Free field boundary conditions for PLAXIS 2D: new improvements and recommendations for practical use

Introduction

One of the greatest challenges in dynamic finite element analysis consists of the need to limit the theoretically unbounded wave propagation domain to a finite domain where waves are not unrealistically reflected at the boundaries. Concurrently, when the source of excitation is external to the model, such as in earthquake-induced loading, it is necessary to ensure that free field motion (i.e. dynamic motion in the absence of soil-structure interaction) is computed near the boundaries. In PLAXIS, free field boundary conditions can play this dual role. To improve their abilities, two modifications were recently introduced to the formulation implemented in PLAXIS 2D, one of them related to the equilibrium condition, while the other concerning the contribution of the boundaries to the damping matrix. The effects of the newly introduced modifications are explored in a case study concerning the response of two adjacent buildings resting on a saturated deposit of moderately loose sand subjected to an earthquake motion causing liquefaction. It is shown that: 

• As intended, the newly revised formulation improves the deformation pattern and excess pore pressure profile obtained near the boundaries.
• For the present case study, it is possible to reduce the model width without affecting the accuracy of the obtained solution, while significantly reducing the run time.

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• Free field boundary conditions for PLAXIS 2D: new improvements and recommendations for practical use.pdf

See also