Civil Engineering Association

Structural Element Approaches for Soil-Structure Interaction

The emphasis within this study regards structural element models for soil-structure
interaction (SSI). The methods are compared and calibrated against an elastic
continuum modelled with solid elements, which in the study is used as the “correct”
solution. Main interest is the influence on results of simplifications in the method
often used today, with springs representing the subgrade (Winkler model). In the
study this model is modified to better capture the soil’s behaviour.
In a Winkler model the springs act independently of each other, while the soil in
reality is a continuous medium that also transfers shear stresses. To achieve a better
behaviour in a structural element model, different kinds of interaction elements are
included, which couple the springs.
The biggest shortcoming, identified in this thesis, for a Winkler model with uniform
foundation stiffness is that the soil around the superstructure is not taken into account.
This can result in major underestimation of the foundation’s stiffness towards the
superstructure’s edges which normally, at the edges, leads to conservative sectional
forces in the ground slab and unconservative ground pressure. It can also result in a
convex settlement profile, where a concave would be more realistic.
Surrounding soil can be taken into account by increasing the foundation’s stiffness
towards the superstructure’s edges, alternatively the different models described in the
thesis can be implemented. The different models are applied in two simple cases, one
representing a footing and the second a slab. The best correlation to the elastic
continuum was achieved by applying an interaction element between the springs in a
Winkler model that only transfers shear deformations. This shear layer model is also
evaluated in 3D for a case study of “Malmö Konsert, Kongress och Hotell”.
A simple method, for practical use in 2D, to determine this shear layer model’s
parameters is developed by the authors. Analyses indicate that the shear layer’s
stiffness can be determined independently of the superstructure’s geometry. Therefore
only the soil’s properties and depth is needed to determine the shear layer’s stiffness.
A relation for homogenous and elastic soil is presented to determine the stiffness. The
relation is based on 2D analysis and is not verified for 3D. Further study and
verification is needed to make the method complete for practical use.