2023
Deformation behaviour of clay due to unloading and the consequences on
construction projects in inner cities
Étude du comportement en déformations de l’argile suite à un retrait de charge et conséquences
lors de projets de constructions en zone urbaine
Katzenbach R., Leppla S.
Technische Universität Darmstadt, Institute and Laboratory of Geotechnics, Germany
ABSTRACT: In the course of construction projects in many cases the soil is unloaded. For example at the construction of excavations
or the deconstruction of existing buildings the soil relaxes due to the reduced stress level. Cohesive soil materials like clay react
strongly time dependent. At all construction projects in the city of Frankfurt am Main, Germany, which belong to the Geotechnical
Category 3 regarding Eurocode EC 7 the deformation of the soil was measured during and after the construction works. It has to be
determined that the settlement relevant soil layer, the tertiary Frankfurt Clay, relaxes time-delayed due to the unloading in the
dimension of centimetres. The new acknowledgement of the deformation behaviour of the Frankfurt Clay due to unloading the subsoil
is presented in detail by typical projects for the development of the city.
RÉSUMÉ : Il arrive régulièrement dans le cadre de projets de construction en zone urbaine que le sol subisse un déchargement. Dans
le cas d’excavations par exemple, le sol se détend suite à une baisse des contraintes appliquées. Le comportement des sols cohésifs
tels que l’argile dépend fortement du temps. Pour l’ensemble des projets de construction à Francfort en Allemagne, ces projets étant
classés en Catégorie Géotechnique 3 selon l’Eurocode 7, les déformations du sol ont été mesurées pendant et après les travaux. Il est à
mettre en avant que le sol de la couche pertinente en terme de tassements, l’argile tertiaire de Francfort (Frankfurt Clay), subit un
relâchement différé dans le temps suite au retrait de charge de l’ordre du centimètre. Les nouvelles reconnaissances sur le
comportement de l’argile de Francfort suite à un déchargement du sol sont présentées ici en détail pour des projets typiques dans le
cadre du développement de la ville.
KEYWORDS: Soil-structure-interaction, Frankfurt Clay, time dependent deformation behaviour, observational method.
1 INTRODUCTION
Since the beginning of the large infrastructure and high-rise
building constructions in Frankfurt am Main the bearing and
deformation behaviour of the Frankfurt Clay has been
scientifically researched intensely (Chambosse 1972, Breth and
Stroh 1974, Amman et al. 1975, Sommer et al. 1990, König
1994, Katzenbach and Moormann 1999, Breth and Katzenbach
2000, Katzenbach et al. 2001, Moormann 2002, Katzenbach et
al. 2002, Dürrwang et al. 2007, Janke et al. 2010, Vogler 2010,
Katzenbach et al. 2011). The first research and experiences
made with the Frankfurt Clay result from new constructions, i.e.
from high-rise buildings and tunnel constructions.
Figure 1. Measured uplift of the surface of the excavation pit at BfG
high-rise building (now: European Central Bank ECB).
Due to the development of the city a lot of deconstruction
activities occur in advance of new construction projects. In the
course of these projects new knowledge about the time
dependent bearing and deformation behaviour of the Frankfurt
Clay is obtained, especially if there is a larger timeframe
between the deconstruction (unloading) and the new
construction activity (reloading). In such cases the uplifting is
not overlapped or compensated by an early reloading. For
example a deep excavation base usually heaves up to several
centimetres as shown in Figure 1 (Amman et al. 1974).
The time dependent deformation behaviour of the over-
consolidated tertiary Frankfurt Clay is presented in detail by 2
large deconstruction projects. The first project is the
deconstruction of the building complex of the Zürich Insurance
consisting of 2 high-rise buildings (Zürichhochhaus I and II)
and the construction of the new high-rise building Opernturm at
the same location. The project is located very close to a tunnel
of the metro system. The second project is the deconstruction of
an up to 14 storeys high-rise building in the historic centre of
Frankfurt am Main. The project is located directly over
2 tunnels and an underground station of the metro system.
2 DECONSTRUCTION OF THE ZÜRICH INSURANCE
HIGH-RISE BUILDINGS
The Zürich Insurance building complex was built between 1959
and 1963. It consisted of 2 high-rise buildings (Zürichhochhaus
I and II) with heights of 70 m and 63 m and annexe to the high-
rise buildings with up to 8 storeys. The complex was founded
on a raft in a depth of 7 m below the surface. In the years 2001
and 2002 the complex was deconstructed down to the ground
