For the installation and the requirements of sealing systems of tunnels the regulations of the guidelines for sealing systems of the Austrian Structural Engineering Association apply in Austria. Practical observations revealed that tunnel sealings which were installed more than 30 years ago show some damage. The causes of this damages are not evident. The present study examines the mechanical properties of the sealing system in the compound of a two-layered tunnel construction. For this purpose representative specimens were made, whose materials and processing have been chosen so that conclusions about the validity of the guidelines of the Austrian Structural Engineering Association are possible. To investigate the influence of the particle size of the seal carrier, two different shotcretes were used. First the effects of the fresh concrete pressure and of a stress rearrangement from the outer to the inner shell on the sealing system were studied in pressure tests. In long-term experiments the creep behavior of the sealing system was observed. Furthermore, the characteristic shear parameters of the shell composite and its resistance to the simultaneous influence of normal and shear forces were determined through shear tests. During the pressure tests no perforations of the sealing system which satisfied the requirements of the Austrian Structural Engineering Association have been found. The long-term tests revealed a strong temperature dependence of the deformation of the sealing system. The shear tests showed that the influence of the roughness of the seal carrier to the shear parameters was low. Furthermore, it was found that there might be a perforation of the sealing in the case of a combination of normal stresses and shear movements.