606
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
The correlations appear to be linear, however this cannot be
true since there must exist an upper and lower boundary for the
cone resistance and pore pressure corresponding to fully drained
and fully undrained. The mean values can instead be fitted by an
expression from Chung et al. (2006), which has been modified
by Poulsen et al. (2012b). The expression is given in (1) and (2)
���
(2)
where is the pore pressure (kPa),
is the cone resistance
(MPa),
is the penetration rate (mm/s),
is the reference
penetration rate equal to 20 mm/s and
a
u
,
b
u
,
c
u
,
m
u
,
a
q
,
b
q
,
c
q
,
and
m
q
are fitting constants.
The value of
corresponds to undrained penetration,
whereas
corresponds to a fully drained penetration.
From Figure 6 and Figure 7, it is not possible to see when the
penetration is undrained or fully drained from the mean values.
For this reason, the constants
a
and
b
must be assumed.
However, in Figure 2 the penetration is close to fully drained for
a penetration rate of approximately 0.5 mm/s. This gives an
estimate of the constants
a
u
+
b
u
and
a
q
+
b
q
. The value of the
other constants can be seen in Table 1, and the fitting curves for
the pore pressure and cone resistance can be seen in Figure 6
and Figure 7.
Table 1. Derived value for fitting constants.
a
b
c
m
Pore pressure,
u
350
-290
1.2
1.1
Cone resistance,
q
5.3
3.8
3.1
0.9
By lowering the penetration rate so that the penetration
occurs as drained, the cone resistance increases. This can be
expressed as (3) (Poulsen et al. 2012b):
(3)
Where
is the cone resistance corresponding to
drained penetration (MPa),
is the measured cone
resistance determined with a penetration rate of 20 mm/s (MPa),
and is a coefficient of drainage. The coefficient of drainage,
can for the Dronninglund silt be set to 1.0-1.7 depended on
whether
is undrained ( =1.7), fully drained
( =1.0), or how close to fully drained it is.
5 CONCLUSIONS
This paper has shown how a change in the penetration rate
affects the measured cone penetration parameters in silty soil.
When using cone penetration tests (CPT) with the standard rate
of penetration of 20 mm/s, the penetration will appear as fully
drained in sandy soils and undrained in clayed soils. However,
for silty soils the standard rate of penetration of 20 mm/s results
in a partially drained penetration. In order to examine which
affect a changed penetration rate has in silty soils on the
measured cone penetration parameters (cone resistance, pore
pressure, and sleeve friction), 15 CPTs with varying penetration
from 60 to 0.5 mm/s have been conducted.
Results from the cone penetration tests conducted with a
penetration rate of 60 and 0.5 mm/s were compared for the cone
resistance, pore pressure and sleeve friction. It was shown that
both the pore pressure and cone resistance gave different results
for a penetration rate of 60 and 0.5 mm/s. The pore pressure
measured with a penetration rate of 0.5 mm/s corresponded to
drained penetration, which resulted in the highest cone
resistance. For the sleeve friction, no correlation was seen.
In addition, a correlation between the mean pore pressure
and mean penetration rate, and mean cone resistance and mean
penetration rate was however seen when plotting the mean
penetration rate in a semi logarithmic plot.
Compared to the normal penetration rate of 20 mm/s, a
decrease in the penetration rate leads to an increase in the cone
resistance due to drainage. The increase can be expressed by a
coefficient of drainage,
, that is equal to 1.0 for fully drained
penetration and 1.7 for undrained penetration. The increase
depends on whether the normal penetration rate of 20 mm/s has
been conducted under undrained, partially drained or fully
drained conditions.
For that reason, it can be concluded that a correlation
between the cone penetration rate and the cone resistance and
pore pressure exists. It is an important factor that the cone
resistance is dependent on drainage condition and consequently
the penetration rate. Particularly if a project requires knowledge
of both the undrained soil parameters and the drained soil
parameters. In this case, it can be useful to know when the
penetration is partially drained and how to convert it to a fully
drained penetration or undrained penetration.
6 ACKNOWLEDGEMENTS
The project is funded by DONG Energy and associated with the
EUDP programme “Monopile cost reduction and demonstration
by joint applied research” funded by the Danish energy sector.
The funding is sincerely acknowledged.
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