Abstract: Cyclic and monotonic loaded offshore structures (e.g. piles, pipelines, cables, and suction-buckets) must be designed and calculated considering the effects of the soil-structure interaction. An advanced constitutive soil model must be used, but a simplified Mohr-Coulomb friction law is applied for the soil-structure interface. The Mohr-Coulomb model is a simple bilinear friction model that cannot take into account monotonic and cyclic interface phenomena. These include hardening, softening, and stiffness degradation under loading. However, we propose two advanced hypoplastic interface models for fine- and coarse-grained soils with intergranular strain concept. The intergranular strain concept has been proven to be an efficient way to model the small-strain deformation and un/reloading behavior of soils. The intergranular strain concept is therefore used to enhance the prediction capabilities of the hypoplastic interface models. Differences of the recent model formulation compared with previous versions are presented and discussed based on simulation. This is followed by an application of different models in finite-element simulations. The new models are used to simulate cyclic direct shear interface tests. Furthermore, the advanced interface model is used for simulation of a torodial penetrometer penetration problem. Several aspects, which are particularly important for offshore structures, are compared with the simple Mohr-Coulomb friction model.

Key words: Hypoplasticity; Interfaces; Cyclic loading; Fine and coarse grained soils

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