1819
Comparison of the safety concepts for soil reinforcement methods using concrete
columns
Comparaison des concepts de sécurité pour les méthodes de renforcement de sol avec colonnes
en béton
Katzenbach R., Bohn C.
Technical University Darmstadt, Insitute and laboratory for geotechnics
Wehr J.
Keller Holding GmbH
ABSTRACT: The rigid inclusion concept is a soil reinforcement method using concrete columns with a small diameter compared to
usual piles. The load bearing behaviour of such systems is presented in comparison to pile foundations and to combined pile-raft
foundations. The safety concept developed in the French recommendations ASIRI for rigid inclusions is divided in two domains
depending on the use of the columns, either to enhance the bearing capacity (analogy to piles) or only to reduce the settlements. The
safety factors considered for the bearing capacity are compared with those in pile standards (Eurocode 7) and with those in different
recommendations for similar reinforcement systems. The particular sensitivity of columns with small diameter is highlighted.
RÉSUMÉ : Les inclusions rigides sont une méthode de renforcement de sol avec colonnes en béton de diamètre faible par rapport aux
pieux usuels. La répartition des charges au sein d’un tel système est présentée en comparaison avec les pieux et les fondations mixtes.
Le concept de sécurité développé dans les recommandations ASIRI pour les inclusions rigides se divise en deux domaines
d’utilisation, soit comme augmentation de la portance (analogie avec les pieux), soit uniquement comme réduction des tassements.
Les facteurs de sécurité considérés pour la portance sont présentés en comparaison avec les normes de pieux (Eurocode 7) et avec
différentes recommandations pour des systèmes similaires. On insiste sur la sensibilité particulière des colonnes de faible diamètre.
KEYWORDS: rigid inclusion, soil reinforcement, safety concept, Eurocode 7, column diameter, sensitivity
1 INTRODUCTION
One of the existing soil reinforcement methods with concrete
columns is the so-called „rigid inclusions“ method, which has
experienced a fast development in the last years, in particular in
France (Briançon et al. 2004). This technique consists of a soil
improvement method using in general non-reinforced concrete
columns with a column diameter of 25 cm up to 80 cm with a
soil displacement method. The rigid inclusions are in general
separated from the structure by a granular load transfer layer
(Figure 1).
Figure 1. Rigid inclusions system in comparison with usual foundation
systems
To some extent, these columns can be compared to usual
piles. But in the case of rigid inclusions, the bearing capacity of
the soil itself is taken into account in the design, like for
combined pile-raft foundations (CPRF), which leads to
considerable savings of concrete in the columns and of steel
reinforcement in the foundation slab.
The technique with rigid inclusions is employed as well
under shallow or raft foundations (e.g. industry halls, water
tanks) as under embankments (e.g. high speed railway lines).
The French recommendations of the national research
project ASIRI („Améliorations de Sols par Inclusions
RIgides“), which have been published in 2012, provide a
harmonization for the (in particular numerical) calculations
(Jenck et al. 2004), for the safety requirements and for the
execution of rigid inclusions. The initial point of the safety
theory is the European standardization, that is the French
applications of the Eurocode 7 for the geotechnical safety
checks, in general based on the pressuremeter design theory in
France (Frank 2009), and the Eurocode 2 for concrete. Two
different application cases are distinguished in ASIRI: the rigid
inclusions can be either used to guarantee the stability of the
structure or only as settlement reducers. These different
application cases either in analogy to foundations systems or to
soil reinforcement methods are reflected in the recommended
safety concept in ASIRI.
2 LOAD BEARING AND DEFORMATION BEHAVIOUR
OF RIGID INCLUSIONS
Shallow
Pile foundation
„
rigid inclusions
“
Combined pile‐raft
foundation
Load transfer layer
Rigid inclusions can both be embedded in a bearing layer or – in
the general case – designed as floating elements in a
compressible soil. The applied load from the structure on the
system is distributed between the soil and the column heads. A
so-called efficiency of the system can be defined from this load
distribution as the ratio of the total load in the column head to
the total vertical load. This definition corresponds to the pile-
raft-coefficient for CPRF (Hanisch et al. 2002). The load
distribution depends on different factors, in particular the
rigidity of the foundation and of the soil, the thickness of the
load transfer layer, the spacing between the columns and the
rigidity of the columns (Okyay 2010).
The interactions between soil and columns in the case of a
vertical load are presented in Figure 2. A negative skin friction
affects the upper part of the columns due to the separation
between slab and columns and because of the compressibility of
the soil to be improved. This leads to a reduction of the load in
the soil over the depth and to an increase of the force in the
columns until the depth where the differential settlement
between soil and column is equal to zero. Below this neutral
plane, the force in the column is transferred to the soil through
