Keywords: 3D case study, finite elements, coupled analysis, retaining system optimisation, modelling-measurement comparison.

The numerical simulation of a large excavation for the construction of a watch production centre is described in this paper. A three dimensional non-linear coupled finite element analysis has been conducted with Z_Soil 3D v5 [1] in order to verify and optimise the designed retaining system as well as to predict carefully the settlements (as many manufactures stand in the surrounds). The excavation (Figure 21.1a) concerns a 145 165 m soil surface with a maximum excavation level of about – 20 m in very soft and saturated clays [2]. The designed retaining system is composed of a slurry wall braced at its top. The bracing leans on a 130 m diameter circular reinforced concrete beam supported by piles which are linked with a buried circular internal slurry wall located at the excavation's bottom.

A similar case [3,4,5] (large dimensions, same soil and hydrological conditions, similar execution steps and retaining system) constructed in the 1970's has been used as a real-scale test in order to define the soil parameters and the hydro- mechanical behaviour including transient effects with the help of a back analysis. A three dimensional mesh of about 10'000 elements has been used. Located near Lake of Geneva, the soil is almost fully saturated and therefore a coupled hydro- mechanical analysis has been made. A plastic "cap" model has been used for the normally consolidated clayey-silt soils. The retaining system components have been modelled with shell, beam and truss elements (Figure 21.1b). After the simulation of the excavation stages on a reference case, different characteristics of the model have been changed: slurry walls thickness and depth, bracing prestress, bracing level, presence/absence of secondary supporting walls, … The comparison of settlements and stress levels in the structure helped the project engineers to design and optimise the retaining system.

The paper begins with an introduction in section 1. The different modelling assumptions are described in section 2. This includes hydrogeotechnical conditions and finite element model characteristics. The main results of the study are then summarised in section 3. A reference case is described in details, followed by a parametric study. Finally, some conclusions are drawn. As the project is currently under construction, a brief comparison between in situ observations and numerical predictions is stressed out (Figure 21.2).

1

"Z_Soil 2002 User manual", Zace Services Ltd, Elmepress International (Lausanne), 1985-2002.

2

"Etude géotechnique", Géotechnique Appliquée Dériaz SA, Genève, 2001.

3

Dysli M., Fontana A., Rybisar J. "Enceinte en paroi moulée dans des limons argileux: calculs et observations". 7ème Conférence Européenne de Mécanique des Sols et de Travaux de Fondations (vol. 3), Brighton, 1979.

4

Fontana A. "Concepts de dimensionnement de deux fouilles genevoises". Publication No 104 la Société Suisse de Mécanique des Sols et des Roches, Genève, 1981.

5

Dysli M., Fontana A. "Déformations autour des excavations en terrain compressible". Ingénieurs et Architectes Suisses 12, 1988.

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