916
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Table 2: Selected sand properties i) column (Weber, 2008) ii) Perth sand
(Buchheister, 2009).
Origin
Column
Perth
USCS classification
SP
SP
Density
ρ
s
[g/cm
3
]
2.65
2.65
Critical state angle of
friction
�
’
cv
[°]
37.0
30.5
Medium grain size d
50
[mm]
0.75
0.23
Coefficient of uniformity [-]
1.4
1.79
Coefficient of gradation [-]
1.0
1.26
Grain shape
semi-angular-
slightly rounded
-
3 SAMPLE PREPARATION
The clay was consolidated in a 250 mm diameter oedometer
with incremental loading up to a total vertical stress of 200 kPa.
The sample was removed from the oedometer container and the
pore pressure transducers (PPTs) were installed. Their locations
are shown in Fig. 1. The sample was then put into the centrifuge
strongbox (diameter 400 mm) and the gap of 75 mm between
container wall and clay model was filled with Perth sand by dry
pluviation without compaction (Fig. 1), resulting in an
axisymmetric sample. In this test, the boundary conditions
cannot be considered to be oedometric with no radial strain, as
the sand/clay interface is not rigid.
Fig. 1: (a) Plan view and (b) cross-section of the model set-up.
4 T-BAR TEST
A T-bar test, the location of which can be seen in Fig. 1, was
conducted in order to determine the profile of the undrained
shear strength in the soft clay. The T-bar (Fig. 2) has a length of
28 mm and a diameter of 7 mm. It was driven at a rate of
0.5 mm/s up to a depth of 140 mm, where a waiting time of one
minute was observed before the tool was pulled back out of the
model.
The undrained shear strength was calculated with the
following equation:
[
u
b
F
]
s
kPa
L B N
(5)
where F is the force recorded for the T-bar penetration, L the
length of the T-bar, B the width of the T-bar and N
b
the T-bar
factor, set in this case at 10.5 [-] (Stewart et al., 1994).
Fig. 2: T-bar (Weber, 2008).
(a)
Fig. 3: Profile of the undrained shear strength obtained with the T-Bar.
5 INSTALLATION OF THE STONE COLUMN
The test procedure and the corresponding results are presented
at model scale with the exception of the forces, which are scaled
to prototype scale. According to the scaling relationships,
stresses are scaled by the factor 1, whereas time scaling from
model to prototype differs by a factor n
2
for diffusion processes
and by n during inertia processes. Forces are scaled by a factor
n
2
(e.g. Schofield, 1980), n being the factor by which earth’s
gravity is increased.
(b)
Standpipe
P4
P7
Fig. 4: Evolution of the pore water pressures during the in-flight
consolidation.
P1 – P3
P5
P6
P2
