Numerical methods and specifically the finite element method have improved significantly since
their introduction in the 60's. These advances were mainly in: 1) introducing higher-order elements,
2) developing effective solution schemes, 3) developing sophisticated means of modeling the
constitutive behavior of geotechnical materials, and 4) introducing iteration techniques to model
material non-linearity. This thesis, on the other hand, deals with the topic of modeling the boundary
conditions of the finite element problem. Typically, the boundary conditions will be approximated
by specifying displacement constraints. such as restraining the bottom boundary of the finite
element mesh against displacements in the horizontal and vertical directions (x- and y-directions).
Where bedrock or dense residual soils underlie the soft foundation soil at a relatively shallow depth,
this is a good assumption. However. when soft soil is encountered for large depths, the assumption
of zero movement constraints for a mesh boundary at a shallower depth than the actual bedrock will
result in a serious underestimation of stresses and displacements. By coupling boundary elements to
the finite elements and using them to model the infinite extent of the foundation soil, a more
realistic answer is obtained. Employing the coupled boundary element - finite element method, four
cases were analyzed and the results compared to values of the pure finite element method. The
results show that the coupled method indeed yielded higher stress- and displacement-values,
indicating that the pure finite element method underestimates stresses and displacements when
modeling very deep soils.
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