152 mm adopted in this
study, a displacement of 3 μm corresponds to an axial stain of
about 0.004%.
Figure 1. Suction and temperature controlled cyclic triaxial apparatus.
3 SOIL TYPE AND SPECIMEN PREPARATION
The material tested is a completely decomposed tuff (CDT)
sampled from Hong Kong. Measured liquid limit, plastic limit
and plastic index are 38%, 25% and 13%, respectively. The
sand, silt and clay contents are 25%, 71% and 4%, respectively
(Ng and Yung, 2008). Following the Unified Soil Classification
System, the CDT is described as silt (ML) (ASTM, 2006).
In order to obtain soil specimens with identical fabric, all
specimens were prepared following the same method. Each
triaxial specimen, 76 mm in diameter and 152 mm in height, is
compacted at initial water content of about 16.3% and dry
density of about 1760
kg/m
3
. In order to produce a uniform
specimen, each specimen is compacted in 10 layers. After the
completion of compaction, the height and diameter of the
specimen are measured by a caliper (readable to 0.01
mm
) and a
PI tape (readable to 0.01
mm
), respectively. The initial suction
of the specimens after compaction is 95±2 kPa as measured by a
high capacity suction probe.
4 TEST PROGRAM AND PROCEDURES
Three series of cyclic triaxial tests were performed to
investigate effects of (i) net stress and matric suctionand (ii)
temperature on
M
R
of an unsaturated subgrade soil. Figure 2
shows the stress and thermal paths adopted in the three series of
tests. The initial state of each specimen is donated by point A in
the figure. Firstly, each specimen is isotropically compressed to
a net confining stress of 30 kPa (A→E) at room temperature (20
℃
). Depending on the test requriements, specimens are then
brought to different suction and temperature conditions at net
confining pressure of 30 kPa. Stages of suction equalisation and
temperature equalisation are necessary to ensure that the entire
specimen reaches the desired suction (
u
a
-
u
w
) and temperature
conditions.
In Series 1 tests, three specimens W0T20, W30T20 and
W60T20 were wetted by decreasing suction from 95 to 0, 30
and 60 kPa (i.e., E→B, E→C and E→D) at 20
℃
, respectively.
To investigate suction effects on
M
R
along the wetting path,
M
R
of these three specimens were measured and compared In Series
2 tests, three specimens D100T20, D150T20 and D250T20 were
dried to suctions of 100, 150 and 250 kPa (i.e., E→F, E→G and
E→H) at 20
℃
, respectively.
M
R
were measured and compared
to study suction effects on
M
R
along the drying path. In Series 3
tests, three specimens W0T40, W30T40 and W60T40 were
wetted from 95 to 0, 30 and 60 kPa at 20
℃
and then heated up
from 20 to 40
℃
at constant suctions (i.e., E→B→B’,
E→C→C’ and E→D→D’). To investigate thermal effect on
M
R
of unsaturated soil, measured
M
R
of W0T40, W30T40 and
W60T40 was studied and compared with that of W0T20,
W30T20 and W60T20 in Series 1. More details of experimental
program are summarised in Table 1.
Figure 2. Stress and thermal path of each soil specimen during stages of
suction equalisation and temperature equalisation.
Table 1. Details of the experimental program.
Series
Specimen
identity
Matric suction
(
kPa
)
Temperature
(
℃
)
Equalization
time (day)
1-wetting
W0T20
0
20
12
W30T20
30
20
7
W60T20
60
20
4
2-drying
D100T20
100
20
4
D150T20
150
20
7
D250T20
250
20
13
3-thermal
W0T40
0
40
15
W30T40
30
40
11
W60T40
60
40
9
Once a specimen had equalised at a given suction and
temperature, it was subjected to cyclic loads to determine its
M
R
. In each cyclic test, applied axial stress was varied with time
following a haversine form while net confining pressure and
temperature was maintained constant. For clarity, variations of
axial stress during the first and last 10 cycles are shown in
Figure 3. The difference between the maximum and minimum
axial stresses is defined as cyclic stress
q
cyc
. According to
AASHTO (2003) standard for resilient modulus test, four levels
of cyclic stress (i.e., 30, 40, 55 and 70 kPa) were considered and
applied to each specimen in succession. At each level of
q
cyc
,
100 cycles of loading-unloading at 1 Hz were applied. More
details are given by Ng et al., (2012).
In each cyclic triaxial test, constant water content condition
is maintained because the dissipation rate of excess pore water
pressure is low compared to the rate of repeated traffic loads in
the field. The pore water pressure was measured at the base and
mid-height of each specimen, as shown in Figure 3. For clarity,
