Short-term deformations in clay under a formwork during the construction of a bridge : A design study

Sammanfattning: During the casting of a concrete bridge deck, the temporary formwork is causing the underlying ground to deform if a shallow foundation solution is used. There are often demands on the maximum deformation of the superstructure when designing the foundation for the formwork.  To keep the deformations within the desired limits, several ground improvement methods like deep mixing columns or deep foundation methods like piling can be used. Permanent ground improvement methods are however expensive, and far from always needed. To reduce the need for unnecessary ground improvements, it is crucial to calculate the predicted deformations accurately during the design phase. The purpose of this thesis was to investigate how short-term deformations in clay under a formwork during bridge construction should be calculated more generally in future projects. Three different calculation models have here been used to calculate the ground deformations caused by the temporary formwork. A simple analytical calculation and two numerical calculations based on the Mohr Coulomb and Hardening Soil-Small constitutive models. The three calculation models were chosen based on their complexity. The analytical calculation model was the most idealised and the Hardening Soil-Small to be the most complex and most realistic model. Results show that the numerical calculation model Mohr Coulomb and the analytical calculation model gives the best results compared to the measured deformation. One of the most probable reasons for the result is that both of the models require a few input parameters that can easily be determined by well-known methods, such as triaxial-, routine- and CRS-tests. The more advanced Hardening soil small model requires many parameters to fully describe the behaviour of soil. Many of the parameters are hard to determine or seldom measured. Due to the larger uncertainties in the parameter selection compared with the other two models, the calculated deformation also contains larger uncertainties.