1008
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
this study only a single grading was used. The particle size
distribution was composed of 89% sand and 11% fines, of
which 7% is silt and the remaining 4% is clay size particles. It
has a liquid limit of 25.5%, a plasticity index of 10 and specific
gravity of 2.7.
2.2
Laboratory testing program
The laboratory testing program included the execution of
Proctor compaction tests (AS1289.5.1.1, 2003) under different
levels of compaction energy (i.e. 15, 25, 35 blows per layer
corresponding to 358, 596 and 834kJ/m
3
). The required amount
of water was added to the sample and the mixture was
thoroughly mixed with a masonry trowel and then left under
constant temperature and humidity conditions for 24h to ensure
a uniform distribution of moisture. The compaction data is
shown Figure 1. Subsequently, the specimens were carefully
trimmed (60
60
25mm
3
) from the compacted soil cylinders
(1L) to minimize disturbance, while the excess of soil was
typically used to determine the water content and suction using
filter paper method and a small tip tensiometer. The procedure
was completed in a matter of minutes to minimize exposure to
air to prevent the loss of any moisture. Thereafter the specimens
were wrapped in cling film and stored inside a plastic bag.
6
8
10
12
14
16
18
15
16
17
18
19
20
21
S
r
=0.67
Reduced, 15bl/layer : E=358kJ/m
3
Standard, 25bl/layer : E=596kJ/m
3
Enhanced, 35bl/layer : E=834kJ/m
3
Dry unit weight,
d
(kN/m
3
)
Water content, w (%)
S
r
= 1
S
r
=0.8
CWDST tests
Figure 1. Compaction curves obtained for the silty sand soil.
2.2.1
CWDST program
A conventional shear box apparatus with a carriage running on
roller bearings and a step motor drive unit capable of applying
constant rate horizontal displacements was used. The apparatus
was equipped with a load cell and two LVDT displacement
transducers for measuring the horizontal shear force and
monitor the horizontal and vertical displacements (accuracy of
0.002kN,
0.0025mm and
0.001mm, respectively). A vertical
load was applied with a lever arm loading system with beam
ratio of 10:1. Data acquisition was controlled by a LabVIEW
program coded “in house” accompanied with a National
Instruments card NI USB-6009 with 8 input channels.
To conduct the tests under CW conditions, an effort was
made to prevent any evaporation. This was achieved by running
the compression and shearing stages of the tests in a
temperature controlled environment (23
2
o
C), and by enclosing
the direct shear box with the assembly in an air tight plastic bag
(Figure 2). Furthermore, to minimize the volume of air around
the specimen, a 1mm thick film of polyethylene was placed in
contact with the specimen and the gaps between the two sliding
halves and the bottom half and base were sealed with silicone
grease. The compacted specimens were extruded into the shear
box and then subjected to a compression stage (vertical stresses
of 38.4kPa, 79.5kPa, and 146.7kPa). Subsequently, the
specimens were sheared at a constant rate of displacement of
0.01mm/min. It is important to note that in a direct shear test,
the shear zone is localised and thin compared to the mass of the
specimen (Shibuya et al. 1997). While suction is likely to be
constant throughout the specimen when a small displacement
rate is adopted due to self-equilibration, water content likely
differs, but on average it would the same as the initial water
content because water is not allowed to flow out and
evaporation is minimised. This rationale is supported by the
slight difference in suction measured using filter paper method
(ASTM D5298, 2003) at the beginning and end of the tests
(<5kPa and <1kPa for specimens prepared at dry and wet of
OMC, respectively) and a small vertical variation of water
content typically less 0.2-0.3% obtained in the sheared
specimen at the end of the test.
Figure 2. Shear box diagram with the system implemented to prevent
evaporation.
3 RESULTS AND DISCUSSION
3.1
As-compacted water content and suction
Figure 3 shows the water retention data for additional
compacted specimens prepared at different energy levels.
Overall, the suction decreases with an increasing water content
varying between 5 kPa to 616 kPa. Although there is no
apparent relationship between suction and compaction energy,
all data points seem to converge to a logarithmic regression line
given by Eq. (1) (
R
2
> 0.95).
( ) -1.56ln(s)+18.50
w s
(1)
This indicates that the hydraulic behaviour of compacted soil
may be independent of the compaction characteristics (i.e.
change in the water content and energy level).
1
10
100
8
10
12
14
16
18
1000
Moisture content, w (%)
Matric suction, s (kPa)
R
2
>0.95
-1.56ln(s)+18.50
Compaction energy:
E
2
=358 kN.m/m
3
E
3
=596 kN.m/m
3
E
4
=834 kN.m/m
3
Figure 3. Post-compaction matric suction data in terms of water content.
3.2
Shear strength behaviour
In general, all the specimens showed a peak followed by a
decrease in shear stress before subsequently attaining a
relatively constant value after 6.5 mm of horizontal
displacement. Although strain softening and dilative behaviour
was clearly evident in the specimens compacted at dry of OMC,
the specimens compacted at OMC and wetter of OMC did not
show a distinct post peak drop and showed a mainly contractive
response. Figure 4 shows the typical displacement plots
obtained for an applied vertical stress of 40 kPa.
3.2.1
Peak and ultimate shear strength envelopes
Figure 5(a) shows the failure envelopes for the specimens
prepared at same energy level but different water contents
grouped by the correspondent applied vertical stresses. Both
peak and ultimate states are represented and the envelopes were
interpolated using the procedure suggested by Vilar (2006).
While both peak and ultimate shear strength increase with
