604
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
2.1
Test site soil stratigraphy
The soil stratigraphy was at the test site identified by means of
two geotechnical boring results. In addition, soil samples were
collected and laboratory tests were executed in order to classify
the soil. Both test results show that the subsoil consists of sandy
silt with clay stripes from approx. 4.5 to 11.4 m below ground
level. Above 4 m, the soil consists of silty sand, and below 11 m
the soil consists of clay with sandy silt stripes. In addition, the
groundwater was encountered at approximately 0.2-0.6 m below
the ground level. Generally, the soil is much layered and
inhomogeneous which makes the soil difficult to classify.
However, in Poulsen et al. (2012a), the soil was overall
classified as sandy silt with clay stripes.
2.2
Cone Penetration Tests
In order to examine how a change in the cone penetration rate
affects the measurements, various cone penetration tests have
been conducted. A total of 15 CPTs with five different
penetration rates were conducted; these were 60, 20, 5, 1, and
0.5 mm/s. All CPTs were conducted with a distance of
approximately 3 m. This was done to make sure that the
drainage of each CPT would not affect the drainage of the other
CPTs. The location of the 15 CPT’s can be seen in Figure 1.
60mm/s
20mm/s
5mm/s
1mm/s
0.5mm/s
577670
577660
577650
577640
6336550
6336540
6336530
Eastcoordinate(m)
North coordinate (m)
Borings
Figure 1 Location of the 15 CPTs with penetration rates of 60, 20, 5, 1
and 0.5 mm/s. The coordinates are given UTM coordinates.
During the execution of the CPT’s, the cone resistance, (
q
c
),
pore pressure (
u
2
), sleeve friction (
f
s
), depth (
d
), and the
penetration rate (
v
) were measured. A more detailed description
of the test site, experimental programme and the validity of the
tests can be found in Poulsen et al. (2012b).
Because of the layered and inhomogeneous soil, the
measured CPT parameters are very fluctuating and hence
difficult to interpret. As a result, the data has been smoothed for
every 50 cm, which was concluded acceptable in Poulsen et al.
(2012b).
3 EFFCT OF PENETRATION RATE IN SILT LAYER
The soil layer classified as sandy silt with clay stripes was
located between 4.5 to 11.4 m below ground level. Only this
layer has been analysed since it is considered to be the silt layer
where the effect of the penetration rate is clearest. As a result,
the following graphs only contain results from 4.5 to 11.4 m.
In the following, it is analysed how a change in cone
penetration rate affects the measured cone resistance, pore
pressure and sleeve friction respectively. As described in
Poulsen et al. (2012b), the soil layer consists of many stripes,
which gives a very fluctuating result for the measured cone
penetration parameters. In order to clearly visualise the effect of
a change in the penetration rate, only the penetration rates of 60
and 0.5 mm/s have been included. This is done as it is the
extreme points corresponding to undrained and fully drained
that are of interest, and the penetration rates of 60 and 0.5mm/s
are closest to these conditions. Consequently, the data from the
CPTs conducted with a penetration rate of 20, 5 and 1 mm/s
have been excluded in the figures.
3.1 Pore pressure
In Figure 2, the smoothed pore pressure from 4.5 to 11.4 m
for the CPTs conducted with a penetration rate of 60 and 0.5
mm/s can be seen.
-200
0
200
400
600
800
1000
5
6
7
8
9
10
11
u
2
(kPa)
Depth (m)
60 mm/s
0.5 mm/s
u
0
Figure 2. Comparison of the smoothed pore pressure conducted with a
penetration rate of 60 and 0.5 mm/s. The figure contains results from 3
CPTs test for each penetration rate. The plotted
u
0
is an average value.
Figure 2 shows that changing the penetration rate from 60 to
0.5 mm/s results in a decreased pore pressure. This is because
the drainage conditions change when the penetration rate is
decreased. From Figure 2, it seems as though the CPT
conducted with a penetration rate of 0.5 mm/s corresponding to
fully drained penetration, since the measured pore pressure is
close to or equivalent to
u
0
.
However, it is not possible to conclude if the CPT conducted
with a penetration rate of 60 mm/s corresponds to undrained or
partially drained penetration. Nevertheless, by lowering the
penetration rate, the penetration changes from undrained or
partially drained to fully drained, which results in a lower pore
pressure.
3.2 Cone resistance
In Figure 3, the smoothed cone resistance from 4.5 to 11.4 m
for the CPTs conducted with a penetration rate of 60 and 0.5
mm/s can be seen.
Figure 3 likewise shows that changing the cone penetration
rate from 60 to 0.5 mm/s results in a change in the cone
resistance. However, a decreased penetration rate results in an
increased cone resistance. The changes observed in the cone
resistance are like the pore pressure caused by changes in
drainage conditions where the penetration changes from
undrained or partially drained to fully drained. This results in a
higher cone resistance.
3.3 Sleeve friction
In Figure 4, the sleeve friction from 4.5 to 11.4 m for the
CPTs conducted with a penetration rate of 60 and 0.5 mm/s can
be seen.
Contrary to the pore pressure and cone resistance, Figure 4
does not show any correlation between the sleeve friction and
cone penetration rate.
