820
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
sand layer, for which an appropriate equivalent resistance needs
to be assessed for the considered design case of the structure.
Where soil and state properties cannot be linearised, an
upper and lower equivalent resistance can be assessed using the
diagrams. The resistance can be then interpolated linearly
between both values. Due to the squeezing effect,, the scaling
factors accounting for changes in relative density, shown in
Figure 5 and Figure 6, may underestimate the actual resistance
of relatively thick layers. Thus, the diagrams proposed by Baldi
et al. (1986) might be used instead.
In order to consider different penetration rates, meaning
partially drained conditions of the sand layer, a fully drained
and a fully undrained equivalent resistance need to be
determined. Given that the drainage and hydraulic boundary
conditions are comparable between the reference test, e.g. a
CPT measurement, and the structure to be designed, one can
interpolate between the two values using one of the approaches
discussed by Danziger and Lunne (2012).
The diagrams can be also used where viscous-type rate effect
matters. In this case the penetration rate used in the soil
investigation should corresponds to the penetration rate of the
structure to be designed.
5.2
Limitations
The diagrams cannot be directly used for multi-layered soils
where sand layers interfere with each other, meaning that the
resistance in the clay is affected by both an upper and a lower
granular layer.
In this study, a stress ratio of
k
0
=0.75
has been used. The
diagrams can be employed to other stress case only, when the
stress state is corrected for the effective mean stress, which
governs the response of the sand layer in the hypoplastic
formulation. Preliminary FE calculations indicate that the
curves are very similar to ones presented in Figure 5 to Figure
7. However, further calculations needs to be performed to
confirm that.
Only one sand type has been considered in the presented
parametric study. Due to the normalisation, the diagrams should
be applicable to other sands as well. FE simulations, in which
other soil properties for the sand have been used, showed
quantitatively similar curves. However, additional simulations
should be performed to confirm the normalisation and the
general applicability of the diagrams to other materials.
Not considered by the hypoplastic model is grain crushing.
At high penetration pressures, grains may crush, which is
accompanied by a change of the soil properties. Although FE
calculations indicate that the diagrams are applicable to other
sands with different properties than the one analysed, the
properties should not change during penetration. Grain crushing
affects the grain size distribution and the limiting void ratios,
meaning that for example also the relative density changes.
Thus, the diagrams cannot be applied when grain crushing is
expected.
6 SUMMARY AND OUTLOOK
In this contribution, the effect of thin sand layers on the
penetration resistance is discussed. By means of FE simulations,
a comprehensive parametric study has been performed varying
thickness and relative density of a sand layer embedded in soft
clay. In addition, the strength of the clay and the vertical
consolidation stress has been systematically varied. The results
are presented in normalised diagrams of which the influencing
factors can be read out. To superimpose different effects, the
corresponding factors need to be simply multiplied. The
plausibility of this approach has been shown and possible
application cases have been discussed.
To overcome some of the limitations and existing
uncertainties, further FE simulations are planned to perform, in
which in particular the stress ratio
k
0
and the soil properties are
varied systematically. It is believed that different soil properties
do not affect the presented diagrams and the effect of the stress
ratio can be represented by an additional normalised curve.
In addition to numerical studies, model and field tests should
be performed to reinforce the approach proposed.
7 REFERENCES
Ahmadi M.M. and Robertson P.K. 2005.
Thin-layer effects on the CPT
q
c
measurement
. Can. Geotech. J. 42: 1302-1317.
Baldi G., Belotti R., Ghionna N., Jamiolkowski M. and Pasqualini E.
(1986),
Interpretation of CPT’s and CPTU’s. 2
nd
Part: Drained
penetration resistance
. 4
th
International Geotechnical Seminar,
Field Instrumentation and In-Situ Measurements, Singapore, 143-
153
Cudmani R. 2001.
Statische, alternierende und dynamische Penetration
nichtbindiger Böden
. Ph.D thesis, Karlsruhe, Germany.
Cudmani R. and Sturm H. 2006.
An investigation of the tip resistance in
granular and soft soils during static, alternating and dynamic
penetration
. Int. Sym. on vibratory pile driving and deep soil
compaction TRANSVIB 2006, Paris, France.
Danziger F. and Lunne T. (2012),
Rate effect on cone penetration test in
sand
, Geotechnical Engineering Journal of the SEAGS & AGSSEA
Vol. 43 No. 4, 72-81
Niemunis A. and Herle I. (1997),
Hypoplastic model for cohesionless
soils with elastic strain range
, Mechanics of Cohesive-Frictional
Materials, Vol. 2, 279-299
Sturm H. and Andresen L. 2010.
Large deformation analysis of the
installation of dynamic anchors
. NUMGE 2010, Trondheim,
Norway.
Sturm H., Lieng J.T. and Saygili G. 2011.
Effect of soil variability on
the penetration depth of dynamically installed drop anchors
. OTC
Brasil 2011, Rio de Janeiro,
OTC 22396
.
Vreugdenhil R. Davis B. and Berrill J. 1994.
Interpretation of cone
penetration results in multilayered soils
. Int. J. Num. Ana.
Methods, Vol. 18: 585-599.
Wolffersdorff P.-A. v. (1996),
A hypoplastic relation for granular
material with predefined limit state surface
, Mechanics of
Cohesive-Frictional Materials, Vol. 1, No. 3, 251-275
