The construction of underground structures in existing tunnel systems poses a number of challenges for planners. The construction records are often inadequate and an assessment must be carried out based on the existing condition. This thesis deals with a branch of the Presserstollen in the Styrian Erzberg, which is part of the Zentrum am Berg (ZaB). The objective is the geotechnical evaluation of an approximately 55 m long adit with a diameter of around 3.5 m, that is lined with steel arches and wood. The work focuses on the invert, because the unclear stability conditions prevented any measurements on or behind the lining. The methods used can be divided into three categories: laboratory tests on loose and solid rock samples, geophysics in the invert and evaluation of the deformation of the steel arches. Grain size distribution curves, abrasivity, electrical resistivity, p-wave velocity and strength parameters (triaxial test, point load test) were measured in the laboratory. In geophysics, electrical resistivity tomography (ERT) and refraction seismics were used to identify fault zones and the depth of the damaged zone. The tunnel deformation was assessed by closely observing and evaluating the deformation visible in the steel arches. The results of the laboratory testing show a clearly schistose rock, which has low strength and, depending on the orientation, shows strong anisotropy in behavior. The geophysical surveys show at least two fault zones that cross the tunnel and a damaged zone with a depth of around 4 m, which is less pronounced at both ends of the section. Two faults also appear in the evaluation of the deformation. These can also be assigned a Clar value. In addition, the lower stress in the footwall of the faults can be clearly seen, as can the transfer of the loads near the face. Overall, there is a high correlation between the methods used. In particular, the observation of the deformation provides a good indication of the geological structures encountered in relation to the effort required. On the other hand, the results are very dependent on personal assessment and experience, which requires a second method to verify the results. In this area in particular, further research results and more precise observation procedures could help to improve the quality of the data.