The estimation of an interference with the vault of an existing tunnel built of stone masonry regarding the effect on its load capacity poses a great challenge. Doing structural calculations mostly fail due to missing input parameters and on the complexity of representing a tunnel, which is subject to diverse influences over the years, in a structural model. In this thesis, the relevant influences and their dependency on the boundary conditions are illustrated. Further, an approach for the determination of the input parameters of a structural model are explained, using the example of the uniaxial compressive strength of a brick.