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Innovative solutions for supporting excavations in slopes
Solutions innovantes pour le soutien d'excavations situées dans des terrains en pente
Lüftenegger R.
GDP ZT-OG, Austria
Schweiger H.F., Marte R.
Institute for Soil Mechanics and Foundation Engineering, Graz University of Technology, Austria
ABSTRACT: The design of support measures for deep excavations is one of the key tasks in geotechnical engineering. The choice of
the most appropriate support system depends on various obvious factors such as ground conditions and excavation depth but
sometimes also on less obvious boundary conditions, for example when construction of ground anchors is not possible because
permission of placing them in neighbouring property is not given. In these cases other options have to be pursued, resulting
sometimes in non-conventional solutions. Examples for such innovative support systems are presented in this paper. In the first case
the arching effect of the retaining structure was used to design an excavation pit without any anchors reaching on the neighbouring
ground, because there was no permission for construction elements there. The 6 meter spanned arches consist of mixed in place
columns (MIP), which rest on supporting walls (also mixed in place columns) oriented in the direction of the slope. In the second
example the behaviour of a serrated sheet pile has been investigated. Comprehensive 3D finite element analyses have been performed
on order to prove that the suggested retaining structures are feasible solutions.
RÉSUMÉ : La conception des mesures de soutènement pour les excavations profondes est une des tâches fondamentales dans la
géotechnique. Le choix du système le plus approprié dépend de plusieurs facteurs évidents comme les conditions de sol ou la
profondeur de l'excavation, mais parfois aussi de contraintes moins évidentes comme par exemple le cas où des ancrages ne sont pas
possibles parce qu'il n'y a pas d’autorisation pour l'installation dans la propriété voisine. Dans ces cas, d’autres options doivent être
envisagées, qui résultent parfois dans des solutions non-conventionelles. Quelques exemples de telles solutions sont présentés. Dans le
premier cas, l’effet de voûte de la structure de soutènement a été utilisé pour la conception d’une excavation qui bordait une propriété
pour laquelle il n’y avait pas d’autorisation pour l’installation des éléments d’ancrage. Les voûtes avec une portée de 6 mètres ont été
réalisées avec des colonnes « Mixed in Place » (MIP) qui prenaient appui sur des parois orientées dans la direction de la pente. Dans
le deuxième exemple, le comportement d’une palplanche dentelé a été examiné. Des analyses par la méthode des éléments finis 3D
compréhensives ont été effectuées afin de prouver que la structure de soutien proposée était une solution réalisable.
KEYWORDS: deep excavation, finite element method, three-dimensional analysis.
1 INTRODUCTION
The design of support measures for deep excavations is one of
the key tasks in geotechnical engineering and, depending on soil
conditions and adjacent infrastructure, many different options
exist. One of the most difficult situations to overcome is when
space for support measures is limited and due to legal reasons
support elements such as ground anchors cannot be built on
neighbouring ground. The obvious solution in these cases,
namely putting struts, is often not very convenient for the
excavation process and sometimes even not possible, e.g. if the
excavation is situated in a slope. These cases require special
attention and two case histories where innovative solutions have
been found are presented in this paper.
2 NUMERICAL ANALYSIS
In order to demonstrate the feasibility of the proposed design
and to assess expected deformations a number of three-
dimensional finite element analyses have been carried out.
These analyses also served as basis for the design of the
structural elements. The finite element code Plaxis 3D
Foundation has been used for all analyses presented in this
paper (Brinkgreve and Swolfs 2007).
It is well established that for this type of analysis simple
linear elastic-perfectly plastic constitutive models are not very
well suited and therefore a more advanced model, namely the
Hardening Soil model, has been employed. This model is a so-
called double hardening model and allows for plastic
compaction (cap hardening) as well as plastic shearing due to
deviatoric loading (friction hardening). The main features of
this model, as implemented in Plaxis, can be summarized as
following.
- Stress dependent stiffness according to a power law.
- Plastic straining due to primary deviatoric loading.
- Plastic straining due to primary compression.
- Elastic unloading / reloading.
- Failure according to the Mohr-Coulomb criterion.
A more detailed description of the Hardenings Soil model can
be found e.g. in Schanz et al. 1999.
3 CASE HISTORY 1 – MIXED IN PLACE COLUMNS
The first example is concerned with an excavation situated in a
slope, just below existing buildings. The owner of one of the
buildings was particularly alerted because he experienced
significant damage to his building in the past due to nearby
construction activities. He did not allow ground anchors to
reach his property. Thus the task was to stabilize the excavation
without ground anchors and at the same time provide sufficient
support to keep deformations, which could lead to damage of
the building located above the excavation, to an absolute
minimum. This could be achieved by arches of 6 meter span
