Geomaterials typically exhibit non-linear elasticity during deformations over significant ranges of shear strain. The design of geomechanical structures is normally carried out both for the ultimate limit state, in which the load-deformation behaviour may not be of significant interest, and for the serviceability limit state, where realistic modelling of the load-deformation behaviour of the soil is essential. A suitable estimation of soil deformations depends on the loading geometry and the soil properties, and consequently the idealization process is very significant in practical design. This idealization concerns both the material behaviour and the model geometry, and a suitable compromise between model refinement and simplicity should be the aim of the design process. Here the idealization procedure is demonstrated for a practical case study for a medium-risk project, in which the formwork of a concrete bridge was placed on soft clay covered with gravel rather than the standard solution using a piled platform for temporary use. The elastic properties of the clay were assessed from triaxial tests and empirical correlations, and the time-dependent deformation response during loading was simulated by a small-strain finite element model in 2D plane strain geometry. The calculated deformations are compared to field measurements, and the suitable idealization of field conditions and model soil types for similar cases of routine design for medium-risk projects is discussed.