1822
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
soils with greater undrained cohesion (Vogt et al. 2005 and
Eurocode 7 - DIN 1054 - 2010).
4.3 Sensitivity of columns with small diameter
Two different design philosophies exist concerning the check of
the bearing capacity of the column in the case of column
diameters greater than 25 cm. The philosophy can be either
without safety check (ASIRI in the domain 2 or CPRF-
guideline, where the system is checked as a global system
including slab, piles and soil), or with safety check in
accordance with the piling standards (ASIRI in the domain 1).
For smaller column diameters with a load transfer layer there is
no recommendation for the use as pure settlement reducers, use
which namely is only applicable for systems with a sufficient
ductility and possibilities of load redistribution (flexible
connection between piles and slab or relatively large column
diameter).
Special attention must be paid to the particular sensitivity
concerning the internal resistance of columns beneath the load
transfer platform under horizontal loadings. Horizontal loads
from the structure lead to shear, bending moments and thus
possibly tension in the column section. Though the concrete
design is regulated uniformly and with an adequate safety factor
for all diameters, the particular importance of the interactions
between the structure and the system made of soil and columns
is not completely described in the current state of the art, in
particular for very slender elements and under special temporary
loading cases.
On the safe side, it can be assumed that the total horizontal
load is taken by the columns, but this may lead to an
uneconomical design. Furthermore, any execution imperfections
in the position of the columns (eccentricity), in the column
diameter or in their inclination can have a considerable
influence in the case of small column diameters and induce
undesigned shear stresses, even in compliance with the given
tolerances. Attention must be paid to the execution stages too,
where the top of the columns can be particularly endangered
due to heavy vehicle traffic.
5 CONCLUSIONS
The soil reinforcement method with rigid inclusions is a soil
stabilization system with columns which are separated from the
structure by a load transfer layer, and which work either by
analogy with a pile foundation or with a conventional soil
reinforcement technique. The distinction between two
application domains 1 and 2, either for an increase of the
bearing capacity (1) or for a settlement reduction (2) reflects
this difference. In the domain 1, the safety checks are similar to
those for conventional pile foundations. In the domain 2, only
SLS checks have to be carried out, including a control of the
internal resistance of the columns.
The use of such column systems in the domain 2 corresponds
to the design philosophy of the CPRF-guideline (Hanisch et al.
2002), in which no verification of the bearing capacity of single
piles has to be done, and where only the stability of the global
system and of course the structural capacity have to be checked.
In the CSV-guideline (DGGT 2002), a higher safety level
than in ASIRI and in the CPRF-guideline is considered for the
external resistance for the relevant small diameters. Indeed, the
bearing capacity of single columns must always be carried out
with the assumption of the total load in the columns, and the
approach as settlement reducer without safety check for the
single columns is not permitted for those diameters.
The internal resistance of columns with small diameters can
be particularly threatened because of the significant influence of
potential imperfections in the execution in this case. In the
current state of the art of the soil-columns-structure interactions,
an increased safety level should be taken into account for
columns with small diameters under horizontal loads.
6 ACKNOWLEDGEMENTS
Special thanks go to Professor Roger Frank of Ecole des Ponts
ParisTech (Navier-Cermes), Bruno Simon from the French
company Terrasol and scientific director of ASIRI and Serge
Lambert from the French company Keller Fondations Spéciales
for the scientific and practical explanations of the
recommendations ASIRI.
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