Keywords: deep excavations, diaphragm walls, numerical methods, beam-on-spring methods, finite element method, ground movement.

The design of a deep excavation with diaphragm walls is a very complex problem that needs considerable knowledge of geotechnical and structural engineering as well as experience in engineering practice. There are many steps involved in this type of works. Going through them the designers need to make multiple decisions in order to include a very complex reality into their calculations. The data and constraints of the works will lead to the choice of a construction method. In deep excavations in urban areas, where buildings and facilities are very likely to be encountered adjacent to the limits of the excavation, concrete diaphragm walls are one of the preferred earth support systems. The dimensions of the wall will depend on soil properties, geometry, allowable movements and construction sequence (temporary and definitive strut levels, excavation phases, ground water drainage, etc.). All these aspects need to be definitely established and refined during the design.

Once the construction method is chosen, the design needs to model the problem through an adequate calculation method. The model has to be chosen so it can consider the complexity of the problem. Simple geometries in well known soils will allow the use of simpler beam-on-spring methods while complex geometries with complex constraints will need the use of two dimensional or even three dimensional finite element or finite differences methods. Soil investigations should be planned in order to obtain the data needed as input for the calculations. No matter how complex the model is, there are some aspects that are very seldom included in the calculations, like the effect of diaphragm wall installation or the effects of grouting in the surrounding area (when a bottom grout slab is used). In some cases these effects can be neglected, while in others they need very careful consideration. Once the construction starts, thorough monitoring is needed in order to verify the preliminary assumptions and to upgrade the design. If needed, remedial measures can be taken.

The paper presents the advantages and limitations of the multiple choices that the designer faces at any phase of the design and construction process. Also, the actual design trends are highlighted. Some case histories are presented, as examples, to illustrate some features of the design process.

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