Holistic approach for the design of single piles and pile groups under cyclic loading
Long term load-displacement behaviour of piles under cyclic axial and lateral loading is essential for the design of pile foundations. The prediction of this behavior is linked to large uncertainties due to the distinct type of cyclic loading as well as the extremely high numbers of load cycles compared to conventional engineering structures. There is a need of a design approach being able to cover the soil behavior under cyclic and high-cyclic loading and the specific behavior of pile constructions. Existing design approaches show significant uncertainties already for monotonic loading. The major objective of the research project is the further development of a design approach based on an existing subgrade reaction method called SRMHYP, that is currently designed for lateral cyclic loaded single piles in sand. This model is planned to cover the following cases at the end of the project: single piles and pile groups, sand and clay, cyclic and high-cyclic axial and lateral lading, soil stiffness for small and large deformation, rate-independent behavior of sand and rate-dependent behavior of clay, effects due to pile installation and set-up. This extended model will be validated by means of centrifuge tests, numerical simulations, and existing results of field tests. All planned experiments, models and numerical simulations are designed for two different model soils, namely a fine uniform silica sand and a kaolin clay representing non-cohesive and cohesive soils. The results of the laboratory tests on sand and kaolin with cyclic loading are used as input parameters for the SRMHYP model, for further development of a stress-strain model for kaolin, for further development of high-cycle accumulation models for sand and clay, and for calibration of hypoplastic and visco-hypoplastic models for sand and clay. The results of the centrifuge tests on cyclic loaded piles in sand and clay under defined conditions are used to validate the SRMHYP model, and additionally the numerical simulations, because the tests are carried out under a realistic stress level. The numerical simulations of piles under cyclic and high-cyclic loading are based on the continuum approach and the FEM. Therefore, the further developed of the stress-strain model for clay and the accumulation models for sand/clay mentioned above are applied. Additional work on the contact model for the contact surface pile/soil as well as work on the user subroutines for the FE-program are necessary. The results of the numerical simulations serve as a basis for the abstraction of the simpler and less computer consuming SRMHYP model. The simulation models are validated by means of results of centrifuge tests and results from field tests.