Predicting the end of service life of an engineering structure, or obtaining parameters from inverse analysis of measured forces or displacements, is a complex task which requires deep knowledge of material behaviour and software development. In this paper it is shown how these two procedures can be carried out by writing a subroutine with the software MATLAB to run with predefined input data, pre- and post-processing a finite element model in PLAXIS 3D. The examples show how it is possible to predict the rest of service life of an airport pavement and to obtain layer stifness parameters from inverse analysis of a three-dimensional deflection bowl. The developed MATLAB routines allow the field of possible PLAXIS 3D applications to be extended considerably.