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Loading behaviour of flexible raft foundations in full scale and centrifuge models
Comportement de radiers flexibles dans des essais grandeur nature et en centrifugeuse
Arnold A.
Dr. Vollenweider Ltd., Zurich & Lucerne University of Applied Sciences and Arts, Lucerne
Laue J.
Institute for Geotechnical Engineering, ETH Zurich
ABSTRACT: Flexible rafts are commonly used foundation systems for different kinds of constructions. A raft is easy to build and to
design even though the dimensioning is not straight forward. Two buildings were equipped to measure the stress distribution between
raft foundations and the adjacent soil, and to measure the deformation of the load-carrying column on top of the foundation in order to
know the load extent. To compare the full scale measurements with another model, centrifuge tests have been conducted in a drum
centrifuge at ETH Zurich. The loading behaviour of different raft foundations has been studied on these two models. This contribution
gives a short summary of the comparison between the measurements gained from full scale and the model tests in the centrifuge. The
conditions in the centrifuge provide an ideal stress distribution between raft and soil, while different influences on a raft foundation in
full scale such as the geometry of the load bringing structure and loading sequences influence the stress distribution in the real world.
RÉSUMÉ : Les fondations flexibles sur radier sont un système de fondations utilisé communément pour différents types de
construction. Un radier est facile à construire et à dimensionner, même si le dimensionnement n'est pas immédiat. Deux bâtiments ont
été instrumentés pour mesurer la distribution des contraintes entre les fondations sur radier et le sol adjacent ainsi que les
déformations de la colonne porteuse située sur la fondation afin de connaitre l'importance de la charge. Des essais en centrifugeuse ont
été réalisés dans la centrifugeuse tambour à l'ETH Zürich afin de comparer les mesures grandeur nature à un autre modèle. Le
comportement sous charge de différentes fondations sur radier a été étudié pour ces deux modèles. Cette contribution donne un court
résumé de la comparaison entre les mesures obtenues grâce aux essais grandeur nature et aux essais dans la centrifugeuse: les
conditions dans la centrifugeuse fournissent une distribution des contraintes idéale entre le radier et le sol, alors que les différentes
influences sur une fondation sur radier, telles que la géométrie de la structure transmettant la charge et les séquences de chargement
ont un effet sur la distribution des contrainte dans une situation grandeur nature
.
KEYWORDS: raft foundations, loading behaviour, system-stiffness, centrifuge tests, full scale measurements
1 INTRODUCTION
Raft foundations are frequently used systems to distribute loads
of different structures into the ground. They are cheap and fast
in construction using simple design assumptions. They provide
a robust system which is not sensitive in terms of settlements –
especially for overconsolidated clays and coarse grained soils.
One may use piled foundations for normally consolidated fine
grained soils to avoid unacceptable settlements.
been studied in model tests and in full scale to improve the
analytical approach by means of investigating the changing
stress distribution due to stiffness variation in soil and structure.
1.1
Analytical models
A short summary of the different analytical models is given
here. The simplest model to obtain a stress distribution between
foundation and soil is to focus purely on the vertical- and on the
momentum equilibrium of force. The approach given in figure 1
does not care about the deformation, which must be identical on
the foundation plate as well as in the soil. Since the deformation
of the foundation system is not regarded, changes in soil- and
structure stiffness are neglected with this method. Thus, those
models provide only a preliminary distribution of the stresses.
Figure 1: Stress distribution between foundation and soil fulfilling the
vertical- and momentum equilibrium (Kany and El Gendy 1996).
Another method deals with a coefficient of subgrade
reaction, based on the approach after Winkler (1867) and
Zimmermann (1888). As given in figure 2 each spring is
independent of the other springs, which results in an unrealistic
distribution of settlements especially at the corners of the
foundation.
Figure 2: Independent springs on the approach of coefficient of
subgrade reaction (Kany and El Gendy 1996).
A third approach is based on the linear-elastic behaviour of
soils after Boussinesq (1885) which has been developed to an
Even though raft foundations are easy to build, the
dimensioning of such structures is not straight forward and
partially to simplistic. The analytical approaches most
commonly used base on equilibrium and linear-elastic
behaviour of soils, which usually provides only an ideal shape
of the stress distribution acting on a foundation.
The stress distribution between raft and soil has therefore
