2562
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
In this study, the effect of smear zone on strength of model
SCP installed in 100mm diameter and 200mm long clay
specimens is investigated using conventional triaxial
compression tests under different confining pressures ranging
from 50kPa to 575kPa. The composite specimen were prepared
by driving a small diameter PVC casing into the sample and
then backfilling the cavity with sand column after removing the
casing. The casing was roughened using sand glued to its outer
walls prior to insertion to replicate the smearing effect. The
SCPs were prepared using area replacement ratio of 6.25 to
64% and compacted using pneumatic compactor. SCPs of
different diameters (25-80mm) were used to investigate the
improvement in the load-carrying capacity of the specimens.
The effect of smear zone on SCP was investigated by observing
the change in pore pressure during consolidation and undrained
shear strength of the composite ground. The test results suggest
that, tress-strain behavior of the clay was influenced by the
presence of smear zone. The natural fabric of the soil was
destroyed adjacent to the SCPs with smear zone which in turn
affected pore pressure response of the composite soil sample.
Shear induced pore pressures were less in soil specimens with
smear-effect, but this difference was not apparent when 80mm
diameter SCP with smear zone was used. In addition, as the
reinforcement area ratio increased, both the stiffness and the
shear strength of the specimen increased. Thus, sand
compaction piles currently stand as one of the most viable and
practical techniques for improving the mechanical properties of
soft clays.
2. EXPERIMENTAL WORK
2.1.
Materials and methods of sample preparation
The test specimens were prepared in 450mm long and 250mm
diameter stainless steel cylindrical mould. Deaired clay slurry
was consolidated on the laboratory floor, first under its own
self-weight and later under surcharge of 211- to 404 kN/m
2
applied in stages on top of the clay surface using a custom
designed pneumatic load frame (Fig.1).
Figure 1. Consolidation set-up on the laboratory floor
Upon completion of the 1-D consolidation, the block of clay
was extruded and trimmed into three 100mm diameter
cylindrical specimens using soil lathe. Up to 3 specimens could
together be prepared using this mould. The experimental
program consisted of 20 tests on composite clay with sand
column. The specimens were held in split cylindrical moulds
and a smooth PVC casing slowly pushed along its length to form
a cylindrical hole at the centre. The hole was backfilled with
fine sand (d
50
=0.3mm) compacted in layers at 90% relative
density using a pneumatic compactor (Fig. 2).
Figure 2. Preparation of composite specimen
Diameter of the sand column varied between 25- and 80mm in
the specimens. This corresponds to an area replacement ratio, a
s
(Aboshi et al. 1979) that ranges between 6.25- and 64%. The
smear zone was created by using a rough casing painted with a
paste of coarse sand (d
50
= 1.3mm) to drill the hole. Thickness
of the smear zone was taken equal to the thickness of the paste.
The effect of smear beyond this zone was ignored. After
preparing the sand column, the ends of the specimen were
covered with a thin circular rubber sheets having a central hole.
Diameter of the hole was slightly less than that of the sand
column so as to only permit radial drainage. Two deaired
porous stones were then placed at the two ends of the specimen
and the entire assembly mounted on the triaxial chamber.Table
1 shows properties of the clay used in this study. The ratio of
the diameter of sand column with smear zone to the diameter of
sand column without smear zone (d
s
/d) was about 1.1 to 1.2 in
all tests, which compares well with the values reported by the
previous researchers (e.g. Indraratna and Redana, 1998;
amongst others). The specimen was enclosed in a rubber
membrane and the chamber filled with water. The soil samples
were then isotropically consolidated under mean effective
stress,
p'
which varied between 50 and 575 kN/m
2
.
Table 1 Properties of kaolin clay
Clay
(%)
Silt
(%)
Liquid
limit (%)
Plastic
limit (%)
Shrinkage
limit (%)
G
s
75
25
49
23
16
2.64
3. RESULTS AND DISCUSSIONS
Consolidated undrained triaxial tests were performed on 200mm
long and 100mm diameter cylindrical samples prepared from
remoulded and reconsolidated commercially available kaolin
clay installed with SCP. Table 2 shows the details of the soil
specimens prepared for testing. In the table, OCR is defined as
the ratio of the isotropic preconsolidation pressure,
p
0
'
to
p'
.
p
0
'
was taken equal to the higher of either
p'
or the mean effective
stress after 1D consolidation,
p'
estimated using the equation
(Wroth 1984):
'
'
v
'
sin .
p
6701
(1)
where
′
is the effective angle of friction (e.g. Schofield and
Wroth 1968). The load-deformation data was analyzed using the
unit cell arrangement proposed by Balaam et al. (1977). In this
method, the column and surrounding clay are assumed to act as
a single element with equivalent distributions of stresses and
strains in composite specimens. Figures 3a-b show results of
deviator stress,
q
plotted against axial strain,
a
. As can be
seen, all samples reached peak deviator stress (q
max
) at 6 to 10%
axial strain. Figures 3a-b also show that the ultimate strength
exhibit transient peaks in some tests. This was expected since
these soil samples were overconsolidated prior to the shearing.
In few tests on normally consolidated clays, q decreased after
passing
q
max
because of instability of the failed samples at high
confining pressure.
Soil
specimen
Final specimen size
Specimen trimming
Slurry consolidation
Sand column
Soil sample
Pneumatic
compactor
PVC casing
pushed into
the sample
