| Popis: |
There are a large number of settlement records in the literature showing that the creep settlement of shallow foundations on sand can increase over decades to a magnitude comparable to the immediate settlement. However, in most of the existing settlement prediction methods, the creep settlement of footings on sand is either ignored or accounted for using an arbitrary empirical multiplier, which has been identified as one of the major reasons for the poor predictability of these methods. The objectives of this study are to establish a method that can explicitly account for the time-dependent behaviour of footings on sand in a realistic manner and to explore its potential use in practice. To achieve the goal, the research included three parts, starting with understanding the time-dependent behaviour of sand that derives the time-dependency of footings on sand using the discrete element method (soil element scale), followed by the formulation and validation of the footing modelling method (footing scale), and the application of the proposed method in soil–structure analysis (structure system scale). At the soil element scale, the creep and aging behaviour of sand in triaxial conditions is studied using the discrete element method. The macro- and microscopic responses of the discrete element models of sand subjected to successive triaxial shear, creep and post-creep shear are studied and analysed. Increases in soil stiffness and strength are observed in the simulations as the consequence of imposing a creep period. The analysis of the stress–dilatancy behaviour reveals that the development of interlocking between sand particles during creep is the cause of these aging effects. At the foundation scale, an elasto-viscoplastic macro-element model that can simulate the time-dependent behaviour of footings on sand is constructed based on the non-stationary flow surface theory. The macro-element model with only three parameters is found capable of replicating the time effects of footings seen in practice, such as post-creep stiffening and settlements induced by unload-reload cycles. The data from more than fifty full-scale footing load tests are employed to validate the macro-element model, which confirms the applicability of the proposed method to foundations on sand under typical working load conditions. Guidelines are provided for the determination of model parameters based on cone penetration test results. Besides, the macro-element model is applied to formulate a settlement correction factor for creep. With the correction factor, the long-term settlement of a footing on sand can be predicted from the observed immediate settlement. At the scale of a structure system, centrifuge experiments are performed to study the soil–structure interaction in 3D-printed framed structures of different stiffnesses supported by spread footings on different soils. The magnitude of load redistribution induced by soil–structure interaction is found to be greatly influenced by the relative structure–soil stiffness. The effect of the time-dependent behaviour of foundations on soil–structure interaction is further studied based on a proposed numerical method that integrates the macro-element model with superstructure analysis. The method is validated with the results from the centrifuge experiments and applied to shed light on the time-dependent soil–structure interaction in the long term. |
