Efficient performance of large infrastructure

Accurate and precise prediction of soil deformations is key to the performance of any construction project and in particular to modern infrastructure projects such as railways for high-speed trains which is a transportation solution in increasing demand. In these projects even small deformations may significantly reduce their serviceability. Soil formations with a complex deformation behaviour are therefore a major challenge when encountered at the alignment of such projects. Since they are often unavoidable, e.g., due to the cost of alternative alignments or the need to establish certain connections or reduce transportation time, a suitable framework for describing them is necessary.
High plasticity overconsolidated clays are one of such complex soil formations which are challenging geotechnical design due to their highly non-linear stiffness which is both stress, stress-path and time dependent. In addition they have a substantial content of the active clay mineral; smectite, which makes them particular reactive to the ion composition of their pore water. They have been the subject of substantial research, but with a general focus on compression and shear behaviour. However, the unloading behaviour of these clays is equally important for instance to cut and cover tunnels or excavations to level out road or rail alignments. The oedometric unloading behaviour of these soils is therefore the focus of this project, both in their natural conditions and with modified pore water composition.
Based on the clay, ”folded Røsnæs Clay”, a series of both incremental loading and constant rate of strain oedometer tests were carried out to consistently map the unloading behaviour under different conditions. This led to the development of a framework of swell modes which provide a conceptual description of the expected unloading behaviour. Additionally these tests revealed that the measured permeability of such clays contain a transient component which influences the response at sudden changes in strain rate. Further more it was established that the secondary deformations of this clay are correlated with both the time-dependent and independent primary deformations.
The potential for using modification of pore water chemistry as a means for changing deformations characteristics of this clay was investigated by resedimenting it in four different water environments and subsequently testing its unloading behaviour with and without changing the dominating cation between potassium and sodium as well as increasing or decreasing the cationic charge of the water. The clay clearly reacts to the water environment by displaying an increased pace of consolidation when dominated by potassium and for increasing charge. Additionally less deformations are accumulated during unloading when the pore water is dominated by potassium. A clear reaction to the resedimentation environment appears, while the subsequent change of pore water has less effect on the mechanical behaviour. However, by using the framework developed for the natural clay behaviour a logical pattern following the observations from the resedimented samples appears.
To enable the use of the experimental observations in engineering design, a series of existing constitutive modelling frameworks were analysed for their capability in representing these observations. Initial conceptual evaluation revealed that the elasticity formulation fundamentally produces an opposite response to that most commonly observed. The most sensible correction for this is through gradual plasticity for which a radial mapping rule proved superior to a bubble model. This is caused by the shape of the yield surface extending beyond the tension cut-off lines at low stresses, which makes the reference to a previous stress state necessary to avoid excessive plasticity in unloading. By calibrating the best performing model to a set of experiments, a correlation between its gradual plastic hardening parameter and the unloading response is proposed.